textproduct: Chicago

KEY MESSAGES

- Periods of rain, heavy at times, are expected through early Friday morning.

- A large footprint of 1" to 3" rainfall amounts across the region may lead to significant rises on area rivers and streams.

- The potential exists for localized corridors of rainfall amounts in excess of 3" this afternoon and evening.

- Dry and seasonably cool conditions will arrive this weekend and continue into early next week.

DISCUSSION

Issued at 306 PM CDT Wed Sep 30 2026

Several small-scale mid-level impulses, emanating from a broader trough shifting across the southern Rockies, continue to eject east-northeast from the Great Plains into the lower Great Lakes this afternoon. The initial impulses have supported a persistent, steady soaking rainfall across much of the area, particularly along and south of the I-88 corridor. The airmass remains unseasonably moist due to a continuous deep northward feed of Gulf moisture. This was confirmed by a derived precipitable water value near 1.7 inchesa near-record high for late Septemberfrom a recent Valparaiso University sounding in northwestern Indiana. Fortunately, a lack of appreciable instability has kept rain rates manageable, generally below 0.25 inches per hour with only localized corridors reaching 0.5 inches per hour. Consequently, no significant hydrological concerns are anticipated for the remainder of the day.

The focus for this persistent rain is expected to shift gradually northward into southern Wisconsin and far northern Illinois this evening. Shower coverage should temporarily ease overnight before the next wave arrives with a cold front on Thursday. A higher concern for torrential rainfall and associated hydrological impacts exists Thursday afternoon and evening as low-level lapse rates steepen ahead of the front. Prefrontal instability will remain focused primarily below the -10C level, which is unfavorable for lightning production but highly conducive to efficient warm-rain processes and low centroid cells given warm-cloud depths reaching 12,000 feet.

We will have to closely monitor the potential for northeastward- training showers near the frontal boundary capable of rain rates exceeding 1 inch per hour. Where repeated heavy rain occurs, localized flash flooding could develop with totals possibly exceeding 3 inches. While footprint amounts over 3 inches will be localized, broader rainfall totals of 1 to 3 inches are expected across the area through Thursday night, likely causing aerial flooding and significant river rises through the end of the week. These threats remain highlighted in our Hydrologic Outlook.

The surface cold front will push south of the area on Friday, bringing an end to the wet pattern as high pressure builds in for the weekend. Dry and cooler conditionstypical for early Octoberwill prevail for the weekend and early next week, with high temperatures in the 60s.

KJB

AVIATION /06Z TAFS THROUGH 12Z FRIDAY/

Issued at 118 AM CDT Thu Oct 1 2026

Key Messages:

- Periods of rain will continue through this evening before rain tapers off.

- MVFR and IFR ceilings and visibilities are expected at times through this evening.

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

Waves of showers will continue to pass through the C90 airspace through the remainder of 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 the Chicago metro during the early-mid afternoon, allowing for prevailing MVFR to IFR conditions to make a return for several hours. This band will likely set up over RFD earlier, with conditions expected to deteriorate there by the mid-late morning. There remains only about a 20% chance that isolated embedded thunderstorms within this broader rain band may affect the terminals during the afternoon (mid-late morning at RFD), so have continued to withhold on any formal TSRA or VCTS mentions in the TAFs. The rain will then taper off from northwest to southeast during the evening, with VFR conditions returning at some point thereafter.

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.

Ogorek

LOT WATCHES/WARNINGS/ADVISORIES

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