textproduct: Chicago

KEY MESSAGES

- Thunderstorms are likely Saturday evening into early Sunday morning. A few storms could be strong to severe, with a threat for large hail, damaging winds, and locally heavy rainfall.

- Cooler temperatures (highs in the 70s to low 80s) along with a returning threat for showers and thunderstorms is expected for early to midweek.

DISCUSSION

Issued at 300 PM CDT Fri Jun 12 2026

Temperatures remain in the upper 70s across the area this afternoon, in the wake of yesterday's frontal passage. This front has also pushed the richer surface dewpoints well off to the east, with dewpoints in the middle to upper 50s today.

By tonight, surface high pressure anchored over southern Indiana will begin to move off to the east, with southwesterly flow establishing across the local area prior to sunrise. As deep surface low pressure begins to eject across northern Ontario by tomorrow afternoon, a trailing cold front will begin to move southeast across the upper Midwest. Ahead of this front, the increasing southwesterly flow will bring a return of richer surface dewpoints in the upper 60s into northern Illinois and northwest Indiana. Though mid-level lapse rate are not overly steep (6-6.5 C/km) the increasing surface moisture should promote MLCAPE increasing to ~2000-2500 J/kg immediately ahead of the cold front.

As an upper shortwave ejects southeast across the northern Plains tomorrow evening, middle and upper flow strengthens enough to promote ~30-35kts of effective shear. The stronger westerlies aloft lag the front somewhat, and keeps the strongest shear largely postfrontal tomorrow. Still, increasing convergence along the surface front is expected to lead to widespread thunderstorm development across central and eastern Iowa by late tomorrow afternoon. It is expected that this will move east and southeast with time, and impact much of northern Illinois and northwest Indiana late tomorrow evening and overnight. Uncertainty remains with respect to the threat for severe weather, with a consensus among high-res guidance of quick upscale growth along the surface front. The propagation of any clusters and the development of cold pools will likely drive the threat locally, and lends low confidence for now in the coverage of any severe threat. This is in line with the latest SPC Day 2 outlook, with the slight risk trimmed to the southwest and now is restricted to the western half of Illinois. Any thunderstorms should quickly exit the area after midnight, as the cold front begins to clear the area off to the southeast.

The passage of this front will usher in cooler temperatures as we head into early next week, with highs remaining in the middle 70s to low 80s each afternoon. Deep upper troughing remains anchored over northern Ontario through much of the upcoming week, promoting broad northwest flow across the Midwest into the Ohio Valley. This suggests the threat for showers and thunderstorms will persist into much of early to middle week across the area. Any threat for severe weather is less certain, with the cooler temperatures keeping instability more limited.

Brown - WFO LSX

AVIATION /18Z TAFS THROUGH 00Z SUNDAY/

Issued at 1220 PM CDT Fri Jun 12 2026

Gusty west-southwest winds persist for all of north central Illinois and northwest Indiana through sunset this evening.

Winds then weaken and become southwest overnight, at less than 10kts. Gusty southwest winds are then expected again for all terminals tomorrow afternoon, with gusts as high as 20-25kts. Mid-level clouds are also expected to increase through the afternoon tomorrow.

Thunderstorm coverage is expected to increase across north central Illinois late tomorrow evening, though this is expected to remain off to the west of the Chicago terminals through 00Z Sunday.

Brown - WFO LSX

LOT WATCHES/WARNINGS/ADVISORIES

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