July closed with eye-catching temperature deficits over a wide swath of the northern U.S. overnight. Chicago's 69.4-degree average July temperature at O'Hare International Airport was the coolest of the past 17 years. But at Midway Airport, the month's 71.0-degree average temperature was the site's coolest in 42 years. Estimates based on the month's temperatures suggest the need for air conditioning was 30 percent below the long-term average.
Cool as it's been in Chicago, in a number of Midwest cities July has never been cooler. Records were established in Rockford; Madison, Wis.; South Bend and Ft. Wayne, Ind.; and Benton Harbor, Saginaw and Flint, Mich. The month's temperature in those cities finished 4.5 to more than 7 degrees below normal.
The month's lack of rainfall was impressive -- and a huge change from the wet spring that kept farmers out of their fields. Only 1.53 inches of rain was measured here in July, less than half the 3.51 inches considered normal. The dry weather has led to browning lawns.
Dear Tom,
You recently said the last time we had a July with no 90s was back in 2000. What happened the rest of that summer?
Samantha Higgins, Chicago
Dear Samantha,
It has been nine years since the city has experienced a July without a 90. In terms of days in the 90s, the summer of 2000 actually started out cooler than the current one which has already recorded three consecutive highs of 94 degrees on June 23-25. In 2000 the city did not record its first official 90 until Aug. 15 when it finally reached 92 degrees. The heat-challenged summer of 2000 went on to produce only three more 90-degree days which occurred consecutively Aug. 31-Sept. 2. The highest temperature that year was only 93 degrees, logged on Sept. 1. The weather remained warm into early October with many days in the 80s, but the mercury never again reached the 90-degree mark.
Thursday afternoon's atmospheric setup was hardly one to produce a major severe-weather outbreak. But a bit of sun peeked through the day's clouds allowing temperatures to rise and winds did vary in speed and direction with height---a situation which caught the eye of forecasters at the Storm Prediction Center. They issued a late-day advisory cautioning that thunderstorms under development across sections of northern Illinois and southern Wisconsin might become capable of producing a few weak funnels. When Doppler radar scans of air movement within an eastbound cluster of thunderstorms over Boone County exhibited strong rotation, a tornado warning was issued around 4:20 p.m. for eastern Boone into McHenry Counties. Within moments of the appearance of circulation on radar, trained ground-based storm observers spotted a funnel cloud above far northwest suburban Capron. It soon extended to the surface where it produced 72 m.p.h. wind gusts that tossed dirt and tree debris into the air. Gusts strengthened to 83 m.p.h. by 4:30 p.m. as the parent thunderstorm, with cloud tops extending to 31,000 feet, continued east into McHenry County at nearly 30 m.p.h. The twister remained small and weak and went on to produce no damage before dissipating.
Pacific Northwest Temperatures
Cool breezes developed in some coastal areas of the Pacific Northwest, lowering temperatures late Thursday. But another record occurred in Seattle (95-degrees) and blazing heat is predicted to continue in interior sections of Oregon and Washington into the weekend.
July 2009 closes as Chicago's coolest in 118 years
The month closes at midnight Friday night and appears likely to finish as Chicago's coolest July in 118 years.The average temperature of 69.4-degrees over the first 30 days is more than 4-degrees below the long term average. It becomes the first July in 139 years of records here which has failed to produce a temperature greater than 86 degrees. An 86 degree high occurred earlier this month on July 6--a reading never exceeded.
Dear Tom,
In Erik Larson's book "Devil in the White City" there are frequent weather references, among them one about unusually warm weather on December 15, 1890. Are these references historically correct?
Jerry Halperin
Dear Jerry,
With the help of Chicago climatologist Frank Wachowski we cross-checked several of the weather references in the book with the original Chicago weather records and they are indeed correct. The day in question, December 15, 1890, the high climbed to a balmy 48 degrees melting the last traces of what remained of the city's seven inch snowpack that fell earlier in the month. We previously answered a similar question about Larson's reference to a funnel cloud that blew through the Midway at Columbian Exposition's on July 9, 1893 and found that reference to be meteorologically correct as well.
As July winds down, the brutal heat responsible for a swarm of record high temperatures across the Pacific Northwest and triple-digit readings in many sections of the West and Southwest is showing no sign of making a move on Chicago. Temperatures here will remain comfortable, with readings near or a bit below typical mid- and late-summer levels. While July's average temperature in Chicago ranks 3rd coolest of the past 81 years--running nearly 4-degrees below normal--Rockford and South Bend and Ft. Wayne, Ind., are all on track to close the books on the coolest July on record. The breadth of the month's cooler-than-normal weather has been, and continues to be, remarkable, literally covering the entire Midwest. Some weather observation stations are reporting July temperature deficits approaching 8 degrees.
Wednesday's 81-degree high in Chicago marked only the 34th time this year the temperature has reached 80 degrees. Records reveal a typical year has logged 49 such days by now--44 percent more than this year's tally.
Dear Mr. Skilling,
I recall a rather cool and cloudy summer, like we are experiencing now, in 2001 or 2002. Can you confirm my recollection?
Mark J. Girolamo, Aurora
Dear Mark,
Memories can be deceiving. It's certainly true that this summer has been cool. As of July 26 (when you wrote to us), it was running 2.7 degrees below normal and it ranked 5th coolest out of 50 years of temperature data at O'Hare Airport. On average, 10 of the annual tally of O'Hare's 17 days at or above 90 degrees should already have occurred, but we had logged only three.
However, the summer of 2002 (through July 26) was hot: 2.6 degrees above normal, 7th hottest, 16 days in the 90s. In 2001, summer ran near normal: 0.4 degrees below normal, 28th warmest, 10 days in the 90s.
Might 2000 have been the cool summer that you recall? With an average temperature of 68.9 degrees and no 90-degree days, it ranked 9th coolest.
Tuesday proved one of July's warmer, more humid days. Midway Airport's 86-degree high equaled the month's warmest reading while O'Hare's 84 degrees fell two degrees short. A line of thunderstorms bubbled to life during the afternoon and evening across the southern suburbs from Streator to Michigan City and La Porte, Ind., emanating from clouds which towered to 50,000 feet. The storms hit a 10-mile-wide corridor paralleling Indiana's Lake Michigan shoreline from Porter into La Porte Counties especially hard over a nearly three-hour period, unleashing repetitive downpours which began around 5:30 p.m., and ultimately led to nearly 5 inches of rain at Indiana Dunes State Park. Burdick, Ind. -- just east of Chesterton -- was drenched by 2.78" while 1.30" was reported in nearby Portage in just over 75 minutes.
Blistering heat in the Western U.S. included record-breaking triple-digit readings at a number of unlikely locations across the Pacific Northwest including 106-degrees at Vancouver, Wash. -- the hottest not only for July 28 but a new all-time high there -- and 106 degrees in Portland, a record-breaker for the date and just one degree shy of its all-time high.
"Wake low" produces damaging pre-dawn winds in west/northwest suburbs
Powerful pre-dawn winds gusting as high as 55 to 60 m.p.h. downed trees and snapped limbs over a swath of the Chicago area Monday night and Tuesday morning extending from west suburban Huntley in the far northwest suburbs west of Algonquin east to Roselle and Waukegan. Dissipating thunderstorms were behind the powerful gusts -- the product of what meteorologists refer to as a "wake low". The winds occur when a region of low pressure develops north of dissipating thunderstorms, strengthening the outflow winds which flow from them. The strongest gusts occurred between 11 p.m. and 1 a.m.
Dear Tom,
How many Julys and Augusts have failed to record a 90?
Norman H. Jannusch, Schaumburg
Dear Norman,
Using the city's official station weather records which began in 1871, Chicago has recorded only six Julys that failed to produce a high temperature of at least 90. This July stands to become the seventh and the first since 2000. Augusts without 90s are a little more common here, having occurred 16 times, most recently in 1986. Further checking reveals that there have been only two years, 1875 and 1915, where neither July nor August recorded a 90. In fact, 1875 is the only year that failed to produce any 90s at all. It looked like 1915 would become the second such year, but a late-season temperature surge pushed the mercury to the elusive 90-degree mark on Sept. 14 for the year's single occurrence.
The cooler than normal July temperatures which have slashed air conditioner bills across much of the Midwest and East Coast appear all but certain to establish a new temperature benchmark by the time the month ends at midnight Friday. With the reading of 86 degrees set back on the 6th likely to stand as the month's warmest since July 1, the month is likely to become the first July in 139 years of official records here which fails to produce a temperature of 87 degrees or warmer. Monday's 84-degree high -- far from an exceptionally warm reading -- was the city's warmest in 12 days.
Several clusters of thunderstorms are likely to sweep the Chicago area Tuesday. Half inch diameter hail accompanied storms in Ogle County Monday evening just northwest of Polo.
Severe storms produce tornado touchdown south of La Crosse, Wis.
Thunderstorms have erupted five of the past seven days somewhere in the mid and upper Midwest. Monday's storms produced a tornado touchdown 37 miles south/southeast of La Crosse, Wis., around 5:18 p.m. Law enforcement officials indicated the twister generated numerous reports of damage there. Storm downpours across southwest Wisconsin -- from the towering 50,000-foot-tall thunderheads responsible for that twister -- were impressive and included 1.62" at Friendship, 1.58" at Muscoda, 1.50" at Mineral Point and 1.46" at Fennimore.
Dear Tom,
In contrast to this summer, I remember some of Chicago's highest dew points when my daughter was born on July 31, 1999. Am I correct?
Ralph Bornhoeft, Green Oaks, Ill.
Dear Ralph,
Your daughter was born just one day after the city sweltered through one of its steamiest days in history. For nearly 16 hours, from the evening of July 29 to midday on the 30th, Chicago dew points remained at Amazon-rain-forest levels of 80 degrees or higher, peaking at an all-time record high of 83. At 9:33 a.m. on July 30 the official O'Hare temperature stood at 90. Combined with that record 83 dew point, the heat index soared to a suffocating 113.
Still humid but slightly drier air moved in during the afternoon of the 30th, allowing the dew point to fall to 69, but with the mercury soaring to 101 the heat index only dropped to 108.
A southwesterly breeze develops Monday, bringing perhaps the warmest readings in three weeks to the area. Chicago's last official high of 86 degrees was recorded back on July 6. It will be close, but that high could be matched or even exceeded in spots across Chicagoland.
A cold front will move slowly through northeastern Illinois Tuesday accompanied by showers and thunderstorms, including some downpours. After that front moves east, cooler and less humid Canadian-source air will flow into the area, and high temperatures will probably fail to reach 80 degrees the rest of the workweek. Thus far, this July is tied with July 1924 as Chicago's fifth-coolest on record since 1871. All of the cooler Julys occurred prior to the 20th century when official observations were taken at various downtown locations near the lakefront.
U.S. weather extremes Sunday
The southwestern U.S. was scorched Sunday with temperatures peaking well above 100 degrees from southern California into Texas. Elsewhere, severe storms featuring funnel clouds, damaging winds and large hail hit the East Coast from Virginia to Massachusetts. Eight people were injured in Readington, N.J., when high winds hit a hot air balloon festival.
Dear Tom,
Why are all planets spherical instead of some square, triangular, etc. Does it have to do with gravity?
Fred Spitzzeri
Dear Fred,
Indeed, it does. Gravity is a force that pulls everything evenly toward the enter of the planet. Over time, this results in a spherical planetary shape.
Even "solid" materials like rock flow like a liquid, albeit incredibly slowly, when they are pulled by a strong, steady force for a very long time. "Softer" solids such as ice respond more readily, and that is one of the mechanisms of glacial motion. Despite gravity, planets are not perfectly spherical. Every feature of the terrain -- mountains, valleys, buildings, even transient features like ocean waves -- constitute a deviation from a perfectly spherical planet.
Another force is also at work. Gravity maintains a planet's near-spherical shape, but rotation introduces centrifugal force that causes it to bulge at its Equator.
Dear Tom,
My thermometer has been registering temperatures in the 60s this July. If global warming is the real truth, why don't we have readings in the 100-degree range by now?
Bruce Ameismeier, Chicago
Dear Bruce,
Given this summer's unusually cool weather (third coolest in 82 years at Midway Airport), your point is understandable. However, Chicago is not the world and it is risky to suppose our cool summer is representative of the global situation, because it is not.
NOAA's Climate Diagnostics Center provides a daily global picture (derived from infrared satellite imagery) of areas of above-normal and below-normal temperatures. The global picture is not what you might expect: Areas currently experiencing above-normal temperatures (such as the Western U.S.) greatly exceed areas running below normal.
Thunderstorms pounded southern Minnesota, portions of Wisconsin, eastern Iowa and northern Illinois Friday afternoon and evening with hail, high winds and driving downpours -- and were expanding into sections of the Chicago area late Friday evening. Doppler radar scans detected tornadic circulations, prompting tornado warnings north of Dubuque in sections of eastern Iowa and south of Platteville in southwest Wisconsin.
Trained spotters in Dubuque reported rapid rotation within a funnel cloud which hovered above the city around 6:30 p.m. Gusts of 65 m.p.h. hit the city a short time later -- while 70 m.p.h. winds and torrential downpours combined to knock out power to the National Weather Service Office in Davenport while flooding downtown streets there. Rockford was hit with 60 m.p.h. winds around 9:10 p.m. Reports of heavy rain were widespread: In just 45 minutes, 2.75" swamped Aurora, Iowa, while 1.10" of rain fell in 15 minutes in Freeport.
The storms, which produced nearly 11,000 cloud-to-ground lightning strokes from eastern Iowa into northwest Illinois in just 6 hours, were produced by thunderstorms towering as high as 54,000 feet where temperatures plummeted to 69 degrees below zero. Little wonder the storms were prolific hail producers. Hailstones the size of tennis balls -- 2.50" in diameter -- lambasted an area 2 miles northwest of Oneida in Iowa's Delaware County. The day's largest hailstones, measuring 4.25" in diameter (grapefruit size) pounded Winneshiek, Iowa. And hail was responsible for severe crop damage along U.S. Route 20 just west of Dubuque.
Dear Tom,
On a sunny spring afternoon, a strange wind pattern in the newly plowed cornfield behind our house raised dust and corn husks into the air in a circular fashion. Coincidentally, thunderstorm winds caused minor damage at our house that evening. Was the first incident a prediction for the evening storm?
Irene and Ed Sandner
Dear Irene and Ed,
Your characterization of the two phenomena as coincidental is correct -- they were not related. The first event was a dust devil. On sunny, tranquil days, a shallow layer of hot air develops above soil that is being strongly heated by sunlight. That air rises and the resultant in-rush of replacement air will sometimes spiral inward, setting into motion the whirlwind that you witnessed. Dust devils develop at ground-level, whereas thunderstorm winds result from the down rush of rain-chilled air that surges outward beneath and ahead of the storm.
Storms headed this way---but Friday has a shot at becoming July's warmest first
An impressive thunderstorm outbreak looms Friday night. The potential for severe weather will have to be monitored. But it's Friday's warm-up which is front and center as the day dawns.Never in the 81 years of weather observations at Midway Airport have Chicagoans found themselves 24 days into the month of July without a reading above 86-degrees on the books. Never, that is, until this summer. But with daytime heating and the healthy assist of strengthening southwest winds, Friday temperatures have a respectable shot at surging past that reading and reaching 88-degrees. The extent of cloudiness filtering the day's sunlight will play an important role in determining just how daytime temperatures go. An 88-degree high would be the city's warmest in the four weeks since June 27 when an 89-degree reading occurred.
Thunderstorms exploded to life in Thursday's unstable atmosphere and doused the evening rush hour over sections of the metro area. Wauconda was hit with 1.67 inches of rain in 20 minutes. Other totals included 1.48 inches at Resurrection High School's Weather Bug rain gauge in Chicago, 1.35 at Harwood Heights where 40 m.p.h. thunderstorms gusts delivered the rain, and 1.26 at Des Plaines and Forest Park. The downpours were selective. No rain was recorded at O'Hare.
The evolving pattern the next 2 weeks is looking wet; regular t-storms could bring 2 to 5-inch totals.
Dear Tom,
The flight times of commercial jetliners flying between Chicago and Los Angeles, cruising at 35 thousand feet, are not affected by jet streams nearly as much as aircraft in the 1950s and 1960s, cruising at 20 thousand feet. Are today's jets flying above the jet stream?
Al Claus, Vernon Hills
Dear Al,
Retired United Airlines pilot Phil Rider tells us today's commercial aircraft are much less affected by unfavorable winds than in the past not because they cruise above jet streams (which occur between 20 and 55 thousand feet) but because of "... better flight planning and the (pilot's) flexibility to choose among more routes and altitudes."
Rider explains that on-board computers can quickly compare flight times over a variety of routes, upper wind forecasts are better and today's flexible air traffic system allows pilots to chose the most advantageous flight path and altitude.
Daytime heating ignited scattered but vigorous thunderstorm development over Chicago's northwest suburbs Wednesday afternoon. The slow moving storms unleashed localized downpours--including the 1.24 inches which drenched west suburban Huntley--at the same time triggering the funnel clouds reported at 1:50 p.m. over Woodstock in McHenry County and around 3:50 p.m. near Ogle County's Rochelle. The funnels--a product of strong thunderstorm updrafts brought on by the fast rate at which temperatures dropped with height Wednesday---also produced small hail, much of it nickel sized. Radar scans at several points during the afternoon indicated the storm's towering parent clouds reached heights of up to 42,000 ft.
In southern Wisconsin's Elkhorn, situated in Walworth County, a thunderstorm cluster stalled producing a deluge which spanned several hours---peaking between 3 and 3:45 p.m. Serious flooding was the result.
Easterly winds off Lake Michigan cooled the air just enough in lakeside counties of northeast Illinois and northwest Indiana Wednesday--that storms were unable to develop.
While haze, low clouds and areas of fog greet area residents as Thursday gets underway, clouds are predicted to break allowing some sunshine to emerge. This should re-heat the still unstable atmosphere, setting the stage for a repeat of the scattered thunderstorm development which drenched part of the area Wednesday.
Dear Tom,
During a big thunderstorm in May, I and my family heard a distinct "pop" or
"click" that was instantaneously followed by a tremendous crash of thunder.
Lightning struck a tree only a hundred feet from the house. What was that
clicking noise?
Richard Johnson, Chicago
Dear Richard,
Lightning expert Ron Holle of Vaisala Inc. of Tucson Ariz., tells us any
source of such a sound requires being within a few hundred yards of the
ground strike point.
Holle explains that "The clicks may have been static discharge from upward
streamers from the house, the ground, or other nearby objects; these are
released just when a flash strikes the Earth's surface. Or, there may have
been a buildup of static charge on parts of the house just before the
strike. Sometimes on (AM) radios, you can hear this buildup as a whine or
series of clicks until the flash strikes the ground."
Several clusters of downpour-generating showers and few isolated thunderstorms are being tracked as we post this at 3:50pm Wednesday--one on the Kane/DuPage County line and still another producing lightning north of McHenry over Wisconsin's Walworth County. The most extensive collection of cells runs from southern Boone into DeKalb and LaSalle Counties. All of these showers and thunderstorms been developing farther inland with time. It's a trend being promoted by westward moving cool air off Lake Michigan which has tended to stabilize the air and limit rainfall in lakeside counties. As of this posting, the tallest and most prolific rain-generating cells are located about 75 miles west of Chicago from near Rochelle to near Compton in Ogle and Lee Counties. Lightning has really been quite limited and had been associated almost exclusively with the Walworth County storm until a series of cloud to ground strokes began in the Ogle/Lee county thunderstorms. Cooling temperatures as we approach and pass sunset will lead to a rapid demise of these cells--with the likelihood scattered thunderstorms will flare again over sections of the Chicago metro area Thursday afternoon with daytime heating.
Track these storms here through this University of Wisconsin-Madison satellite imagery:
http://www.ssec.wisc.edu/~tomw/wisgifloop.html
Several cold air funnels have been reported within showers and thunderstorms which have been forming northwest of Chicago in Wednesday afternoon's unstable atmosphere. Cold air funnels tend not to be damaging--often spinning beneath towering cumulus clouds without touching the ground. Thunderstorms--some towering as high as 31,000 ft.--but topping out at 29,000 ft. as of this 2:45pm posting---have been developing and dissipating across sections McHenry County--some producing nickel size hail. Doppler radar indicates there has been slow southward development---even as remnant showers attempt to move southeast with with northwesterly upper steering winds. Thunderstorms weaken as they move into counties immediately adjacent to Lake Michigan as they encounter cooler air there. Weather Bug sensors are indicating temperatures are in the mid to upper 60s near the lake and in the vicinity of this afternoon's showers and thunderstorms--but in the low 80s at the warmer inland locations which haven't been subjected to cooling rainfall and storm outflows. A funnel cloud was reported around 1:55 pm near Woodstock also in Mc Henry County--something which may happen from time to time with any of these thunderstorms. The situation doesn't support damaging or an organized outbreak of tornadoes.
Temperatures are falling with height at a faster than normal pace which means the atmosphere is unstable. The set-up is just the latest ramification of the cooler than normal air which northwest upper steering winds have continued to deliver and replenish so often this summer. Daytime heating encourages air near the surface of the earth to warm, rendering it buoyant. This heated air then ascends and cools prompting the thunderstorm development we've been observing much of the afternoon. It's a situation likely to spawn additional (though scattered) thunderstorms over at least sections of the Chicago area into evening. The thunderstorm development is being further enhanced by the convergence of ground level winds along inland-moving easterly winds off Lake Michigan. The cooling these winds promote across the counties closest to the lake has been--and is likely to continue--inhibiting thunderstorm development by stabilizing the air. Only sprinkles or light showers--remanants of the west and north suburban thunderstorms---are able to make it into these lakeside counties.
Indications are areas west and north of the Chicago--including DeKalb, Kane, McHenry, Boone, Ogle and Lee Counties will be affected from time to time by these thunderstorm clusters--any of which may spawn some cold air funnels and produce hail and some downpours.
We'll have more on WGN News at 5:30 pm and 9PM tonight--and also a look at a surge of warmth Friday which appears poised to bring us July's warmest temperatures to date. That warmth may be followed by vigorous if not some possibly severe thunderstorms later Friday night into a portion of Saturday.
Tom Skilling
Chicago's official high temperature Tuesday topped out at 80 degrees--only the ninth time readings have reached 80 or higher this month--as driving downpours drenched sections of northwest Illinois and eastern Iowa, with some areas reporting 3 inches or more of rain. Hardest hit was the area surrounding Dubuque and Galena. Totals included 3.10 inches at Asbury and Maquoketa, both in Iowa, and 2.24 inches in Dubuque. The totals equaled a full July's rainfall at many locations and began just after daybreak. Downpours increased, becoming heavy by late morning and through much of the afternoon. The big rains set up in moist, highly unstable air beneath diverging branches of stronger than usual late July jet stream.
The Chicago area is not alone in stunning lack of summer 90s.
Cooler than normal weather has been the rule this summer not only in Chicago, but in much of the eastern half of the nation. While Chicago has logged only about a third of its normal 90s to date, Boston and New York have both recorded 25 percent or less.
The cool summer has impacted Great Lakes water temperatures. Each lake
is running 5 to 9 degrees below levels observed at this time a year
ago. Chicago's water temperature near Navy Pier is 5 degrees off last
year's 74-degree reading.
Dear Tom,
What, if any are the effects of tides on the Great Lakes?
Vince Venturella Carmel, Indiana
Dear Vince,
The Great Lakes, like all bodies of water, are affected to some extent by the gravitational pull of the moon and sun and therefore do experience tides. As on ocean, these tides occur twice daily but, unlike their ocean counterparts that are miniscule and are totally masked by short-period water-level fluctuations caused by wind and changes in air pressure. On Lake Michigan, these tiny tides range from about a half to 1.5inches and are unnoticeable.
Dr. Dave Schwab of the Great LakesEnvironmental Research Lab in Ann Arbor does acknowledge the existence of a larger Lake Michigan tidal swing in the Bay of Green Bay where local geography can generate about a four-inch tide.
Chicago isn't alone with cooler than normal July temperatures. Monthly temperature deficits--a number of them quite impressive--are on the books for July at every major Midwest reporting station. A nationwide analysis of July temperatures from NOAA's Climate Prediction Center indicates nearly half the Lower 48--from the northern Rockies across the Midwest and up and down a wide section of the East Coast--have experienced sub-par temperatures. While Chicago's July 1-20 average temperature is running 5.4-degrees below the area's 138 year average, other July deficits include 6.5-degrees at Marquette, Mich., 6.1-degrees at Cincinnati, 5.7-degrees at Grand Rapid, Mich. and 3.7-degrees at Des Moines, Iowa.
The cool air has had consequences. In its weekly update on U.S. crops, the USDA reported only 26 percent of Illinois' corn crop has reached the pollination stage--the lowest level at this point in a season in 12 years. At the same time, estimates of air conditioning usage based on temperatures to date suggest levels 58 percent off the normal for July and 31 percent off typical summer levels.
Area residents have seen the second fewest 80 degree or higher July temperatures in 81 years---only nine of them! Weather records reveal it's hard to turn the tide on cool patterns once they've dominated this much of the summer season. Of 13 years with comparably cool temperatures through July 21, only 3--just 23 percent---have managed warmer than normal August temperatures.
Dear Tom,
Hurricane forecasting is supposedly better now than ever before, but Hurricane Katrina in 2005 makes me wonder if that is really true. What are your thoughts?
James Van Horn
Dear James,
Hurricane forecasts are more accurate. Dr. Hugh Willoughby, a Distinguished Research Professor in the Department of Earth Sciences at Florida International University, says, "The chances of dying in a hurricane have been reduced by a factor of 100 during the course of the 20th Century."
Indeed, hurricane forecast accuracy is improving at an average rate of one to two percent per year in recent years. Hurricane Katrina in 2005 was very well forecast, even three days in advance. The horrific death toll in Louisiana and Mississippi speaks to lack of adequate hurricane preparedness, not to forecast accuracy.
Unseasonably cool weather continued to grip the Midwest and upper South Sunday, with highs failing to reach 80 degrees as far south as Tennessee. Nashville recorded a record low high of 77 degrees, while Louisville did the same with a high of just 73. Readings fell into the 30s in northern Minnesota Sunday morning, with Embarrass registering a frosty 33 while International Falls logged a record-breaking 37. Since Friday, the highest temperature Chicago could muster was 72 -- great for mid-May, but rather chilly for mid-July.
In stark contrast, intense heat is baking the Southwest. Death Valley recorded its ninth straight day of 120-degree-plus temperatures Sunday with a high of 126. That California desert site hosts the nation's all-time record high of 134 established on July 10, 1913. Record highs also tumbled in Texas, with Corpus Christi at 98, Brownsville at 100 and McAllen at 106.
Readings will gradually warm into the 80s this week in Chicago, but the below-normal temperature trend is expected to continue and 90s aren't due anytime soon. Scattered showers and storms also will affect the city this week, with the most widespread and heaviest activity expected late week.
Dear Tom,
I was reading my mom's old Nancy Drew book that mentions a cloud formation caused by "orographic uplift." What is that?
Chloe Brougham, 5th grader
Dear Chloe,
Orographic lift is a rising air flow caused by mountains. As air moving with the prevailing wind flow encounters a mountain range, it is forced to rise. This causes it to cool and condense, forming clouds and eventually rain if the air contains sufficient moisture. This process, known as orographic lift, is a major precipitation-producing mechanism. In the foothills of the Rockies, east winds push air up the eastern slopes, producing thunderstorms in summer and foggy, drizzly, rainy and snowy weather in winter. Orographic lift accounts for much of the precipitation in some of this planet's rainiest locations including Mt. Waialeale in Kauai, Hawaii; Mawsynram, India; and Lloro, Columbia.
July begins climb to regain summer status
So far, July 2009 has been a cool one with temperatures averaging well below normal. Though no 90 degree days appear to be in the immediate picture, there is growing confidence that readings will return to normal midsummer levels in the middle 80s this week. Though Sunday's mid-70s high will be warmer than Saturday's 72-degree reading, high temperatures will still be nearly 10 degrees below typical mid-July levels. Under clearing skies and a light wind regime, lows Sunday night are expected to dip into the middle 50s threatening Monday's record low of 53 degrees established in 1970.
As the week progresses, the center of the cool air will slide slowly east and southerly winds will bring an increase in both warmth and humidity. Early in the week precipitation will be limited to scattered hit-or-miss afternoon and evening thunderstorms. A more significant system is expected by week's end bringing with it a threat of more widespread thunderstorm activity.
Dear Tom,
You recently mentioned that the summer of 1947 was cool, much like this summer, but didn't it turn hot in August?
--A.J. Simmons
Dear A.J.,
It certainly did. The summer of 1947 was running cool with only six days of 90 degrees or higher on the books through the end of July. Every month since February had logged below-normal temperatures. August appeared to be following suit, opening on a chilly note with a record low of 52 and an afternoon high of only 78 on Aug. 1. However, the weather pattern changed dramatically and blistering heat swept into the city. August 1947 stands as the city's hottest August on record with an average temperature of 80.2 degrees and the third-warmest month on record here. The month had 18 days with highs of 90 degrees or higher, including four days in the 100s.
What a summer! Many Chicago area residents are just shaking their heads -- some pleased by the lack of heat, others disappointed at the failure of hot weather to gain a foothold here. Extremely rare mid-summer lake-effect rains were pouring down on sections of La Porte and Berrien Counties in Indiana and Michigan Friday evening -- just the latest meteorological twist in a summer of topsy-turvy weather across the region.
July has slipped to the coolest to date here in 42 years -- its 68.7 degree average temperature running nearly 5 degrees behind the long-term (138-year) average. Friday's 70-degree high was the first time in 53 years a July 17 temperature failed to rise above 70 -- you'd have to travel back to a 64-degree high 85 years ago to find a July 17 that was cooler. In Rockford, Friday's 67-degree high broke the record for the date, becoming the coolest July 17 high on the books. The reading was Rockford's fourth record-low daytime maximum to fall since June 30.
July's average Chicago highs rank among the two lowest in 50 years at O'Hare
The average high for July's first 17 days has been 77.5 degrees -- the second coolest in the 50 years of O'Hare Airport weather records dating back to 1959. Only 1967's 76.2-degree tally has been cooler.
Dear Tom,
We're in a dry climate here in Arizona, with temperatures in the 100s but low humidity. However, in your Chicago weather reports I never see a reference made to humidity levels. Why?
Elise Pearce
Dear Elise,
Much of Arizona lies in the Sonoran Desert, whose arid climate is so radically different from Chicago's that words like "humid" (and "hot") have very different implications in the two areas. We do use humid in our Chicago forecasts, but only when the dew point temperature nears or exceeds 70 degrees, a level rarely achieved in desert cities like Phoenix or Tucson.
In Chicago, "hot" appears in weather forecasts when readings rise to the lower 90s or higher, but in Phoenix that term is used only in reference to temperatures near or above 110 degrees. Here's a typical Phoenix forecast: "Mostly sunny, with a high near 104" -- no mention of heat.
It's not often a set of mid-to-late July days produce temperatures which fail to break above 70-degrees. Yet that's what's predicted for the Chicago area Friday and Saturday. A mass of unseasonably cool air, which only days ago was 1,100 miles north of Chicago over Canada's chilly Hudson Bay, is riding well developed northwest winds associated with a buckling North American jet stream into the city. It's a development likely to deliver a good deal of instability cloudiness and passing light showers in addition to highs of 68-degrees Friday and 70-degrees Saturday--the city's coolest mid-to-late July two-day spell in 28 years. On only three other occasions over the 81 years in which temperatures have been recorded at Midway Airport have back-to-back days at this point in a summer failed to break above 70-degrees.
Thunderstorms erupted over sections of the Chicago area late Thursday. Radar scans put maximum cloud tops at 36,000 ft. Winds gusted to 60 m.p.h. as the storms swept across south suburban Crete Thursday evening. Earlier, 55 m.p.h. had been clocked west of the city in Glen Ellyn and 38 m.p.h. winds raked Midway Airport. Small hail accompanied some of the storms and downpour just at Plainfield totaled 1.17 inches in just 22 minutes right before 7 p.m. Valparaiso, Indiana was doused with 0.88 inches of rain in 20 minutes.
A buckling jet stream is behind the cool-off here and heat plagued Oklahoma and north Texas, where a powerful storm hit Thursday lowering Oklahoma City's temperature from the day's high of 99-degree to 82-degree by nightfall. Coming days will see the country's hot air shift west into the Rockies and Southwest while May level temps take over across into the weekend in the Northeast U.S.
Dear Tom,
I heard that the first leg of I-355 was finished early during the summer of 1988 because of good weather. How did that summer compare with other summers?
Tom Muelleman West Chicago
Dear Tom,
The summer of 1988 provided a mixed bag for road crews. On the plus side it was a drought summer with very few rainy days to halt construction. During a typical Chicago summer measurable rain falls on about 30 days, an average of 10 each month, but the summer of 1988 had measurable rain on only 17 days--4 in both June and July, and 9 in August--so there was little down time because of rain. On the negative side was the summer's intense heat that created brutal conditions for outdoor work. Officially at O'Hare International Airport there were 47 days with highs of 90 degrees or higher including 7 days with triple-digit heat.
The cool summer of 2009 is poised to deliver another round of below normal, May-level daytime readings Friday and Saturday.
Thursday's predicted 82-degree high may be the last 80-degree reading likely to occur in the Chicago area until Tuesday. With extensive cloudiness expected to accompany the abnormally cool incoming air mass, readings will be hard-pressed to break above 70 degrees. Temperatures at such levels this time of the year are truly rare--81 years of weather observations at Midway Airport show only six instances in which back-to-back highs in late July have been as cool as the 68-degree and 70-degree highs predicted for Friday and Saturday. A buckling jet stream is behind the predicted cool-down. Northwest winds stacked vertically from the ground tens of thousands of feet aloft assure the flood of cool air from Canada will be hard to stop.
Widespread cloudiness and the fact that Midwest days are nine hours shorter than those in the Arctic this time of year are among the factors helping southbound air masses to grow cooler as they sink into our area.
Dear Tom,
Denver, in the rain shadow of the Rocky Mountains, would be much wetter if the mountains weren't there. How would their absence change Chicago's weather?
Richard Waller
Dear Richard,
Oriented perpendicular to the prevailing westerlies, the Rocky Mountains massively influence the climate of Chicago and, indeed, of North America. The western highlands inhibit the eastward movement and moderating influence of mild, moist Pacific air; to their east, the Rockies encourage the north/south transport of arctic and Gulf air.
In their absence, the more extreme aspects of Chicago's climate would be greatly diminished. The city would experience milder winters and cooler summers, less abrupt day-to-day temperature swings, far fewer severe thunderstorms. Gone: the big winter storms ("Panhandle Hooks") that swirl into the city from their birthplace in the lee of the Rockies.
Here are some unofficial Chicago area rainfall tallies from this morning's pre-dawn rains from our Weather Bug observation network:
O'Hare 0.68"
Lake Geneva, WI 0.66
Valparaiso, IN 0.65"
Chicago (Robert S.
Abbott School) 0.60"
Darien 0.58"
Rockford 0.55"
Highland, IN 0.54"
Wilmette 0.54"
Wadsworth 0.52"
Trevor, WI 0.47"
Kenosha 0.47"
Carpentersville 0.47"
Long Grove 0.46"
Park Ridge 0.45"
Itasca 0.45"
Munster, IN 0.42"
Glenview 0.42"
Niles 0.41"
Lombard 0.40"
Des Plaines 0.40"
River Grove 0.39
Lake Villa 0.39"
Alsip 0.39"
Chesterton, IN 0.39"
Morton Grove 0.39"
Gary, IN 0.38"
Henry, IL 0.38"
Marseilles 0.37"
LaGrange 0.37"
Waukegan 0.37"
Janesville, WI 0.35"
Burr Ridge 0.35"
Oak Lawn 0.35"
Westchester 0.35"
Lombard 0.35"
Elgin 0.33"
Mundelein 0.33"
Lansing 0.33"
Lombard 0.33"
Elmhurst 0.33"
Naperville 0.33"
May-level temperatures arrive as a pool of cool mid-summer Canadian air settles into the area late this week into the weekend. We'll have more on that predicted cooling on our WGN News programs at 5:30 and 9pm. See you then!
Tom Skilling
Thunderstorms--some potentially strong and capable of generating wind-driven downpours and hail--greet at least some residents of the Chicago area as Wednesday dawns. Atmospheric moisture levels have surged to Gulf Coast levels just west of the area overnight. Up to 1.84 inches of evaporated moisture is available for thunderstorms to tap. An expeditious retreat of these storms is predicted by mid morning, clearing the way for emerging sunshine and the most humid surge of air this month. Dew points, a measure of atmospheric moisture, are to rise above 70-degrees for first time in two weeks--levels comparable to those found on the Gulf Coast. At the same time, sunshine is to send temperatures to within striking distance of 90-degrees. And, despite predictions of May level temperatures with an impressive late week and early weekend cool-off here, weather records reveal hot temperatures aren't history just yet. In 81 years of observations at Midway Airport, only one year---1967--- has failed to produce at least one 90-degree high beyond July 15.
Flooding rains drenched sections of Minnesota Wednesday. Pillager, Minn.--not far from Duluth---was swamped by 6.30 inches of thundery rainfall. Near Brainerd in the northern section of the state, 5-inch rains were common---much of it falling in just 2 to 3 hours.
Dear Tom,
The hurricane season in the Atlantic Basin seems to be quiet this year. What is the prediction for the rest of the season?
T. L. Oksana, Dundee, Ill.
Dear T.,
Forecasters at the Climate Prediction Center anticipate a near-normal hurricane season. They ascribe a 70 percent chance to the occurrence of 9 to 14 named storms somewhere in the Atlantic Basin, of which four to seven will be hurricanes (sustained winds of 74 m.p.h. or greater).
However, the outlook for the 2009 hurricane season is a tough call because of the presence this year of "competing climatic factors." On the one hand, a 20-40 year cycle of enhanced hurricane activity began in the Atlantic Basin in 1995. Conversely, an El Nino event is now under way. El Nino suppresses hurricane activity by increasing vertical wind shear (change of wind speed with height) over the tropical Atlantic Ocean.
Here's an update from the WGN
Weather Center on the latest thinking on the threat for severe
weather in the Chicago area. As indicated earlier, this threat
first materializes in the hours toward dawn Wednesday. The latest suite of
computer projections and our in-house analysis of the situation puts the focus
for any turbulent weather from 4 to 8 a.m.Wednesday morning.
Though clearly
outside the period of peak solar heating, an influx of warmer, more humid air
predicted to be ongoing by then will have the atmospheric moisture content (the
"precipitable water" values) surging to 1.84" of evaporated water at the same
time a pocket of especially strong jet stream winds approaches from the west.
Air sinks immediately below the front quadrant of such band of strong upper
winds commonly referred to by meteorologists as a "jet streak". But, this large
scale sinking of air actually enhances the upward motion of the air just ahead
of such a feature--which is where the Chicago area will be situated Wednesday
morning.
The 4 to 8 a.m. window would be the period in which the ascension of
air would be at its peak and, therefore, potentially the most supportive of
thunderstorm formation in the increasingly moist air expected to be pouring into
the area. In addition, the faster than usual vertical temperature decline--in
other words the instability of the atmosphere-- projected by computer models as
well as energy calculations (so-called "CAPE" values--an acronym for "Convective
Available Potential Energy") 2500 joules per kilogram--1000 is widely viewed as
the severe thunderstorm threshold--appear likely to be supportive too. Many of
the critical values we examine for severe weather generation are fairly
comparable to early this past Saturday when predawn thunderstorms roared across
part of the Chicago area generating 1"+ downpours and hail.
We'll be updating this on tonight's 5:30 and 9PM WGN News programs.
An
impressive line of thunderstorms developing from South
Dakota into Minnesota as we post
this at 3pm Tuesday appears to be the first stage of the thunderstorm cluster
expected to reach at least sections of the Chicago area early Wednesday and Doppler
scanned cloud tops there have already reached 48,000 ft. The weather system
producing these storms has a history of large hail production--having produced
4.25" diameter (softball size) hail in sections of Wyoming and South Dakota Monday evening.
Tom
Skilling
Tuesday's sunshine and comfortable temperatures and humidities belie the severe weather threat predicted to come together after midnight and into the predawn hours Wednesday. That's when strong fast-moving thunderstorms are to sweep across the Chicago area from the west, possibly delivering downpours, hail and potentially strong winds. The vigorous, jet stream-borne disturbance expected to ignite these storms was behind an eruption of severe weather Monday afternoon across the northern Plains and was still in progress as night fell. Several storms generated cloud tops scanned by radar to have heights of 67,000 feet. The worst of these storms unleashed grapefruit to softball-size hail that broke windows in buildings and cars at Hulett, Wyo. Another bombarded St. Francis, S.D., with hailstones the size of baseballs. At least five reports of twisters were filed with the Storm Prediction Center across four states, from Montana to the Dakotas, a region covered by five severe weather watches-two for tornadoes and three for thunderstorms.
This summer's days have generated much larger temperature deficits than the nights
Chicago is in the midst of its coolest July open in 40 years. The month's average temperature through late Monday was 68.5-degrees---four degrees below the long-term (50-year) average at O'Hare International Airport. The month has produced fewer than half the 80-degree or higher temperatures during the same period a year ago. The cool readings have had a beneficial effect. It's estimated these lower temperatures may have reduced the need for air conditioning by more than half.
Dear Tom,
I just read that we should expect an El Niño winter in 2009-10. Does this mean warmer temperatures for the upcoming winter season? Are we looking at more or less snowfall?
Dan Becker, Gurnee
Dear Dan,
Indeed, an El Niño event (abnormal warming of water in the equatorial belt of the Pacific Ocean westward from South America) began in June, is likely to strengthen in the autumn and persist through winter. That's the word from the National Weather Service's Climate Prediction Center.
El Niños have little impact on U.S. temperature and rainfall patterns during the summer and early autumn, though they can help to suppress Atlantic Ocean hurricane activity.
Late autumn and winter is another story. At Chicago, "El Niño winters" are usually milder and less snowy than normal, sometimes dramatically so if the El Niño event is especially strong. We'll keep you posted.
You keep looking as a forecaster for signs that heat's imminent. They just aren't there--at least at this time. That certainly doesn't mean we'll escape the summer with no hot weather at all. But the beautiful weather of late, free of a single spell of oppressive heat, is--putting it mildly---a bit remarkable! After all, if it's going to get hot here in the Midwest, this is the time of year it happens---and there are simply no obvious signs temperatures are likely to surge--other than a brief mid to upper 80 spell Wednesday. The heat continues in a pool hovering over the southern Plains and, at least up to this point, is showing no signs of staging a move on the Chicago area or this section of the Midwest anytime soon.
The stats on this summer's temperatures are really quite impressive. July's opening 12 days have seen half the number of 80+ highs of the same period a year ago and average 4.1-degrees below normal and 3.9-degrees behind the same period a year ago. While hot weather afficianados lament the cool summer weather, others relish it and appreciate the savings on summer air conditioning. Estimates based on temperatures put air conditioning use in July less than half normal (47%) and only 78% normal since June 1. The necessity for air conditioning usage this summer is estimated to be running 26% behind the same period a year ago.
Wet soils may be playing an important role in the cool summer pattern--though there are no doubt other factors at work as well. A feedback develops when soils are wet--and record spring rainfall here assured moisture levels were elevated heading into the warm season. As hot air makes a move on the region, moisture is returned to the atmosphere from the wet soils and transpired back into the air by plants with roots in these wet soils---a process which leads to clouds and thunderstorms which, in turn, mixes down cooler air and moderate the advancing heat. Our Frank Wachowski reported to us just last week that July's opening 9 days were the month's cloudiest in 40 years, confirmation that cloud cover has been way up and sunshine way down.
And, even as the southern Plains continues broiling in wilting triple digit heat, our section of the Midwest is not---staying instead emminently comfortable with easterly lake breezes to blow well into Tuesday and limiting shoreline temperatures to the 70s..
We're analyzing computer projections which include the onset of an unseasonably cool mid-summer air mass later this week into the coming weekend. We'll have more on that air mass and look the potential for some active or severe thunderstorms Tuesday night and possibly again midday Wednesday on our evening television shows (at 5:30 and 9 p.m.) and right here on our WGN weather blog.
Hope your Monday is going well!
Tom Skilling
The latest attempt at a warm-up in Chicago looks like it will be short-lived. Canadian-source high pressure will hold Monday, continuing a cooling weak northeast wind off Lake Michigan. As the high moves east, winds pick up from the southeast and finally become southwest later Tuesday, allowing warm, moist air to feed into northern Illinois. The approach of a cold front from the northwest will trigger a band of showers and thunderstorms which should hit the Chicago area Tuesday night and early Wednesday. Cool dry high pressure will follow the cold front and hold sway over the Midwest and Great Lakes into next weekend. The southern third of the United States will continue to swelter--in the 90s across the Southeast and more than 100 degrees in the Southwest.
Cool temps not really that unusual here
While this summer has been cool, a closer look at Chicago's 139 years of records shows that the overall average temperatures during the first half of summer 2009 actually barely fall in the coolest third with 91 warmer and 47 cooler. In the 51 years of observations at O'Hare Airport, 2009 ranks as the 14th coolest.
Dear Tom,
I received an e-mail saying Mars will be very close to the Earth on Aug. 27 and will appear to the naked eye to be as large as the full moon. Is this true?
Carol Notley
Dear Carol,
It's an e-mail hoax, and it keeps popping up. Containing phrases like, "The Red Planet is about to be spectacular" and, "On August 27th Mars will look as large as the full moon" and, "No one alive today will ever see this again," that e-mail has garnered international attention -- but it's just not true.
It all started when, on Aug. 27, 2003, Mars orbited within 35 million miles of Earth, its closest approach in just under 60,000 years. At that time, Mars did appear as large as the full moon -- but only when viewed through binoculars or a telescope at 75-power magnification. Mars made another close approach to Earth in December 2007: 55 million miles. These approaches had no effect on Earth.
While the southern half of the country continues to bake, cooler Canadian-source high pressure looks to dominate Chicago and northeast Illinois weather during
the week ahead.
The weather pattern along the Gulf Coast states and the southwestern U.S. is expected to change very little, so daily highs in the 90s over the Southeast and the lower 100s over the Southwest should persist.
But across the northern tier states, including Illinois and Indiana, average highs are forecast to remain at or slightly below mid-July normals.
Cool summer in city
Chicago's average temperature from June 1 through July 10 has been 67.5 degrees. This is the coolest since 1992, which followed the eruption of Mt. Pinatubo in 1991 north of Manila. That eruption spread a layer of dust aloft around the world that cut back on the amount of solar radiation. This year's average summer-to-date temperature is 2.1 degrees cooler than the normal of 69.6 degrees and only 0.9 degrees above the 66.6 recorded in 1992.
This year's average temperature as of July 10 was 2.1 degrees cooler than the normal of 69.6 degrees and only 0.9 degrees above the 66.6 recorded in 1992.
Dear Tom,
For gardening purposes, there are light requirements for plants for which I cannot find answers. When is the hottest time of the day, and when does the hottest time of the day give way to less intense sun?
--Linda Davis
Dear Linda,
Daytime air temperatures usually lag a few hours behind the intensity of sunlight. On any given day, assuming no air pollution and no cloudiness, the most intense sun is always when the sun is highest in the sky. This occurs in Chicago at about noon (1 p.m. daylight-saving time). In the summer, the hottest time of the day in Chicago is usually within an hour of 4 p.m., but, depending on the weather, the time of the highest temperature can vary greatly. On rare occasions, it can even occur at night. By convention, the maximum temperature is the highest reading attained during the 24-hour calendar day.
July's opening days have hit Chicagoans with a double meteorological whammy: subpar temperatures and far less sun than normal. The area has seen only 35 percent of its possible sunshine from July 1 to 10. Not since 1969 has the period hosted so little sunlight. A typical July sees 68 percent of its possible sunshine.
Saturday morning's clouds and scattered showers give way to sunshine and declining humidities by afternoon. A pre-dawn cold frontal passage is allowing drier Canadian air to move slowly into the area -- an air mass that will dominate the weekend and produce warm midsummer temperatures.
But while seasonable readings are predicted here, a fourth consecutive day of brutal heat is to blaze across the central and southern Plains. In Oklahoma, Friday thermometer readings topped out at 112 degrees at Gage and 111 degrees at Enid, Medicine Lodge and Clinton. Warnings for excessive heat cover sections of states from Arizona to Texas, Louisiana, Oklahoma and Arkansas.
Dear Tom,
On April 25, Chicago's temperature dropped 28 degrees in 14 minutes. What is the greatest change on record?
Ryan Thomas, Chicago
Dear Ryan,
Lake breezes aside, sudden temperature declines around here are not confined to any particular time of the year, but they do show a marked preference for the spring, and Lake Michigan is the culprit.
Very cool air approaching Chicago from the north travels the full 310-mile length of Lake Michigan, whose water in the spring is still cold. Such air arrives at Chicago with unabated chill. If the city happens to be enjoying a warm spring day, the arrival of cold air from the north sends temperatures plunging.
This happened on May 9, 1963 -- the date of Chicago's most stunning short-term temperature drop. Beginning at 1:47 p.m., the temperature at Grant Park plunged 22 degrees (from 84 to 62) in 150 seconds.
The Chicago area has been cleared of the severe weather threat----morning and early afternoon showers and south suburban thunderstorms have stabilize the atmosphere by introducing rain-cooled air. An extended break in precipitation is underway which will last through the afternoon and evening, removing any threat of weather troubles for those with plans to be outdoors. But, thunderstorms may not be entirely over across the metro area. Scattered thunderstorms could may re-develop late Friday night in the humid environment predicted here night over 30 to 40% of the metro area. And, despite their limited areal coverage and the fact they are to occur beyond the period of peak heating, a few of the heavier thunderstorms may still be capable of localized downpours, gusty winds and even some hail. Forecasts of the atmosphere's ability to generate upward motion of the humid air predicted in the area, as reflected in predicted CAPE (Convective Available Potential Energy) values from various models plus predictions of late Friday night atmospheric stability, still appear supportive of at least some thunderstorm development. In addition, surface winds are to converge along a southeastbound cold front which enters the area toward morning. When winds converge at the surface, air if encourage to rise and cool---a development which fosters cloud and thunderstorm formation. If I had to put times on any thunderstorm threat, I would suspect a few isolated thunderstorms could bubble up over part of the metro area between 10 and 1 am--but that the period from 1am to 5 am is the one to be most closely watched for thunderstorms. Showers may extend beyond 5am into mid morning Saturday before clearing takes place and a rain-free period begins.
A short explanation of how we've gotten to where we are as we post this at 3 pm Friday. Among thunderstorms' functions in nature is to exercise a measure of control over extreme heat. As products of the tallest clouds on earth, thunderstorms are able to "mix" cooler air down to the ground from great altitudes quite efficiently with their rains and downdrafts. The process slashes the vertical temperature decline which so critical to additional thunderstorm development. There are instances in which this cooling can be overcome by daytime heating once storms clear an area. That had appeared a stronger possibility earlier today than it does now. New data makes clear the diminishing area of showers, some with thunder in some west and southwest suburbs, which passed between mid morning and 1:30 pm this afternoon, proved more effective at stabilizing the atmosphere than first predicted. The extended break in rainfall currently underway and predicted to continue through this evening over most of the area, is one result. The probability that organized severe weather won't develop through this evening and early tonight is another.
The rains which fell fairly lightly across the city earlier today were actually leftover from some impressive thunderstorms which moved out of Iowa overnight and this morning. Clouds tops were collapsing and the areal extent of the rain was shrinking by the time it arrived in the Chicago. But to our west, where the rains were heavier and better organized, there have been some impressive rain totals. Unofficial totals off our Weather Bug network indicate Marseilles had 1.02", Henry, Il 0.57", Rockford 0.52" Minooka 0.30", DePage Airport 0.29" , Sandwich 0.25" and DeKalb 0.22".
We'll have more on the thunderstorm threat later tonight and the full weekend weather outlook (a bit of instability could lead to isolated t-storms over several sections of the Chicago area Sunday afternoon) and a the longer range weather picture on tonight's 5:30pm and 9Pm News programs.
Severe thunderstorms threaten to rake sections of the Chicago area Friday. The warmest temperatures here in two weeks this afternoon are to heat an atmosphere dripping with 2 inches of evaporated moisture, initiating the process of thunderstorm formation. Air rises and cools when that happens, especially when temperatures decline with height at a faster-than-usual rate. The atmosphere is labeled unstable by meteorologists in this situation. Add an unusually energetic jet stream with winds diverging overhead and the stage is set for atmospheric fireworks. That same roster of conditions late Thursday generated 52,000-foot-tall thunderstorms that bombarded north-central Iowa with golf-ball-size hail and wind-driven downpours. Weather watches may become necessary in the Chicago area later Friday and Friday night as the volatile situation unfolds.
Where are the 80s?
Thursday's official Chicago high fell just short of 80 degrees--hitting 79. There have only been 20 days with readings 80 degrees or warmer at O'Hare International Airport (31 is average). It's the lowest tally of 80-degree-or-higher days here in 35 years--since 1974 when July 10 arrived with only 19 days of 80s on the books.
Dear Tom,
A Chicago suburb recently published a newsletter advising that a viaduct or highway underpass offers safety from an approaching tornado. Isn't this ill advised
Carole Jacobsen, Carpentersville, Ill.
Dear Carole,
Your question comes up frequently, and the answer deserves repeating: It's a very dangerous thing to do. Contrary to popular belief, expressway underpasses do not offer safe shelter from the high winds of tornadoes or severe thunderstorms.
Because of channeling and funneling, winds actually blow stronger when they sweep through expressway underpasses and beneath bridges, thereby increasing the risk of injury from airborne debris.
Misconceptions about the safety of underpasses probably originated with a widely circulated video of motorists rushing to an underpass in order to avoid a tornado that was crossing Interstate 35 near Wichita, Kansas, on April 26, 1991.
Severe weather threatens the Chicago area Friday and Friday night--though any storms which hit are likely to hit the area in fairly distinct clusters separated by MANY rain-free hours. At the moment, it's the late day/Friday night storm threat which concerns us most, since it will be able to tap daytime heating and is to take place in an atmosphere dripping with an impressive 2" of evaporated water. It's been my experience the 2" precipitable water values are often in place and have served as a proxy indicator of some of this area's most active severe weather outbreaks. That's A LOT of water to have floating around for developing thunderstorms to tap. Add to that a powerful, spring rather than summer intensity jet stream overhead, and the reason for some concern about the predicted atmospheric set-up is obvious.
The picture on just when thunderstorms might sweep the area Friday is becoming clearer and I thought I'd share with you some of our thoughts on this as we continue our afternoon/evening analysis. Two periods emerge as being at elevated risk--the first mid and late morning when a rapidly diminishing area of initially strong thunderstorms (strong to our northwest, to be precise) is expected sweep in from the west and northwest, exiting midday or shortly thereafter----the second and potentially most worrisome in terms of severe weather, could begin with a few t-storms erupting with peak heating later Friday then build into more organized storms Friday night. The University of Illinois WRF model runs suggest atmospheric energy as indicated by the CAPE index could reach or exceed 2000 joules/kilogram toward 9 to 10 pm Friday evening--well over the 1,000 joules per kilogram often viewed as the severe weather threshold. If true, the second potential squall line could be a nighttime affiar.
A noteworthy caveat on Friday's storm risk is revolves around the strength of the initial storm band--most likely a morning phenomenon. Late model trends suggest this cluster--or what remains of them---will be crossing the area mid and late morning in a far weaker state than the powerhouse complex of storms from which it is to originate overnight in Minnesota and far western Wisconsin. Weakening storms would NOT cut seriously into daytime warming and would therefore not hinder secondary storm development later in the day. But if any storms arrive here Friday morning stronger than currently predicted, they might well interfere with late day storm re-development. So, this will have to be watched.
We'll have much more on the Friday storm threat on our WGN weather segments at 5:30 and 9 pm, here on the wgntv weather blog and with Steve Cochran on WGN radio at 6:05 pm.
Tom Skilling
Dear Tom,
A friend and I have long debated the meaning of a 30 percent chance of rain. Can you help?
Randolph Penna
Dear Randolph,
Regardless of its accuracy, a weather forecast fails if the user does not understand the forecaster's words. The proper interpretation of a 30 percent chance of rain (assuming the forecast verifies perfectly) is that you will have rain on your head three out of ten times that you hear such a forecast.
The forecaster may believe rain will cover 100 percent of the area if the rain arrives, but his confidence that it will arrive is only 30 percent. Alternatively, the forecaster might have great confidence that rain will occur, but he believes it will be scattered showers affecting only 30 percent of the area.
Regardless of the forecaster's rationale, the meaning for you is always the same: The chance of rain on your head is 30 percent.
East winds off Lake Michigan had temperatures on a downward trajectory Tuesday afternoon. Instead of warming, readings slipped nine degrees at O'Hare International Airport over 5 hours---falling slowly from 77 degrees at noon to 68 by 5 p.m. An incoming overcast and a few sprinkles combined with the air flow off the cool lake water, which still hovers in the 60s, reversed the typical surge in temperatures that occurs through mid and late afternoon. The resulting May-level temperatures spill into a second day Wednesday with lakeshore readings unlikely to escape the 60s because of extensive cloudiness expected to limit any warming sun. Cool winds off the lake have a stabilizing effect on the atmosphere, slowing the upward motion of air that might otherwise produce a few thunderstorms. This will deflect the day's southeastbound thunderstorms well west and south of Chicago, leaving sprinkles that could build to a few showers. The dominance of cooler than normal temperatures has produced Chicago's coolest July in 25 years.
Wildfires in Alaska
Visibilities from Prince William Sound into Alaska's interior have been limited in recent days as smoke from 64 active wildfires burning in the state settled over the region. In the state's Interior, Fairbanks---where temperatures reached the 80s Tuesday---reported visibilities slashed to just 5 and six miles. McGrath, Alaska recorded an 88-degree high.
Dear Tom,
What is the difference between isolated and scattered thunderstorms?
Doug Porter, Utica, Ill.
Dear Doug,
In meteorological jargon, scattered and isolated describe the percent of the forecast area experiencing thunderstorm rainfall at any given moment.
Scattered thunderstorms are those whose areal coverage 10-50 percent and whose occurrence across the landscape displays no organization (such as lines or clusters), and they randomly cover 10-50 percent of the forecast area. Isolated thunderstorms are "loners," well removed from any others and affecting less than 10 percent of the area.
The terms scattered and isolated refer only to areal coverage and do not address other thunderstorm issues (such as storm severity, lightning production or rainfall intensity). The parent thunderstorm of the devastating Plainfield tornado of Aug. 28, 1990, that claimed 27 lives was an isolated storm.
July is off to cool start, averaging more than 5 degrees below normal. The month's first six days have come in at just 67.3 degrees, well short of the 50 year average of 71.6 at O'Hare International Airport. This makes the opening six days of July 2009 one of the eight coolest July openings since weather observations began on the Northwest Side in 1959. Monday's high of 86 degrees and the 81-degree high on Sunday provided quite a contrast to July's cool trend. Monday was the warmest day of the month to date.
But 80s aren't likely to be repeated Tuesday. The re-emergence of northeast winds off Lake Michigan brought on by Canadian high ridges southward into the Chicago area, pressure will slash Tuesday afternoon's highs by at least 12 degrees even as blistering heat expands into the Plains. The developing clash in coming days between the two widely varied air masses will promote the formation of thunderstorm clusters--expected to initially track to the west and south of Chicago, brushing the area with several showers Tuesday and Wednesday. But by Thursday, the growing dome of heat threatens to nudge storms farther north, potentially affecting Chicago.
Latest stats confirm summer among the coolest/wettest on the books here
The three month meteorological summer period, which gets underway June 1, has averaged 67.5-degrees--more than degree below normal and ranks among the coolest third of all summers on the books here since 1871. The 7.43 inches of rain nearly twice the 4.34 long term average and places the period since June 1 among the wettest 9% on the books.
Dear Tom,
The National Weather Service website provides two short-range radar images: base and composite reflectivity. What is the difference?
Dave Makarski, Arlington Heights
Dear Dave,
Radars transmit a beam of energy, some of which is reflected back to the radar site when the beam strikes an object (such as a raindrop). Reflectivity, a measure of the amount of energy that returns to the radar, is greatest when raindrops and cloud particles are both large and numerous.
The radar beam is first transmitted at 0.5 degree above the horizontal, making a full circular sweep, followed by additional sweeps at progressively greater tilt angles. Base reflectivity refers to reflectivity when the beam is elevated 0.5 degree; its data indicate rain reaching the ground. Composite reflectivity displays in a single picture the highest reflectivity values from all beam tilt angles; it describes the storm's total water content.
A sunny and warm Sunday afternoon turned showery late as rains developed along a wind-shift line moving southeast out of Wisconsin. Though the showers were low-topped and produced little thunder and lightning, they did spawn numerous funnel clouds west of the city along a corridor between Interstate Highway 39 and the Fox Valley. These funnels were "cold air-type funnels," the kind not associated with severe thunderstorms that almost always dissipate without producing any damage.
After a chilly 4th of July, temperatures rebounded into the comfortably warm lower 80s Sunday, a level expected to repeat here through midweek. Hot weather has been noticeably absent from the city since a streak of 90s in late June, but the latest suite of computer forecasts hints at a brief surge of hot weather that should reach the city by Friday. The downside to the expected warm-up will be a threat of showers and thunderstorms that will continue to add to the city's growing 2009 precipitation total that has now reached 26 inches -- nearly three-quarters of the city's normal annual total of 36.27 inches.
Dear Tom,
On average, how many days a year does Chicago experience fog?
Alice H.
Dear Alice,
On many cool, calm mornings around sunrise, parts of the Chicago area, especially in the suburbs, will experience ground fog as the air cools to saturation. The fog usually burns off quickly, creating only brief inconvenience. There are many other days where light fog is present from a variety of causes resulting in reduced visibility. However, there are only an average of about 12 days each year when Chicago is socked in with dense fog (visibility one-quarter mile or less) that seriously impacts transportation. Dense fog is most common here during the cold season, frequently occurring when warm, moist air passes over cold snow-covered ground. December through March hosts a majority of the area's dense fog occurrences with each month averaging about two days.
Chicago's 4th of July weather was a bummer: cool and rainy. The official high of 69 degrees at O'Hare marked the first time since 1997 (66 degrees) that the holiday failed to break 70, and the 0.20 inches of rain was the most since 1.72 inches fell in 1995. The only positive note was that most of the rain ended during the early evening, a few hours before scheduled holiday fireworks. Warmer weather is on the horizon as a persistent dome of hot air that has been baking the central and southern Plains and lower Mississippi Valley with record-breaking heat appears to be making a move toward the area. Triple-digit heat shattered 4th of July temperature records in Texas Saturday, led by a 107-degree high at McAllen. Chicago-area temperatures are expected to surge into the 90s by Thursday and again on Friday for the first time in two weeks. Gulf-level dew points in the 70s will accompany the heat, assuring that recently silent air conditioners will be humming again. The heat will also bring another round of thunderstorms -- adding to area rain totals that are nearly 9 inches above normal for the year.
Dear Tom,
Why does the air sometimes smell like "rain"?
Laura Farr, Berwyn
Dear Laura,
Theories abound about the "smell of rain", but so far there is no definitive answer. Many feel that the dominant vegetation of a region contributes to the odor. The reaction between the moisture in the air and certain volatile substances on plants appears to be a partial explanation. Pine forests, for example, release terpenes (substances found in perfumes or medicines) into the air. Some olfactory specialists believe that moisture, warmth and low pressure, all harbingers of rain, enhance our sensitivity to smell while hastening the release of fragrant molecules from plants. Once the rain begins, the drops can kick up ground particles that add to the aroma. Some suggest that raindrops push earthy smelling gases produced by streptomyces bacteria out of the soil.
Eastbound low pressure, which first developed late Thursday with an eruption of thunderstorms across the western Plains, is behind Saturday's gray, cool weather so reminiscent of much of this area's spring and early summer. Blazingly hot air charged with a huge supply of tropical moisture is fueling Saturday's cloud and shower-producer here. But, its heaviest downpours are to drench Missouri, Downstate Illinois and Indiana with thundery deluges that may deposit local rains of 4 inches or more. More conservative totals appear a good bet in the Chicago area. An average of 30 widely varied computer rainfall estimates suggests precipitation here may average 0.61 inches -- though individual projections range from as little as 0.02 inches to as much as 2.61 inches. Summer rains are fickle and often widely varied, which supports the huge spread in projected rain totals. With clouds expected to limit temperatures to levels more than 10 degrees below normal and winds off the lake likely to limit shoreline highs to the 60s, Saturday may end up the area's coolest in 12 years.
Dear Tom,
I remember a very chilly 4th of July in the middle or late 1960s. At our picnic we were all huddled in blankets. Can you pinpoint the year?
C.J. Phillips, Chicago
Dear C.J.,
The year was 1967 and Chicago's high that day was only 64 degrees, a value more typical of late April or early May. It remains the fourth coldest Independence Day on record, out-chilled only by a 62-degree high in 1920, and 63-degree highs in 1882 and 1909. The day was generally dry, though a few sprinkles were noted. Skies were gray and overcast, and cool winds blew from the north and northwest. The month opened hot in typical July fashion with highs around 90 degrees, but the mercury plunged following the passage of a cold front. July 5 was equally chilly with the official high climbing only one degree to 65. Summer weather finally returned by July 8 with readings rebounding into the 80s.
Friday afternoon's predicted 80-degree temperatures over much of the metro area will mark the first time this week it's truly felt like summer. Many Chicagoans have openly voiced disappointment over the recent succession of lackluster daytime highs which have been more reminiscent of May than late June and early July. But increased sunshine Friday is to allow temperatures to surge. It's a move that finally returns temperatures here to seasonable levels.
Storm clouds loom Friday night into Saturday over sections of the Midwest. Thunderstorms that flared late Thursday over the Plains (with cloud heights towering as high as 57,000 feet and prompting a series of severe weather watches) are targeting sections of Iowa, Missouri, Downstate Illinois and Indiana.
A suite of computer projections has shifted these storms progressively farther north in recent days. Individual projections of potential Chicago rainfall late Friday night into Saturday varies widely across 15 models. The average of these forecasts calls for a total of 0.52 inches. A consensus of these forecasts places the axis of potentially heaviest rainfall across central Illinois and Indiana.
Much improved weather is due Sunday with sunshine allowing temperatures to surge back into the 80s in all immediate Lake Michigan shoreline locations where upper 70s are likely. Evidence that hot weather is preparing to stage a comeback later next week continues to mount. A dome of hot air is predicted to become established over the nation's mid-section by Thursday---a development which may well produce the Chicago area's next round of 90-degree temperatures. The heat could lead to increased rainfall. Rainfall estimates in the 1-2 week range here passed two inches in several computer projections, evidence a sporadically stormy "ring of fire" pattern could take shape, sending a succession of thunderstorm clusters running along the northern flank of the predicted dome of hot air across the Chicago area with some regularity.
Dear Tom,
What is "aphelion"?
Susan Bosserman Fairview, NC
Dear Susan,
Our starry-eyed sky guru, Triton College astronomer Dan Joyce, informs us that today is Aphelion Day, the one day in the year on which the Earth passes farthest from the sun. That's right: farthest. Surprising, but true. We're farther from the sun during summer than during winter (but remember, it's winter in the Southern Hemisphere now).
The average Earth-sun separation is about 93.2 million miles, but the path the Earth traces as it orbits the sun is that of an ellipse, not a circle. When the Earth arrives at the end of the ellipse most distant from the sun, as it does today at about 9 p.m. CDT, the sun will be 94.4 million miles distant. Joyce says that point, in the parlance of astronomy, is aphelion. At perihelion, the point of closest approach (Jan. 3), the Earth-sun separation is 91.3 million miles.
The latest suite of computer model forecasts--and from more than one meteorological agency--are worrisome in terms of July 4 weather here. A number of the latest runs appear to heighten concern that sections of the Chicago area is in line for rainfall Friday night into Saturday. Thundery downpours have been predicted for the early weekend downstate--part of a swath of significant precipitation which has been expected extend from Missouri across sections of downstate central and southern Illinois and Indiana. In at least sections of that area, rains have been predicted to top 2" in the Friday night/Saturday period. But now (as of early Thursday afternoon), several models are aggressively shifting the eastbound storm onto a more northerly storm track increasing rain prospects over at least portions the Chicago area with rain accumulations trending heavier the farther south one travels. Hitting the northward shift in the wet Friday night/Saturday storm's movement are the Canadian, European and Navy global forecast models. Less impressive are the lighter, more scattered rains projected here by the Weather Service's GFS and WRF models. These models continue to highlight southern Illinois and Indiana as areas likely to be at the epicenter of the heaviest early weekend rains--south of a Quincy to Terre Haute line by in large.
If you've been following our weather programs this week, you know this is hardly a completely surprise---it's been a development we've viewed as a wildcard scenario for some time. We're in the midst of evaluating the new data and model runs and will have more here and on our 5:30pm and 9 pm programs and on our WGN radio reports this afternoon--and on the Chicago Tribune weather page Friday.
What has happened to summer? That was the question from many area residents Wednesday amid May-level 60-degree temperatures. The day's high of 65-degrees marked the chilliest open to a July here since 1930 and was one of the three coolest July 1 readings on the books in 139 years of weather records since 1871. Summer temperatures at that level are truly rare. Of 7,452 meteorological summer (June through August) highs on the books since 1928 at Midway, only 184 of them--just 2 percent---have registered temperature as cool or cooler.
Scattered lake-enhanced rain showers amid the Wednesday's chill lowered cloud bases in the downtown area, obscuring the tops of skyscrapers while producing periods of upper 50-degree temperatures.
While Chicago missed the July 1 record low maximum of 61-degrees set 1904 and 1924, record low daytime highs occurred at Rockford (65-degrees) and downstate at Lincoln where the high was just 70-degrees.
Thundery downpours could be part of the holiday weekend downstate
Thunderstorms expected to erupt in the Plains later Friday threaten to track east/southeastward into sections of downstate Illinois and Indiana where they may produce downpours totaling 2 or more inches for a portion of the upcoming July 4 holiday weekend.
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