textproduct: Chicago

KEY MESSAGES

- Locally heavy rainfall could result in some isolated flooding through mid-late evening.

- Chilliest night of the fall is expected Friday night with widespread lows in the 40s away from the Chicago urban heat island.

- Extended period of dry, pleasant fall weather expected Friday through potentially all of next week.

DISCUSSION

Issued at 100 PM CDT Thu Oct 1 2026

Earlier discussion regarding this afternoon and evening's heavy rain threat (issued at 952 AM)...

Have opted to issue a flood watch for portions of northern IL into Lake County Indiana for this afternoon and evening.

A cold front stretching from near Milwaukee to north central Missouri will move slowly southeastward through tonight. An unseasonably moist air mass is in place ahead of this front with an axis of 70F+ dewpoints noted pooling just ahead of the boundary across southeastern IA. The 12z ILX sounding had a PWAT of 1.77" with SPC mesoanalysis depicting a corridor of 1.9-2.1" PWATs pooling near the front from NW IL into SE IA and northern MO. This is about as moist of an air mass as can occur this time of year in this part of the country.

As the cold front moves slowly southeastward, a mid-level shortwave trough over northwest MO is expected to shear east- northeastward into northern IL, providing enhanced ascent coincident with the timing of the cold front moving into northern IL. The incredibly high dewpoints for early Oct should contribute to some weak low level instability near and ahead of the front despite the poor lapse rates. The weak low level instability coupled with progged strong omega all below -10C, should result in a very favorable warm rain process set-up, which tends to result in very efficient heavy rain production.

Given this afternoon's heavy rainfall looks to set up not far from where the axis of heaviest rainfall (>1") fell yesterday, there is increased concern for some flooding problems developing this afternoon into this evening. The latest 3-hour flash flood guidance is around 1.75", though naturally lower than that in the highly urbanized areas of the Chicago metro area. There has been a very consistent signal in every run of the HRRR since 6z with an axis 3 hour rainfall totals of 2.5 to nearly 4" of rain this afternoon ahead of this front, which is given the rather low flash flood guidance, would be quite problematic.

Remainder of the discussion (late this evening onward)...

While the heavy rainfall threat is likely to diminish late this evening, some light rain may linger through midnight north of I-80 and only gradually end from the north south of I-80. It is possible that light rain may not end south down to the Hwy 24 corridor until just after sunrise Friday.

Other than the chance of some lingering light rain near Hwy 24 early Friday morning, dry weather is expected through the weekend and potentially most of next week as well. Skies will gradually clear from north to south Friday. Despite increasingly sunny conditions, northerly winds behind the cold front should result in cooler temperatures and noticeably less humid conditions Friday.

Clear skies and light winds should set the stage for the chilliest night of the fall for most Friday night. Widespread 40s are expected outside of Chicago's urban heat island. Couldn't even totally rule out an isolated 39 in the typically chillier cold air drainage areas.

A moisture starved cold front will sweep across the area Sunday. After a couple of days with highs in the 60s Friday and Saturday, it could briefly warm up into the lower 70s Sunday ahead of the front. Behind the front, highs in the 60s are expected the beginning of next week, gradually moderating back toward 70 by midweek. All in all, dry and very pleasant fall weather is on tap after the rain ends tonight/very early Friday morning.

- Izzi

AVIATION /18Z TAFS THROUGH 00Z SATURDAY/

Issued at 1226 PM CDT Thu Oct 1 2026

Key Aviation Messages for the current TAF period:

- Steady rain through this evening that will result in lower cigs/vis down to IFR conditions with a slight chance for LIFR conditions.

- Southwest winds are expected to turn to the north behind a front with the heavier rain this afternoon

Rainy conditions are expected to prevail through the day today. Winds remain out of the southwest and just under 10 knots. As a front over northern Illinois (currently over RFD) moves southeast, rain rates are expected to increase and winds will flip to the north. There is a little uncertainty on the exact wind direction as they may flip between NNW and NNE at times. However, there is higher confidence that the periods of steadier, heavier rain will be accompanied by lower cigs and vis down to at least IFR conditions. There is a chance for LIFR conditions, but too low for formal mention in the TAF. Lastly, there is a slight chance (less than 20 percent) for an isolated lightning strike this afternoon, but confidence remains too low for mention in the TAFs.

Rain will gradually taper off from northwest to southeast through the evening. Drier, VFR conditions are expected after midnight with northerly winds between 10 to 15 knots prevailing through the day tomorrow.

DK

LOT WATCHES/WARNINGS/ADVISORIES

IL...Flood Watch until 10 PM CDT this evening for ILZ006-ILZ010- ILZ011-ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ103-ILZ104- ILZ105-ILZ106-ILZ107-ILZ108.

IN...Flood Watch until 10 PM CDT this evening for INZ001.

LM...Small Craft Advisory from 4 AM to 10 PM CDT Friday for the IL and IN nearshore waters.