textproduct: Chicago

KEY MESSAGES

- A few showers may graze the US-24 corridor tonight and tomorrow morning.

- Dry weather with seasonable temperatures expected to end the work week.

UPDATE

Issued at 825 PM CDT Wed Sep 9 2026

Made a couple of minor tweaks to the forecast through daybreak Thursday, mainly to add a mention of patchy fog after midnight outside the core of the Chicago metro area and to trim the northward extent of pops a bit prior to daybreak across our south.

A weak, somewhat diffuse cold front currently stretching from southern WI and northwest IL back across southern IA will continue to push slowly southeast across the area through tonight. Winds near and ahead of the front have already diminished to 5 kts or less after sunset, so there's not much push or boundary layer mixing evident. Thus with light winds, little mixing and dew points still in the upper 60s/around 70 ahead of the front, would expect the potential for patchy fog to develop across parts of the area especially after midnight through daybreak. Visibility guidance generally favors areas west/south/southeast of Chicago for at least some patchy fog, while areas south of I-80 appear most favored for dense fog. Main limiting factor perhaps is an area of dense convective cirrus spreading northeast from eastern KS/MO, which may have some impact on how dense or widespread fog becomes, but did add a patchy fog mention for all but the IL/WI border area and warmer core of the Chicago urban area overnight - and will monitor trends overnight.

The only other change of note was to trim the northward extent of pops across our southern cwa through daybreak, based on high-res guidance generally keeping precip along/south of about U.S. 24 (roughly Peoria-Watseka-Wolcott) prior to sunrise.

Ratzer

DISCUSSION

Issued at 101 PM CDT Wed Sep 9 2026

Showers continue to taper in coverage across the area this afternoon in the wake of expansive composite outflow racing southward toward southern Illinois and Missouri. Gradual clearing of clouds has commenced allowing for temperatures to finally begin rising this afternoon. The expectation remains for highs to reach the mid to upper 70s before gradually falling after sunset. A few sprinkles may continue from time to time south of I-80 and especially US-24 through the evening.

Another shortwave over the central Rockies is expected to shear rapidly eastward into the lower Missouri River Valley later tonight and into tomorrow likely resulting in an uptick in coverage of showers and thunderstorms over Missouri. As this activity pushes east into Illinois, it is possible that the northern flanks of the showers could scrape across our far southern CWA late tonight into Thursday morning but the vast majority of this rain and all of the associated thunderstorm activity should stay south of our area.

By Thursday night, the center of the high pressure ridge should set up over the area. This should set the stage for nearly ideal radiational cooling with clear skies and calm winds. Have nudged low temps downward Thursday night, especially in the typical cool spots where lows could dip into the 40s. The urban heat island should keep the heat of the city of Chicago in the lower 60s for low temps. Can envision areas of shallow fog developing by daybreak, but will withhold mention in the forecast for now. Friday looks like a spectacular early autumn day with highs around 80, low humidity, and sunny skies.

The weekend forecast overall looks ok, though forecast confidence is a bit lower with some chance it could rain a bit at some point. Another trough is expected to pass well north of the area over the weekend with a weakening cold front reaching the area Saturday or Saturday night before probably washing out altogether. Cannot totally rule out a few showers with that front, but given weak frontal convergence, antecedent dry air mass, and minimal upper forcing, the more likely scenario looks to be dry weather for most. Southerly winds in advance of this front should allow temps to warm more solidly into the 80s on Saturday. Another trough is expected to move into the region early next week bringing what looks to be a somewhat better chance of showers and thunderstorms in the Monday/Tuesday time frame. There is still model variability in the timing and track of that trough, so forecast confidence is still on the lower end of the spectrum.

-Izzi / Borchardt

AVIATION /00Z TAFS THROUGH 06Z FRIDAY/

Issued at 627 PM CDT Wed Sep 9 2026

Only aviation forecast concerns for the 00Z TAFs are for some patchy MVFR ceiling potential this evening, and some concern for MVFR/IFR fog to develop after toward morning mainly south/west of the terminals.

A slow-moving and somewhat diffuse cold front was analyzed from roughly KMTW-KDSM at 6 PM, and this front will continue to drift slowly southeast across the terminals late this evening/overnight. A combination of satellite and surface observations indicates some patchy 2000-2500 foot MVFR stratus/stratocu lingers along the I-88 corridor west of Chicago and across the southwest/south suburbs. Coverage/footprint upstream of all TAF sites except GYY appears to be low enough to just maintain a SCT mention this evening. Maintained a TEMPO mention at GYY for potential 2500-3000 foot for a couple of hours. This should eventually shift east/south of the terminals later this evening. Late tonight, there is some guidance support for MVFR/IFR fog development mainly south and west of the terminals, with perhaps DPA/GYY having the greatest threat of some fog prior to sunrise. Additional thick high clouds remain upstream of the area at this time however, which cloud slow/prevent fog development, so confidence is somewhat low on direct impacts to these terminals. Will keep an eye on trends this evening for the possible need to add a BR mention with later updates.

Wind-wise, the wind shift across the cold front to our northwest is somewhat subtle, with westerly winds predominant. A more substantial shift to light northwest or north-northwest looks to hold off until later tonight/predawn Thursday however. Winds will likely turn northeast across the Chicago area after sunrise Thursday morning, with a more easterly lake-assisted push later in the afternoon though speeds look to remain generally 10 kts or less.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...None.