textproduct: Chicago

KEY MESSAGES

- Fog, including areas of dense fog, will persist through mid- morning across portions of northern Illinois and northwestern Indiana. The greatest coverage of dense fog is expected to remain near/south of I-80 and near/east of I-55.

- Widespread rain is likely across the region in the late Wednesday through Thursday time frame.

DISCUSSION

Issued at 337 AM CDT Mon Sep 28 2026

Calm winds and mostly clear skies have allowed for fog to develop in portions of northern Illinois and northwest Indiana once again early this morning. Webcam observations suggest that a lot of this fog is shallow and patchy in coverage at this time, but a more expansive bank of thicker fog is evident on GOES Nighttime Microphysics and Fog imagery channels across southeastern portions of our forecast area. This fog will continue to expand in coverage prior to sunrise, and visibilities of 1/4 mile or less should become more common with time as well, so will be issuing a Dense Fog Advisory this morning for the affected areas.

After the fog burns off by the mid-late morning, today will shape up to be another pleasant late September day with broad high pressure over the region. Yesterday, dew points mixed out fairly easily and air temperatures correspondingly overperformed forecast guidance slightly in areas outside of Lake Michigan's influence. With the same air mass still in place over the region today, would expect something similar to occur once again with temperatures climbing into the mid-upper 70s away from the lake's shadow, while temperatures in close proximity to the lake struggle to climb above 70F. Another night of mostly clear skies and light/calm winds should then set the stage for another round of fog potential in our forecast area tonight into Tuesday morning. Forecast guidance is particularly bullish on this fog developing within the footprint of the marine- influenced higher mixing ratio air that will have settled in over much of the Chicago metro and adjacent counties prior to sunset, so for starters, have added a patchy fog mention to the forecast grids for these areas Tuesday morning.

Longwave troughing over the western CONUS will facilitate the development of a frontal zone across the central CONUS by midweek. The slower eastward progression of this frontal zone that ensemble guidance has latched onto in the past day or two means that the same air mass will persist in our area through at least Tuesday, and quite possibly through Wednesday as well. Tuesday should thus be another dry day here with slightly warmer temperatures compared to today and yesterday. Should temperatures overperform raw model guidance once again (which currently seems more likely than not), then many of our southern locales could climb into the low 80s Tuesday afternoon.

Temperatures on Wednesday will depend on how far east the aforementioned frontal zone and an associated corridor of expansive rainfall get. Southeastern portions of our forecast area will likely remain dry through the daytime on Wednesday and should thus see another day of warm temperatures with highs in the upper 70s to the low-mid 80s. Meanwhile, ensemble guidance remains mixed on whether the rain shield will spread into the northwestern half or third of our forecast area. If the rain remains north and west of our area like the GEFS and GDPS are generally favoring, then mid-upper 70s highs should be attainable once again in our northwestern locales on Wednesday, even with more extensive cloud cover. If a more southerly/easterly solution in line with most of the EPS's members were to verify and the rain (and possibly a few accompanying storms) were to spread into our northwestern counties during the daytime on Wednesday, then the mid 70s forecast high temperatures from the NBM for these areas are likely overdone, quite possibly by 10+ degrees.

Eventually, the frontal zone and the main axis of the associated rainfall will slide far enough eastward to engulf our whole forecast area sometime on Thursday as an influx of combined Pacific and Gulf moisture is drawn northeastward into the Great Lakes region. How much rainfall we'll see and how quick the rain will be to skedaddle out of our forecast area remain points of considerable uncertainty with our forecast with the various ensemble solutions still at odds on the extent to which the remnants of Pacific Hurricane Polo will phase with a cut-off upper low over the Desert Southwest. This is something that will need to be monitored closely, though, with good ensemble agreement on precipitable water values at least approaching, and possibly exceeding, 2" -- something that has only occurred a couple of times at the beginning of October in our neck of the woods since the mid 1990s, per sounding climatology at ILX and DVN. Either way, expect cooler temperatures to return for the end of the week into this coming weekend after the frontal zone has passed through the region.

Ogorek

AVIATION /12Z TAFS THROUGH 18Z TUESDAY/

Issued at 635 AM CDT Mon Sep 28 2026

Key aviation forecast messages:

- Areas of dense fog early this morning, mainly away of the TAF sites.

- Another round of fog likely across the Chicago metro early Tuesday morning, some of which may become dense.

A large area of dense fog has developed near and south of GYY across portions of east central IL and northwest IN. Patchier coverage has developed across the northwest suburbs to just east of Rockford. In contrast to yesterday, fog coverage across northeast Illinois has not increased as we approach sunrise and suspect that notable VSBY reductions will struggle to expand into the Chicago sites. Opted to maintain brief TEMPOs for MVFR VSBYs at ORD/DPA/RFD for an hour or two just in case something still attempts to develop.

Light NNW to variable winds will eventually settle into a prevailing northeast direction mid to late morning around 5-10 kt reinforced by a weak lake breeze early in the afternoon.

Winds ease again with sunset under clearing skies which will set the stage for yet another night of fog development. Model guidance remains persistent in developing dense fog across a larger footprint of the Chicago metro area in the 6-9Z timeframe that persists through early Tuesday morning. Due to this signal, have maintained or introduced TEMPOs for VSBYs in the 1-3 SM range (lowest outside of Chicago), however, even lower VSBYs and CIGs (potentially LIFR/VLIFR) may become warranted with later updates if confidence increases.

Petr

LOT WATCHES/WARNINGS/ADVISORIES

IL...Dense Fog Advisory until 10 AM CDT this morning for ILZ023- ILZ033-ILZ039-ILZ107-ILZ108.

IN...Dense Fog Advisory until 10 AM CDT /11 AM EDT/ this morning for INZ001-INZ002-INZ010-INZ011-INZ019.

LM...None.