textproduct: Chicago

KEY MESSAGES

- Areas of fog, some locally dense, may impact the start of the commute this morning.

- Periods of rain are expected Tuesday night through Friday, with the greatest coverage on Thursday.

DISCUSSION

Issued at 335 AM CDT Tue Sep 29 2026

While slower than previous model guidance suggested, shallow fog is beginning to expand in coverage early this morning across portions of the area outside of Chicago, some of which is dense. The general expectation is for areas of fog to persist through the first part of the morning commute, with some fog potentially drifting into portions of Chicago toward daybreak (though confidence in this remains on the lower side). Any fog that does develop should erode quickly between 8-10 AM CDT.

GOES water vapor imagery depicts the high-amplitude upper level pattern ongoing across the CONUS early this morning. Narrow ridging located across the Mississippi Valley and western Great Lakes is squeezed in between deep upper troughing and associated closed low centered over southern CA/NV, and the remnants of the slow moving Nor'easter currently drifting northeast across New England.

In between the western trough and the upper ridge overhead there is an expansive plume of deep moisture lifting across the Great Plains toward the Upper Mississippi Valley, which is expected to be further moistened with the arrival of the tropical remnants of Polo which recently made landfall across central Baja California. This will set the stage for a multi- day stretch of rainfall across the region, beginning potentially as early as Tuesday night here locally into portions of northwest Illinois as warm advective showers blossom atop a low- level frontal zone. Shower coverage then gradually expands across much of the area on Wednesday, including the Chicago metro. Confidence in the timing of the various rounds of greater shower coverage remain on the lower side, with some breaks possible at times.

The combination of both Gulf and Pacific moisture will feed anomalously high moisture content into the region. In fact, modeled PWATs approaching or even exceeding 2" is toward the upper echelon of climatology per DVN/ILX RAOBs for this time of year. This would support the potential for heavy rainfall and an associated localized flooding threat. Do think this may be mainly tied to any isolated embedded thunderstorms that potentially redevelop over a narrow more focused corridor, though confidence in if and where that occurs remains low. The aforementioned closed upper low across the SW CONUS will be slow to merge back into the northern stream upper jet, which may lead to showers persisting into the day on Friday as well before the axis shifts fully south of the area Friday evening.

Temperatures through late week will be dependent upon shower coverage each day with temperatures in the lower to mid 70s favored in areas with greater shower coverage and upper 70s to around 80 in areas south of the rainfall axis (likely south of I-80).

The upper levels then transition toward a zonal to quasi-zonal pattern for the weekend into early next week. While a few low- amplitude waves likely move within the flow aloft, any associated precipitation is currently favored to remain mainly north of the area. North to northeasterly surface winds will support cooler temperatures to start the first full week of October with highs in the 60s and lows in the 40s to around 50.

Petr

AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/

Issued at 1259 AM CDT Tue Sep 29 2026

Key Messages:

- Visibility reductions from fog are likely overnight until mid-morning

Fog has begun to develop in portions of northern Illinois and northwest Indiana at the beginning of the 06Z TAF period and will continue to expand in coverage and become more dense over the course of the night amidst light/calm winds and clear skies. Forecast guidance continues to hit the Chicago metro terminals hard with LIFR/VLIFR visibilities and ceilings by daybreak. It's possible that this may be overdone at some locations with the urban heat island effect currently keeping most of the initial fog development outside of the heart of the Chicago metro, but conditions are certainly supportive of at least MVFR visibility reductions occurring at some or all of our Chicago metro TAF sites through mid-morning. There is less support for significant visibility reductions at RFD, but areas of fog have already started to show up near RFD in certain GOES satellite channels, so it nevertheless appears likely that sub-VFR flight categories will also be attained there prior to daybreak.

The fog should burn off by 15Z or so, resulting in prevailing VFR conditions for the remainder of the TAF period. Sustained winds from mid-morning onwards will generally fall in the 5-10 kt range and largely come from a south-southwesterly to south- southeasterly direction. A few showers could be observed towards the very end of the TAF periods for RFD, ORD, and MDW. Though, with the better chances for showers coming beyond the end time of the current TAF period, will let any precipitation mentions be addressed in forthcoming TAF packages.

Ogorek

LOT WATCHES/WARNINGS/ADVISORIES

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