FIRST, THE WEDNESDAY WEATHER HEADLINE

–Another beautiful day, Wednesday, with a preliminary high of 75 at O’Hare and 77 at Midway.

–It’s the warmth before the BIG COOL DOWN which hits with a cold frontal passage early Thursday afternoon–a frontal passage which is to bring a jarring wind shift to the north with gusts picking up to 30 to 40 mph later Thursday and Thursday night and a near 20-deg temp pullback between tomorrow’s early afternoon high of 72 and the predicted 55 deg high Friday

–GALE WARNINGS are out on Lake Michigan where waves, which start out as mere ripples Thursday night, pick up to 6 to 12 feet by late Thursday evening–waves which include white caps and put on quite a show on Chicago’s lakefront.

–The powerful winds and sharp temp pullback to the coolest levels in 5 months–since last May–with back to back highs in the 50s Friday and Saturday—are the product of steering winds aloft shifting in from Canada and carrying a strong 1041 mb (30.74″) Canadian high pressure around which an impressive pressure field is to generate gusty, full fetch NORTH winds down the length of Lake Michigan.

–The chilly winds riding down 280 miles of comparatively warm lake water (the average southern Lake Michigan water temps is now 63-deg on the south end of the lake) set the stage for lake effect clouds and even sporadic rain showers Thursday night into Friday.

–Is this the last of the warm weather, you may ask?  Not at all. A temp rebound early next week is likely to bring 70s back to the area next Tuesday and Wednesday before another chunk of cool air sweeps in Thursday into the following weekend.

–My WGN meteorological colleague has checked Chicago’s 151-year observational record and finds that 74 of the past 151 years—49% of them—have generated at least one additional 80—some years more.

–And only 3 of those 151 years have produced 70-deg temps from this date forward–yielding a 98% climatological probability we’ll see 70s again.

–But days are shortening, and this brings with it more frequent incursions of cool air. Mark has found 120 of 141 years—thay79% of them–produce a freezing or lower temp in the city at some point by November 5th.  And inland temps may flirt with frost producing levels late Friday night into Saturday.

–Still looks cool for the Chicago Marathan kickoff Sunday morning with temps around 40 at race time then with a 7 to 15 mph W or SW wind as the race gets underway.

HERE THE LATEST CHICAGO METRO FORECAST

TONIGHT: More clouds than last night and milder. A few sprinkles or a spotty light shower or two possible–but impacting only 20% of the area. Low 54.

THURSDAY: Peeks of sun give way to more clouds. Mild into early afternoon–but turning windy and cooler. Growing prospects for some showers–most numerous in the afternoon when nearly 60% of the area may see some rain. Winds shift north and increase to 20 to 30 mph mid and late afternoon. High 72–but falling back to the 50s many areas by nightfall.

THURSDAY NIGHT: Windy and noticeably cloudy. Cloudy spells, especially along Lake Michigan with scattered lake effect rain showers. Wind gusts topping 30 mph are likely. Low 43.

FRIDAY: Windy with the coolest daytime temps since last May. Mixed sunshine–but clouds off Lake Michigan may produce scattered lake effect rain showers in lakeside counties. High 55.

SATURDAY: Mostly sunny, breezy and cool. High 56.

SUNDAY: Increasing cloudiness, breezy and not quite as cool–but still below normal for the season. High 62.

MONDAY: Partly cloudy. High 63.

TUESDAY AND WEDNESDAY: Increased cloudiness, breezy and warmer. Scattered showers possible Tuesday night and Wednesday. High Tuesday 72. Wednesday’s high 75.