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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
