textproduct: Chicago

KEY MESSAGES

- Periods of rain, heavy at times, expected late tonight through early Friday morning. Significant rises on area rivers and streams may result.

DISCUSSION

Issued at 241 PM CDT Tue Sep 29 2026

Early this afternoon, GOES water vapor imagery shows an expansive plume of deep Gulf moisture (PWATs ~1.5"+) streaming northward from the Plains across the Upper Midwest and into Ontario. This broad latitudinal transport is driven by a strong south- southwesterly low-level mass response east of a northern- stream trough traversing the Canadian Rockies and a southern- stream system advancing into the Desert Southwest. As the northern- stream trough moves rapidly east across southern Canada through midweek, it will push a surface cold front west- southwestward into the Midwest and western Great Lakes. Meanwhile, as the Desert Southwest trough moves toward the Great Plains through Thursday, it is forecast to eject several smaller-scale impulses east-northeast over the Corn Belt into the lower Great Lakes. These waves, combined with a persistent northward feed of Gulf moisture into the frontal boundary, will set the stage for multiple rounds of heavy rain Wednesday through Thursday night.

Initial rain showers driven by warm air advection may move into portions of northwestern Illinois as early as later this evening. However, the first primary wave of rain is expected to shift into northern Illinois and parts of far northwestern Indiana late tonight (likely after 4:00 AM) and persist through the morning. The threat of lightning with this activity appears low (but non-zero), but locally heavier downpours may accompany it. Scattered shower activity will likely continue into Wednesday afternoon and evening. The primary area of focus may begin to shift northward toward the Wisconsin state line, though the exact extent of this northward shift remains uncertain. Substantial cloud cover and periods of rain will keep temperatures in the 60s to around 70 across most of northern Illinois on Wednesday.

The axis of rainfall, potentially accompanied by some thunderstorms, is expected to shift back south into the area Thursday into Thursday night as the surface frontal boundary drops southward. The presence of near-record precipitable water values near 2 inches (rare for early October per sounding climatology at ILX and DVN) suggests periods of moderate to heavy rainfall will accompany this system. Rainfall amounts of 2 to 3 inches (with locally higher totals) are possible across the area. This threat will be highlighted in a Hydrologic Outlook (ESF).

The surface cold front will shift south of the area on Friday, bringing an end to the wet pattern as high pressure builds in for the weekend. Conditions for the weekend look dry and coolermore typical for early Octoberwith temperatures topping out in the 60s.

KJB

AVIATION /00Z TAFS THROUGH 06Z THURSDAY/

Issued at 629 PM CDT Tue Sep 29 2026

- VFR prevails through tonight. Deteriorating conditions with periods of rain Wednesday morning and afternoon.

Winds will briefly back out of the southeast this evening at under 10 kts before returning out of the S/SSW overnight. Precipitation will gradually overspread the area late tonight and into early Wednesday morning, first in the form of VFR sprinkles before quickly transitioning to steady rainfall. As the steady rain arrives, a brief period of gusty southwesterly winds (perhaps into the mid 20 kt range) will be possible as the remaining dry sub-cloud layer saturates.

Cigs will then rapidly lower to low-MVFR/IFR through the morning along with MVFR/IFR vsbys. The greatest potential for IFR conditions exists near RFD, but have also included TEMPO IFR conditions at DPA and ORD with the outgoing 00z TAFs.

Once this morning/early afternoon batch of rain moves east of the region, intermittent showers/drizzle will continue. A northward-moving boundary will eventually scour out the low cigs, first at GYY, then at MDW and ORD through late afternoon/early evening, although there is uncertainty regarding when this will take place. An additional round of showers will then likely develop late Wednesday evening and overnight--just beyond the end of the extended ORD/MDW TAF periods.

Carlaw

LOT WATCHES/WARNINGS/ADVISORIES

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