CHICAGO — A pool of cool, unstable air settling over the comparatively warm, moderately humid air here at ground level sets the stage for clusters of showers and t-storms through mid-week, rains which could produce sporadic downpours and generate some locally heavy rainfalls.

Placing cold, unstable air on top of a mass of warm air encourages air near the ground level to become buoyant and want to ascend and cool. Such cooling saturates the air. It’s a formula for precipitation formation. Modeling suggests the vertical temp drop argues strongly for embedded t-storms at times—much as has happened this morning in parts of the Chicago area with some dramatic lightning and loud claps of thunder in affected areas. 

Modeling puts the probability of thunder at 30% overnight but increases it to 70 to 80% at times on Tuesday and Tuesday night. 

Model rainfall projections suggest TOTAL RAINFALL through Wednesday night could range from as little as 0.30″ to scattered local totals approaching 2″. We’re still in warm season precip territory in which rainfalls vary widely. But where there are downpours, the rain could come down heavily at times.

Monday marked the 6th consecutive day of ABOVE NORMAL TEMPS—and this warmer-than-normal pattern has a way to run. The past weekend—the first nearly FULL weekend of astronomical autumn (which began at 1:50 a.m. CDT Saturday morning) came in more than 8 degrees warmer than the preceding weekend

Current forecast guidance produces ABOVE NORMAL AVERAGE FULL-DAY TEMPS each of the coming 14 days. The presence of moisture in the lower atmosphere will tend to suppress nighttime cooling which would otherwise occur well into next week. So instead of normal low and mid 50-deg nighttime lows, forecast minimum temps will hold in the 60s allowing daytime temps, restricted the next several days by clouds and precip to take off as we get into less cloudy skies and increased spells of mixed sun later this week through the weekend and into next week.

Daytime temps will be impacted by a continuation of easterly low-level winds off Lake Michigan thanks to a large Canadian high pressure all but locked in place by an atmospheric blocking pattern. 

While last week finished 2-deg above normal, this week is likely to finish 7 to 8-deg above normal and the week that follows may generate a 12-deg temp surplus—though the warmth may tail off a bit the back half of next week even though temps are to continue their ABOVE NORMAL WAYS.

At present, it seems more than likely daytime highs will reach into the 80s barring a change in forecast reasoning this coming Saturday, Sunday and Monday and very possibly into mid next week. 80s aren’t all that uncommon this late in the season. Since 1942, when official observations in Chicago moved to Midway then O’Hare, 81% of those 81 years have produced 80-deg high temps beyond Sept 25 (today’s date). While this weekend is likely to average 10 to 15-deg above normal, high temps appear likely to fall short of the record levels in the low 90s for the period.

TONIGHT: Partly cloudy this evening with several spotty showers. More clouds and increased chances for showers and possible t-storms later tonight. Rain is likely to have impacted 60% of the area by morning. Mild for the season. Low 64.

TUESDAY Through WEDNESDAY”: More cloudy than not with clusters of showers and t-storms—some capable of downpours. Temps remain above normal for the season with rain chances 80% Tuesday and Tuesday night and 60% Wednesday. High Tuesday 72—a bit lower lakeside. Low Tuesday night 63. High Wednesday 70—60s on area beaches.

Full forecast details and more at the WGN Weather Center blog

THURSDAY: Opens with clouds and some possible sprinkles or spotty showers. But turning partly sunny, still mild. High 73—a few degrees lower lakeshore.

FRIDAY, SATURDAY & SUNDAY: A good deal of sunshine with steadily warming temps with day-to-day easterly lake breezes gradually decreasing in strength and inland penetration. High Friday 77 increasing to 82 Saturday and 84-Sunday—but 70s along Lake Michigan.

MONDAY: A good deal of sunshine, unseasonably warm for the season. High 85—but mid-70s to 80 lakeshore.