WHAT AN EXTRAORDINARY RUN OF LATE SEASON WARM WEATHER HERE IN CHICAGO. Chicagoans were treated to a 14TH CONSECUTIVE DAY OF ABOVE NORMAL TEMPS.

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 21^st 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.

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 IS 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. 

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 BEYOND THOSE TWO DAYS INTO THE LATTER POTION OF NEXT WEEK THROUGH THE FOLLOWING WEEKEND AND INTO THE WEEK WHICH FOLLOWS.

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.