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 /06Z TAFS THROUGH 12Z WEDNESDAY/

Issued at 114 AM CDT Tue Aug 18 2026

Expect VFR conditions through the morning with plenty of cirrus transitioning to increasing mid cloud cover. SW winds under 10 knots around 5 knots will increase to 10 to 15 knots by late morning, then gust to around 20 knots through the afternoon.

Ongoing convection over Nebraska and less-so over southeast Minnesota should steadily weaken with eastward extent this morning. However, SHRA originating from a residual mid-level cloud deck may survive across northern Illinois (particularly RFD) this afternoon.

An elongated mid-level wave and trailing cold front will drag across southern Wisconsin early this evening and northern Illinois and northern Indiana late evening into early Wednesday morning. There remains some question as to how far south convection develops early to mid evening owing to increasing capping with southward extent over northern Illinois. While isolated to scattered SHRA are expected during this time (VCSH in current TAF), coverage of TS remains low enough to maintain PROB30 (roughly 23-03Z at ORD/MDW). Will need to continue refining the time window while determining if confidence is high enough to convert to a TEMPO TS, but a more concrete time window may not be apparent until much later today.

The cold front will shift winds NW under 10 knots early Wednesday morning amid VFR conditions.

Kluber

LOT WATCHES/WARNINGS/ADVISORIES

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