Brrr! The big cooldown moves in towards the end of the week

WEDNESDAY WEATHER HEADLINE FIRST

-Another fine day Wednesday with provisional highs of 75 on O’Hare and 77 on Midway.

– It’s the warmth before the BIG COOL DOWN that hits with a cold frontal early Thursday afternoon – a frontal that’s expected to bring a confounding northerly wind shift with gusts of up to 30-40mph later Thursday and Thursday evening and a nearly 20 degree pullback in temperature between tomorrow’s early afternoon high of 72 and Friday’s predicted 55 degree high

— GALE WARNINGS are cruising on Lake Michigan where waves that start out as bare waves Thursday night grow up to 6-12 feet late Thursday night — white-capped waves and quite the show on Chicago’s lakefront.

–The strong winds and sharp pullback in temperature to the coolest levels in 5 months – since last May – with back-to-back highs in the 50s on Friday and Saturday – are the result of directional winds pushing up from Canada and strong 1041 mb (30.74″) Canadian anticyclone around which an impressive pressure field is said to generate gusty, full-haul NORTH winds along Lake Michigan.

– The cool winds riding down 280 miles of comparatively warm lake water (southern Lake Michigan’s average water temperature is now 63 degrees at the southern end of the lake) set the stage for lake-effect clouds and even sporadic rain showers Thursday night into Friday.

– Is this the last warm weather you might be wondering? Not at all. A rebound in temperatures early next week is likely to bring the 70s back to the area next Tuesday and Wednesday before another chunk of cool air moves in the following weekend on Thursday.

–My meteorological colleague at WGN reviewed Chicago’s 151-year observational record and found that 74 of the last 151 years — 49% of them — produced at least an additional 80 — a few more years.

-And only 3 of those 151 years have produced 70 degree temperatures from that date – giving a climatological 98% chance that we will see the 70’s again.

– But the days are getting shorter, and that means more frequent bursts of cool air. Mark has found that 120 out of 141 years – or 79% of them – will result in a freezing point or lower temperature in the city by November 5th. And inland temperatures could flirt with frost-inducing levels Friday night through Saturday.

– Still looking cool for Sunday morning’s Chicago Marathan opener with temps in the 40’s at race time and then 7 to 15 mph W or SW winds when the race gets underway.

HERE’S THE LATEST CHICAGO METRO FORECAST

TONIGHT: More clouds than last night and milder. A few splashes or a spot or two of light showers are possible – but only affect 20% of the area. Low 54

THURSDAY: Sunbeams give way to more clouds. Mild until early afternoon – but windy and cooler. Growing chances of some showers – most numerous in the afternoon, when nearly 60% of the area may have some rain. Winds turn north and increase to 20 to 30 miles per hour in the afternoon and late afternoon. High 72 – but by nightfall many areas revert to the 50’s.

THURSDAY NIGHT: Windy and noticeably cloudy. Cloudy spells, particularly along Lake Michigan with scattered rain showers with lake effect. Wind gusts of over 30 km/h are likely. Low 43

FRIDAY: Windy with the coolest daytime temperatures since last May. Mixed sunshine – but clouds off Lake Michigan may produce scattered lake-effect rain showers in lake districts. high 55

SATURDAY: Mostly sunny, windy and cool. high 56

SUNDAY: Increasing cloudiness, breezy and not quite as cool – but still under the season. high 62

MONDAY: Partly cloudy. high 63

TUESDAY AND WEDNESDAY: Increased cloud cover, more breezy and warmer. A few showers possible Tuesday night and Wednesday. High Tuesday 72. Wednesday High 75.