EXPLAINER: August 2007 Archives

With only hours to run in the June through August meteorological summer season, the area is in the midst of a dry spell destined to become Chicago’s longest in a month. No rain is predicted here until spotty t-storms threaten far northern sections of the metro area late Labor Day (Monday). Up to now, the longest rain-free period in the past month was the 7 day stretch from July 28-Aug. 3.
The frequency with which rain has fallen in the past three months has been impressive. The summer’s 15.85” official tally at O’Hare ranks 9th wettest of the past 137 years and accumulated in rains which fell measurably on 43 days—far beyond the 29 days considered “normal.” Thunderstorms occurred on 26 summer days—more than the usual 15.
September opens Saturday with 2 hours less daylight than back on June 21, the start of summer and the northern hemisphere’s longest day.
--By Tom Skilling, WGN-TV Chief Meteorologist

There have been two cooler daytime highs this month—68° on August 18 and 19. But, neither reading occurred amid the abundance of sunshine predicted Thursday. That highs will struggle to make it to 73° today is a sign that seasonal cooling is underway and occasionally strong enough to make it all the way into Chicago. The cooling will accelerate over the coming month. While week to week daytime temperature declines here in August’s closing week average 1°, the rate at which readings pull back will increase to more than 3° per week by late September.
Warm weather enthusiasts shouldn’t be discouraged yet. Daytime highs return to the 80s for the Labor Day weekend. Weather history indicates an average of three of the 24 daytime highs at or above 90°occur beyond this date.
Phoenix, Arizona broiled at 113° Wednesday, the 29th day this year over 110°—a new record!

--By Tom Skilling, WGN-TV Chief Meteorologist

Chicago’s 17th official 90° temperature of the year is on the books. Tuesday’s 91° high at O’Hare was August’s sixth 90°+ high and the city’s warmest temperature in three weeks. Other metro area thermometer readings Tuesday included 93° at Northerly Island and 95° at Gary, which was the metro area’s hot spot. In combination with the day’s muggy 70°+ Gulf Coast-level dewpoints, heat indexes at the two sites topped out at 104° and 105° respectively.
The heat provoked waves of powerful thunderstorms, which at one point, stretched from Kansas to Upper Michigan—some 60,000 ft. tall and accompanied by 80 mph wind gusts. At Crandon, WI (in upstate Forest County), t-storm winds downed trees and flipped a mobile home. Preliminary reports indicate the massive squall line may have spawned at least six twisters—four in Iowa and one in Nebraska.
-By Tom Skilling, WGN-TV Chief Meteorologist

The Chicago area flirts with 90 degrees Tuesday for the first time in a week and only the sixth time this month. By almost every measure, meteorological summer 2007 has been a warm one. With just 4 days left in August and the meteorological summer season set to close at midnight this Friday, readings are running 1.7 degrees above normal, daily temperature averages have finished above normal 53 of the past 87 days (61% of the time) and overall air conditioning usage is estimated to have increased nearly 20%. Yet, despite all these warm weather indicators, the number of days at or above 90 degrees is actually off long term averages by 20%. A 91 degree high today marks the 17th 90 degree+ day this year—20 is considered normal.
At 3:30 p.m. on this date 17 years ago, the most powerful U.S. tornado to occur in August formed near Oswego and began a deadly 16.4 mile rampage through Crest Hill and Plainfield. By the time it had dissipated, 29 people were dead and more than 300 injured.
--By Tom Skilling, WGN-TV Chief Meteorologist

The city can expect two more rain-free days to clean up and dry out before the next round of precipitation threatens here Tuesday night and Wednesday. With only light to moderate rainfall expected, this August should end as Chicago's fourth wettest since records began in 1871. Not so at South Bend, Ind., where the month's 8.88" is the most in any August, while Rockford's 13.82" total elevates this August to the all-time wettest of any month there.
With clear skies expected Monday night, early rising Chicagoans with a view to the west will be able to see the first half of a total lunar eclipse. The full moon will gradually disappear as it moves into the Earth's shadow with the eclipse reaching totality at 4:52 a.m. Unfortunately, Chicagoans will miss the moon's emergence from totality, which according to Dan Joyce, astronomer at Triton College's Cernan Earth and Space Center, will not be visible here, a result of moonset and the onset of twilight.
--By Steve Kahn, WGN Weather Center Meteorologist

The great rains of August 2007 have ended, at least for a few days. After three weeks of downpours that brought widespread flooding and historic monthly totals to many areas, nature’s rain machine is shutting down here for a while. Drier air will be making inroads into the area giving a chance for the thoroughly soaked ground to dry out and flood-swollen area rivers to recede.
However, the book on August precipitation is not yet complete. With O’Hare’s August rainfall total at a robust 9.70” and many other locales tallying more than a foot of rain, monthly totals are still likely climb as the month’s final round of precipitation is expected to arrive Tuesday night and Wednesday.
The hurricane season is approaching its most active period. Though the tropics remain quiet in the wake of Hurricane Dean, the latest forecasts still call for a very active season and new tropical cyclone development could quickly initiate.

--By Steve Kahn, WGN Weather Center Meteorologist

After a run of tumultuous weather this week, Illinois state climatologist Dr. Jim Angel has confirmed what many flood and storm-plagued northern Illinoisans suspect. August rainfalls averaged across all of northeast Illinois, many in excess of 12" and well beyond the 4.62" considered normal in Chicago, are the heaviest since regional records began in 1895. An average of 11.32" has fallen in an area from Boone to LaSalle County eastward -- including the Chicago metro area. That total surpasses the previous record of 11.02" set in August of 1987.
Individual weather station totals run as high as 16" at southern McHenry County's Huntley, 15.05" at Elgin and 14.19" in rural DeKalb County. Chicago's 9.36" in August at O'Hare Airport is the 2nd wettest of the past 137 years.
The month's tally of thunderstorm days -- 14 -- equals the record for the most August thunderstorm days set back in 1940.
--By Tom Skilling, WGN-TV Chief Meteorologist

Mike Frankowski shares this remarkable photograph taken in South Elgin around 1:30 am Friday morning. Great shot, Mike!
--By Tom Skilling, WGN-TV Chief Meteorologist

Photo courtesy of Mike Frankowski

The Chicago area is no stranger to powerful thunderstorms. But the expanse and breadth of Thursday afternoon and evening’s outbreak was enough to make even veteran meteorologists, like my colleague Steve Kahn, who for more than 40 years worked for the National Weather Service here, sit up and take notice. Rare is the storm outbreak that produces damage and flooding from one corner of the metro area to the other. Yet, that’s just what happened yesterday. Steve compares the scope of the damage produced by Thursday’s storms to an Aug. 26-27, 1965 squall line which, unlike the latest surge, hit in the middle of the night. The area’s strongest wind gusts reached 80 m.p.h. at Manhattan and 75 m.p.h. at Elgin. But 60+ m.p.h. wind gusts swept virtually every corner of the region. The storms roared across the area at nearly 60 m.p.h. and towered at one point to 64,000 ft.—more than 12 miles into the atmosphere.

--By Tom Skilling, WGN-TV Chief Meteorologist

A second wave of powerful t-storms erupted late Wednesday battering sections of the area with 60 m.p.h. winds while unloading as much as 1.60” of rain near SW suburban Marseilles. Waves of storms to Chicago’s north in recent days have left La Crosse and Madison, Wisc. reeling with 12.23” and 13.33” August rain tallies—amounts which have exceeded all previous August records and now stand as new all-time monthly precipitation highs.
Wednesday evening’s storms, which generated dazzling cloud to ground lightning—4,500 cloud to ground strokes within a 200 mile radius of Chicago in just ten minutes—followed Chicago’s first 90° high in 15 days.
Predawn t-storms Wednesday sent 50 m.p.h. winds roaring across the area and out over Lake Michigan. The waves generated there were reflected from Michigan/Indiana back to Chicago’s shoreline, giving rise to a series of minor seiches (lake-level fluctuations.
--By Tom Skilling, WGN-TV Chief Meteorologist

Dan Strickland, on vacation with his family (which includes our own Jackie Bang) in Green Lake, Wisconsin, photographed this spectacular cloud to ground lightning discharge around 2 a.m. Wednesday morning. The powerful storms, which appeared as a fast moving bow-shaped line on radar displays--the signature for strong and potentially damaging winds, swept across southern Wisconsin and northern Illinois generating powerful wind gusts, driving downpours and stunning lightning displays, as pictured here. Once out over Lake Michigan, the squall line appears to have generated a "seiche" activity--rapid lake level fluctuations--- Wednesday morning along Chicago's lakefront--notably near Montrose Harbor. One of our viewers reported the 6" of water she had waded into while walking her dog suddenly disappeared only to return shortly afterward--the sort of oscillation in lake levels typical with seiches. The strong winds which gush out of a line of storms like those which swept the area this morning, produce waves on the lake which hit the Michigan/Indiana shoreline and reflect back to the Illinois side of the lake as a seiche.
Thanks to Dan Strictland for the stunning photo--and have a great vacation!!

-Tom Skilling


Photo courtesy of Dan Strickland

Thunderstorms, among this summer’s most powerful to date, lambasted sections of South Dakota, Nebraska and Iowa overnight late Tuesday. Reports of mammoth 6-8” diameter hailstones near Dante, S.D., which if verified would rival the all-time record 7” diameter hail which punched holes in roofs in Aurora, Neb. on June 22, 2003, underscored the remarkable intensity of Tuesday’s storm outbreak. With cloud tops which mushroomed to 72,000 ft., reports of 80 m.p.h. gusts near Rolfe and 4.50” of rain at Meridan—both in northwest Iowa—came as no surprise.
The eastbound storms threatened to push the record August rain tallies at LaCrosse, Wisc. (11.95”) and Madison (10.49”) to all-time monthly totals. Ironically, while these cities have never tallied bigger monthly totals, Duluth, Minn.—only several hundred miles to the north—has recorded only 0.24” this month, its driest August ever.

--By Tom Skilling, WGN-TV Chief Meteorologist

With soils saturated and a number of streams and rivers in flood, area residents are likely to cast nervous glances at each new cluster of thunderstorms in the days to come. Were August to proceed through its remaining nine days without an additional drop of rain, the month -- with 6.40" on the books to date -- would rank 13th wettest of the past 137 Augusts.
There hasn't been a wetter 20-day August open at the city's official rain gauge since 1990 when 7.25" had fallen by this date. Only seven other Aug. 1-20 periods since 1871 have been wetter. But the story is an even wetter one in areas around the city. WeatherBug rainfall sensors in northwest suburban Palatine have tallied 13.05", while 9.82" has fallen at Highland Park and 9.33" in Glencoe.
Hurricane Dean, headed for what could be a calamitous landfall Tuesday in Mexico's Yucatan Peninsula, was declared a "catastrophic" Category 5 at 8:35 p.m. Monday evening when sustained winds reached 160 m.p.h.
--By Tom Skilling, WGN-TV Chief Meteorologist

The National Weather Service has northeast Illinois under a Flash Flood Watch today. Remnant moisture from Tropical Storm Erin feeding into strong southwest flow forced aloft over a warm front continues to produce a parade of showers and thunderstorms across northern Illinois.
"Training" storms developing and moving repeatedly over the same area gave rainfall totals of 1-3" or more Sunday, and similar action is expected today over much of the Chicago metro area. Two-day rainfall totals could exceed 5 inches in some locations.
Small stream, street/road and basement flooding are the most immediate problems anticipated, but already swollen larger streams and rivers could flood out of their banks in the coming days as the runoff quickly feeds into the hydrologic system.
Elsewhere around the nation, response to major flooding from 10-15" downpours was ongoing in Oklahoma, La Crescent, Minn., and La Crosse, Wis.
--By Paul Dailey, WGN Weather Center Meteorologist

With a warm front poised just to the west, widely scattered showers and thunderstorms will threaten the Chicago Air Show later this morning and afternoon. Tonight and Monday morning strong storms with heavy downpours could develop. Localized two to three-inch rains may cause flooding of streets and underpasses as well as some streams. T-storms will be in the forecast off and on through Friday, and rainfall totals for the week could exceed 4” at some rain gage sites. Heat and humidity will return to Chicago with 90°+ readings as early as Tuesday. Meanwhile, Hurricane Dean will feast on the warm waters of the Caribbean Sea as it passes south of Cuba on a westerly trek toward the southern Gulf of Mexico. Dean seems to be following a similar course as Hurricane Gilbert, which was the strongest hurricane to date in 1988. As it approaches the Yucatan Peninsula Monday, Dean’s sustained winds may peak out well over 160 m.p.h.
--By Paul Dailey, WGN Weather Center Meteorologist

Smoke off distant Idaho and Montana forest fires produced the red tint to the moon which caught the attention of many Chicagoans Friday night. The smoke rode westerly winds aloft and arrived at the 18,000-foot level.
Friday's 79 degree high was the second coolest daytime reading of the month and followed a morning low of 64 degrees at O'Hare -- coolest in nearly two weeks.
Eastbound t-storms erupted in the Plains and western Midwest Friday, towering to 60,000 feet. The storms pounded Rapid City, S.D., with 2.75"-sized hail and nearly 2" of rain. The stormy environment shifts into the Chicago area later Saturday and Saturday night.
In the tropics, Jamaica appears to be Hurricane Dean's next target. The storm -- a Category 4 with 145 m.p.h. sustained winds late Friday -- underwent explosive development once in contact with the bathtub-warm waters of the Caribbean. Dean is on a path similar to 1988's Gilbert, in which peak gusts reached 218 m.p.h.
--By Tom Skilling, WGN-TV Chief Meteorologist

A swath of intense wind damage produced by t-storms Wednesday night was concentrated in a 3 to 5 mile band wide from East Chicago and Gary in Lake County, Indiana southeast to Hobart and Merrillville and across Porter County. That’s the conclusion of a team from the National Weather Service which surveyed the area Thursday and attributed the damage to powerful straight line winds rather than a tornado. The team noted trees and corn were blown down uniformly in a southeast to south direction and lacked the rotary distribution which would have occurred in a tornado. Peak gusts were estimated as high as 100 m.p.h.
The storms have ushered the coolest air in nearly four weeks into the area. Chicago’s 77° high Thursday was the lowest daytime temp in 26 days. The cool-off sent downstate temps plummeting in the wake of thunderstorms—but not before Paducah reached 105°, its hottest reading in 4 decades.
-By Tom Skilling, WGN-TV Chief Meteorologist

T-storms, which went on to produce flooding 1”+ downpours, hail and 70 m.p.h. wind gusts, erupted rapidly Wednesday evening. Lightning data tells the story. The evening’s fast-evolving storms began as towering cumulus clouds with radar-scanned tops near 39,000 ft. around 7 p.m., then mushroomed to a height of 60,000 ft.—apparently energized in part by heat generated by the city and converging winds near the lake. Lightning discharges grew exponentially—exploding from just 54 cloud to ground strokes in ten minutes around 7 p.m. to nearly 3,500 by 9 p.m.
Rainfall grew heavier as the storms swept across the city’s South Side totaling 1.14” in under an hour near the 5000 block of South Archer. Local 1.50”+ totals innundated NW Indiana where hail and powerful winds prompted t-storm warnings. A radar-detected tornadic circulation in Porter County led to a tornado warning while a 69 m.p.h. straight line gust was clocked at Gary.
--By Tom Skilling, WGN-TV Chief Meteorologist

It’s been wet this month and additional thundery rains are on the way. The rains have kept lawns green, fostered the best corn and soybean crops on area farms in memory and shielded the Chicago area from the string of hot days which have broiled much of the nation’s midsection. St. Louis, where the mercury has climbed above 90° every day this month, has accumulated more 90° temperatures in the past two weeks than Midway Airport has seen all summer. Yet, despite all the rain and the limited number of truly hot days, the month has managed an average of 78.0° at the O’Hare —a reading 5.7° above the long term average and warm enough to qualify as the third warmest open since 1959.
The tropics continue to heat up. Tropical Storm Dean threatens to become a major 110+ mph hurricane before this weekend and Tropical Storm Erin may form in the Gulf and head toward Texas. Trop. storm watches were hoisted there Tuesday evening.
--By Tom Skilling, WGN-TV Chief Meteorologist

A line of powerful, swift-moving t-storms threatens to rake sections of the Chicago area Tuesday morning after battering the Dakotas and Minnesota with 60 m.p.h. gusts and hail the size of golf balls. Hailstones produced by the 55,000 ft. tall storms covered the ground to a depth of 3” at Fort Ripley, Minn. More than 100 reports of severe weather had been logged late Monday evening by the Storm Prediction Center.
The squall line which could sweep parts of northern Illinois early Tuesday, is referred to as a derecho by meteorologists—a feature noted for its bow-shaped appearance on weather radar screens and its ability to endure for hours, producing wind damage over hundreds of miles of terrain. Another derecho Sunday night into Monday morning, left a swath of damage from the Dakotas south to Missouri.
Blistering heat—like the 107° record high at Rapid City, S.D. Monday—is fueling the t-storm eruptions.
--By Tom Skilling, WGN-TV Chief Meteorologist

After a sunny and warm Monday, the overnight hours will likely feature a blast of thunderstorms similar to those that raked the area early Sunday morning packing wind gusts to 50 m.p.h. and brief but heavy downpours. With searing triple-digit heat anchored from the southern Midwest to the Gulf Coast and a northwesterly jet stream across the upper Midwest, Chicago lies in a favorable path for repeat episodes of thunderstorms arriving from the northwest.
Weather across the Chicago area will remain unsettled through the week as a frontal system separating hot, steamy weather to the south from cooler, drier conditions on its northern flank wavers across the region. Meteorological setups like this favor the development of nocturnal thunderstorms in the Midwest.
The tropics are becoming active as mid-August approaches. In the Pacific, Hawaiians are nervously watching the approach of powerful Hurricane Flossie.
--By Steve Kahn, WGN Weather Center Meteorologist

With weather patterns remarkably similar to last week’s, Chicago is in line for a continuation of the recent warm, muggy weather punctuated by several periods of showers and thunderstorms. With dew points flirting with steamy Gulf-Coast-70º levels, episodes of heavy rainfall are possible as waves of t-storms amble through the area. The week promises to be an unsettled one as a frontal system oscillates across the region, generating a lot of cloudiness and triggering periodic thunderstorms.
Searing heat continues to bake the South, where triple digit readings shattered records from Texas to South Carolina Saturday. Baton Rouge, Louisiana led the “heat parade” with a scorching 104º.
With a little luck night skies may be clear enough here to allow viewing of the spectacular Perseid meteor shower which peaks Sunday night with a possibility of viewing more than 100 meteors each hour.
--By Steve Kahn, WGN Weather Center Meteorologist

It's back to the 90s this weekend over much of the metro area, extending to 14 and 15 the number of consecutive above-normal days here. August's warmer-than-normal trend -- nearly 6 degrees above average -- has pushed the month's ranking to 3rd warmest at O'Hare since 1959.
Saturday's light easterly winds protect the narrow corridor within a few miles of the lake from the area's hottest temperatures as readings there hold to the mid 80s. But on Sunday, the heat is to extend up to and over the shoreline as SW winds take hold.
Several potential complications arise Sunday. Oppressive levels of humidity are predicted with dewpoints surging into the mid 70s -- similar to tropical rain forests. In addition, the atmosphere is to become incredibly unstable -- i.e. temperatures are to plummet vertically at a much faster pace than usual. Add to the mix converging winds along a cold front, a vigorous jet stream above and atmospheric energy levels at the upper end of the spectrum, and powerful storms may erupt over parts of the area.
--By Tom Skilling, WGN-TV Chief Meteorologist

Friday brings the area’s first break in daily thunderstorm eruptions in a week. A final t-storm late Thursday evening—with a cloud top to just 20,000 ft.—produced surprisingly heavy rain. An inch of rain fell over sections of suburban Elgin before the cell weakened as it traversed DuPage County. Doppler radar tracked a modest circulation aloft with the storm for almost 45 minutes. And, what appeared to be a funnel cloud was photographed around 9:15 p.m. in near Elgin. But, in the end, the downpour-generating t-storm produced only a handful of cloud to ground lightning strokes and any circulation never made it to the ground.
Storm downpours have hit one section or another of the Chicago metro area in nine separate clusters since Sunday. Total rainfall in some west and north suburban locations has reached 4 to 7-inches—as much or more than a full August. Not all areas have been as wet. Midway Airport has recorded just 1.26” for the period.

--By Tom Skilling, WGN-TV Chief Meteorologist

The muggiest air of 2007, which first reached the area Sunday, exploded Wednesday for a fifth consecutive day into downpour and funnel-cloud producing t-storms. Doppler radar velocity measurements identified tornadic rotation around 2:45 p.m. Wednesday afternoon in northern Kane County. A flurry of funnel reports ensued from Maple Park to Schaumburg, prompting the activation of tornado sirens. A quick 1”+ of rain hit in sometimes blinding downpours. No touchdowns occurred beneath the 40,000 ft. tall storms and flooding proved the major culprit. The same westward moving lake breeze front responsible for the mid afternoon storm eruption produced the day’s second thundery cloudburst—this one focused on Sugar Grove and Aurora in southern Kane County. Rainfall rates there reached 3.50”/hour with radar storm estimates topping 3”. Significant flooding was reported in the area on Illinois Route 47.
--By Tom Skilling, WGN-TV Chief Meteorologist

Flooding of historic proportions gripped the Rockford and Belvidere areas Tuesday forcing residents from their homes and vehicles in the hardest hit locations. Those with whom we talked Tuesday characterized the latest cloudburst as potentially as bad—if not worse—than last September’s Labor Day cloudburst. That devastating flood was the area’s worst in at least 30 years.
The latest storms, which struck in Tuesday’s predawn hours, drenched Belvidere with 6.30” while saturating Rockford with 5.25”—accumulations which produced 100+ and 75-year floods in those communities respectively according to Illinois state climatologist Dr. Jim Angel. Rainfalls of 4+” extended east across a wide swath of Lake and McHenry counties, forcing road closures for a time early Tuesday.
---By Tom Skilling, WGN-TV Chief Meteorologist

The most humid air in a year erupted into thunderstorms for the third time Monday blasting sections of the Chicago area with 40 mph gusts, prodigious cloud-to-ground lightning and rains of up to 4”. Even before Monday’s rains hit, Chicago’s Aug. 1-6 rainfall (1.46”) ranked the 19th heaviest for the period since records began 136 years ago. Blinding rains and, at one point Monday evening more than 3,000 cloud-to-ground strokes in 10 minutes, walloped some south and western suburbs. Western DeKalb County—7 miles southwest of DeKalb—proved ground zero for the heaviest storms. There 3.82” of rain fell between 3:50 p.m. and 6:50 pm—2.90” of it in just 30 minutes.
Blistering heat downstate—this summer’s hottest yet—provided the fuel for the storms. Temps topped out near the century mark at St. Louis (99°), 99° at Centralia and 100° at Mt. Vernon. The heat index at Taylorville reached a staggering 113°.

-By Tom Skilling, WGN-TV Chief Meteorologist

Northeast Illinois was served up a severe weather mix Sunday, and more of the same could be in store for the next few days. A nearly stationary front will be oriented west-east through southern Wisconsin just north of the Illinois border, and southerly winds will maintain hot, humid conditions over the metro area most likely into Thursday.
Early Sunday morning, strong thunderstorms spread into northern Illinois dumping from a half inch of rain along the Interstate 88 corridor to as much as 3 inches along the Illinois-Wisconsin border. Then later in the afternoon, very strong thunderstorms fired primarily along and south of Interstate 80 with torrential downpours of 1-2 inches per hour prompting flash flood warnings. Outflow winds gusted to 70 m.p.h. at Crete in southern Cook County, and a small tornado reportedly touched down in Romeoville.
Timing is difficult, but the next organized surge of storms may be due later tonight and Tuesday.
--By Paul Dailey, WGN Weather Center Meteorologist

Overnight showers and thunderstorms should gradually diminish during the morning hours. As the storms moved out of Iowa and NW Illinois, some “trained” (repeatedly passed over the same area) resulting in localized flooding where rainfall totaled well over an inch. The storm-triggering warm front is expected to move north of Chicago this afternoon, and as it does, clouds will break and winds shift to the south. The front should lie just to the north, orienting west-east across southern Wisconsin, the first half of the week.
Chicago will thus be situated on the northern edge of a hot, humid air mass centered over southern Missouri and Arkansas. An Extreme Heat Warning is in effect today and an Extreme Heat Watch for the remainder of the workweek in the St. Louis area. Meanwhile, Chicago will experience daily flirtations with 100° afternoon heat indices.
--By Paul Dailey, WGN Weather Center Meteorologist

Resurging heat, with implications to Chicago's weekend weather, fueled waves of powerful thunderstorms which unleashed a half foot of rain on sections of South Dakota late Friday. The mammoth storms towered to heights of 54,000 feet and flared as hot, humid air was lifted into an unusually strong mid-summer jet stream. Shifting wind direction with height generated storm rotation -- always an ominous development. Grapefruit-sized hail bombarded Rapid City, S.D., breaking windows, and at least two small tornadoes touched down briefly near St. Francis, S.D., and Sparks, Neb. Rainfall Friday reached 5.92" at Chamberlain and 2.36" at Pierre -- both in South Dakota—3.10" at Pine Bluffs, Wyo., and 2.61" at Fessenden, N.D.
The warm front responsible for the Plains storms is eastbound. It reaches Chicago late Saturday night. Increasing clouds and SE winds off the lake today threaten potentially prolific lightning-producing thunderstorms overnight.
--By Tom Skilling, WGN-TV Chief Meteorologist

Thursday’s 91° high was the year’s 13th daytime reading at or above 90°, and the third time this summer area residents have been treated to three consecutive 90° highs. Peak readings in this most recent hot spell of the season first topped 90° on Tuesday, then reached 92° Wednesday and 91° Thursday. This marks only the 6th time in the past four decades at Midway Airport that an August has opened with back to back 90s.
Meteorological summer has averaged 72.8° in Chicago—virtually identical to the same period a year ago and 1.6° above the 137-year long term average. That’s qualifies the 2007 season as Chicago’s 38th warmest since 1871. While humidities pull back Friday into Saturday, heat resurges strongly Sunday afternoon into next week—a development sure to push our summer temp ranking even higher.

--By Tom Skilling, WGN-TV Chief Meteorologist

Though it’s become hot in recent days, the area has certainly dealt with more debilitating heat in the past. Chicagoans were saddled with triple-digit heat indexes as the mercury soared to 97° a year ago. But, gentle lake breezes and fairly moderate humidity levels, including low and mid 60° dew points instead of Gulf Coast level 70°+ dew points, have spared the area the full impact of the heat up to this point. Make no mistake, working outdoors hasn’t been easy. But, truly humid air—like that expected to take hold later this weekend into next week—would already have pushed heat indexes to dangerous 100°+ levels.
Thursday marks the sixth consecutive day with high temps warmer than the day before. It’s also the 13th of the past 15 days to feature a lake breeze. Pollutants have been building. An Air Pollution Action Day has been declared Thursday. Ozone levels reached 102 parts per billion in some areas late Wednesday.
--By Tom Skilling, WGN-TV Chief Meteorologist