textproduct: Chicago

KEY MESSAGES

- Below average temperatures expected for the remainder of the work week.

DISCUSSION

Issued at 155 PM CDT Mon Sep 21 2026

A strong (1031 MB) high over Lake Superior is helping produce rather brisk northeast winds which have been ushering in cooler temperatures and gradually lower dewpoints. A closed mid-upper level low seen on water vapor imagery is moving into western IA early this afternoon. This upper low is expected to slowly weaken as it pushes east into northern IL by midday Tuesday. The unseasonably moist air mass that helped feed heavy rainfall totals over the weekend is being shunted off to the south of the area. However, cold mid-level temps (around -15C) associated with this weakening upper low should be enough to result in scattered showers this afternoon across IA. This shower activity is expected to attempt to spread east into northern IL late tonight into Tuesday morning as the upper low pushes this way, however, the showers are expected to encounter a gradually drier air mass as they move east of the MS River. Have maintained some 20-30% chances for showers late tonight into Tuesday AM, especially across our western CWA.

This shortwave trough is progged to continue eastward toward the East Coast by Wednesday into Wednesday night, likely aiding in intense coastal cyclogenesis. A Rex Block is expected to develop over southeastern Canada into the Great Lakes later this week as a strong, large cut-off coastal cyclone becomes quasi-stationary to the south of a strong mid-upper high over southeastern Canada. For our area, this should result in an extended period of somewhat cooler temperatures and very little threat of precipitation from mid week through the upcoming weekend. One small possible exception to the dry weather could be Wednesday morning into the Chicago Metro area when there could be a few sprinkles or very light lake effect rain showers. Thermodynamics are marginal and with fairly dry low level air mass, the chances appear pretty small so didn't introduce any precip into the grids at this time, but did nudge pops up to around 10%. Otherwise, look for temperatures that will likely slowly moderate late in the week into the weekend with temperatures probably warming back into the 70s.

- Izzi

AVIATION /00Z TAFS THROUGH 06Z WEDNESDAY/

Issued at 641 PM CDT Mon Sep 21 2026

There are no significant aviation forecast concerns for the current TAF period.

Patchy MVFR ceilings are still being observed at and near ORD and RFD at press time and may linger in the area for a while longer this evening. Additional periods of MVFR ceilings appear to be possible at times tomorrow, though prevailing VFR conditions are still favored at all of our TAF sites over the course of the TAF period.

Light rain will likely persist at and near RFD through this evening. An additional wave or two of rain showers may pass over or near RFD again tomorrow, and sprinkles or a brief period of rain showers may also occur at ORD and MDW towards the end of their current 30-hour TAF period, but will let the potential need for additional precipitation mentions in the TAFs be addressed in forthcoming TAF issuances.

Otherwise, expect persistent northeasterly winds throughout the TAF period with gusts to around 20 kts at times.

Ogorek

LOT WATCHES/WARNINGS/ADVISORIES

IL...Beach Hazards Statement through Wednesday evening for ILZ006- ILZ103-ILZ104.

IN...Beach Hazards Statement through Wednesday afternoon for INZ001- INZ002.

LM...Small Craft Advisory until 7 PM CDT Wednesday for the IL nearshore waters.

Small Craft Advisory until 10 AM CDT Wednesday for Calumet Harbor IL to Burns Harbor IN.

Small Craft Advisory until 7 PM CDT this evening for Burns Harbor to Michigan City IN.