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 /18Z TAFS THROUGH 00Z SUNDAY/

Issued at 120 PM CDT Fri Jul 31 2026

A low-pressure system with features more characteristic of fall will drift over the area throughout the TAF period. Expect several periods of SHRA mixed with summertime convective elements.

An initial axis of RA/SHRA has already spread across the terminals early this afternoon. At the Chicago terminals, visibility and ceilings are expected to remain VFR. At RFD, periodic MVFR visibility/ceilings are anticipated, with IFR conditions also possible over the next several hours.

An arc of convection expected to soon develop from southern IA to central MO will pivot across northern IL tonight, affecting RFD through much of the night and the Chicago terminals late evening and overnight. Periodic SHRA with embedded convection is possible, with the current PROB30 for TSRA likely requiring one or two targeted windows with TEMPO TSRA. For ORD/MDW, TSRA is possible anytime from roughly 04-13Z, but will not occur every hour during this time.

SHRA coverage and intensity will diminish mid to late morning Saturday. IFR ceilings expected south and west of the terminals will expand farther north during the morning, particularly with added low-level moisture off Lake Michigan. While climatologically uncommon in early August, prevailing IFR ceilings may be required in the TAF through the morning hours. Additionally, BR and spotty SHRA/DZ will lower visibility into MVFR levels.

Diurnal influences combined with marginal low/mid-level rates should result in a secondary pivoting arc of low-centoid SHRA and possible TS over or just south of the Chicago terminals Saturday afternoon. Have included PROB30 TS as the exact location of this arc of precip remains unclear. However, any SHRA/TS will produce torrential rainfall rates with low-end IFR to LIFR visibility. The main precip arc is expected to begin shifting south of the terminals very late in the TAF period (early evening).

S winds 10 to 15 knots at TAF issuance will vary between 150 and 200 degrees throughout the afternoon. Winds will then gradually back and increase through the rest of the period, shifting E around 15 knots by midnight, and NE 15 to 20 knots with gusts to around 25 knots Saturday afternoon.

Kluber

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.