textproduct: Chicago

KEY MESSAGES

- Periods of rain are expected through Saturday night with a chance of thunderstorms, highest storm chances overnight and again Saturday afternoon.

- The potential exists for locally heavy rainfall with localized pockets or corridors of 2"+ rainfall totals.

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

DISCUSSION

Issued at 314 PM CDT Fri Jul 31 2026

The first round of showers has moved into the area this afternoon but have remained mainly light to moderate as far as rainfall rates go (under 0.5"/hr). The rain and widespread cloud cover have also helped cool temperatures back into the 70s.

This initial band of showers may decrease in coverage for a few hours here late this afternoon before coverage increases again heading into the evening as the center of the low gets closer to the area (currently centered over Iowa). Instability is slow to increase with this system which suggests that the lightning coverage will remain low through this evening though a rogue lightning strike or two can't be ruled out prior to 9 PM. The potential for isolated embedded thunderstorms then increases from west to east through the late evening and continues overnight (30-40% chance). Based on the favored track of the low and a collective assessment of the available model guidance, the threat for localized flooding this evening through the night is greatest north of I-80 and toward northwest Illinois and the WI/IL state line.

There may be a brief lull in coverage in the morning on Saturday (though that is not consistent among all the guidance), potentially mixed with reduced visibility in drizzle and/or fog. Confidence is higher that shower coverage increases through midday near and south of a Waukegan to Rochelle line, especially across much of the Chicago metro area. This is the period we are most concerned about a renewed threat of lightning as well as heavy rainfall (rates in excess of 1-2"/hr) which may lead to localized flooding. Concern is greatest in urban areas and in any areas that experienced flooding from storms earlier in the week (if showers/storms happen to stall or redevelop over the same areas). We will also have to monitor for potential funnel clouds south of the Illinois/Kankakee River Valley where there is an area of higher low-level instability closer to the center of the surface low. Heading into the evening hours on Saturday the northern deformation axis of the low (and associated area of greater shower coverage and higher rainfall rates) will likely focus south and east of I-55 into northwest Indiana and east central Illinois.

Confidence in if and where localized pockets or corridors of rainfall amounts in excess of 2-3 inches occurs in the local area remained too low to issue a Flood Watch with the afternoon forecast update. Have instead issued a Hydrologic Outlook (ESF) add a bit more visibility to the flooding potential in addition to our graphical messaging. Will continue to monitor trends closely in case a Flood Watch is needed with later updates.

While some lingering showers can't be ruled out into portions of northwest Indiana into Sunday, cooler and dry conditions are then favored during the first part of next week with gradually increasing temperatures back into the 80s by TUE-WED. An upper level wave and attendant surface front may swing through the area toward midweek bringing renewed shower/storm chances though confidence in the details remains low in that timeframe.

Petr

AVIATION /00Z TAFS THROUGH 06Z SUNDAY/

Issued at 645 PM CDT Fri Jul 31 2026

Showers will expand in coverage and intensity through late this evening and overnight. Cigs have fallen quickly to low-MVFR at RFD, and these lower cigs will work east across the rest of the TAF sites this evening. Have introduced IFR cigs at RFD late tonight into tomorrow morning where confidence is highest, and may need to drop cigs to IFR in future issuances at the Chicago terminals if confidence continues to increase.

An arcing band of embedded thunderstorms is currently crossing the Mississippi River and will pivot towards RFD after 03-04z this evening and then into the c90 airspace after 07-08z. Have introduced VCTS mentions at all terminals tonight to account for this, and have maintained the inherited PROB30s for on-station TSRA. Any storms would likely produce torrential rainfall and be capable of dropping vsbys even briefly to 1/2 sm.

The main TS threat will diminish around or just after daybreak Saturday with showers persisting. Another period of TS will be possible in the afternoon, for which PROB30 TSRA groups have been maintained. Once again, any storms (and even more robust showers) will be capable of producing sharply-reduced vsbys and very heavy rainfall.

While showers and storms will continue into Saturday evening, the coverage of activity will begin to diminish.

Carlaw

LOT WATCHES/WARNINGS/ADVISORIES

IL...Beach Hazards Statement from 3 AM CDT Saturday 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.