textproduct: Chicago

KEY MESSAGES

- Threat for severe thunderstorms and flash flooding Tuesday afternoon/night.

DISCUSSION

Issued at 252 PM CDT Mon Aug 17 2026

While the weather is pleasant and quiet for a change today, attention quickly shifts to Tuesday afternoon and evening as a mid/upper trough digs into the Great Lakes. As this occurs, a belt of enhanced mid and upper level flow will sag southward into the Midwest and steer a series of mid-level impulses right across our area as a surface cold front drops southward into southern WI into northern IL.

The kinematic environment is expected to be quite favorable for organized severe thunderstorms, owing to the 45-55 kt of mid- level westerly flow developing through the base of the Great Lakes trough. The main uncertainty at this time regarding the severe thunderstorm threat across our area is afternoon destabilization. There is some potential for storm development this evening across parts of the northern Plains to shift east- southeastward across the Corn Belt tonight, and then across our area in a weakening fashion late Tuesday morning/early afternoon. If this were to pan out, afternoon destabilization could be limited ahead of the front due to extensive cloud debris and remnant showers, hence leading to a lower severe threat into the evening. This potential thus keeps confidence in the severe threat over our area low at this time. Nevertheless, if ample destabilization occurs in the afternoon, conditions will be favorable for the development of severe thunderstorms, including supercells. The main hazard from these looks to be wind gusts in excess of 60 mph. The threat of a tornado or two is low, but non-zero. Locally heavy rainfall may also accompany these storms into the evening, and could result in a localized flash flood threat, particularly if multiple cells move over the same area.

The surface cold front will shift south into central IL and IN into Wednesday. In its wake, a cooler and less humid airmass will filter into the area on northerly winds. A period of pleasant and dry weather is expected to persist for a couple of days with this airmass as a surface high shifts over the Great Lakes on Thursday. High temperatures are forecast to remain in the mid to upper 70s both days under partly cloudy skies. Confidence in the details then decreases late week into the upcoming weekend as additional waves move within the upper-level northwest flow pattern, with a potentially more robust and slower moving wave bringing increasing shower/storm chances for the weekend.

KJB

AVIATION /00Z TAFS THROUGH 06Z WEDNESDAY/

Issued at 625 PM CDT Mon Aug 17 2026

VFR conditions will prevail through most of the TAF period. Winds will become light/variable this evening and then will gradually become southwesterly through the overnight. Some gusts to around 20 kts are expected midday tomorrow and into the afternoon.

There is a threat for showers and thunderstorms in the area, mainly late Tuesday afternoon and into the evening hours. Additional areas of sprinkles/light showers will be possible towards late-morning/midday. Confidence in general precipitation trends remains low at this time given a wide disparity in model guidance, along with the potential for cloud cover to stunt insolation and afternoon instability. Continue with PROB30 groups for TSRA during roughly 20-01z timeframe.

Carlaw

LOT WATCHES/WARNINGS/ADVISORIES

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