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 /00Z TAFS THROUGH 06Z SUNDAY/

Issued at 639 PM CDT Fri Jun 12 2026

Aviation forecast concerns for the 00Z TAFs:

- Low confidence in timing/coverage of scattered TSRA late Saturday/Saturday evening.

Quiet, VFR weather conditions are expected through at least midday Saturday. Breezy west winds will diminish to less than 10 kts and turn southwest with sunset this evening. Winds will pick up again after sunrise Saturday morning, becoming gusty 20-25 kts again especially by afternoon.

A cold front will slowly approach the area from the northwest Saturday afternoon and evening, eventually pushing through the terminals sometime Saturday evening/overnight, though there is some spread within current model guidance as to the exact timing. In addition to timing differences with the front, there is quite a spread in various high-res guidance on scattered thunderstorm timing and coverage head of the front. While some guidance brings storms into the terminals as early as ~20Z Saturday afternoon, high-res ensemble guidance suggests somewhat later timing (23-00Z onward) for higher probability(40-55%) of TSRA. Given the low confidence in individual models/runs have trended toward the ensemble mean timing with PROB30 mentions after 23Z for RFD and 00Z for the Chicago metro terminals. May eventually end up needing TEMPO or prevailing mention for a time, though confidence is too low at this time.

Otherwise, VFR conditions are expected through the period, other than possible brief IFR/MVFR in TSRA.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

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