EXPLAINER: July 2007 Archives

Heat has expanded its grip on U.S. weather and is still spreading as August 2007 gets underway Wednesday. Chicago’s 91° Tuesday, the hottest here in three weeks and the 11th time highs have reached or exceeded 90° this year at O’Hare, was but one of myriad 90°+ temperatures scattered across 42 of the Lower 48 states. Highs of 105° at Miles City, Montana and 103° at Williston, North Dakota, established new records at each site. Bismarck, North Dakota’s 102° just missed a record.
Wednesday marks the fifth day Chicago’s temperatures have increased. Peak readings surged from 80° Saturday to 85° Sunday, 89° Monday and 91° Tuesday. Hot as Wednesday’s predicted 93° high is, it lags behind the 99° on this date a year ago. With the three-month meteorological summer period now 2/3’s over, the season ranks 12th warmest since O’Hare Airport observations began in 1959.

--By Tom Skilling, WGN-TV Chief Meteorologist

Six of the next 7 days will see 90s over at least sections of the metro area, more than the two 90s which occurred here all of last August. It’s a period predicted to include several daytime highs likely to tie or exceed Chicago’s highest 2007 temperature to date—94° recorded on July 8 and 9. Light easterly lake breezes blow for a sixth day Tuesday. But with Lake Michigan’s temperature at 77°—the year’s highest—the cooling impact of these winds is a fraction of just a month or two ago. Monday’s lake breeze restricted Loop highs to 83° while O’Hare topped out at 89°—the warmest in the three weeks since July 9.
August 2007 gets underway at the stroke of midnight Tuesday night. The month is home to 25% of the city’s 90°highs. Since weather observations began at Midway Airport in 1928, Augusts have hosted an average of six 90s.

--By Tom Skilling, WGN-TV Chief Meteorologist

A host of meteorological indicators are converging on a weather scenario that points to a hot week for the Chicago area with temperatures expected to approach or top the 90 degree mark on most days.
After several false alarms, a portion of the large dome of hot air that has been baking the West much of this summer is finally making inroads on the Midwest and should arrive for what appears to be an extended stay by Tuesday. Any cooling from Lake Michigan will be felt only along the immediate lakeshore as a hazy sun beats down on the city in the light wind regime.
A frontal boundary will approach the region by late in the week, offering up a threat of some cloudiness along with some showers and thunderstorms that could keep the mercury just below the 90 degree threshold on Friday.
The week's hottest weather could arrive Saturday as gusty southwest winds push the boundary to the north and send the mercury toward the middle 90s.

The mercury has not hit 90 degrees in Chicago since July 8 and 9 when readings peaked at a season’s high 94 degrees on back-to-back afternoons. Since then the official number of 90 degree-plus days this year has been stuck on 10 at O’Hare as temperatures have been limited to 70s and 80s during what is historically the hottest time of the year.
All that is likely to change this week as readings are expected to surge into the 90s on several days. And while clouds and thunderstorms are expected to put a damper on temperatures by the end of the week, a renewed and potentially prolonged surge of hot weather could return here by August's opening weekend.
The tropics remain quiet, but August marks the start of hurricane prime-time in the Atlantic. That region has been free of tropical cyclones since T.S. Barry's demise June 2, but forecasters are keeping an eye on a low pressure area in the central Bahamas that could develop into the season’s third named storm: Chantal.
--By Steve Kahn, WGN Weather Center Meteorologist

Saturday becomes the 19th consecutive day with a high below 90°. While this ninth weekend of meteorological summer promises to be eminently comfortable, its temperatures are likely to fall below normal thanks in part to Saturday’s NE winds. Five of the past eight weekends have fallen below seasonal norms. It’s a trend at odds with last year. Today’s date marked the beginning of Summer 2006’s longest hot spell of six straight 90s—including a 99° high!
Though not as humid as in recent days, moisture in the lowest 4,000 feet of the atmosphere fosters cloud formation in the cooler hours (i.e. at night and in the early morning). Lighter winds in Sunday’s predawn could foster the development of fog patches, likely to burn off as temperatures warm in the afternoon.
Big rains which swamped sections of Wisconsin with over 5” of rain Thursday night diminished as they settled into Illinois. Still, some north and west suburbs reported up to 0.80” of rain.
--By Tom Skilling, WGN-TV Chief Meteorologist

Sections of two southwest suburbs -- Burr Ridge and Homer Glen -- were damaged by powerful microbursts spawned in predawn thunderstorms Thursday. The storms, accompanied by dramatic lightning and loud thunder claps, hit around 5 a.m. Microbursts occur when comparatively dense, rain-cooled air plunges to earth then fans out in all directions, generating strong straight-line winds. National Weather Service survey teams estimated that winds in Homer Glen, which ripped part of a roof off a building, may have reached 100 m.p.h.
Blinding rains with the heaviest storms produced eye-catching totals. New Buffalo, Mich., which was hit by the first in a series of downpour-generating thunderstorms Wednesday night, reported 5.50” of rain, while sections of Knox in Indiana’s Starke County were flooded by 4.76”. Chicago’s southeast side was hit with 4.10”, and Chesterton and Hammond, Ind., tallied 3.46” and 3.39” respectively.
The Fox Valley and areas west of Chicago didn't escape the big rains. Kane County's Elburn reported a whopping 4.22" between 7 a.m. and noon Thursday.
--By Tom Skilling, WGN-TV Chief Meteorologist

Temperatures surged to 85° here Wednesday marking the 26th time this year that the mercury reached or exceeded 85°. It was Chicago’s warmest temperature in a week. Each of the preceding five days had finished cooler than normal.
At a time of year known for 90° days, the absence of heat has been noteworthy. The metro area hasn’t recorded a 90° temperature since July 9—more than two weeks (17 days) ago! If July 2007 closes at midnight next Tuesday without another 90° day, this becomes only the sixth year since 1928 at Midway Airport in which the final three weeks of the month failed to produce a single 90°.
The heat wave which has gripped the Plains with triple-digit temperatures for days is breaking. Readings were in free-fall late Wednesday from Montana north to Canada’s Prairie Provinces, where 106° to 115° highs Tuesday were off more than 40-degrees in spots.
--By Tom Skilling, WGN-TV Chief Meteorologist

Chicago winters have warmed an average of 5 degrees since the 1970s, but summers haven’t kept pace in the production of truly hot days. When overall summer temperatures are averaged and compared, readings haven’t fallen. But, as hot weather enthusiasts may have suspected, the pace at which 90°-plus days have occurred is off. Since 2000, the city has hosted only 143 daytime highs of 90° or higher at Midway. By comparison, the 1930s saw 343 days with 90s, the 1950s hosted 276, and the 1940s featured 252. If 90s continue occurring at the pace observed so far, the 2000-2009 period would be the lowest yielding decade in terms of summer 90s since the 1930s.
Blistering heat broke records again Tuesday from the northern Plains to the shore of Hudson Bay. An automated Canadian weather station at Rockglen, Saskatchewan recorded an unofficial high of 115°. Had that reading been official, it would have exceeded Canada’s highest temperature ever: 113° set on July 5, 1937.
--By Tom Skilling, WGN-TV Chief Meteorologist

Dedicated hot weather enthusiasts may grumble, but for many Chicagoans the past five days—the 5th coolest July 19-23 period since observations began in 1959 at O’Hare—have been a pleasant change. The summer had produced an ever-expanding roster of 90s at an impressive pace up until the recent cool weather’s arrival several weeks ago. Ten were on the books by July 9. Then the 90s stopped. The effect on this summer’s temperatures has been dramatic. The season has slipped from the 27th to the 43rd warmest of the past 137 years—a drop of 16 slots over the past 15 days.
Haze continues to increase slowly in coming days as humidities edge higher, and the area remains in an extremely disorganized surface wind regime. Winds aloft are a bit better organized but blowing into the area from the east. This carries more sunlight-filtering clouds into the area Tuesday and Wednesday. Cooling aloft lays the foundation for a few scattered thunderstorms Wednesday/Thursday.
--By Tom Skilling, WGN-TV Chief Meteorologist

As the northern Plains states bake under 100°-plus heat and high humidity, Chicago will experience close to a “typical” mid-July day today with high temperatures reaching into the mid 80s.
The upper-air jet stream pattern will change very little in the next couple days with a huge ridge of high pressure out west and a deep low-pressure troughing effect over the eastern United States. This will result in Heat Advisories over the Dakotas and just a slow increase in humidity and a possible introduction of scattered showers or thunderstorms in the forecasts here.
The jet stream flow will modify slightly toward the end of the work week, but the primary storm track for low pressure centers will be west-east across southern and central Canada. We anticipate just one “cool” frontal passage in northeast Illinois during the week ahead on Friday, ending the chance of thunderstorms and dropping humidity significantly next weekend.
--By Paul Dailey, WGN Weather Center Meteorologist

Early this week, cool dry Canadian high pressure will slowly recede to the east, taking its unseasonably low 50° dew points (temperatures to which the air must be cooled to condense, and a good guide to overnight low temps under clear skies) with it. By Monday, under the influence of this high, much of the eastern half of the country will be rain-free. However, rains will continue along the Gulf Coast and southern Texas where downpours of over 10 inches Friday into early Saturday triggered extensive flooding of rivers, streams and highways and numerous flood warnings by the Austin-San Antonio National Weather Service Forecast Office. Southerly flow will gradually draw this moisture north up the Mississippi Valley into Illinois and Wisconsin. By week’s end, a broad area of showers and thunderstorms will exist from the northern Plains east into the Great Lakes and south to the Gulf of Mexico.
--By Paul Dailey, WGN Weather Center Meteorologist

Chicago’s in the midst of uncommonly cool mid-summer weather. That’s why few air conditioners will be humming Saturday and a light jacket may feel good again Saturday night, especially in cooler locations away from the city.
Daybreak temperatures outside the city early Saturday hover within a few degrees of the 51° record low set 37 years ago in 1970. The “chill” follows Friday’s 75° high—the fourth coolest July 20 on record and a reading nearly 10 degrees below normal.
Historically, only 20 percent of meteorological summer (June through August) days have recorded highs 75° or lower at O’Hare Airport since weather observations began at the Northwest Side site in 1959.
By the end of this weekend, the Chicago area will have gone 13 consecutive days without a 90° temperature. That’s happened between July 10 and 22 only eight times in 79 years, dating back to 1928 at Midway Airport.
--By Tom Skilling, WGN-TV Chief Meteorologist

Friday’s predicted 72° high, the area’s coolest in 3 weeks, falls on the date which has hosted more 90° highs than any other. Since 1928, temperatures on July 20th have surged to 90° or higher 30 times, the most of any day of the year. By contrast, highs have been as cool or cooler than today’s 72° only five times on this date. Fewer than a quarter of July’s high temperatures have been this cool.
Wednesday evening’s downpours have pushed July rainfall above 5” in parts of the metro area. But at O’Hare, July rainfall just 20 days into the month equals the average full July tally of 3.50”. That qualifies this month as the 20th wettest of the past 137 years.
Blazing heat broke records out West Thursday. Great Falls, Montana hit 104° while Casper, Wyoming topped out at 99°.

--By Tom Skilling, WGN-TV Chief Meteorologist

The Chicago area, which had escaped flooding rains during thunderstorm eruptions to the west and south Monday and Tuesday evenings, couldn’t escape Wednesday evening’s storms. Rains fell so heavily in parts of the area, motorists were forced to the shoulder. Lightning displays were dazzling. Nearly 20% of cloud-to-ground strokes were so-called “positively charged” lightning discharges, which are often of higher amperage (i.e. more energetic) and therefore more prone to setting fires and causing injuries. Police report a moving vehicle was struck by lightning in Riverwoods setting it on fire. Miraculously, there were no injuries.
The t-storms responsible tapped the most humid air of 2007 to date and towered to 59,000 ft. At peak intensity, 4,700 cloud to ground lightning discharges were measured in just 10 minutes Wed. evening. By 9:30 p.m. Algonquin had recorded 2.70” of rain.
--By Tom Skilling, WGN-TV Chief Meteorologist

A series of downpour-generating thunderstorms drenched an area from eastern Iowa into central Illinois and downstate Indiana Tuesday, only flirting with the Chicago area but stranding motorists for a time near Mattoon. Rainfall with the southeastbound t-storms was eyecatching, totaling 5.70” just south of Cedar Rapids and 5” near Shellsburg—both in Iowa. In central Illinois, 4.40” walloped Windsor, while nearby Neoga recorded 3.95”. Mattoon’s rainfall totaled 2.93”. Even sections of the Chicago area didn’t completely escape the rain. Pontiac recorded nearly an inch (0.98”), while Oak Brook received 0.63”.
The storms erupted on the eastern flank of a punishingly hot mass of air which baked the Plains a third day. Only the rain-cooled storm outflow deflected the heat away from Chicago. Readings topped out at 106° at Pine Ridge, S.D. and Valentine, Neb.
--By Tom Skilling, WGN-TV Chief Meteorologist

The thunderstorms which rake the metro area at times the next few days are blunting the eastward advance of blistering triple-digit heat baking sections of the Plains and Midwest. While downpour-cooled t-storm outflows off Tuesday’s thunderstorms limit Chicago area highs to the 80s, temperatures soared to 104° Monday at Rapid City, South Dakota and Chadron, Nebraska, 103° at Bozeman and Miles City, Montana, 95° at Dickinson, North Dakota and 92° at Des Moines.
With such hot air to our west and some of the year’s most humid air headed this way, the atmosphere is in an explosive state Tuesday. Similar conditions to Chicago’s west produced powerful thunderstorms from the Dakotas southeast to Iowa Monday, some more than 10 miles high. Hudson, Iowa was pelted with softball size (4.25” diameter) hail while a number of other areas in the state reported golf-ball size hail.
--By Tom Skilling, WGN-TV Chief Meteorologist

Chicago lies in an area of severe storm potential later today as a warm front approaches from the southwest. This front is expected to orient in a position extending from northern Iowa into northwest Indiana and become quasi-stationary for the next few days, slowly vacillating back and forth from positions just south of Chicago to north along the Illinois-Wisconsin border. With cool northwest flow aloft and converging warm, moist low-level flow along the warm front, unstable meteorological conditions will persist over northeast Illinois into Thursday. With radiational cooling on top of cloud layers, nighttime periods could be especially conducive to heavy rain and flood-producing conditions as slow-moving thunderstorms could develop and “train” (one storm after another tracking over the same area).
Forecasts have backed off a warm-up next weekend with latest guidance indicating increased influence of cooler Canadian high pressure.
--By Paul Dailey, WGN Weather Center Meteorologist

Chicagoans have a couple more days of near normal high temperatures in store, and then a gradual warm-up begins which may see readings climb into the mid 90s by the end of the workweek. The cold front which moved through northeast Illinois Saturday afternoon rain-free will reach southern Illinois today, then very slowly move back north as a warm front. There will be many extended rain-free periods, but showers and thunderstorms will be in the forecast Monday through Thursday. As the frontal boundary approaches, there will be a noticeable rise in humidity most likely starting Tuesday.
By Friday the upper air pattern over the western Great Lakes is expected to evolve significantly from primarily NW to a weak west-east flow positioned north of the U.S.-Canadian border. Southwesterly flow into the Midwest will then allow the dome of heat over the Plains to spread eastward and persist through much of the following week.
---By Paul Dailey, WGN Weather Center Meteorologist

Veteran Mac racers will tell you it’s been some time since the Chicago to Mackinac race kicked off amid 20 m.p.h. sustained winds. That’s what’s predicted as the race gets underway at noon Saturday—making today’s open potentially the windiest in years. The gusty winds precede a cold front which is racing southeastward beneath an abnormally strong July jet stream bearing winds as high as 125 m.p.h. at its core. These powerful upper-level winds (at the 18,000 to 36,000 ft. level) threaten to lift Saturday’s moderately humid low 60° dew point air, producing scattered-coverage t-storms in the process. Computer energy calculations, which take into account moisture, the rate the temperature declines with height and the tendency for wind directions to vary through the atmosphere, suggest some storms may become active. They may occur almost anywhere, probably becoming most numerous in areas just west and south of Chicago.
--By Tom Skilling, WGN-TV Chief Meteorologist

You wouldn’t know it from Friday’s comfortable weather but Chicago is entering a period of the year known for its 90° highs. Friday’s highs are predicted to remain in the 70s—specifically, 77° at O’Hare. That’s “cool” for this time of year.
The “normal” high—the smoothed average of the highs observed on this date from 1971-2000—is 84°. That puts today 7 degrees below normal.
Nearly a quarter of the city’s 90°-plus temperatures—488 of the 1,876 on the books at Midway Airport since 1928—have occurred between July 11 and 31. Daily highs reach or exceed 90° nearly a third of the time in July’s back half. Weather records indicate Chicagoans encounter the best odds of experiencing a 90° high on July 20 with a 38 percent historic probability, followed by July 19 with a 37 percent chance.
Thursday’s 80° high was the 42nd reading at or above 80° to date this year—the most here by July 12 in 16 years.
--By Tom Skilling, WGN-TV Chief Meteorologist

Wednesday’s 77° was Chicago’s coolest in 10 days and 6° below normal. It’s little wonder area residents have taken notice of the temperature downturn. Sub-normal readings have been in short supply this season. Temperatures since June 1 have averaged 72.4° at O’Hare—well above the site’s 69.5° average since 1959. It’s the warmest start to a summer season here in two years and more than 3° warmer than a year ago.
A showery disturbance embedded within NW steering winds sweeps into Chicago Thursday. A fairly rainy period could dominate from mid morning until mid-afternoon before precipitation becomes more scattered and finally exits the area tonight.
Rains here pale with those which have fallen in the Plains. Oklahoma City has measured 35.03” this year —the heaviest on record since 1891.
--By Tom Skilling, WGN-TV Chief Meteorologist

The most comfortable weather and lowest humidities in almost a week greet Chicagoans Wednesday. The change follows another wild afternoon of thunderstorms over parts of the metro area Tuesday. An eastbound squall line, which included t-storms with radar-scanned cloud tops that reached 55,000 ft. into the atmosphere, unleashed temperature-slashing 60 m.p.h. winds in west suburban Sugar Grove and parts of the city. Damage, including downed trees and power lines in Franklin Park, bore the markings of a possible microburst. The storms’ lightning was dramatic. Up to 1,700 cloud-to-ground strokes occurred Tuesday afternoon within a 225-mile radius of Chicago.
The heat won’t be missed. Warmer than normal temperatures since June 1 have boosted air conditioning usage 54% over the same period a year ago.

--By Tom Skilling, WGN-TV Chief Meteorologist

Thunderstorms, some more than 10 miles high, erupted with a vengeance Monday afternoon in the muggy, energy-charged atmosphere dripping with nearly 1.75” of evaporated moisture. The powerful winds which gushed from the storms reached 57 mph at Kenosha, 60 m.p.h. south of Algonquin and 50 m.p.h. at Romeoville. Sections of McHenry County were walloped by 3” of rain. But, nowhere were the storms’ effects more severe than in DeKalb, hit first by temperature-crashing 50+ m.p.h. gusts, 3/4-inch diameter hail and then a blinding cloudburst which deposited as much as 4.25” of rain—more than typically occurs over a full July—in just 80 minutes time. The deluge closed roads, forced evacuation of one Northern Illinois University dorm, left cars stranded and basements flooded. At their height, the storms unleashed more than 2,400 cloud- to-ground lightning strokes in a single 10-minute period within a 240 miles radius of Chicago.

--By Tom Skilling, WGN-TV Chief Meteorologist

It’s supposed to be hot in July, but heat can bring problems when it’s too hot and too persistent. Long-lasting and excessive heat mounts to the level of disaster when it is accompanied by drought—and that has been the ongoing story across the western U.S. A multiyear drought, stretching to five years in some states, has set the stage for extreme wildfire problems. High temperatures, low humidity, gusty daytime winds and lightning strikes that sometimes extend beyond the reach of the limited rain areas of isolated afternoon mountain thunderstorms have set numerous wildfires. Unfortunately, computer models offer no significant change. Intense heat is forecast to persist at least into late July, and moisture will remain critically short.
Chicago’s 94° peak reading on Sunday topped Miami’s 91°. However, despite Chicago’s hot weather, the city did not come close to breaking the July 8 record of 99° set in 1955.
--By Richard Koeneman, WGN Weather Center Meteorologist

Sunday’s afternoon temperature, 96º, marks the highest reading registered in Chicago since 97º was logged on August 2, 2006. It’s also within shooting distance of the official record high for the date, 99º (in 1955).
However, it has been hotter on July 8. A review of Midway Airport temperature data from 1928 to the present reveals that the city sizzled at 106º on that date in 1936, but that reading is not considered to be an official temperature because the University of Chicago was the city’s official weather observation site in 1936. The University thermometer, “cooled” by a lake breeze, stalled at 95º that day.
While summer is in progress here, it’s winter down under. Vostok Station, Antarctica—620 miles from the South Pole—holds the world’s low-temperature record (-129º, on July 21, 1983), and its expected minimum on Sunday morning is -88º. That is 184° colder than Chicago’s Sunday temperature!
--By Richard Koeneman, WGN Weather Center Meteorologist

Much warmer air rides southwest winds into northeast Illinois this weekend. Dermatologist Dr. Bryan Schultz advises taking extra precautions to protect exposed skin under mostly sunny skies both Saturday and Sunday. Still near its zenith, the sun will release the highest concentration of intense sunlight and strong ultraviolet rays observed so far this summer.
With readings reaching the upper 90s Sunday, the heat index is expected to also reach 100° for the first time this summer. Showers and thunderstorms will precede and accompany a cold front Monday, which will not only bring welcome rains, but also cooling northeast breezes off Lake Michigan. A brief surge of warm, humid air later Tuesday into Wednesday could bring strong t-storms and heavy downpours ahead of a potent cold front. Under cool Canadian high pressure, peak late-week readings are expected to hold in the middle 70s—some 5-10 degrees below normal.
--By Paul Dailey, WGN Weather Center Meteorologist

An acrid veil of smoke, haze and fog lay across the Chicago area Wednesday evening in the wake of the holiday fireworks, and in the process it provided a text-book example of smog. That word was coined in 1905 by London physician Harold Des Veaux to describe natural London fog contaminated by smoke.
In the 102 years since then, smog has come to be a synonym for any kind of visible air pollution in urban areas and now, increasingly, in rural areas.
Chicago’s 4th of July smog lingered through the calm, humid overnight hours, not dissipating until winds picked up after sunrise Thursday morning.
Looking ahead, an offshoot of the western heat wave expands across the Midwest this weekend, sending Chicago’s temperatures to the middle 90s on Sunday.- That would be 2007’s hottest to date and highest since 97º on Aug. 2, 2006.
--By Richard Koeneman, WGN Weather Center Meteorologist

Summer is vacation time and the weather in July, averaged across the United States, is more conducive to outdoor activities than in any other month.
Coincidentally, thunderstorm activity also peaks nationally in July, and thunderstorms bring lightning—the most frequently occurring of all hazardous weather phenomena. This country experiences 22 million lightning ground strikes annually.
Geographer Dr. George Kimble once remarked that “Swinging a metal-shafted club overhead has been the last mortal act of many a wet-weather golfer.” It’s a thought intended not just for golfers but for everyone because today’s splendid weather will coax hundreds of thousands of us outdoors—but isolated afternoon thunderstorms will be lurking as well, and their limited-coverage rainfall is less likely to drive us inside.
--By Richard Koeneman, WGN Weather Center Meteorologist

Active, potentially severe, thunderstorms across the area diminish from Chicago northward today as warm, but drier and more stable air works in during the course of the afternoon. The severe threat shifts to areas well south and east of the city during the afternoon.
Elsewhere, heat continues to build across the nation, and only New England remains pleasantly cool. Across the West —already parched and hot—the heat is intensifying to blistering levels. Afternoon temperatures are forecast to rocket to 120-122º in the hottest desert areas of Arizona, California and Nevada, and that is intensely hot even for areas accustomed to very high temperatures. Compare those readings to these all-time heat records: 134º at Death Valley, California, on July 10, 1913 (U.S. record); 136º at El Azizia, Libya, on Sept. 13, 1922 (world record).
--By Richard Koeneman, WGN Weather Center Meteorologist

Today’s high in the mid 80s ends an unprecedented five-day (June 28-July 2) string of afternoon readings in the 70s. It’s never been that cool for that many days in the late-June/early-July period since records began at Midway Airport in 1928.
And the warmth this afternoon strengthens to genuine heat by the weekend. Massive rains that have swamped the Southern Plains in recent weeks call to mind the world’s one-month precipitation record that is held by Cherrapunji, India. Located at an elevation of 4,309 feet in Assam, India, Cherrapunji is subject to warm, moist monsoon winds that sweep inland from the Indian Ocean, depositing buckets of rain as the air ascends toward the Himalayan Mountains. An incredible 366.14 inches of rain came down at Cherrapunji in July, 1861. That world record still stands, even after 146 years.
-By Richard Koeneman, WGN Weather Center Meteorologist

Holidays never come with a guarantee of favorable weather. Consider the case of this year’s Fourth of July holiday period.
Chicago will find itself in a stormy transition zone between intensifying heat to the west and persistently cool temperatures to the east, overlaid by upper-level winds blowing from northwest to southeast.
That means any thunderstorms that might be triggered by daytime heating in the northern Plains will migrate southeast to the Chicago area—and computer models indicate that the Dakotas, Minnesota and eventually Wisconsin will be favored areas for storm development Tuesday and Wednesday.
By no means, however, are steady, all-day rains expected here. It is usually the nature of thunderstorms to be in and out of a given location rather quickly, and most hours during the holiday period will be rain free, despite the ongoing thunderstorm threat.
--By Richard Koeneman, WGN Weather Center Meteorologist