textproduct: Chicago

KEY MESSAGES

- Expect scattered showers on Wednesday associated with a system that will bring temporarily much cooler conditions in its wake through the weekend.

- The colder air mass moving across the mild waters of Lake Michigan will result in a prolonged period of lake effect showers (mixed with snow at times) focused into northwest Indiana Wednesday night into Saturday.

DISCUSSION

Issued at 212 PM CST Tue Nov 10 2026

Through Tonight:

Expect calm and relatively cool conditions coming off the pleasant, unseasonably mild afternoon. High-level clouds overspreading the region from the northwest will likely prevent temperatures from getting a bit cooler than they otherwise have given the setup. This in turn may preclude much in the way of shallow ground fog development. Will defer to our evening shift for another look at the fog potential, also considering that near-term guidance has been much too aggressive with simulated fog coverage in recent nights. If conditions end up more favorable than current thinking for fog, the best chance appears to be for locales near/southeast of I-55 outside of Chicago, particularly in portions of northwest Indiana.

Wednesday through Tuesday:

On Wednesday, a compact, vigorous clipper-like shortwave, and associated surface low will track across the region. Top-down saturation of antecedent dry air mass will likely take some time to erode before scattered showers can fully break through. Initial radar echoes into interior northern IL and portions of central IL Wednesday morning may end up being virga and sprinkles. The progged surface low path from southeast IA early Wednesday and then across central IL and IN during the afternoon should tend to keep the most robust shower activity closer to the low path (ie. favoring areas near/south of I-80). Modestly steepened mid- level lapse rates with the arrival of colder air aloft could yield a few lightning strikes near/south of US Highway 24 in the afternoon.

The departing shortwave will phase in with an amplifying upper trough across the eastern CONUS Wednesday night into Thursday, which will allow a much cooler airmass to descend into the region through the end of the week. 850 mb temps will bottom out in the -4 to -8 Celsius range Thursday PM through Friday night. While embedded mid-level vort lobes rounding the resultant closed upper low may bring a few showers farther inland through the day on Thursday, focus continues to be on the persistent signal for a robust band of lake effect showers to set up over Lake Michigan, potentially into or through Saturday morning. This continues to seem reasonable at this distance given favorable lake-induced thermodynamics and prolonged northerly fetch. While occasional westward wobbles in the position of the band may occur, recent model cycles have continued to favor the banding focusing mainly into northwest Indiana late Wednesday night onward.

The brisk full-fetch northerly flow is expected to propel a marine modified (milder with higher wet bulb temps) air mass fairly far inland during the coldest periods (nighttime and early to mid morning) Thursday night into Saturday. In addition, the lowest forecast 850 mb temps over Lake Michigan are not nearly as cold as those of November 9-10, 2025, and antecedent ground temps will also be notably warmer. The boundary layer thermal profile as such appears likely to be fairly marginal in the heart of the lake induced convergence band(s). Therefore, confidence is on the lower side regarding how far inland a primarily rain p-type will extend, and where p-type may mix with or even change to wet snow. Ultimately, while there has been some ensemble support for slushy accumulations into portions of interior northwest Indiana, an impactful wet lake effect snow does not appear to be the most likely scenario at this time. With all of this said, we will continue to monitor trends closely. Winds will also be strong near the lakeshore in Indiana and adjacent northeast Illinois, with gusts up to 35-40 mph at times during this period.

In addition to the much cooler daytime temperatures (40s) Thursday through Saturday, inland areas away from the lake effect clouds will likely see temperatures fall solidly below freezing Thursday night through Saturday night. Long-range ensemble guidance indicates the colder pattern will be fairly short-lived as upper ridging builds back into the region early to mid next workweek, bringing a return to above normal temps for mid-November.

Castro/Petr

AVIATION /06Z TAFS THROUGH 12Z THURSDAY/

Issued at 1058 PM CST Tue Nov 10 2026

Winds will be light/variable to calm overnight. Some patchy shallow BR will be possible, mainly near GYY, where surface dewpoints remain a bit higher. Latest thinking is that thickening mid and high cloud cover will curtail more of a persistent dense fog threat and have commensurately TEMPO'd in some 4 SM vsbys to account for any ephemeral dips in vsbys.

A vigorous disturbance will move across the region during the day on Wednesday. Strong ascent coupled with the presence of some upper-level instability suggests fairly widespread coverage of showers should develop towards late-morning and into the afternoon. Previously coordinated adding TEMPO groups for showers and have largely maintained the previous timing in the outgoing 06z TAFs. Activity will initially have to fight sub- cloud dry air, but once this erodes, MVFR vsbys in heavier showers appear likely. Additionally, there is about a 20 percent chance of some isolated storms, but these chances are too low to mention in the TAFs at this point. The main window for any heavier precipitation may only be 2-3 hours, but will continue to cast a slightly wider net at this time range.

Precipitation will largely end by late afternoon although some spotty showers may continue into the evening. Winds will turn northwesterly as a cold front pushes through the region. At some point, it looks like gusts will increase but the exact timing remains uncertain and have relegated these to the end of the extended ORD/MDW TAFs. Currently, MVFR cigs look a bit too spotty to include in the TAFs Wednesday night.

Carlaw

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory from 6 PM Wednesday to 4 AM CST Saturday for the IL and IN nearshore waters.