textproduct: Chicago

KEY MESSAGES

- Spotty sprinkles/light showers possible in some areas today.

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

DISCUSSION

Issued at 159 AM CDT Tue Sep 22 2026

Latest surface analysis places a broad ~1030 mb surface high from the northern Great Lakes eastward into Quebec. The western periphery of the high is progged to strengthen across the northern Lakes today, in association with strong subsidence and building mid-level heights in the wake of a positive-tilt short wave digging southeast across the northeastern CONUS. Farther to the southwest, a slow- moving closed upper-level low was evident in GOES vapor imagery over eastern IA. This feature is forecast to continue to drift slowly east-southeast across northern IL through this evening and eventually into central IN Wednesday morning, forming a Rex block (upper level high poleward of an upper level low) which will only gradually shift east through mid-week.

Current region radar imagery depicts showers (with even some isolated lightning strikes) associated with the aforementioned circulation over eastern IA and vicinity. These showers will gradually drift east with the circulation today, although they'll be encountering gradually drier air with eastward extent. Would expect at least some spotty precipitation to spread into the forecast area however, and have added some slight chance pops (15-20% - highest west) developing from west to east today/early this evening though coverage may be isolated in many areas under otherwise mostly cloudy skies. Spotty, light shower chances should diminish from west to east across northern this evening as the mid-level circulation continues to drift east, and across northwest Indiana overnight, though probably can't completely rule out a few stray sprinkles from the western fringes of this disturbance even into Wednesday morning. As previous day shift noted, there is a bit of a signal in guidance for slightly better low-level lapse rates over Lake Michigan on Wednesday which could support some lake-effect sprinkles or light showers into northeast IL, though delta-Ts are marginal (10-12C) and inversion heights are near/just below 5 kft. Thus chances continue to appear fairly low and kept only sub-15% pops without a precip mention near the lake for this possibility.

Otherwise, with the relatively strong surface high off to our northeast, breezy/blustery (gusting 20-25 kts at times, especially near the lake) northeast winds will continue across the area into Wednesday before diminishing more substantially. This will maintain dangerous swim conditions (and current Beach Hazard Statements in effect) for our Lake Michigan beaches, as well as cooler than normal temperatures through mid-week. Daytime highs are expected to remain in the 60s with overnight lows in the 50s in most spots, though some late night 40s are possible away from the lake where breaks in the clouds or more widespread scattering occurs.

Eventually, the Rex block is progged to break down in the Thursday and Friday time period, allowing upper-level ridging to extend into the upper Mississippi Valley/western Lakes. This will allow for a modest warming trend into the low 70s and upper 60s, lighter winds and possibly more sunshine. Medium-range guidance continues to depict increasing precip chances west of the Mississippi by Friday however, which may at least spread mid/high-level cloud cover back across the region. Current global models/ensembles suggest precip may try to push into the forecast area by late Saturday/Sunday, though with guidance developing a deep trough/closed low to our east along the Atlantic coast, the eastward progression of any substantial precipitation threat may be somewhat blocked until beyond the end of the current forecast period.

Ratzer

AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/

Issued at 1235 AM CDT Tue Sep 22 2026

Forecast concerns include...

Mvfr cigs through mid/late morning and again early Wednesday. Low chances for showers through the period. Gusty east/northeast winds.

Mvfr cigs continue to develop across northeast IL and southern WI early this morning and are expected to continue through at least mid morning, though there remains uncertainty for how prevailing they will remain. Given current trends, opted to carry mvfr cigs and will monitor trends. There is reasonable consensus for cigs to lift by midday into this afternoon and then guidance is suggesting mvfr cigs will redevelop late tonight into early Wednesday morning.

A slow moving upper wave currently over eastern IA is expected to move across the terminals later this morning. While confidence is low, there appears to be a mentionable chance for showers during this time period and included prob shower mention, though whatever does materialize may end up being isolated. There will be a better chance for showers across northwest IL and at RFD. Additional low chances for showers are possible this evening and again overnight into Wednesday morning. Confidence is too low to include shower mention at this time.

East/northeast winds will remain gusty at times through the period, mainly for the Chicago terminals overnight and again late tonight, and for most of the are during the daylight hours today. cms

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.