textproduct: Chicago

KEY MESSAGES

- Isolated shower or storm possible (15-20% chance) this afternoon around Lake Michigan, and specifically northwest Indiana this evening and tonight.

- Dangerous swimming conditions at Porter County beaches today will include all Indiana and Cook County Lake Michigan beaches late tonight and Sunday.

DISCUSSION

Issued at 231 PM CDT Sat Aug 22 2026

It's a pleasant and quiet Saturday across the area with early afternoon temperatures in the upper 70s to around 80 and diurnally-driven cumulus starting to fill in overhead. Up north, a handful of showers and a couple of thunderstorms have recently blossomed along the eastern WI lakeshore with higher coverage found farther north near the Door Peninsula and UP of Michigan. These developed as a potent vort max and digging trough axis swing across the upper Midwest. The cold advection aloft has allowed for an ample reservoir of instability to build around the region while plenty of broad forcing is around to drive scattered convection. Coverage will attempt to spread south over the next few hours and a few isolated showers/storms may attempt to go up over portions of far northeast IL and northwest IN before the end of the afternoon. A considerable amount of mid level dry air, however, should greatly limit overall coverage and make it largely difficult for cores to produce lightning.

This evening, a secondary vort/backdoor cold front will drive down the windward side of the trough providing continued opportunities for showers and storms to develop over the lake during the night. The backed wind field should steer any evening/overnight activity into northwest Indiana. An isolated shower or two may even linger past daybreak into northern Porter County. In the forecast, maintained slight chances for rain and thunderstorms over and around the lake this afternoon which then focus into northwest Indiana for the evening and night. In addition to the precip potential, the added cooling aloft brought on by that secondary vort will further steepen up lapse rates over the mild lake waters such that waterspouts may develop over the lake this evening through early Sunday morning.

With the trough ejecting to our east tomorrow, surface high pressure will slide into the region and bring quiet conditions to the early part of next week. Afternoon temperatures are forecast in the middle to upper 70s on Sunday and Monday and around 80 for Tuesday. A synoptic system is favored to move across the upper Midwest during the middle of the week and bring shower and storm chances to the area in the Tuesday night to Thursday window. A lot of uncertainty still revolves around this system, including if there may be any appreciable severe threat, so we'll leave it at that for now and keep an eye on model trends over the coming days.

Doom

AVIATION /00Z TAFS THROUGH 06Z MONDAY/

Issued at 603 PM CDT Sat Aug 22 2026

Upper-level cyclonic flow centered over the central Great Lakes and the approach of a surface high pressure system from the northwest will serve as the primary influences on aviating weather through the TAF period. Gusty northwesterly winds this evening will quickly subside after sunset. Will have to watch for a 1-2 hour period of northeasterly winds at ORD/MDW/GYY in the 01-03Z timeframe associated with remnant outflow from weakening convection over Lake Michigan, but feel the threat of occurrence is too low for any mention in the TAFs. Otherwise, mixing after daybreak will facilitate occasional northwesterly gusts of 15-20kt before they subside at the end of the TAF period. Like today, a scattered to broken cumulus deck based near 6000 feet is expected to develop tomorrow afternoon.

Borchardt

LOT WATCHES/WARNINGS/ADVISORIES

IL...Beach Hazards Statement from 4 AM CDT Sunday through Sunday evening for ILZ103-ILZ104.

IN...Beach Hazards Statement from 4 AM CDT Sunday through Sunday evening for INZ001.

Beach Hazards Statement through Sunday evening for INZ002.

LM...Small Craft Advisory from 4 AM to 7 PM CDT Sunday for Wilmette Harbor to Calumet Harbor IL-Calumet Harbor to Gary IN.

Small Craft Advisory until 10 PM CDT Sunday for Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.