textproduct: Chicago

KEY MESSAGES

- Periods of rain, at times heavy, will continue through this evening. Additional rainfall up to 1-2" (and possibly narrow corridors in excess of 2") may lead to localized flooding and river rises.

- Dry and seasonably cool conditions are expected this weekend into early next week.

UPDATE

Issued at 952 AM CDT Thu Oct 1 2026

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.

- Izzi

DISCUSSION

Issued at 348 AM CDT Thu Oct 1 2026

Regional radar imagery shows an expansive area of showers ongoing across the region early this morning, with the earlier focused axis of torrential rainfall in eastern Nebraska having since scattered out for the most part. A broader area of steady rainfall, with embedded higher rainfall rates, is beginning to set up across northeast Iowa, far northwest Illinois, into southwest Wisconsin initially this morning. This favors mainly lighter rain and somewhat intermittent coverage for much of the area through the morning before this activity ultimately drifts back south into northern Illinois toward mid-late morning.

Meanwhile a separate area of renewed heavier shower development will likely occur in response to the phasing of a disturbance lifting northeast from the southern KS/MO border with a separate disturbance moving into southern IA, eventually reaching the area toward the afternoon. In contrast to yesterday, there is a brief window when a narrow corridor of weak instability (up to 250-500 J/kg MLCAPE) may develop in response to this feature as it moves through the area into early evening. The presence of weak instability and near-record moisture content (2"+ PWATs) would support higher rainfall rates, potentially locally in excess of 1"/hr. While guidance has been fairly consistent in the cold front progressing steadily across the area today, any training of heavier showers and isolated embedded thunderstorms may still yield localized corridor(s) of rainfall amounts in excess of 2". The threat of flooding would be mainly tied to these narrow corridors of higher amounts. However, confidence in if and where these ultimately set up remains low and accordingly, plan to hold off on any additional flood-related headlines. Widespread rainfall amounts of 1-2" will still lead to some rises on area rivers and streams.

In the wake of this evening's cold front, persistent northerly winds and cold air advection will lead to a seasonably cool end to the work week on Friday with high temperatures in the 60s, lowest along the lakeshore.

The upper level pattern transitions toward a more zonal WNW to ESE orientation this weekend into next week. Temperatures remain near seasonable to seasonably cool with lower and more comfortable humidity. Several low-amplitude waves will move within the upper jet to our north and swing mainly dry cold fronts across the area at times.

Petr

AVIATION /12Z TAFS THROUGH 18Z FRIDAY/

Issued at 711 AM CDT Thu Oct 1 2026

Key Messages:

- Periods of rain this morning will transition to a steady rain this afternoon (mid-late morning at RFD) before rain tapers off this evening.

- MVFR and IFR ceilings and visibilities are expected at times through tonight. A brief period of LIFR could not be ruled out either.

- A northerly wind shift will occur behind a cold front this afternoon.

Initially, waves of rain showers will continue to pass through the C90 airspace through this morning. Primarily VFR conditions will be favored at the Chicago metro terminals during this time, though brief MVFR visibility reductions may occur at times beneath the steadier showers.

A more solid band of rain will set up over RFD by the mid-late morning before shifting over the Chicago metro during the early-mid afternoon. Prevailing MVFR to IFR conditions are expected to occur for several hours as this band of steady rain sits over the terminals. The rain will then taper off from northwest to southeast during the evening, with VFR conditions returning at some point thereafter.

A brief period of LIFR also could not be ruled, particularly this afternoon at the Chicago metro terminals. The HRRR forecast model, in particular, has had several recent runs with a narrow band of deeper convective showers/possible storms with notable rainfall rates setting up somewhere over the Chicago metro this afternoon. If such a scenario were to verify, then temporary visibility reductions to 1/2 mile or less and lightning would become more likely at the Chicago metro terminals. However, most other models currently do not favor such a scenario and generally keep the heaviest rainfall and greater instability south of the terminals, so held off on depicting this realistic worst case scenario in the 12Z TAFs. If more model support for this scenario crops up this morning or if observational trends start to point towards such a scenario occurring, then these TAFs will need to be trended in a more pessimistic direction.

Lastly, during today's rainfall, a cold front will drop across the area from northwest to southeast. This will result in a wind shift from southwesterly to northerly winds between midday and the mid-late afternoon. Confidence in the exact timing of this wind shift still remains somewhat low, but the wind shift should come within 1-2 hours of the forecasted wind shift times in the TAFs.

Ogorek

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory from 4 AM to 10 PM CDT Friday for the IL and IN nearshore waters.