textproduct: Chicago

KEY MESSAGES

- Dense fog may develop again early this morning within about 25 miles of Lake Michigan, with highest chances in the northern Chicago suburbs. Any dense fog that does form will quickly burn off by mid-morning. - The dry and unseasonably mild weather will continue through the upcoming weekend.

DISCUSSION

Issued at 157 AM CDT Wed Oct 21 2026

GOES nighttime microphysics RGB shows fog developing across Lake County IL into extreme southeast WI. Given the fog is developing a couple hours earlier than last night and with similarly favorable conditions to early Tuesday morning, seems likely that dense fog will continue to develop and expand through sunrise. The northern and northwest suburbs seem most likely for dense fog development, but given the low dewpoint depressions, calm winds, and clear skies, fog will probably expand south into the heart of the metro too, though confidence is low in how far south it develops. A dense fog advisory will likely be issued sooner rather than later and will probably need to be expanded as fog expands. Similar to Tuesday morning, fog should burn off by around 14z.

Very little day to day change in the weather is expected through the upcoming weekend. A very weak southwest gradient today may limit the inland penetration of the lake breeze this afternoon compared to Tuesday. This would have implications on areal coverage of fog tonight, as the added moisture from the marine layer has been contributing to the fog development. Temperatures will likely be a couple degrees warmer today.

Another weak surface high will track across the upper Great Lakes in SE Canada Thursday into Friday, likely resulting in a very weak easterly gradient favoring a deeper inland penetration of the weak lake breeze Thursday and possibly Friday as well. This could tee things up for a bit more areal coverage of dense fog Fri and Sat morning.

The omega block and the amplitude of the upper ridge is still progged to break down early next week which at least opens the door to the possibility of a cold frontal passage and perhaps some rain at some point early next week. Medium range guidance often struggles with the breaking down of blocking patterns and there is still considerable ensemble spread, so the magnitude and timing of any potential return to more seasonable temperatures is still a low confidence item. As is any potential increase in rain chances.

- Izzi

AVIATION /06Z TAFS THROUGH 12Z THURSDAY/

Issued at 1213 AM CDT Wed Oct 21 2026

Key Aviation Messages for the current TAF period:

- Fog, locally dense, is possible prior to sunrise, mainly at Chicago Terminals north of a KARR to KMDW line.

Winds are expected to be light and calm through the night. There is another chance for fog to develop this morning. It has already started to develop around and north of KENW where obs are already reporting 1/4SM. While fog development is not a certainty, if/when it does occur, it can be locally dense with vis potentially near zero. Similar to yesterday, vis improvement should be fairly quick after sunrise.

Light southwest winds are expected through much of the day before becoming more southeasterly toward sunset. Once the fog clears, VFR conditions are expected through the end of the TAF. There is another chance for patchy fog tomorrow morning, but the chances are not as great as recently.

DK

LOT WATCHES/WARNINGS/ADVISORIES

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