September's arrival signals that meteorological autumn is under way. Weather records closed overnight on one of the area's truly peculiar summers. It ended with a string of daytime highs that failed to reach 70 degrees at a time of the year when upper 70s are common. An August that closes with five back-to-back days during which the temperature failed to break out of the 60s hasn't occurred here in 118 years. The summer season hosted 18 such days---more than triple the average of five such days.

Summer short on sun

But perhaps one of this summers most lasting legacies was its lack of sunshine. Summer 2009 goes down in the books as the cloudiest on record. Never since sunshine records began 115 years ago has a June-through-August period hosted less of the possible sunshine. Sunshine is measured in minutes and the summer's total was 42,887 of the season's possible 80,626 minutes of sun, or 53 percent of possible sunshine---67 percent is considered normal.

Days continue to shorten in September, taking a toll on temperatures. 120 minutes of sunshine disappears by month's end and normal highs fall from 78 to 69-degrees.


TROPICAL UPDATE

Powerhouse Hurricane Jimena with near Category 5 strength 155 m.p.h. sustained winds continues churning toward the Baja California peninsula. Cabo San Lucas is among the areas under a hurricane warning.
    

 

Dear Tom,
What are the largest and smallest tropical cyclones (by area) on record?
Tom Polivka Algonquin
 
Dear Tom,
Based on a National Hurricane Center study by Dr. Chris Landsea and updated by Neal Dorst, it appears that the largest tropical cyclone on record was Typhoon Tip which roamed the Northwest Pacific in October 1979. The storm had gale force winds (39 m.p.h. or higher) extending 675 miles out from the eye. The massive storm also holds the record for this planet's lowest sea level air pressure of 25.69 inches of mercury measured 520 miles northwest of Guam on October 12. The smallest tropical cyclone is thought to be Tropical Storm Marco that packed gale force winds that extended only 12 miles from the storm's center of circulation as it moved across the far south Gulf of Mexico on Oct. 6-7, 2008.
 

Chicago's high temperatures the last week of August have featured a steady string of early-October-like 60s, a level of late-summer chill not encountered here since 1965. After a potentially record-breaking low in the 40s Monday morning, the mercury may eke out a afternoon high of 70 degrees as one of the cloudiest, coolest meteorological summers in recent years comes to an end.
With high pressure firmly entrenched across the Midwest, the city can look forward to a stellar week of weather featuring sunny and warmer days as September begins. Temperatures will steadily climb through the week ahead, finally reaching the 80s by the weekend for the first time since Aug. 19.
No rain is expected in the foreseeable future, giving area soils a chance to dry out after last week's soaking.

Frost, freeze in Minnesota
A record-low 32 degrees arrived Sunday at International Falls, Minn., while Duluth logged a record 38. Frost advisories were posted for the Upper Midwest again Monday.

Dear Tom,
Wasn't the period from Aug. 17-24, 1947, a very hot one? Didn't the highs reach at least 95 every day?
Brian Beecher, Villa Park
Dear Brian,
August 1947 stands out as the city's hottest August on record, and the Aug. 17-24 period certainly helped it gain its lofty status.  The high temperatures did not quite reach 95 every day (Aug. 21-22 topped out at 94) but the other six days recorded highs of at least 97 degrees. The final day, Aug. 24, was the hottest of all with the mercury peaking at 100. Not only were the days hot, the nights were warm and muggy. With the exception of a morning low of 68 degrees on the morning of Aug. 17, overnight lows never dropped below 75 degrees.  Four record highs still remain from that period:  98 degrees on the 18th and 20th, 97 on the 23rd and 100 on the 24th.

The summer that wasn't may end with a record chill. As another shot of unseasonably chilly air settles over the Midwest, clouds will develop across the Chicago area Sunday. Coupled with brisk north winds and temperatures more typical of mid-October, the last Sunday of meteorological summer 2009 will have a definite fall-like feel. Lake-effect showers may bring some light rain or sprinkles to areas near the lake. Sunday's highs should struggle to reach the middle 60s and with low temperatures expected to tumble into the middle 40s early Monday, one of the city's longest-standing temperature records (47 degrees on Aug. 31, 1872) may be threatened. Temperatures will slowly rebound to seasonable normals later in the week as winds gradually become southerly. The area will get a chance to dry out after the recent soaking that brought more than 5 inches of rain to the hardest-hit locations. Except for the possible lake-effect showers Sunday, dry weather should prevail until week's end when the next frontal system approaches.

Dear Tom,
In a book about the St. Louis Cardinals' Gashouse Gang, the author mentioned that St. Louis experienced 30 consecutive 100-degree days during the 1934 season. Is that true, and was it also hot in Chicago?
---Alex Miller

Dear Alex,
The summer of 1934 was during the "Dust Bowl" and it was beastly hot in St. Louis; but not quite as hot as the author implied. St. Louis officially experienced 23 days of 100 degrees or higher that summer plus many more days in the 90s. There were eight consecutive 100s from July 18 to 25, including the season's highest reading of 110 degrees on July 24. It was also very hot in Chicago and most of the Midwest. Temperatures at Midway Airport topped the century mark 12 times that summer, including an all-time high reading of 109 on July 23 and six straight 100s from July 20 to 25.

Before this weekend is over, lake-effect rain showers will have swept sections of the Chicago area and a 137-year low temperature record may be replaced.
Powerhouse northwest upper winds--roaring into the Midwest at more than 100 m.p.h. at jet stream level (30,000 feet)--drive a sprawling Canadian high pressure into the area--a development which leads to unseasonably early frosts in northern Minnesota and Wisconsin in coming nights. Air as cool as this often ends up supporting more than the usual amount of cloud cover--especially in the warmer hours of the day.  Because of a steep temperature decline with height, generous morning sunshine heats the lower atmosphere which encourages air to rise and cool. Cottony cumulus clouds develop through this process and build into widely scattered shower producers. More numerous showers of the lake-effect variety--expected to first come ashore in southwest Michigan and sections of northern Indiana later Saturday or Saturday night--are likely to gradually build westward into the Illinois/Wisconsin shorelines by Sunday as winds veer north then north/northeast off the lake.
Lake clouds diminish Sunday night as the core of this weekend's cool air mass settles across the area. Periods of calm and the clearing skies may permit lows to fall below the 1872 record of 47 degrees by Monday morning.
It's anything but cool in the Southwest where excessive heat advisories are in effect. Temperatures Friday soared to 121 degrees at Death Valley, 118 degrees at Palm Springs, Calif., and Bull Head City, Ariz., 113 degrees at Phoenix and 109 degrees at Las Vegas.

Dear Tom,
What does a half-inch of rain mean?
Pat Broz
Dear Pat,
When we say that one-half inch of rain has fallen, we mean a blanket of water exactly 0.50 inches in depth would cover all horizontal surfaces if none of the rainwater had soaked in, run off, evaporated or been blocked by nearby or overhead obstructions such as vegetation, wires or buildings.
Rainfall is measured and reported in hundredths (0.01) of an inch, an increment too tiny to measure directly. Rain gauges therefore employ the principle of multiplication. In a standard gauge, rainwater falls into a circular collection area (a few inches in diameter) whose area is exactly ten times that of an accumulation tube into which it drains. Each one-hundredth inch of rainfall therefore stands one-tenth inch deep in the accumulation tube, and that water is easily measured with a ruler graduated to tenths of an inch.

Rain will continue pouring down on many sections of the waterlogged Chicago area Friday morning. Rains will finally lighten up and scatter Friday afternoon, but area rivers are on the rise; and the 1 to 3 inches of additional rain expected overnight at many locations comes on top of stunning late Thursday storm totals that included 4.63 inches in Genoa, 3.38 inches in Northbrook, 3.19 inches in Waukegan, 2.83 inches in Palatine, 2.83 inches in Glenview and 2.81 inches in Darien. The latest deluges pushed August rain tallies to 7.20 inches at South Elgin, 5.90 inches at Flossmoor and 5.64 at Darien.

The system behind rains here drenched eastern Iowa Thursday. In New London, just across the Illinois line in southeast Iowa's Henry County, 8.34 inches fell---7.20 inches of it in just four hours. That's equivalent to receiving a fifth of Chicago's annual precipitation in that short period of time.

By late Thursday, clouds and precipitation had dropped temperatures across the northern suburbs into the 50s---levels more than 20 degrees below normal and more typical of early November.
 
Summer 2009 on its way to become Chicago's cloudiest on record
 
Meteorological summer has just 4 days to run and appears on track to become the cloudiest here since sunshine data began in 1894. Veteran Chicago weather observer Frank Wachowski reports the summer has hosted only 53 percent of its possible sunshine---far below the 67 percent considered normal. Only three other summers come close----1915, 1960 and 1992 with 54 percent of their possible sun.

Dear Tom,
My husband says that Chicago and Oak Park have never been hit by tornadoes. Is he right?
Kelly McCreary
 
Dear Kelly,
No portion of the Chicago metro area is immune from tornadoes and it should be noted that four twisters just occurred in the area during the recent severe weather outbreak on  Aug. 19, 2009.  At least a dozen tornadoes have been documented  within the city of Chicago, including one in the downtown area in May 1876. The tail end of the deadly Oak lawn tornado traveled through the South Side on April 21, 1967 and another twister tore up portions of the South Side on March 4, 1961. Your husband is correct that Oak Park has not recorded a significant tornado strike, but twisters have touched down in nearby areas like Maywood, Melrose Park, Bellwood and North Riverside.
 

August's infamy as Chicago's wettest month will be on full display Thursday. The area remains precariously positioned at the heart of a region from Iowa east to Indiana that was swept Wednesday by waves of soil-saturating rainfall. Additional downpours, some of them thundery, are to continue into Friday. Rainfall hit 3.50 inches about 3 miles southwest of Rockford on Wednesday and 2.42 inches in Rockford itself while the near 1-inch tally (0.87 inches) at Chicago's O'Hare International Airport marked that site's heaviest one day rainfall in more than two months. The flash flood watch hoisted late Wednesday continues through at least 7 p.m. Thursday and rainfall estimates off 23 computer projections Wednesday suggest coming downpours by week's end may produce totals approaching 4 inches. One forecast off the European Center's global model places potential 5-day totals in parts of northwest Illinois in excess of 8 inches. That area's August rain tallies are already at 6 to 8 inches--more than 3 inches above normal at a number of locations including Dubuque, Cedar Rapids and Burlington, Iowa.
 
The heavy rain threat comes into being as air piles up in the midst of converging winds along a stalled front. The process forces moisture-laden air to rise and cool. Upper atmospheric conditions encourage the air to keep rising. As winds accelerate into the jet stream, air from below is drawn aloft.  Moisture levels near 2 inches make the situation especially dicey since thunderstorms form when this happens and produce uniquely heavy downpours.

TROPICAL UPDATE

Poorly organized, northwest-bound Tropical Storm Danny was churning the Atlantic northeast of Haiti and the Dominican Republic with 45 m.p.h. winds late Wednesday. The storm is to turn north in coming days---but perhaps not soon enough to avoid flirting sections of the East Coast. North Carolina and New England residents have every reason to be cautious. The storm is predicted by some hurricane models to possibly come ashore in New England after brushing North Carolina's Outer Banks.
 

Dear Tom,
How did the word "depression" get associated with hurricanes?
David Ligare
 
Dear David,
When an area of disturbed weather in the tropics begins to acquire a circulation but has top wind speeds of less than 39 m.p.h. it is classified as a tropical depression. The term depression is related to the low barometer readings at the center of the circulation which is depressed when compared to the surrounding air pressure. According to the American Meteorological Society's Glossary of Meteorology, the word depression can be applied to any area of low pressure or trough not just to tropical systems. If the tropical depression starts to strengthen, it becomes a named tropical storm when sustained winds reach 39 m.p.h. and a hurricane when speeds reach 74 m.p.h.
 
 

Tuesday's 80s are long gone, as absent as the sunshine and southwest winds that helped propel city highs to 83 degrees at O'Hare International Airport and the lakefront and 82-degrees at Midway Airport. A southward moving, wind-shifting cool front that ignited thundery downpours Tuesday across sections of southern Wisconsin, including the Milwaukee area, is to stall south of Chicago and hold there the remainder of the workweek. Winds converge along such a front producing a pileup of moist air that is forced to ascend and cool. Waves of showers and embedded thunderstorm erupt in this environment with some regularity and threaten in coming days to produce multiple deluges across a narrow swath of terrain from Iowa across northern and central Illinois and Indiana. The recurrent nature of these rains in combination with light winds steering the storms aloft and an atmosphere dripping with 1.50 to 2 inches of evaporated moisture just south of the front raises the specter of additional slow moving downpour-generating thunderstorms capable of some impressive rain totals.

What happened Tuesday in Wisconsin illustrates what can take place in this type of atmospheric setup.  Thunderstorms swamped River Falls with 2.45 inches while unleashing 1.30 inches on sections of Milwaukee and 1.00 in north suburban Port Washington.

Three day rain projections for the Chicago area vary widely among the 20 most recent computer estimates generated by a suite of forecast models Tuesday ---they range from as little to  0.52 to as much as 2.98 inches.

The return of clouds and the prospect they will dominate the next three days has been a common theme this summer. Since June 1, Chicago has received just 55 percent of its possible sunshine---far less than the 67 percent considered normal---and enough to make the summer season to date Chicago's cloudiest in the 17 years since 1992.
 

Dear Tom,
A friend who lives near Glasgow, Montana, tells a wild, unbelievable weather story. He claims, once upon a time, the temperature in Glasgow shot up into the 90s in the middle of the night. 90s at night? Maybe in Las Vegas, but in Montana?
Ron Smithson

Dear Ron,
Believe him. The event in question took place on Sept. 9, 1994, in Glasgow. It was an odd atmospheric event known as a "heat burst" and it occurs near thunderstorms.

Thunderstorms usually produce cooling gusts of rain-chilled wind, but on rare occasions a parcel of dry air is pushed from an altitude of 20 or 30 thousand feet down to the surface, warming by compression all the way down. On that Sept. 9, Glasgow's temperature at 5:02 a.m. was 67 degrees. A thunderstorm heat burst shot the reading to 93 degrees at 5:17 a.m., but by 5:40 a.m. it was back 68 degrees.
 

Mike Kellems sent us this beautiful sunset photo taken on the east shore of Stone Lake in La Porte, Indiana Saturday evening.

 Thanks for the great shot Mike!


 

It gets harder for the atmosphere to warm this time of year. By late August, shorter days deliver just 80 percent of the solar energy that arrived when summer began in June. The area's longest day, June 20, had 107 more minutes of daylight than will occur Tuesday. But gusty south winds take up some of the slack, transporting warmth and moisture into the area in the day's abundant sunshine. Though Tuesday's predicted 88-degree high isn't unusual, it comes on the heels of the coolest three-day late August stretch in five years. Highs Friday, Saturday and Sunday reached 73, 68 and 74 degrees respectively after nighttime lows slipped into the 40s inland--close to record levels. Monday recovered to 79 at O'Hare International Airport. But the combination of sun and southerly wind Tuesday propels highs to the mid and upper 80s---the warmest levels here 10 days.

Stage set for waves of thundery rainfall, 1-2 inch totals by week's end

Large sections of the Chicago area remain dry in August. At O'Hare, just 2.14 inches of rain has fallen all month---the driest in 10 years. But clashing air masses could change that in coming days. Upper winds will parallel a southbound front, slowing it to a crawl Wednesday. Clusters of storms will flare along it. North suburban and Wisconsin-border communities appear first in line for storms late Tuesday, and clusters of storms should settle into more of the Chicago area by Wednesday morning. More waves Thursday into Thursday night could reignite thunderstorms here.

Tropical Update

Tropical forecasters late Monday were monitoring an area of disturbed weather in the Atlantic 300 miles east/northeast of the Leeward Islands. The system has the potential for development and could become Danny. 


 
 
 

Dear Tom,
How big is Lake Michigan and how does it affect the weather?
Janet
 
Dear Janet,
Lake Michigan is the third largest Great Lake (after Superior and Huron) and the world's sixth largest freshwater lake.  It boasts more than 1,600 miles of shoreline and covers more than 22,000 square miles. It is more than 300 miles long and nearly 120 miles wide. The average depth is about 280 feet and its deepest part (925 feet) is mid-lake east of Manitowoc and Two Rivers, Wis. The lake has a great deal of influence on the surrounding area's weather. Among the most important; it keeps areas near the lake cooler in summer and warmer in winter, extending the growing season by several weeks as compared to inland areas. It causes more cloudiness and generates both lake effect rain and snow in the cold season but often suppresses thunderstorm development in the warm season.
 

Wally Baluk sent us this picture of storm clouds in Henderson, Nevada.

Thanks for the picture Wally!

Temperatures should warm into the 80s Monday and a 90-degree high is possible Tuesday as southerly winds invade Northeast Illinois for a two-day stay. The upper-air steering-level winds that have been persistently out of the northwest for days have temporarily shifted more west to southwest. This change will allow the northern edge of very warm air that has been resting over the southern Plains to finally reach the Midwest. The west to southwesterly flow aloft extends to the Northeast coast and into the northern Atlantic, accelerating the movement of weakening Tropical Storm Bill tracking just to the south of Newfoundland and then east out into the northern Atlantic later Monday.

Cooler temperatures return midweek
The passage of a cold front Wednesday signals the shift of the upper-air pattern back to a northwesterly flow which looks to be a semi-permanent fixture the remainder of this week and next. There are indications the front may stall in central Illinois, which could mean extensive cloudiness over Chicago and a good chance of showers or thunderstorms -- especially in southern sections -- Thursday into Friday.

Dear Tom,
What was the most destructive hurricane that ever occurred?
Luke Budil, Grayslake
Dear Luke,
Dealing solely with U.S. hurricanes, Katrina is the hands-down winner in terms of dollar damage, estimated at more than $84 billion when it devastated New Orleans in August 2005. The Galveston Hurricane that roared ashore in September 1900, leads in loss of life, with fatalities estimated from 8,000 to 12,000.
Expanding the question to tropical cyclones around the world, the Bhola Cyclone that struck Bangladesh (then east Pakistan) in November 1970, ranks as one of the world's all-time deadliest storms. Making landfall on Nov. 12, that killer storm packed 125 m.p.h. winds and sent inland a devastating 15-20 foot storm surge that inundated the area's low-lying landscape. The incredible death toll: at least 500,000 but possibly as high as one million.

The recent upper air pattern has featured a strong northwesterly flow steering cool Canadian-source high pressure into the Great Lakes and Midwest. A brief respite is in store as the flow becomes south to southwesterly Monday, allowing warm and increasingly more humid air from the southern plains to surge northeast into the Midwest. Chicago's temperatures should warm well into the 80s Monday and most likely reach into the lower 90s Tuesday. The warmer air mass will hold more moisture which will combine with an approaching cold front to trigger showers and thunderstorms later Tuesday night and Wednesday.

Hurricane Bill triggers heavy rains/flooding and high surf along northeast coastline

Squeezed between a cold front approaching from the west and moisture-laden easterly winds from Hurricane Bill just 150 miles off the coast Saturday, strong thunderstorms developed and dumped 3 to 7-inch downpours causing extensive flooding from Maryland north to Maine. Surf was expected to hit 14 to 18 feet along the Massachusetts/Rhode Island coastline later Saturday night and early Sunday. The storm is forecast to move northeast and weaken Monday as it tracks over the cool northern Atlantic waters east of Newfoundland.

Dear Tom,
It used to be that Death Valley was exempt from the daily national temperature extremes as it was usually the hottest. Now that has changed and it is listed as the hottest. Why the change?

Don Anderson, Elburn

Dear Don,
Nothing has changed, except that it's summer and Death Valley is broiling. The National Weather Service, which provides the daily listing of national temperature extremes, has strict eligibility guidelines: Only locations in the contiguous 48 states are considered (thus omitting Alaska and Hawaii); the site must lie at an elevation below 8,500 feet (lofty mountains are exempted); the report must come from an area with a population of 1,000 or more. However, if the temperature in a sparsely populated location (like Death Valley) tops 120 degrees, the population requirement is waived.

Summer 2009's weather has certainly taken some interesting twists and turns -- the latest, abnormally cool temperatures and the potential for unseasonable lake-effect rain showers. Lake clouds and precipitation form when temperatures drop 20 or more degrees in the first mile of the atmosphere at the same time incoming winds spend enough time over water to grow moist. Both conditions are met Saturday and Saturday night into Sunday morning -- but the key to determining just where any lake rains may fall is linked to the wind direction. North to northwest winds began blowing down the length of the lake into sections of northwest Indiana overnight, making that area the prime candidate for the first of this weekend's lake-effect rain showers as Saturday gets underway. But winds are to veer north/northeast Saturday afternoon and night, which should permit lake clouds and any rain showers to begin shifting slowly westward. These showers are likely to come and go -- with dry periods between any rainfall. And, it's unlikely these rains will affect inland areas like the Fox Valley, DeKalb or Rockford. But residents of shoreline counties of Illinois and Indiana should be aware that some showers could be on the way. It's possible a few lake-effect showers may even make it as far south as eastern Will, Newton and Jasper counties.
Huge swells emanating from northbound Hurricane Bill -- the tallest of them 14 to 18  ft. high--are to pound areas of the Massachusetts and Rhode Island coastlines Saturday. These areas protrude farthest east into the Atlantic and are therefore most vulnerable to the storm's wave action.
   
Friday's 73-degrees: August's chilliest and coolest in a month
Friday's 73-degree high at O'Hare Airport was the site's coolest temperature in more than a month -- since a 71-degree high July 19. Saturday's predicted 70-degree high is a late September-level reading. Strong warming could see Chicago temperatures flirting with 90 degrees by Tuesday.

Dear Tom,
I was born on Labor Day, Sept. 7, 1953, and my mother likes to remind me of how hot it was (no air conditioning) just before I was born. What caused the heat wave and when did it finally break?
Beth Swanick
Dear Beth,
The days leading up to your birth set new standards for late-season heat in Chicago.  With the jet stream far to the north in Canada and high pressure anchored over the eastern United States, Chicago was locked in a record-breaking heat wave.  The city recorded 11 consecutive days of 90 degrees or higher from Aug. 24-Sept. 3 that included back-to-back highs of 101 on Sept. 1-2. The hot weather established six straight record highs from Aug. 29-Sept. 3, and all remain in effect today.  The heat wave finally broke on Sept. 4 when a cold front passed, dropping the high temperature to pleasant 76 degrees.

The photos below illustrate the extent of tornado damage in Elburn -- the result of an EF1-strength twister that occurred just north of Elburn at about 6 p.m. on Wednesday, according to a National Weather Service survey team.

Matthew Cumberland took these photos Thursday, who spoke to a resident who was at home when the twister struck. Cumberland said before the air raid sirens went off, that resident's "glass patio table blew up and shattered his back window. At that time he knew it was at the point he needed to take shelter in the basement. The home owner was gracious enough to let me take photos of the damage around the home, and I wanted to share these with you."











Jason Rourke took these photos about a half-mile north of the intersection of Illinois Routes 47 and 38 on Thursday.





Thanks Matthew and Jason for your timely and informative photos!

Here are some great photos of Thursday's rainbow, starting with Noelle Bender of Aurora, who took this photo at 7 p.m. Thursday, Aug. 20.



Mike and Chris, also from Aurora, sent us this wonderful picture. If you look closely to the left, you'll see a faint "double rainbow."



Finally Kim Lucarz, another Aurora resident, seemed to discover the "end of the rainbow" right in front of her!



Thanks to all for the great photos!

Here are a few great cloud-and-sky related photos that we've received from our viewers. The first two are from Angela Fink of Wheeling, who captured these very unusual cloud formations north of Lake Geneva during the storm event of Aug. 9.





Tem Hunter sends us this great photo of Chicago's skyline and the clouds beyond.

We received dozens of great viewer photos from this week's storms. If you have a weather photo you want to share, join our WGNtv.com Community at http://community.wgntv.com, upload your weather photos and select "Weather Photos" as the category.

Thanks to all of you who are already part of the WGNtv.com Community!

Friday's thick, sporadically showery overcast and the northwest winds blowing beneath it set the stage for one of Chicago's coolest air masses in more than a month. The sub-72-degree highs predicted over much of the area Friday and Saturday would mark the first time a set of comparably cool late August days has occurred here in 15 years.

Computer trajectory forecasts, which track the path which air masses follow into the area, indicate Friday's late September-level low 70 degree maximum temps originated 860 miles to the north over Canada's Ontario Province only yesterday. An anomalously strong late summer northwesterly jet stream is behind the southward flood of cool air which aided the eruption of powerful mid-week thunderstorms. National Weather Service survey teams confirmed Thursday  four tornado touchdowns from those  storms---the most damaging of them was the EF2 intensity twister with 120 m.p.h. top winds which ravaged a swath 60 yards wide and 2.5 miles in length in Chesterton, Indiana. Two weaker twisters dipped into northern Kane County near Elburn and still another touched down 3 miles north of Watseka in Iroquois County.
   
Swells produced by mammoth Hurricane Bill fanned out across 2,800 miles of the Atlantic Thursday, from the Southeast U.S. Coast east to Puerto Rico and the Leeward Islands.  The storm, huge by hurricane standards, was producing tropical strength winds which extended 260 miles from the storm's center late Thursday. At the time Bill was located 975 miles southeast of Cape Hatteras, North Carolina. If Bill's eye was placed over Chicago, its 39+ m.p.h. tropical storm force winds would extend from the Mississippi River east to Detroit, and hurricane force (74+ m.p.h.) winds from west to Rockford to Grand Rapids, Mich. Though the storm is to churn north and avoid direct contact with the U.S. mainland, huge waves are to batter the Mid-Atlantic and Northeast Coasts and the Canadian Maritimes this weekend.

Dear Tom,
The hurricane season has finally come to life, but might we hope that it will be a quiet season because it has taken this long for it to get going?  
William Stanton, Naples, Fla.

Dear William,

Unfortunately, hurricane statistics from past years do not support that rationale. Dr. Max Mayfield, former Director of the National Weather Service's National Hurricane Center tells us, "It's not unusual to have a late start, and the truth is that there is no good correlation between when the season actually starts with that first named storm and how much activity we ultimately have."

 Hurricane Andrew is a case in point. That catastrophic storm, first of the 1992 season, was not named until Aug. 17. It smashed across southern Florida with category 5 intensity (156+ m.p.h. winds) and became the nation's second costliest hurricane with damage set at $26.5 billion (behind only 2005's Katrina, $81 billion).
 

This video of Wednesday's devastating Chesterton, Indiana tornado was forwarded to us late Thursday from its photographer Paul Ronjak. A National Weather Service survey team investigating the widespread damage in the northwest Indiana community confirmed Thursday that a tornado--not straight line winds---had caused the destruction in Chesterton. The team categorized the damage as consistent with an EF2 twister possessing peak winds of 120 m.p.h. Its damage path was 60 yards wide and 2.5 miles long, according to NWS investigators. Paul Ronjak tells us this video of the storm was shot near Indian Boundary Road and Rt. 49 as the storm churned into Chesterton downing huge trees and ripping roofs off buildings. It clearly shows the rotating wall cloud beneath which the tornado developed. A Weather Bug sensor in Chesterton had measured a 105 m.p.h. gust as the storm hit Wednesday evening before going off line. MANY THANKS to Paul for sharing this with us! It's a remarkable piece of tape! Tom Skilling

Robert Bober sent us this pictures of downed trees in Chesterton. Thanks Robert!

Justin J. Bobin shares these photos of towering cumulonimbus (thunderheads) he took right after damage was reported with the storms which swept Chesterton, Indiana.  Justin tells us the photos look northwest toward Michigan City! Radar scans revealed these storms' cloud tops towered to heights of 53,000 ft.

Great shots, Justin!  You're great for sending them along to us!
 
Tom Skilling
 
Photos courtesy of Justin J. Bobin

Our friend---and talented photographer---Amanda Pickett captured these towering cumulonimbus clouds from her vantage in Winfield, Indiana.  A great shot, Amanda!  THANKS for sharing this with shot with us!
 
Tom Skilling
 
 
Photos courtesy of Amanda Pickett, Winfield, Indiana
 

Storm chaser Matt Piechota shares these 3 p.m. Wednesday shots of tornado-induced damage just east of Elkhart, Illinois southeast of Lincoln. Matt tells us:

"We were chasing the super cell and were traveling east of Rt. 10 and ran into the damage path of the tornado from about 20 mins before we got there..
 We took some pictures and continued to the storm and ended up seeing two 2 tornadoes."
 
Great shots!  Thanks Matt!
 
Tom Skilling






 









Photos courtesy of Matt Piechota
 

Paul Curtis of Hobart, Indiana sent us this picture of the  storms retreating to the east towards Porter County

Matthew Cumberland, in describing this storm, tells us:

"I had just entered Huntley IL as Tornado Sirens started to blare. This was a false warning I believe, but always safe than sorry. It was a fast moving system, and low lying scud clouds that could have resembled funnel clouds. However, I did manage to snap a few photos until my chasing partner received a flat tire and had to call off the chase. I wanted to share some photos for you, as well as inform you I had revisited the tornado reported area of 47 and Barrington Rd."
 
Your shots are always terrific, Matt!  Thanks for sharing them with us!\
 
 
Tom Skilling
 



Photos courtesy of Matthew Cumberland
 

Jason Cales has sent us these shost of Wednesday's Chesterton, Indiana storm taken from about a mile away.  Great shots, Jason!! THANKS for sharing them!
 
Tom Skilling

 


Photos courtesy of Jason Cales, Chesterton, Indiana

This spectacular shot of Wednesday's devastating Chesterton, Indiana storm comes to us from Chuck and Michelle Cota.  This appears to be a wall cloud---the isolated lowering of the thunderstorm cloud base beneath which the rotary winds of a tornado often spin up. Sections of Chesterton suffered serious damage in the storm.  A Weather Bug wind sensor there registered 105 m.p.h. before failing---and National Weather Service meteorologist Paul Merzlock indicates a compact circulation was evident with the storm on Doppler radar---an indication of rotation. Thanks for sharing this remarkable photo with us Chuck and Michelle! We hope you're safe in the wake of Wednesday's storm!
 
Tom Skilling

 
Photo courtesy of Chuck and Michelle Cota, Chesterton, Indiana
 
 

It was just last week when we lamented the lack of rain from mid-July through mid-August.  The first half of this month started out with about half the rain we normally see.  We made up for that a bit yesterday, some areas more than others.  O'Hare's .31" or rain yesterday brings the months total to 1.91".  That's better but still about an inch below average for the month so far.

Here are some heavier rainfall amounts from Wednesday according to our WeatherBug network:

1.90"  Flossmoor

1.11"  University Park

1.00'  Burlington

.98"   Markham

However, the rain was accompanied by some severe weather.  Spotters reported tornadoes near Elburn and Chesterton.  There were almost a dozen tornadoes reported across the midwest according to the Storm Prediction Center in Norman, Oklahoma. 

We are not out of the woods yet.  The SPC has a portion of our area outlooked for a slight risk of severe weather today.

 

 Teams of National Weather Service meteorologists will fan out in parts of Illinois, including the Chicago area, Thursday to survey damage produced by a swarm of powerful thunderstorms late Wednesday. The storms unleashed devastating, possibly tornadic winds in Downstate Williamsville--15 miles northeast of Springfield. By the time the storm had passed, the northwest quadrant of the community lay in ruins. Sangamon County Sheriff Neil Williamson reported 16 of the 38 damaged structures, which included homes, a church and several businesses, were destroyed by the winds which hit around 3:15 p.m.

 In the Chicago area, Doppler radar indications of a strong circulation near Elburn in Kane County just after 6 p.m. were followed by a report of a tornado touchdown by storm spotters. A number of downed trees and power lines forced closure of Illinois Highway 47 between Illinois Highways 38 and 64. Rainfall was torrential with a number of the storms. Mendota in LaSalle County was swamped by 2.49 inches of rain in just 30 minutes while 1.90 inches poured down on Flossmoor.

Wind gusts reached 80 m.p.h. at Streator and 60 m.p.h. at Pontiac, downstate Decatur and Joliet. An unconfirmed 105 m.p.h. gust registered by a Weather Bug wind sensor in Chesterton, Indiana coincided with reports by police there of extensive damage including downed trees and roofs blown off buildings. National Weather Service Doppler radar scans revealed a small but vigorous circulation in the area at the time of the measurement.

Dear Tom,
I've heard that fireflies are decreasing in numbers, but in early August the fields south of Davenport were aglow with them. Why do they flash? Does the weather affect their flashing?
Steve Pond, Chicago

Dear Steve,

Firefly numbers worldwide have declined precipitously in recent years, but the cause is unknown. All 2,000+ species of fireflies have one thing in common: bioluminescence, the ability to produce visible light, but does the weather have anything to do with it?
 
Dr. James Case of the Department of Ecology, Evolution and Marine Biology at the Univ. California, Santa Barbara, says, "Behaviorally, luminescence in fireflies functions in mating, with each species tending to have a distinct communication flash code." The timing and sequence of flashes enable females and males to communicate with each other in preparation for mating, and weather has nothing to do with it.
 

Chuck Hagen sent us these pictures of Chuck tells us:

"Once again a busy day of severe weather as these shots depict. today in Iroquois County near Clifton, Illinois, the storms brought heavy rains as well as some cells with heavy rotation prompting tornado warnings to the area. Although these shots were of a rotating cell, no funnels or tornados touched down in the immediate area luckily."

Thanks for the photos Chuck!

Justin and Brittany Wren and Jordan Olson sent us this picture taken around 5 p.m. Wednesday in Coal City, IL. 


Photo courtesy of: Justin and Brittany Wren and Jordan Olson, Coal City, IL

 

Matt Baranowski sent us this picture of the pouring rains in Chicago Ridge.

The week's second outbreak of thunderstorms looms late Wednesday into Wednesday night---and even may linger into portions of Thursday. Not only is the amount of moisture available for thunderstorm formation predicted to surge (doubling in the 24 hour period from 0.85 inches at 1 a.m. Wednesday to 1.87 inches at 1 a.m. Thursday), but the surprisingly muscular jet stream, with its 100 m.p.h. winds more typical of more energetic fall and winter jet streams, is expected to settle south from Canada into the Midwest.  Just how these strong upper-level winds interact with the abundance of tropical moisture will have a significant impact on the strength of thunderstorms which spring to life.

When thunderstorms tower into the fast winds aloft, they are often fast-movers, capable of tapping upper level wind energy and transferring it to the surface as strong gusts. The presence of a jet stream also supplies a series of impulses that can lift and cool the air, initiating thunderstorm formation. The variation of wind speed with height when jet streams are present can encourage thunderstorms to spin and contribute to their intensity.

Summer rain totals are notoriously widely varied---and there is no reason to believe that won't be the case with the incoming storms, expected to first reach the Chicago area later Wednesday, then to sweep across the area in several distinct waves Wednesday night into Thursday. While estimates of rainfall vary from 0.39 to as much as 2 inches the next few days, an average of the ten most recent computer rain projections comes out to 1.18 inches. 

Dear Tom,
A few years ago you stated we were in a 25-year period of numerous hurricanes and each year would be well above average, yet only the 2005 season was above average. Are you still predicting above-average activity?
Benjamin Hoekstra 


Dear Benjamin,

National Weather Service researchers have learned that hurricane activity in the North Atlantic Basin fluctuates alternately through periods of diminished and heightened hurricane numbers, each approximately 20 to 40 years long. A period of increased activity that began in the middle 1990s is likely to persist into the 2020s. That prediction addresses hurricane activity averaged over the entire active period, not specific years. Tropical cyclone activity since 1995 has averaged 15 named storms per season and almost every season (not just 2005) has exceeded the long-term annual average of 10 named storms.
 

Douglas Hanbury of Peoria sent us these great shots.
Thanks Douglas!

Mary Wallace sent us this beautiful shot. Thanks Mary!

Bill Kondellas was at the Cubs game Sunday and sent us this shot. Thanks Bill!

Photo courtesy of Bill Kondellas

Smooth sailing today with more sun & less humidity.  Tomorrow is another story.  The Storm Prediction Center has outlooked the southern half of our area for a slight risk of severe weather.  A warm front will be pushing northward toward Chicago during the afternoon and good trigger some strong storms.

Snakes, not storms, are the problem down in Texas.  The southern third of Texas is in an "extreme" to "exceptional" drought.  There has been an increase in poisonous snake bites that may be attributed to the dry conditions.  It seems the snakes normal sources of hydration have evaporated so they now are venturing onto watered lawns.  A rattlesnake can bite twice.  There is the initial bite then the bite in the wallet.  A vile of antivenom can cost over $1000.

 Sunshine returns Tuesday in the wake of drenching thunderstorms which, for the hardest hit sections of the Chicago area, meant two-day rains in excess of 2 inches. The thundery rains marked an end to the driest mid-July to mid-August period here in the 75 years since 1934. Heavier two-day totals in the metro area included 2.85 inches in Sandwich in DeKalb County, 2.08 in Naperville and 1.96 at Lisle. But those totals paled in comparison to those tallied Monday after a series of thundery cloudbursts swamped in north-central Indiana. That region bore the brunt of Midwest storm outbreak Monday. Parts of the Goshen area were hit by seven waves of downpour-generating thunderstorms that drenched a few locations with more than a half-foot of rain in just 12 hours---more than what typically falls in all of August. Near Leesburg, Ind., 6.45 inches topped rain gauges while 5.43 inches was measured near Syracuse, 4.79 inches in Goshen and 4.25 near Warsaw, Ind. Rainbows were spotted over many sections of the Chicago area as the setting sun interacted with a band of retreating rains.

Bill gaining strength

Hurricane Bill continued strengthening far at sea in the tropical Atlantic Monday. The storm, situated 975 miles east of the Cape Verde Islands, is headed for major hurricane status---but is on a track that would keep it away from the U.S.

Dear Tom,
Summers in the 1930s were very hot. What was the tally of "century days" in
those years?

Paul Sarewich, Chicago

Dear Paul,
When it comes to 100-degree occurrences in Chicago, the decade of the 1930s yielded a bumper crop of 100-degree days that is unrivaled by any other decade in the city's 139 years of documented weather history. From 1930 through 1939, 37 "century days" were registered at Midway Airport. And in the 1930s two blistering summers stand out: 1934 and 1936. Each of them produced 11 days of 100-degree heat that were without precedent in two dangerous aspects, duration and intensity---six consecutive 100-degree days, July 20 to July 25, 1934;
and eight consecutive days, July 7 through 14, 1936. The daily highs during the 1934 period were 103, 108, 104, 109, 107, 105 and in 1936 the highs were 102, 106, 100, 106, 107, 100, 102, 104.

Photos courtesy of Chuck Hagen, Oak Lawn, Illinois
 

Here is a shot from River North in Chicago:

Photo courtesy of Tom Negovan, River North-Chicago

----------------------------

Rainbow over the WGN's Chicago studios:

Photo courtesy of Joanne Stern

--
Rainbow over U.S. Cellular Field after Sox game supporting Children's Brain Research (CBRF)
 
 
Photo courtesy of Sean Jucas

Lynn Wier from Chicago Heights sent us these pictures taken last Monday of storms over Green Bay in Wisconsin's Door County.

Thanks for the shots Lynn!

More showers and thunderstorms are on tap for Monday as a cold front moves through the region. With ample moisture available, Monday's storms could trigger a repeat of Sunday's intense storms that brought quick bursts of heavy rain to the Chicago area.
An inch of rain fell in just 25 minutes near Aurora. Most locations received more rain in a few hours on Sunday afternoon than they had recorded during the entire first half of August.
While heavy downpours were the main story, strong gusty winds brought down a large tree near Kankakee and downed large branches and signs near Montgomery in Kane County. Wind gusts were clocked at 62 m.p.h. just offshore of Navy Pier.

Claudette landfall expected
Tropical Storm Claudette was expected to make a Florida Panhandle landfall with top winds of 50 m.p.h. early Monday, spreading heavy rain and gusty winds into Alabama. An associated twister damaged several homes in the Cape Coral area on Florida's Gulf Coast.

Dear Tom,
What is the latest date that the first named Atlantic storm has formed?
Steve Hanan
Dear Steve,
Atlantic Basin storms have been named since 1950, and since then the latest starting season was 1967 when Arlene was named on Aug. 30. Arlene never made landfall and moved north through the open Atlantic mainly as a tropical storm, but it briefly became a Category 1 hurricane with top winds of 85 m.p.h. Runner-up for a late-season start was 1977 when Anita was named on Aug. 29.  Anita quickly grew into a monster storm, slamming into the Mexican coast between Brownsville and Tampico on Sept. 2 as a Category 5 hurricane with top winds of nearly 175 m.p.h. The current season, which started with Tropical Storm Ana on Aug. 15, marked the latest start to a season since 1992 when the infamous Hurricane Andrew was named on Aug. 17.

Parched gardens, browning lawns and cracked ground are a testament to just how dry it's been in the Chicago area the last two months. Despite a record wet year in 2008 and a soggy spring and early summer, the Chicago area has turned seriously dry. To date, August has produced a scant 0.60 inches of rain, after the driest July in 18 years produced just 1.53 inches. All that may change Monday as a cold front moves into the area, poised to trigger thunderstorms in an air mass saturated with nearly 2.25 inches of atmospheric moisture. Beyond that, a series of weather disturbances promises several more rounds of thunderstorms Wednesday and Thursday.

Tropics come alive

The 2009 Atlantic hurricane season took off with a bang Saturday. Tropical storms Ana and Bill developed and are moving toward the Caribbean. By Thursday, Ana is forecast to be approaching the Florida Keys just below hurricane strength and Bill is expected to strengthen into a hurricane near the Virgin Islands and Puerto Rico.

Dear Tom,
As we head toward fall with shortening days, I contend that the sun sets later as you go north. Am I correct?

Ginger Bolte, Arlington Heights

Dear Ginger, At this time of year, you are correct. On Sunday, the sun will set at 7:49 p.m. in Chicago, while to the north, our neighbors in Green Bay will not experience sunset until 7:55 p.m. Heading up to Marquette, Mich. the sun sets two minutes later than at Green Bay. As we get into October and the hours of daylight continue to decrease, sunset occurs earlier as you travel north of Chicago. By Dec. 1, sunset will be at 4:21 p.m. in Chicago, seven minutes after setting in Green Bay at 4:14 p.m. and 17 minutes after setting in Marquette. 

As Saturday dawns, this weekend won't be as steamy as last, but it is to share an important similarity--its peak temperatures are likely to flirt with 90 degrees both Saturday and Sunday. Temperatures at that level have occurred only six times this season at Midway Airport and four times at O'Hare International Airport--far short of the average of 18 days of 90 degree or warmer temperatures typically on the books by now.
The Illinois EPA has declared Saturday as an Air Pollution Action Day given surging ozone and particulate levels within the stagnant air mass moving into its fifth day here. It's only the third time this season the agency has warned that pollutants may reach advisory levels.

Northern Wisconsin hit
A compact thunderstorm-generating system walloped northern Wisconsin Friday. More than 7 inches fell southeast of the Twin Cities at Arkansaw, and Washington Island at the north tip of Door County was drenched by 4.70 inches of rain in just 1 hour 45 minutes. 

Dear Tom,
While on a trip to Tennessee last May we drove through southern Illinois. In the area from Mt. Vernon to Fulton, Ky., we noticed many trees uprooted, broken in two, twisted and scattered along the road. What kind of storm produced such widespread damage?
Roy Martin
Dear Roy,
You witnessed the aftermath of one of the worst squall lines to pass through southern Illinois in more than a decade. On May 8, 2009, the long-lived, fast-moving thunderstorm complex known as a derecho left a widespread trail of destruction from southern Missouri and southern Illinois all the way to the western Carolinas. Winds peaked in excess of 100 m.p.h. and the damage was likened to that of a hurricane because of the vast extent of the damage path. Several tornadoes were reported in addition to the very strong winds. Four people perished in the storms and at least a dozen were injured.

The high pressure that first pulled into town Tuesday maintains its dominance over the local weather scene for a fourth day Friday. Winds aloft remain weak. That all but guarantees a sun-filled day Friday with temperatures well into the 80s--and even near 90 degrees. Friday's rain-free skies extend a dry spell that began in mid-July and during which only 0.81 inches of rain fell in the last 30 days. That's just 23 percent of the long-term average of 3.58 inches for the July 16 through Aug. 14 period and a fraction of the 6.25 inches which fell during the same period a year ago. It ranks as the driest mid-July to mid-August period here since the Dust Bowl year of 1934 and the area's sixth driest over the 139-year observational record in Chicago.

Air quality is diminishing as this air mass and its light winds linger. With a slow increase in ozone and particulate levels predicted in coming days, the Illinois EPA has issued 2009's first Air Pollution Action Day declaration for Saturday and cautions that Friday's air quality is likely to be unhealthful for sensitive groups. Similar advisories are in place in northwest Indiana and eastern Wisconsin, from the Illinois border north to Door County. 

Far out in the Atlantic, a vigorous tropical wave which has moved off Africa in recent days and was situated near the Cape Verde Islands Thursday may become the 2009 Atlantic hurricane season's first named storm--Ana. It would be the latest in the season an Atlantic storm with a name beginning with "A" has formed since Hurricane Andrew in 1992.

Dear Tom,
Now that the weather has gotten hot why has Lake Michigan turned so chilly?
Mehul Jethva, Joyce and Ted Husak, Brett Konken
 
Dear Readers,
It seems counter-intuitive, but when the city turns hot, Lake Michigan water temperatures usually drops. A process called upwelling is to blame. As the lake warms during the summer the sun's rays heat the water closest to the surface while the deeper water remains cold. Hot weather arrives in the city on brisk south and southwest winds that push the warm surface water out into the lake toward the east shore, allowing the colder bottom water to upwell and replace it. As a result between August 7-10  Chicago's shore water temperature dropped from 71 to 63 degrees while the water at Michigan City jumped from 55 degrees to 67.
 

The last couple of days temperatures have been increasing by a couple of degrees.  This weekend most of the Chicagoland area will see near 90 degree temperatures and increasing humidity.  The Illinois Environmental Protection Agency (EPA) has an air quality alert for Friday August 14th 2009 and Saturday August 15th 2009.  This means the air could be unhealthy for sensitive groups.

Air Quality Index

A Guide to Air Quality and Your Health

Introduction

Increasingly, radio, TV, and newspapers are providing information like this to local communities. But what does it mean to you ...if you are planning outdoor activities that day? ...if you have children who play outdoors? ...if you are an older adult? ...if you have asthma? This booklet will help you understand what you can do to protect yourself from air pollution.

Local air quality affects how you live and breathe. Like the weather, it can change from day to day or even hour to hour. The U.S. Environmental Protection Agency (EPA) and others are working to make information about outdoor air quality as easy to understand as the weather forecast. A key tool in this effort is the Air Quality Index, or AQI. EPA and local officials use the AQI to provide you with simple information on local air quality, the health concerns for different levels of air pollution, and how you can protect your health when pollutants reach unhealthy levels.

Back to contents

What is the AQI?

The AQI is an index for reporting daily air quality. It tells you how clean or polluted your air is, and what associated health effects might be a concern for you. The AQI focuses on health effects you may experience within a few hours or days after breathing polluted air. EPA calculates the AQI for five major air pollutants regulated by the Clean Air Act: ground-level ozone, particle pollution (also known as particulate matter), carbon monoxide, sulfur dioxide, and nitrogen dioxide. For each of these pollutants, EPA has established national air quality standards to protect public health.

Back to contents

How does the AQI work?

Think of the AQI as a yardstick that runs from 0 to 500. The higher the AQI value, the greater the level of air pollution and the greater the health concern. For example, an AQI value of 50 represents good air quality with little potential to affect public health, while an AQI value over 300 represents hazardous air quality.

An AQI value of 100 generally corresponds to the national air quality standard for the pollutant, which is the level EPA has set to protect public health. AQI values below 100 are generally thought of as satisfactory. When AQI values are above 100, air quality is considered to be unhealthy-at first for certain sensitive groups of people, then for everyone as AQI values get higher.

Back to contents

Understanding the AQI

The purpose of the AQI is to help you understand what local air quality means to your health. To make it easier to understand, the AQI is divided into six categories:

Air Quality Index (AQI) Values	Levels of Health Concern	Colors
When the AQI is in this range:	...air quality conditions are:	...as symbolized by this color:
0 to 50	Good	Green
51 to 100	Moderate	Yellow
101 to 150	Unhealthy for Sensitive Groups	Orange
151 to 200	Unhealthy	Red
201 to 300	Very Unhealthy	Purple
301 to 500	Hazardous	Maroon

Each category corresponds to a different level of health concern. The six levels of health concern and what they mean are:

• "Good" The AQI value for your community is between 0 and 50. Air quality is considered satisfactory, and air pollution poses little or no risk.
  
  
• "Moderate" The AQI for your community is between 51 and 100. Air quality is acceptable; however, for some pollutants there may be a moderate health concern for a very small number of people. For example, people who are unusually sensitive to ozone may experience respiratory symptoms.
  
  
• "Unhealthy for Sensitive Groups" When AQI values are between 101 and 150, members of sensitive groups may experience health effects. This means they are likely to be affected at lower levels than the general public. For example, people with lung disease are at greater risk from exposure to ozone, while people with either lung disease or heart disease are at greater risk from exposure to particle pollution. The general public is not likely to be affected when the AQI is in this range.
  
  
• "Unhealthy" Everyone may begin to experience health effects when AQI values are between 151 and 200. Members of sensitive groups may experience more serious health effects.
  
  
• "Very Unhealthy" AQI values between 201 and 300 trigger a health alert, meaning everyone may experience more serious health effects.
  
  
• "Hazardous" AQI values over 300 trigger health warnings of emergency conditions. The entire population is more likely to be affected.

Back to contents

AQI colors

A specific color is assigned to each AQI category to make it easier for you to understand quickly whether air pollution is reaching unhealthy levels in your community. For example, the color orange means that conditions are "unhealthy for sensitive groups," while red means that conditions may be "unhealthy for everyone," and so on.

Back to contents

How is a community's AQI calculated?

Air quality is measured by monitors that record the concentrations of the major pollutants each day at more than a thousand locations across the country. These raw measurements are then converted into AQI values using standard formulas developed by EPA. An AQI value is calculated for each pollutant in an area (ground-level ozone, particle pollution, carbon monoxide, sulfur dioxide, and nitrogen dioxide). The highest AQI value for the individual pollutants is the AQI value for that day. For example, if on July 12 a certain area had AQI values of 90 for ozone and 88 for sulfur dioxide, the AQI value would be 90 for the pollutant ozone on that day.

Back to contents

When and how is the AQI reported to the public?

In large cities (more than 350,000 people), state and local agencies are required to report the AQI to the public daily. When the AQI is above 100, agencies must also report which groups, such as children or people with asthma or heart disease, may be sensitive to the specific pollutant. If two or more pollutants have AQI values above 100 on a given day, agencies must report all the groups that are sensitive to those pollutants. Many smaller communities also report the AQI as a public health service.

Many cities also provide forecasts for the next day's AQI. These forecasts help local residents protect their health by alerting them to plan their strenuous activities for a time when air quality is better.

The AQI is a national index, so the values and colors used to show local air quality and the levels of health concern will be the same everywhere you go in the United States. Look for the AQI to be reported in your local newspaper, on television and radio, on the Internet, and on many state and local telephone hotlines.

• AQI in the Newspaper
  Newspapers in many U.S. cities, and some national newspapers, carry AQI reports each day. Here is one example:



• AQI in Television and Radio Weather Reports
  Many local television or radio weathercasters use the AQI to provide air quality information in your area. Here's the type of report you might hear:




Tomorrow will be a code red air quality day for Center City. The cold winter air, morning traffic, and wood smoke are expected to cause particle pollution to rise to unhealthy levels. People with heart or lung disease, older adults, and children should avoid strenuous activities.



• AQI on the Internet
  EPA's AIRNow web site (www.epa.gov/airnow) contains general information about air pollution plus real-time and forecast air quality data. It also contains facts about the health and environmental effects of air pollution, steps you can take to protect your health and to reduce pollution, and links to state and local air pollution agency web sites.

One good byproduct of our cool summer is a relatively quiet severe weather season.  According to the Storm Prediction Center, there have been 21 deaths from tornadoes so far this year.  That compares to 122 at this point last year.

It was two years ago that an EF5 tornado struck Greensburg, Kansas.  The storm decimated the town.  To view a full length documentary on the Greensburg tornado, check this out.

 

 

Another day of unlimited sunshine and warm temperatures is predicted Thursday. The air mass responsible is stagnating with time---all but trapped by extremely light winds that extend vertically through tens of thousands of feet of the atmosphere. The lack of significant air movement allows pollutants to build, and cooler onshore breezes along Lake Michigan assures vertical mixing of the air which typically takes place with daytime heating is limited.
 Officials of Wisconsin's Department of Natural Resources issued an air quality watch through the weekend for counties along the lake in anticipation of a buildup of ozone and fine particulates that could reach unhealthful levels. It wouldn't be surprising to see a similar situation develop across northeast Illinois.

 Temperatures tied the 100-degree record high at Minot, N.D., Wednesday beneath the western flank of the same sprawling air mass that covers the Midwest. Strengthening southerly winds in place across the Plains are to shift eastward in the days ahead, reaching the Chicago area by Sunday. It's a development predicted to boost weekend temperatures and humidities, possibly producing the second consecutive Sunday with a 90-degree high.
 

Dear Tom,
I'm not complaining because I love this cool summer, but how many hours of temperatures in the 90s have we had in July and August?
Peggy Pannke-Smith Park Ridge
 
Dear Peggy,
Hours of 90 degree temperatures have been few and far between this summer. Frank Wachowski supplied the data from Midway Airport and reported that there were no hours of 90s in July and so far in August just seven, recorded last weekend, Aug. 8-9. Wachowski noted that we did have 20 hours  spread over four days in June so the total number of hours for the entire summer is just 27. Compare that to some of the city's hottest summers where we have logged as many as 48 days in the 90s and you realize just how little heat there has been this year. Of course, summer is not over yet and more 90s are expected, possibly as early as next weekend.
 

Thanks to Matthew Kulowitch for sharing this great shot of  Saturday's thunderstorms that rumbled much of the day in areas from Iowa across Wisconsin into Lower Michigan, skimming along the Wisconsin- Illinois border. This shot was taken on a flight from Detroit to central Wisconsin at an altitude of about 14,000 feet.

Photo by Matthew Kulowitch

August is a month known for the slow movement of its weather. Jet streams, which guide the planet's weather, often pull north into Canada this time of year, allowing U.S. weather systems to languish as steering winds diminish. This can be a blessing or a curse. If wet weather is occurring, thundery rains can stall, recurring over roughly the same area for an extended period of time---leading to huge rainfalls. That definitely isn't the setup in the coming week. High pressure has mired in the light upper-wind regime here---a setup likely keep ground-level winds light while sidetracking rains until next week. That's not good news in a month that has only tallied 1.30 inches of rain---not even two-thirds normal. Instead, an abundance of sun is predicted in an air mass sure to grow hazier and dirtier in coming days because of the lack of air movement. Computer model wind forecasts suggest breezes by Thursday night from the ground up to 40,000 feet above the surface will be blowing at less than 10 m.p.h., as calm an atmospheric state as occurs here. Without wind to disperse pollutants, a buildup of haze and pollution will become more evident.


Lake breeze front ignites some south suburban downpours Tuesday
 
  Thunderstorms erupted along an inland moving lake breeze front late Tuesday morning and afternoon. Though limited in areal coverage, the storms unleashed downpours on select south suburban areas---even as northeast winds over the city and north lakeshore areas kept skies clear, offering spectacular views of the towering occasionally 53,000 ft. tall thunderheads to the south.

The word from our astronomer Dan Joyce is that the clear skies predicted in coming nights will afford area residents the chance to catch the annual Perseids Meteor Shower which peaked in Wednesday's predawn.  Dan says 40 to 50 meteors may be visible per hour Wednesday night and that at least some can be expected up to August 17.

Dear Tom,
 
You recently mentioned a hurricane near Hawaii. Isn't that rare and why isn't it being called a typhoon?
Leon Panofsky
 
Dear Leon,
To be classified a typhoon, a tropical cyclone (in the northwest Pacific) must develop or move west of the 180th meridian (International Date Line), so the tropical cyclones affecting Hawaii are called hurricanes. The threat of a landfall in Hawaii is considerably lower than in the Caribbean or along the U.S. Gulf and Atlantic coasts. The storms in that part of the Pacific seldom make a direct hit on the islands, usually just skirting them with fringe effects. Since 1950 only two storms have made a direct hit, both on Kauai: Hurricane Dot in 1959 and Hurricane Iniki in 1992. Recently in 2003 Hurricane Jimena passed within 120 miles of the Big Island's South Point bringing 50 m.p.h. winds, 15 foot waves and considerable flooding.
 

Doug Grewe from Palatine sent us these pictures taken this past Sunday in Walworth, Wisconsin.  Doug tells us: "These shots were taken while we were on our traditional hayride. The clouds were making some interesting shapes. All photos taken by my brother in law Paul from Elk Grove Village."

Even though Chicago's meteorological summer so far ranks among the coolest 17% on record, that doesn't necessarily mean that the global warming issue can be put to rest.  July has been a particularly cool month for some. According to the National Climatic Data Center, six states recorded their coldest July on record.  Those states included Illinois and Indiana. 

However, for the whole world, July was the second warmest ever recorded.  Not only that but the global average temperature climbed 0.41 Celcius from June to July, marking the largest one-month increase in the 31-year global temperature record.  Find out more in this article from USA Today.

The trend toward more comfortable temperatures and lower humidities (expected to dominate the remainder of the workweek) accelerates Tuesday as winds shift to the north-northeast. It's a development that will usher air which has traveled nearly the length of Lake Michigan into the city. With the lake's surface water temperatures averaging 67 degrees, air temperatures will peak in the mid-70s--a far cry from Sunday's steamy low 90s and Monday's moderately muggy 89-degree high at Northerly Island. Ground-level winds are likely to converge along Tuesday's inland moving "lake-breeze" front--a setup that encourages air to rise and cool. With almost an inch of evaporated water still in Chicago's atmosphere, several scattered showers or thunderstorms could flare along the lake-breeze front--but areal coverage of any rainfall should involve just a fraction of the Chicago area.
 
Squalls associated with Tropical Storm Felicia, once a hurricane, were bearing down on Hawaii late Monday. Huge swells were reported. Tropical storm watches have been posted for all but the Big Island and while high winds aren't expected, forecasters were warning residents and visitors of potentially flooding rains near some of the mountains.

Dear Tom,
I'm a dispatcher at United Airlines and I see all kinds of weather across
the country. In Phoenix they have dust storms called haboobs. I recently
learned a new term, derecho. What other interesting terms can you share with
us?
Matt Duxbury

Dear Matt,
How about sastrugi, sharp ridges on snow drifts, and its singular form,
sastruga? And then there's aftersummer, a period of unusually warm, quiet
weather in the autumn, and foreshore, the part of a lake shore that lies
between the low and high water marks. A psithurism is the sound of wind in
trees and frazil is slushy ice crystals that form in supercooled water that
is too turbulent, as in gushing streams, to permit the formation of an ice
sheet. In Australia, there's Fremantle Doctor, a strong sea breeze (like our
lake breeze, but stronger) and Cock-eyed Bob, a typhoon. In Britain, they
call squally and tempestuous weather a blunk.

The stormy period that began Sunday afternoon should end later today as a cold front pushes south of of the Chicago area. The Storm Prediction Center posted severe thunderstorm watches Sunday afternoon and evening along the cold front from northern Kansas through northwest Missouri, a good portion of Iowa, northern Illinois, and southern Wisconsin into extreme northern Indiana as well as most of Lower Michigan. Iowa seemed to be hardest hit with many reports of one-and-a-half to two-inch hail along with extensive wind damage. Trees and power lines were down in many locations across southern Wisconsin and Lower Michigan.
Locally heavy downpours were reported in a few spots across northern Illinois. Cloudiness from the storms cut off heating and restricted Chicago area highs to the lower 90s, holding heat indexes to around 100 degrees. Highest readings were at Northerly Island where the high peaked at 93 degrees with a heat index of 103.

Fair skies expected the rest of the week
The upper-air pattern will favor an extended period of fair skies and rain-free weather over Chicago beginning Tuesday and continuing into next weekend.

Dear Mr. Skilling,
What are the lowest and highest temperatures on Aug. 13 (my birthday) and also in all of August?
Maggie Westover, age 7
Dear Maggie,
You'll be 8 on Aug. 13 (happy birthday!) and you were born in 2001. It was a pleasantly cool day in Chicago on Aug. 13, 2001. Afternoon temperatures across the area remained in the middle 70s (the normal high on the 13th is 82 degrees).
We cracked open Chicago's record books and found that the city's temperatures on Aug. 13 have been as high as 101 degrees (in 1936, at Midway Airport) and as low as 51 degrees (1982 and 2004, at O'Hare). Chicago's extremes in all of August range from 104 degrees (Aug. 6, 1934, Midway Airport) to 42 (Aug. 28, 1986, at O'Hare).
August is arguably Chicago's most summery month. July is slightly warmer, but August weather tends to be more tranquil: more sunshine, longer rain-free periods and lots of warm days.

Wow! The amazing photos from Sunday's storms just keep rolling in! Except where noted, all photos are courtesy of the people listed below -- and we give everyone who sent photos a big thank you! Keep the great pictures coming!

In Gurnee, Steve R. described the photo below as "a dramatic storm sweep overhead, but almost no rain."



Katie M. of Antioch took this photo below at a residence on Fox Lake.



Jennifer B. took this photo of an "incredible front" Sunday afternoon as her and her husband drove along North Shore Drive in Lake Delavan, Wis.

Here's another batch of excellent photos from Sunday's storms. Except where noted, all photos are courtesy of the people listed below -- and we give everyone who sent photos a big thank you! Keep the great pictures coming!

The photo below was taken in Richmond by Wally L. He described the storm as "pretty neat" even though it didn't produce the amount of rain that places in Wisconsin received.



Jim and Emily F. took the photo below just after 5 p.m. just north of Rockford, Ill.



The photos below come courtesy of 13-year-old Garrett S. His mom took the photos below as the storm front approached in McHenry.





Patrick B. of Deerfield relates an interesting story as he took this picture near Deerfield City Hall on Sunday evening. "With all the heat in the afternoon, whatever came down in the rain looked liked it dried up and started gathering energy again.  The interesting thing was that [this cloud] looked humongous but with a definite form."

Here's more great photos we've received that captured the unusual cloud formations from Sunday's storms. Except where noted, all photos are courtesy of the people listed below -- and we give everyone who sent photos a big thank you! Keep the great pictures coming!

Kevin C. of Delavan, Wis., took this picture just northeast of Delavan Lake.



The photo below was taken at the Newport Fire Department in Wadsworth, and sent to us by David C., as the storm rolled in.



Mike C. took this photo of clouds that went through his farm just outside of Woodstock, Ill.



Here's another photo courtesy of Tom and Kathy C. as the storm front rolled into Spring Grove.



Dave H. captured these ominous-looking low clouds in Antioch, Ill.

We've received some great photos from today's storms which we'll be posting tonight and tomorrow. Many of the photos revealed some eye-catching cloud formations. Here's a sample of some great photos we've received so far. Except where noted, all photos are courtesy of the people listed below -- and we give everyone who sent photos a big thank you! Keep the great pictures coming!

Kevin K. took this picture while heading back to Chicago from the Wisconsin Dells. He described it as a "crazy storm" while driving into it...



Anne D. took the photo below from her front yard in Lake Geneva, Wis.



Serena H. took the cloud photo below in Woodstock, Ill.



Brandon B. and his family took this dramatic picture below in Kenosha. Brandon said he "saw this cloud mass coming -- then had some strong winds, thunder and lightning."



As the front moved through McHenry, Jennifer L. took the snapshot below.

Dangerously high levels of heat and humidity finally reach Chicago and northeast Illinois today. Strong southwest winds continue to feed the humid air into the metro area that first hit with the overnight rains early Saturday. Mostly sunny skies today should allow the still very intense sun to heat pavement and buildings over the most populated areas, and strong southwest winds will blow this heat toward Lake Michigan, negating any chance of a lake breeze. Thus the highest temperatures this afternoon could well occur in the city near the lakefront. The record high of 97 set back in 1913 may well be broken at the official O'Hare observing site, with even higher readings possible at Midway Airport and Northerly Island.
Similarly highest heat indexes may also occur over the city.

Showers/t-storms Monday, fair skies the remainder of the week.

A cold front will move through northeast Illinois Monday preceded by scattered showers/t-storms and followed by high pressure, initially cooler temperatures and lower humidity, and generally fair skies the reminder of the week.

 

Dear Tom,
My birthday is on Aug. 9 and since 1987 it seems that we've had at least some rain on that day almost every year. Is that true or am I imagining it?

Samantha Moore

Dear Samantha,

Chicago weather historian Frank Wachowski confirms that Aug. 9 has been rainier than usual in recent years, but he added that you have also celebrated many dry birthdays. The average probability for measurable rain (0.01 inch or more) on any August day is 29 percent, but since 1987 the probability of rain on Aug. 9 has been 41 percent (measurable rain on 9 of the 22 days). It rained on your birthday last year (but just a trace) and in 2007 and 2005. It was dry in 2006. For the most part, the rains have been light. Since 1987, the heaviest amount that fell was 0.47 inches in 2001. In official records since 1871, the heaviest Aug. 9 rain was 1.60 inches in 1952.

The hottest weekend temperatures in two years -- including back-to-back highs in the 90s -- take hold this weekend, threatening two temperature records. With lows expected to hold to around 80 degrees in the city Saturday night, the 96-year-old record for the warmest Aug. 9 minimum temperature appears in jeopardy; and Sunday's record high of 97 degrees may well be in reach as well. Saturday's predicted 93-degree high is to become the city's first reading above 90 in six weeks. And the combination of the hot daytime air and high humidities (including Gulf Coast-level 70-degree dew points) is likely to produce the first set of triple-digit heat indexes of the summer.
Thunderstorms may rumble in parts of the area into Saturday morning, initially restraining the onset of heat. But, the incoming dome of warmth tightens its grip Saturday, producing warmth so expansive that it is to "cap" or halt thunderstorm development. Storms instead are to flare in waves across Wisconsin and Michigan where three-day rain totals exceeding 3 to 4 inches may occur in the hardest, most frequently hit areas. That region is also threatened by severe storms.
The one wild-card this weekend involves the potential for daytime heat to breach the "cap," allowing thunderstorms to form. Such storms could be formidable, given the moisture and energy available. It appears the best chance for thunderstorms settling into the area occurs Sunday night into Monday when downpours may occur.

Dear Tom,
What is the difference between a hurricane, typhoon and a cyclone? Do we ever hear about typhoons and cyclones here?
John Flanagan, St. Charles
Dear John,
In the context of your question, hurricanes, typhoons and cyclones are regional specific names for tropical cyclones. They are called hurricanes in the Atlantic, eastern and southwest Pacific and Caribbean, typhoons in the northwest Pacific, and cyclones in the Indian Ocean. While cyclones and typhoons do not affect this country, they become big news here when they cause problems in other parts of the world. Cyclone Nargis was a huge story when it devastated Myanmar on May 3, 2008, as was Typhoon Thelma when it killed 6,000 in the Philippines in 1991. In a non-tropical sense the term cyclone is also loosely applied to tornadoes, waterspouts, dust storms and sometimes just to any strong wind.

Special thanks goes out to Henry, age 6, for sending us this storm photo he took in the southwest suburbs as storms rolled in mid-July earlier this year. Henry's grandmother Terri says that he "watches [WGN weather] every night and took this picture at his home" to share with everyone here in the Weather Center. On behalf of Tom and everyone here, we want to say a special thank you to Henry: Keep sending those great shots in, and keep up the amazing work!



Photo courtesy of Henry, age 6

Cary Kociuba of Mount Prospect shares these thunderstorm cloud photos he took while on vacation last week in north-central Minnesota. Thanks Cary!







Photos courtesy of Cary Kociuba, Mount Prospect

It's been two years since a weekend has done what this coming weekend is predicted to do---produced back to back 90s. Blazing heat, which has been parked west and south of Chicago virtually all of the summer, is on the move.  The transition to 90-degree warmth threatens to be a stormy one for sections of the area as winds converging along a northbound warm front lead to a pile-up of air likely to produce humid updrafts which build into thunderstorms. With 2 inches of evaporated moisture expected to saturate the atmosphere Friday night into Saturday and the heat surging into these storms from the south, the stage is set for active thunderstorms which could be prolific lightning producers. Though rainfall amounts are likely to vary widely, totals should increase most noticeably north of Chicago.  A number of locations toward the Wisconsin line and north could end up with 1 to 2-inch multi-day rainfalls if late Thursday computer model estimates prove accurate. Areas farther south could see lighter and more scattered totals---some missing out on rain altogether.

Highs of 93 and 98-degree are predicted Saturday and Sunday---the latter a possible record-breaker and the highest daytime high in Chicago in over three years. A 98-degree or higher temperature has occurred only 180 times in the past 81-years at Midway Airport. Sunday's high could be within striking distance of 100-degrees. It's been more than 4 years since a triple digit high temperature has been recorded in Chicago. The most recent was a 104-degree high recorded July 24, 2005 at Midway Airport.

Dear Tom,
Before leaving for college in fall of 1971 I remember August being a very cool and damp month. Am I correct? How cool was the entire summer?
Stan Kijek
 
Dear Stan,
August, 1971 did feature a few very cool days which may have influenced your memories but there were also a great many warm days. The month had three days with sub-70 degree highs; 69 on the 3rd, 67 on the 4th and 68 on the 26th, but also featured eight days in the 90s including back-to-back 95s on August 9-10 and a three day stretch with highs of 96,92 and 95 from Aug. 20-22. It was also rather dry with less than two inches of rain. Overall, the summer of 1971 recorded a surplus of hot weather, logging 35 days that reached 90 degrees higher including two days of 101 on June 27 and 28. A typical Chicago summer produces about 17 days in the 90s at O'Hare International Airport and about 24 at Midway Airport.
 

Grassy surfaces across Chicago have taken on a burned-out appearance and gardeners are noticing cracks in once waterlogged ground. As remarkable as it seems, given the incredibly wet start to this year's growing season, the area is dry and in need of rain. It's not an unusual problem this time of year. Summer sunlight is so intense moisture problems often arise at this point in the season. A week of normal rainfall this time of year sees 1.05 inches fall; that's short of the 1.52 inches required to replenish moisture lost through evaporation. Not only has rainfall the past three weeks not come close to normal, totaling a paltry 0.28 inches (just 11 percent of the long-term average), the just completed mid-July to early August period has been the driest in 63 years. The situation may turn around---and quickly in some locations---as thunderstorms erupt ahead of this weekend's blast of heat. Rainfall is unevenly distributed in the warm season because so much of it is generated by thunderstorms, which concentrate downpours over comparatively small area.

The atmospheric setup that appears to be coming together Friday and Saturday will see the air's moisture content surge past 2 inches---as much water as the atmosphere holds here. At the same time, the flood of hot air into the Chicago area may well destabilize the atmosphere, encouraging air to rise. It's a setup known to produce thunderstorms. The wild card will be whether the depth of the hot air grows quickly to the point that a storm-thwarting "cap," i.e. layer of warm air, develops aloft and retards storm development. That may happen Sunday---but could permit storm development ahead of the hottest air's arrival Friday afternoon into Saturday morning--and perhaps over parts of the metro area Saturday afternoon and evening. 

Dear Tom,
How does the cool summer of 2009 compare to other recent cool summers we've endured, such as 2004 and 1992?
Tim Lucole, Deerfield

Dear Tim,
With an average temperature of 70.0 degrees as of July 31, it's certainly accurate to characterize this summer as "cool." In fact, it ranks 8th coolest out of 82 years of temperature data (1928-2009) at Midway Airport.

However, this might be a premature characterization because meteorological summer runs through August 31 and one-third of the season is yet to occur. It's difficult to keep temperatures down in August. Indeed, a pattern change suggests much higher readings in a few days.

From June 1 through July 31, this summer's daily high temperatures have averaged 78.1 degrees (versus a normal of 82.7 in that period) and it ranked third coolest; 2004 (79.6 degrees) was 8th coolest and 1992 (78.8) was 4th coolest.
 

Dave Skrzyniarz sends us these photo. In the e-mail accompanying it, Dave tells us:
"Strange pics on Monday at 4:30----a jet left this trail thru the cloud deck---it actually looked like it emerged from the clouds and left this mark.  Several other jets also had the same effect -- making the clouds appear to be smoke!"
 
Fascinating shot, Dave---THANKS for sharing it with us!
 
Tom Skilling

Powerful thunderstorms sweep into the Peoria area--part of a Tuesday outbreak which produced damaging winds, flooding rains from Missouri to Kentucky
 
Thunderstorms unleashed 60 to 70 m.p.h. wind gusts on sections of downstate Illinois and Indiana as well as Missouri and Kentucky Tuesday morning. Rob Shreiner, a paramedic with photographic skills evident in this set of photos, sends us these eye-catching shots of the ominous skies which accompanied the storms as the roared into the Peoria area.  Rob tells us: 
 
"I was in Peoria, IL this morning as this storm rolled in at approx 8:36 AM today.
 
Amazing formations as they were rolling down at me as I was taking the images.  Gusts present and the heavy rain did not begin until approx 10 minutes after this came in.
 
Only adjustments were auto contrast, slight sharpen and obviously crop, all taken w/ 16-35mm wide angle lens, so that's why all the poles tilt inward."
 
Thanks for sharing these with us, Rob! 
 
Tom Skilling











Photos courtesy of Rob Schreiner, Wadsworth, Illinois

The moderate warmth that has dominated so much of Chicago's summer 2009 to date continues another two days. Wednesday and Thursday highs are to reach the lower 80s. But, big changes loom this weekend as long absent heat begins its charge into the Chicago area on 30+ m.p.h. wind gusts which are expected to be in place here Saturday afternoon and like to continue Sunday. The rapidly expanding dome of hot air threatens ignite clusters of active thunderstorms to the west of Chicago over sections of the Plains and western Midwest in coming days. These storms may sweep into the Chicago area Friday night into Saturday morning. The cool outflows which occur with such storms must always be monitored--they've been known to interfere with hot air's movement. But, with the incoming hot air mass expected to become "capped" and unable to produce thunderstorms here Saturday afternoon and evening, mid 90s seem a good bet before the sun sets Saturday and readings may even increase to the upper 90s Sunday. Temperatures at that level this weekend would be the warmest here in three years and mark the first time a weekend has produced back to back 90+-degree highs in just over two years.

Powerful storms storms topple trees, power poles downstate; Kentucky hit by 6-inch-plus rains
 
The veil of high clouds across the Chicago area Tuesday blew off the tops of powerhouse thunderstorms responsible for damaging winds and driving rains from northern Missouri across central and southern Illinois and Indiana. Gusts to 67 m.p.h. raked Indianapolis and hit 66 m.p.h. in west Lafayette in Indiana. In downstate Illinois, gusts of 60 m.p.h. swept Springfield while Lincoln and Mattoon recorded 55 m.p.h. gusts and Bloomington was swept by 49 m.p.h. gusts. Utility poles and trees snapped at many locations. Meantime, rainfall reached 6.46 inches at Grand Rivers, Kentucky and Louisville's 4.52 inches was a record calendar day rain for the month of August.

Dear Tom,
Last summer severe storms swept the Chicago area and Wrigley Field was evacuated. Were any tornadoes reported that evening?
Monica Salgado Chicago
 
Dear Monica,
A year ago on the evening of August 4 a violent squall line raced across the Chicago area producing wind gusts as high as 94 m.p.h., intense lightning and thunder and widespread flooding from torrential rainfall. The storm complex known as a DERECHO left a long legacy of downed trees and power lines all the way from northwest Illinois to Lake Michigan. In the city, tornado sirens blared as the stands at Wrigley Field were evacuated. Most of the storm damage was due to high straight-line winds but there were three confirmed tornadoes; an EF2 (winds to 135 m.p.h.) in Griffith, Indiana and EF1 twisters (winds to 110 m.p.h.) in Bloomingdale and Bolingbrook.
 

It may seem like we skipped summer this year.  We have had only three days so far with highs in the 90s.  This July ranked as the second coolest since the official weather station for Chicago moved inland from the lakefront in 1942. 

Summer will surge back though this weekend.  If we hit the predicted 95 on Saturday, that would make it the hottest day since August 2nd, 2006 when O'Hare hit 97.

Today won't be quite as hot but it could be the warmest day since June 25th when we hit 94.

For more information on how to handle the upcoming mini-heat wave, click here.

The unrelenting hot air which has seared the southwestern half of the country while eluding Chicago the better part of this summer appears likely to make a move on the Midwest this weekend. It's part of a massive pattern change which could deliver long absent summer heat to the Chicago area.  The onset of the heat is still 4 to 5 days away, though Tuesday's predicted 88-degree high offers a modest taste of what may be to come.  It would qualify as the area's highest readings in five weeks.
  
A comfortable interlude, including winds off the lake, dominates Wednesday and Thursday before the first phase of the weekend warm-up ignites possible Friday thunderstorms. The key to just how hot temperatures end up here may well turn on thunderstorm clusters likely to develop at the northern periphery of any expanding dome of hot air. Cool thunderstorm outflows, critically important because of the forecast challenge they represent and their ability to stem the flow of hot air into an area, are currently predicted to stay north of Chicago Saturday.  If true, the stage appears set for what could become the first Chicago weekend in nearly years to produce a set of 90-degree highs. Our currently predicted high of 96-degrees Saturday is a reading which would rank among the hottest since 2006.

44 of the past 50 years have seen more than this year's three 90s
 
Chicago's paltry three days at or above 90-degrees this late in the season is rare. Only two of the past 50 years at O'Hare have had fewer 90s by now--while 44 years in the past half century have recorded more.

Dear Tom,
What causes hail to vary in size?
Leon J. Hoffman, Chicago

Thunderstorms produce hail of varying sizes for several reasons, but a key element is the strength of the storm's updraft, the powerful column of ascending air that feeds heat and moisture into the storm. "Thunderstorms that produce larger hail have stronger updrafts." That's the word from severe weather researcher and native Chicagoan Brian Smith.
"Hail starts as small ice particles supported thousands of feet aloft in the updraft, growing larger layer by layer by collecting droplets of supercooled water that adhere to them and freeze. Like the motion of an elevator, hailstones repeatedly ascend in updrafts, descend in downdrafts, then ascend again. Giant hailstones, baseball-sized or larger, usually occur in so-called supercell thunderstorms whose tremendous updrafts are rotating, circulating the hail until it finally grows too heavy and falls out."

Nick Liveris shares this fascinating sequence of shots with us.  He tells us:
 
"I took these yesterday on Sunday, while flying from Las Vegas to Midway.  We flew around a thunderstorm, somewhere in the plains halfway into the trip.  The pilot mentioned we were flying around some "weather."  The other shot is of the grand canyon, and a shot of Chicago's skyline from the plane just before sunset.  I also included a random shot of Las Vegas shortly after our afternoon take-off, where I was told it was about 105 degrees!  I also included a random shot of Lake Mead, which was created by the Hoover Dam and was a refreshing 83 degrees when boating."
 
What a terrific series of shots!  We took a good chunk of the trip you, Nick!  Thanks for sharing these shots with us!
 
Tom Skilling
 

Nearly six weeks have elapsed since the mercury cracked 90-degree levels in Chicago, but that is about to change. The persistent pattern that brought record July coolness to parts of the Midwest and all-time record heat to South Texas and the Southwest is finally showing signs of breaking, allowing hot weather to return to the Midwest. The heat will tease the area Monday as a burst of southwest winds pushes the mercury into the upper 80s along with a noticeable increase in humidity. That could trigger a few thunderstorms along the Wisconsin border Monday morning and some additional storms by evening.
The midweek period will feature more of the comfortable weather that dominated July, but the latest computer models show an increased probability of temperatures rising into the 90s here by next weekend. If that occurs, four of the next seven days will feature above-normal temperatures -- the same number that the city mustered during the entire month of July. Chicago has logged only three days in the 90s this year: three 94s on June 23-25.

Dear Tom,
Why haven't there been any hurricanes yet this year?
Amanda Lonis
Dear Amanda,
The tropical Atlantic has been very quiet so far this season. Not only haven't there been any named storms, but forecasters have had only a smattering of disturbances to investigate. The developing El Niño may be part of the reason. During an El Niño episode, hurricane frequency tends to decrease in the Atlantic Basin. The reason is an increase in the winds aloft which creates more wind shear, a condition that tears storms apart and disrupts their formation. With no named storms through Aug. 3, this season becomes the latest starting Atlantic season since 1992. However, that season more than made up for its late start. The first named storm which finally formed on Aug. 16 developed into Category 5 Andrew, an historic storm that devastated far south Florida.

July 2009 was not only cool and cloudy, but also dry. The month's high temperature was just 86 degrees, logged on July 6. This marks the first time since weather records began here in 1871 that July had so low a maximum temperature.  Sub-par rainfall totaled just 1.53 inches --ending a five month string of above-normal precipitation and making July the driest since 1991. Sunshine was deficient, not only in July but in June as well. Since June 1, the amount of possible sunshine here has averaged just 52 percent, the least since the 49 percent recorded in 1992.
 

Heat may be on the way
 
August's opening week will feature seasonally warm weather in the lower 80s, though a surge of warmth could send readings into the upper 80s Monday. Real heat may make inroads into the city by Saturday for the first time since June 25 when Chicago recorded its third consecutive official high of 94 degrees at O'Hare International Airport.
 

Dear Tom,
Sometimes dew forms all night. Can it be measured? If so, how?
---Norm Victorson

Dear Norm,

The National Weather Service's weather observation program does not include measurements of dew. However, the amount of water contributed by dew can be determined, and a variety of techniques exist to do the job, though with varying degrees of accuracy. Dew can be measured most easily and accurately by weighing it. A piece of sod (or other material representative of the soil surface in the area for which dew measurements are desired) is placed on a scale that records the weight increase as dew accumulates. Unfortunately, accurate measurements of dew are not possible in densely vegetated areas. For example, little or no dew forms under a dense forest canopy, but it does accumulate on leaves at the top of the canopy.