textproduct: Chicago

KEY MESSAGES

- Periods of rain this afternoon through Saturday evening with a chance of thunderstorms.

- The potential exists for locally heavy rainfall with areas of 2"+ rainfall totals.

- High waves and dangerous swimming conditions at Lake Michigan beaches late tonight through Monday morning.

DISCUSSION

Issued at 151 AM CDT Fri Jul 31 2026

Low pressure will move southeast across IA today then across south/central IL tonight and across the Ohio River valley Saturday as it slows. Showers and at least a chance for thunderstorms will spread across the area this afternoon. As the low approaches later today, the southern cwa may remain in the warm sector allowing for a better chance of thunderstorms, a few of which may be strong to locally severe. Though it appears the best chance of any severe thunderstorms will remain southwest of the local area.

Periods of rain will continue tonight into Saturday morning along with a continued chance for thunderstorms, which current timing trends would suggest the best chance in the evening into the early overnight of Saturday morning. The rain during this time period will likely be moderate to heavy at times with precipitable water values in the 2 inch range. With the system slowing as it moves southeast, at least scattered showers are expected to continue Saturday and these may continue into Saturday night, especially across northwest IN. Models differ quite a bit for storm total rainfall but amounts in the 1 to 3 inch range look reasonable, highest amounts across the northern cwa. There may be some locally higher amounts as well.

A prolonged period of northeast winds is expected, gusting to 30-35 mph across southern Lake Michigan. This will allow for high waves and dangerous swimming conditions at Lake Michigan beaches late tonight through early Monday morning and plan to issue a beach hazard statement with this morning's forecast.

High pressure will bring dry weather back to the area early next week with precip chances possibly returning late next week. cms

AVIATION /12Z TAFS THROUGH 18Z SATURDAY/

Issued at 644 AM CDT Fri Jul 31 2026

Key Messages:

- SHRA develop west-to-east this afternoon.

- Increasing threat for embedded thunderstorms with periods of heavy rain and MVFR to IFR conditions late this evening into Saturday.

- SSE winds shift easterly and increase later into tonight.

A weather disturbance will gradually shift east-southeastward across the Midwest into the lower Great Lakes regions today into Saturday. As it does, rain will develop eastward across the northern IL terminals into this afternoon (potentially as early as 15-16Z at RFD and 17-18Z around the Chicago terminals). Rain showers will then become more pronounced across the area into tonight as the center of the system begins to shift into northwestern IL. Steepening mid-level lapse rates will also support increasing chances for embedded non-severe storms with heavy downpours, particularly during the late evening and overnight hours.

At this time, we made no major changes to the going PROB30 mention for TS and lower flight categories tonight. However, if confidence continues to increase, I could foresee the need to convert the PROB30 to a prevailing mention of VCTS (or TS) with +SHRA for at least a few hours after 04Z. Otherwise, expect the threat for showers (and possibly a few lingering storms) to linger on Saturday as the parent storm system slowly moves across the area. MVFR to IFR conditions will also persist as a result.

Otherwise, south-southeasterly winds today will shift easterly and increase tonight as surface low pressure begins to shift into western IL.

KJB

LOT WATCHES/WARNINGS/ADVISORIES

IL...Beach Hazards Statement from late tonight through Monday morning for ILZ006-ILZ103-ILZ104.

IN...Beach Hazards Statement from Saturday morning through Monday morning for INZ001-INZ002.

LM...Small Craft Advisory from 3 AM Saturday to 3 AM CDT Monday for the IL nearshore waters.

Small Craft Advisory from 9 AM Saturday to 3 AM CDT Monday for the IN nearshore waters.