It’s not an exaggeration to refer to the current warm spell—particularly the last four days of it— as “July in October”——-a meteorological “achievement” at a time of the year in which normal temps typically come in nearly 20-degrees cooler!  We’ve lost nearly 4 hours of daylight and the sun treks across the sky 30% lower than it did back on June 21st.  Sunlight that arrives at an angle and is shining down us four fewer hours than at it peak back on June 21st is weaker and delivers less energy.  So for nature to delivering us mid to upper 80-deg temps is an achievement.

Hotter Chicago area high temps Tuesday included 91 at Roselawn, IN; 90 at Channahon; 89 at Hobart and Gary IN, Oswego, North Aurora, Warrenville: and 88 at  Tinley Park, Morris, and Valparaiso, IN.

It appeared as the day got underway that record or near record early October temps were likely to occur Tuesday over all or parts of 15 states—from Minnesota east to New England. This is a dome of warm air which covered a HUGE SWATH of real estate.

Tuesday, with its high of 87-deg, finished 4-deg off the day’s 1954 record of 91 and a stunning 19-deg ABOVE NORMAL.  It’s definitely NOT EVERY DAY we reach this level of warmth in ANY MONTH—let alone October. 

Further underscoring the RARE OCTOBER 3rd TEMP TERRITORY we’ve occupied here in Chicago—ONLY 3 OTHER OCT 3’s of THE 153 ON THE BOOKS DATING BACK TO 1871 have been AS WARM OR WARMER. Those occurred in 2018 (88-deg), 1997 (87-deg) and 1954 (91-deg).  

And we did it under a 100% SUNNY SKY for a 4th consecutive day.  Veteran NWS observer Frank Wachowski notes that this year marks only the THIRD TIME SINCE OCT 1957—that the opening three days of an October have been 100% sunny.  The month is VERY YOUNG—yet we’ve received 175% the typical Oct 1-3 sunshine to date.

Clearly, this can’t go on.  We’ve sat within an extraordinary DOME OF WARM AIR which has been all but trapped in place thanks to an ATMOSPHERIC BLOCKING PATTERN which has prevented its movement. The BLOCKING PATTERN IT TO BREAK DOWN, replaced by a “WAVY JET STREAM” which by late Thursday and Friday, will be blowing into Chicago from the northwest and transporting much cooler air into Midwest from Canada. 

Scattered showers and even a few t-storms could “dot” the 20 or 25% of the area later Wednesday with rain chances surging to 70% by morning Thursday—then exiting the area. 

The SEA CHANGE PREDICTED IN UPPER AIR STEERING WINDS is to guide MARKEDLY COOLER AIR INTO CHICAGO—with the coolest temps in 5 months (i.e since early last May) on the way—including daytime highs restricted to the 50s this weekend and nighttime lows in the 40s.

The air mass coming in will be cool and unstable.  In other words, the air aloft will be so cool, it comes with a good deal of cloud cover in tow and instability showers—showers which “bubble up” as air ascends into the chilly air aloft cooling to saturation—with rains most widespread Friday.  But instability lingers Saturday. So morning sun and any “heating” it produces, may lead to clouds redeveloping and potentially becoming at least spotty shower or sprinkle producers Saturday.

HERE’S SOME GOOD NEWS. While the change to cooler air will be jarring by and during the coming weekend given the warmth we’ve been through, the cool air mass is TO BE TRANSIENT—in other words, it’s to keep moving along.  So a TEMP REBOUND APPEARS IN OUR FUTURE next week. The week is to begin cool—but it’s quite likely, if current forecast trends verify, that temps will rebound to the low 70s Wed and Thursday.  And ABOVE NORMAL TEMPS MAY EXTEND BEOND THOSE TWO DAYS INTO THE LATTER POTION OF NEXT WEEK THROUGH THE FOLLOWING WEEKEND AND INTO THE WEEK WHICH FOLLOWS.


TONIGHT: More clouds, unseasonably mild temps continue.  Low 71—but mid 60s inland.

WEDNESDAY: A good deal of cloudiness, windy and warm with a 5th day in the 80s. Several widely scattered showers and possibly a t-storm impact 20 to 25% of the area in the afternoon and evening. Some gusts to 30 mph likely by afternoon. High 83.

WEDNESDAY NIGHT: More cloudy than not. Several scattered showers lead into a period of more numerous showers and a few possible t-storms toward morning. Low 66.

THURSDAY: Opens cloudy and showery—isolated thunder possible. Then partial clearing, breezy and cooler. High 72.

FRIDAY: Early sun gives way to cloudy skies and showery weather in the afternoon and Friday night.  Breezy and markedly cooler. High 61.

SATURDAY: Coolest weather in 5 months—since early May. Mixed sun morning sun then building clouds, breezy and cool. Scattered showers possible in the afternoon. High 54.

SUNDAY: Clouds increase again after possible mixed morning sun. Continued breezy and cool. High 58.

MONDAY: Partly cloudy, breezy and cool. High 65.

TUESDAY: A good deal of sunshine, warmer. High 68.