textproduct: Chicago

KEY MESSAGES

- The dry and unseasonably mild weather will continue through the upcoming weekend.

DISCUSSION

Issued at 136 PM CDT Wed Oct 21 2026

An anomalously strong mid-level ridge over the central CONUS will anchor omega blocking across much of North America over the next several days. Meanwhile, an elongated surface ridge extending from the Lower Mississippi River Valley to the Canadian Maritimes is in the processes of congealing with another expansive high pressure shifting toward the Upper Great Lakes. As a result, quiet weather with unseasonably warm conditions will continue through the upcoming weekend. As was observed the past few mornings, generally clear skies and light/calm winds should promote shallow fog across primarily interior northern Illinois late tonight through sunrise Thursday.

The blocking pattern will slowly break down early next week as the ridge flattens and a southwestern upper-level low begins to meander toward the central CONUS. A series of smaller troughs on the periphery of a developing longwave trough across the north Pacific and western Canada will eventually pick up the southwestern low. Consensus guidance suggests that the influence of the low on our area could occur as early as Monday night, but more likely Tuesday and Wednesday. Cooler and more seasonable conditions would accompany this system.

Kluber

AVIATION /00Z TAFS THROUGH 06Z FRIDAY/

Issued at 641 PM CDT Wed Oct 21 2026

Main aviation forecast concern is the potential for patchy IFR/LIFR fog to develop mainly northwest of Chicago early Thursday morning.

Surface high pressure ridge has shifted east of the area across lower MI/IN early this evening. Light south-southwest surface winds at the terminals are expected to quickly become light/variable or just calm early this evening, though there may be a brief (1-hour)period of light east winds at ORD/MDW/GYY as a weak lake breeze decays. The pressure gradient will remain weak into Thursday, keeping winds light/variable through the morning before becoming light (5-6 kts) northeast and eventually east in the afternoon/evening.

Clear skies and light/calm winds tonight will allow for ideal radiative cooling conditions, allowing shallow fog to develop in spots once again. This appears most likely across southern WI/northeast IA and far northern/northwest IL KRFD) where surface dew points remained in the mid-upper 40s during peak mixing Wednesday afternoon. While guidance is not too aggressive with lower visibilities, have maintained a 2SM IFR tempo and MIFG in the KRFD TAF, and will monitor overnight for tactical lower vsbys. The drier low-level airmass to the southeast looks to be less conducive to fog development with southeastward extent. Have maintained a 3SM MIFG tempo at DPA, with VFR conditions for the other Chicago area terminals.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

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