WGN Weather Center Blog

History proves an invaluable guide on the time frame for frost here. Residents of west suburban areas see frost first each fall—often sometime in the next week or two. By contrast, heat generated within the city delays frost’s arrival until the final week of October. In fact, the Loop proper may not expect to see frost most years until the first week of November.
The cool air behind Thursday morning’s frigid temperatures —including 18° at Embarrass, Minn.—threatened protected areas of northern Indiana and southwest Lower Michigan with patchy frost Friday morning.
Chicago’s 62° high Thursday was its coolest daytime reading since May and only the third afternoon high which has failed to reach 70° since June 1. There’s not been another year since records began in 1871 that has come even close to producing so few sub-70° temperatures in the June 1 through Sept. 30 period. The average is 22.

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Wind-driven rains reached the city at 3 p.m. Wednesday, then fell without interruption for four hours through the evening rush. The rains erupted along a vigorous cold front, behind which chilly Canadian has spilled into the area.
Wednesday’s rain totals included 0.58” at O’Hare and Midway, 0.56” Arlington Heights, 0.55” Berwyn and 0.51” in west suburban Oak Brook. It was only the fifth time since June 1 that this area has recorded a half an inch or more of rain on a single calendar day. The heaviest rains hit to the south. Kankakee County’s Bonfield was doused by 0.85” and downstate Cahokia (near St. Louis) reported 1.59”. Thunder accompanied the downpours there.
September 2005 is set to conclude with this area’s seventh consecutive monthly rain deficit. The 2.65” on the books is the single largest monthly precipitation tally since January.

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Rain reaches the city by lunchtime, then falls much of the afternoon. But, with maximum amounts of 0.75” predicted—and most area totals under 0.50”—the incoming precipitation represents but a brief break in the ongoing seven month old drought. The rain has been so limited, Chicago’s 2005 growing season now ranks third driest among the 135 on the books since 1871. Only 12.25” which has fallen at O’Hare since March 1—just over half normal and 10.64” below the long-term average.
Skies were completely cloud-free Tuesday allowing 100% of this area’s possible sunshine for the first time in five weeks.
An autumn chill descends on the area Wednesday night with 30s possible in the coldest outlying areas. The cool air contrasts mightily with record heat Tuesday on the Gulf Coast. San Antonio reached 102° Tuesday while Houston baked at 100°, with a 110° heat index.

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Hurricane Rita, once a behemoth in the world of tropical cyclones, exited the U.S. with a meteorological whimper late Monday. Gone were the 175 m.p.h. winds which once sliced the tops off 30 foot waves in the Gulf of Mexico late last week, then hurled a devastating 15 ft. storm surge against the Louisiana Coast, submerging much of the state’s southwestern lowlands in the process. The storm’s entanglement with a 125 m.p.h. jet stream—once predicted not to occur—spared the Deep South what could have been a prolonged, catastrophic rain. By early Monday, what was left of the system’s fragmented center raced past Chicago only 80 miles to the south, having produced more than 2” of rain at downstate Scott Air Force base before dousing sections of Upstate New York with up to 4” of rain before exiting by way of the St. Lawrence Seaway.

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For the first time in weeks, there are no active tropical cyclones in the Atlantic. Even the remains of Hurricane Rita are cooperating. Instead of following the forecast and stalling with flooding rainfall totals, moisture with the system is being swept northeastward away from the Gulf states. Chicago’s rain on Sunday can be traced by the southerly flow of moisture from the decayed hurricane.
Over the next few days, the already well-defined jet stream strengthens, with fast-moving upper systems sending cold fronts racing eastward at regular intervals. Most systems are backed by Pacific high pressure, with the guarantee of near-normal temperatures most of the time.
However, Wednesday’s cold front will be a little different. Cold air with this system has origins in northern Canada, not the Pacific, so temperatures Wednesday night through Thursday night will be the coldest of the season, with some lows dipping into the 30s in the colder suburbs.

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A series of frequent cold fronts should continue for the next week and beyond. The first of these arrived last Thursday, and Fridays high of 66° was the first sub 70° day since last May 28. The next front sweeps through later Sunday along with a good chance of afternoon thunderstorms. Cooler air with this front has origins in the Plains, with the resultant push of high pressure bringing afternoon temperatures close to the normal of 70°.
Later in the week, the very respectable jet stream begins to buckle, with the upper flow eventually dropping down from northern Canada. As a result, colder air behind Wednesday’s frontal passage will be bracing, with Thursday’s high temperature struggling to reach 60°, and a chance of snow as close as Lake Superior. As a bonus, each front brings a good chance of rain and continued progress to ease the ongoing drought that has gripped northern Illinois since last spring.

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It’s been a night of unimaginable fury—even terror—on the Gulf Coast as Hurricane Rita has roared ashore. Anyone who’s experienced dark hours in such a storm will tell you that the uninterrupted roar of hurricane force winds is frightening almost beyond words. The added sound of the building which surrounds you coming apart becomes a constant reminder of one’s mortality.
A Category 3 hurricane such as Rita will never happen here in Chicago, given the absence of warm, tropical ocean waters able to sustain it. But, if it could happen, this is what might take place. Lake Michigan would rise 15 to 20 feet, flooding the city—while 20 to 40 foot waves battered anything which hadn’t been submerged. Add a F2 intensity tornado (one bearing 115-157 m.p.h. winds), which swings by its Midwest targets in under 20 seconds—but in this instance for five or more hours straight. With such forces at work, it’s little wonder hurricane damage is catastrophic.

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Autumn, 2005 had no sooner begun at 5:23 p.m. Thursday, than the heavens opened, slamming the northwest suburbs hardest. Hail, some of it the size of quarters, peppered an area from Palatine and Algonquin east to Winnetka. The wind gusts which swept Palatine downed trees 40 feet tall and eight inches in diameter while multiple downpours left 1.52” at
Arlington Heights and 1.50” at Wheeling. The late day storms followed a flurry of morning cloud-to-ground lightning strikes and loud thunder which punctuated Thursday morning’s rush hour.
Unseasonably warm air helped fuel Thursday’s storms. Highs hit 87° at O’Hare, 89° at Midway and 93° at south suburban Kankakee.
An autumnlike chill settled over areas to Chicago’s west prompting the first frost warnings of the season in northern Minnesota and Wisconsin as well as Upper Michigan.

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Severe weather erupted late Wednesday afternoon across central Minnesota as a wave of violent thunderstorms blasted through the Minneapolis area. Hail up to the size of grapefruits fell at Dassel, Minnesota west of Minneapolis and a possible tornado damaged several houses at Andover, just north of Minneapolis.
The storms formed ahead of a cold front at the nose of a prod of hot air that sent temperatures soaring into the 90s and low 100s from the Gulf Coast into Iowa and southern Minnesota.
The hot air brought record midsummer-level heat to the Plains while Chicago on the eastern edge of the heat dome topped out in the upper 80s.
This same cold front will reach Chicago today bringing the potential for strong thunderstorms here, especially in the afternoon. Cooler weather will follow on Friday as clouds and gusty northeast winds hold temperatures to near 70º.

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It isn’t often a 90° temperature occurs here the day before autumn begins. But, Wednesday’s predicted 89° high—a reading 16° above the normal—will come close, and may surge within a few degrees of the 92° record high for the date set in 1970. In the 135 years of weather records since 1871, a 90° temperature has occurred only 26 times beyond September 21 at the city’s official observation sites.
Record breaking heat has taken hold in the Katrina-devastated Gulf Coast as well as areas close by over the past week. Rescue operations in and around New Orleans have been forced to proceed in near-triple digit heat—readings well above the mid 80s normal there this time of year. A series of record highs were recorded Tuesday including 102° at Little Rock, Arkansas and in Shreveport, Louisiana. Lake Charles, to the west of New Orleans, broiled at 100° while Vicksburg and Greenwood, Miss. reached 99°.

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Sunrise with Geese, Ryan Szekeres

Ryan comments, "This was taken behind the Adler at 6:58 A.M. today. Nothing complicated. I just set the camera down on the concrete to make sure it was stable and let it do all the work! The geese didn't mind me taking their picture one bit."

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"Eldorado City of Gold" By Reidar Hahn

Here is a photo Friday night taken by Reidar Hahn at sunset from the top floor of Wilson Hall at Fermilab of the Chicago Skyline.

Hahn comments, "Two weeks a year the sun sets at just the right spot on the horizon to reflect off the windows of high-rise buildings in downtown Chicago. It happens during the week before Fall starts (Autumnal Equinox) and the week after spring starts (Vernal Equinox). At these two times the reflection of the sun off the windows line up with Wilson Hall.

The weather also has to be just right as well. The air has to be clear and just as the sun sets there can be no clouds on the western horizon. It's not the clearest or sharpest shot, because it shot with a big telephoto lens through 35 miles of 'Chicago atmosphere' "

About this picture's photographer:
Many thanks to Reidar Hahn, a member of Fermilab's Information Resources Department, who photographed and then forwarded this image to us. Reidar and his Information Resources colleagues at Batavia, Illinois-based Fermilab have worked with us in the production of our annual Fermilab/WGN Tornado and Severe Weather Seminar held each April.

--Tom Skilling

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A microburst early Monday may have been responsible for scores of downed trees across sections of McHenry County near Woodstock, Union and Crystal Lake. Some eye witnesses reported funnel-like cloud formations around 8 a.m. just before damage occurred. These may have been gustnadoes, which swirl on the leading edge of some thunderstorms, but are structured differently than true twisters. Others reported winds roaring like a freight train—an occurrence not limited to tornadoes. High winds of any type roar when forced into a confined space by trees and openings between buildings.
The same storms swept Chicago producing 0.73” at Midway Airport, where 1.49” has been measured since Thursday—the heaviest rainfall in nearly 8 weeks. Other powerful storms walloped central Illinois late Monday, producing 1.61” at Peoria, 1.53” at Galesburg and 1.49” in Springfield.

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The Autumnal equinox Thursday signals a more fall-like weather pattern. A cold front will whip through northeast Illinois today preceded by gusty winds and a line of showers or thunderstorms that could produce some severe weather. Thursday storms will precede another cold front which is expected to stall south of Chicago. This could leave the metro area the target of a front that may meander north and south, with alternating southerly and northeasterly winds giving a taste of summer or fall and a persistent threat of precipitation.
Tropical storms Philippe and Rita are each expected to strengthen to hurricane force during the next 24 hrs.—Philippe moving toward Bermuda far off the east coast, while Rita takes a more threatening route south of Florida into the Gulf of Mexico. By next weekend Rita could curve north with it’s heavy rains spreading up the Mississippi Valley mixing with the front over this area next weekend.

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As the midday sun drops lower in the sky, each day loses almost 3 minutes of daylight from the day before. Thursday will signal the autumnal equinox as the sun at its zenith is directly over the equator. Chicagoans are noticing the shortening days, but the coming week will still seem like summer with temperatures averaging some 10° above normal.
Scattered showers or thunderstorms are expected ahead of a cold front late Monday night and early Tuesday. Thursday through Saturday showers or thunderstorms will be in the forecast as first a cold front and then a warm front traverses the area. Timing of these storms, especially those later in the week is very difficult.
As Ophelia weakens and moves into the north Atlantic, a new tropical storm is developing east and north of the Lesser Antilles. Daily dangerous heat indices well above 100° are expected this week in New Orleans area as recovery work continues.

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Hints of fall color are on display this weekend. Northern Minnesota is home to the most extensive color, with 30 to 50 percent of its trees in the midst of annual change.
Drought and the summer’s unseasonably mild temperatures are expected to impact this year’s colors and may well alter the timetable for their appearance. But, the Midwest as a whole is still weeks away from this year’s peak color display. The peak usually arrives in the Chicago area toward mid-October.
Area residents were treated overnight to some of the coolest temperatures since July with 40s in the western suburbs. The chilly night follows the area’s heaviest rain in nearly two months a night earlier. Midway Airport’s 0.76” was the most there since 0.86” fell back on July 26. Other heavy area totals included 1.08” at Park Forest, 1.68” at Watseka and 0.82” at Valparaiso, Ind.

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It was time for some cool weather. Thursday’s 70° high was not only the coolest here since the 70° two months ago on June 17, it was the first daytime reading in September which was below normal. Still, the reading marked the 110th consecutive day in Chicago with a peak reading of 70° or higher—a new record. The 110 day streak which began back on May 29 sneaks by the previous record of 109 straight days at or above 70° set between May 24 and September 9, 1959.
Thursday’s cool daytime readings followed chilly lows of 48° at west suburban Aurora and Rochelle. The country’s chilliest air occurred out West. A low of 27° produced frost in the Grand Canyon early Thursday while Flagstaff—an hour away by car—bottomed out at 30°.
Warmth returns here with southerly winds this weekend.
-Tom Skilling

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The area’s contact with comfortable autumn air moves into a second day Thursday. Highs hold to the 70s, as was the case Wednesday with its 76° peak afternoon reading—the first time in nearly three weeks (20 days) that the mercury here has failed to reach 80°.
Cool weather—in particular, sub-80° daytime highs—have been in unusually scarce supply this warm season. Since June 1, Chicago’s highs have remained below 80° on only 18 days—a fraction of the 41 observed over the same period during the past 135 years. It’s a number which is only 44% the long term average and the least of any year here since 1871.
Midway Airport’s unimpressive 0.15” rain early Wednesday was the city’s first precipitation at the site in 24 days, tying 1964 and 1999 for the unenviable title of fifth longest rain-free period on the books since 1928.

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Not since the 99° high at Midway Airport on Sept. 29, 1953 has a temperature been as hot as Tuesday’s 95° this late in the year here. The reading tied as the 4th highest of 2005. O’Hare’s official 94° high was 20° above normal and only 4° from the 1939 record. It was the area’s fourth consecutive day of 90s, something which has happened in a September here an average of once every six years—and it’s quite possibly the last string of multiple 90° highs we’ll see this year.
A total of 62-daytime highs have pushed above 90° after September 14 in the city since 1928—including the area’s latest recorded 90°+ temperature (94°) on Oct. 6, 1963. But, on average, Chicagoans have encountered only one additional 90° high from this date forward—and in only 40% of all years on record.
September 2005 is now 8.4° above normal—5.7° warmer than the same period a year ago.

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A fourth consecutive day of 90° temperatures—more 90s than occurred in all of 2004-—is predicted Tuesday. But, with soil moisture-levels low in response to this year’s extreme drought, there’s less moisture returning to the atmosphere through evaporation—and that holds humidities lower than they might otherwise be. It’s a development which spares area residents the full measure of discomfort that long periods of 90° temperatures can bring. This “heat” is also occurring with the sun on a late season trajectory across the sky each day. The lower sun angle reduces sunlight intensity and no doubt contributes to the comparative tolerability of the current round of heat. But, the number of remaining truly hot days this time of year is finite.
It’s been ten years (since 1995) since Midway Airport has recorded more 90s than the 35 which will be on the books here after today.

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A very uncomfortable day lies ahead with a heat index in the mid 90s. Hazy conditions will result from an accumulation of particulate matter in the air. The Illinois Environmental Protection Agency on Sunday has issued the 10th air pollution alert this summer for Chicago and the 13th of 2005. An approaching cold front early Wednesday should clear out the air pollutants—preceded by very welcome though most likely brief showers or thunderstorms, and followed by a much cooler, less humid air mass. More seasonable but somewhat unsettled conditions will prevail the latter half of the week. Longer-range forecast models still indicate no relief in sight from the extreme drought conditions that persist over northern Illinois. A severe solar magnetic storm continues, and some residents got a rare glimpse of the northern lights over the weekend. Still a Category 1 hurricane, flooding and high surf are forecast as Ophelia moves up the eastern seaboard.

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While 90° high temperatures are expected the first three days of the week in Chicago, Hurricane Ophelia feints movement toward the mainland off the Carolina coast, threatening copious rains over portions of the southeast U.S. And the drought continues over northeast Illinois, with the best and maybe only good chance of showers in the Chicago area occurring ahead of a cold front Tuesday night and early Wednesday. It is hard to get rain in the middle of a drought, and the possibility of a significant rain (an inch or more) with this system is fairly slim—so the extreme drought will likely persist for the foreseeable future.
Because of the drought and the minimal number of strong low pressure systems, severe weather has been reduced over northeast Illinois this summer. However, history says about a fifth of the severe weather season remains, so whenever a vigorous low pressure system surges through this fall, there will still be a threat of severe storms and tornadoes.

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