textproduct: Chicago

KEY MESSAGES

- Scattered thunderstorms are possible late this afternoon into the evening (50% chance), some of which may be capable of damaging wind gusts and heavy downpours which may lead to localized flooding.

DISCUSSION

Issued at 330 AM CDT Tue Aug 18 2026

All is quiet weather-wise across northern Illinois and northwest Indiana early this morning with temperatures in the 60s and increasing high cloud coverage. Attention is instead on monitoring upstream features which will play a role in our shower and storm chances later today. Overall confidence is on the lower side in how things unfold today owing to weaker forcing mechanisms for ascent initially in the afternoon and less favorable diurnal timing of the cold frontal passage this evening. This has led to continued variability from model run to run in the location and coverage of storms later this afternoon through the evening.

There are currently a few different thunderstorm complexes to our west and northwest, including southeast Minnesota, southeast South Dakota and eastern Nebraska tied to mid-level impulses moving within the broader northwest flow aloft. The Minnesota storms have been struggling to hold together for several hours and are likely to continue decaying with southeast extent across southern Wisconsin. The complex of storms in Nebraska remains more organized early this morning but is expected to eventually weaken with eastward extent into Iowa and northern Missouri. However, it is possible that a remnant MCV feature within the northern stratiform rain area persists as it crosses Iowa this morning into the area by this afternoon. This paired with a mid-level vort may lead to renewed shower development this afternoon near and north of I-80. Modeled soundings suggest that the layer of steeper lapse rates will be fairly narrow through mid-afternoon suggesting that instability will be limited and keep the lightning threat lower initially.

Farther to the northwest a cold front is currently diving southeast across the SD/ND border and northwest MN. As this moves into central Wisconsin later this afternoon it may lead to increasing thunderstorm coverage as the mid-level flow strengthens to 45-55 kt. This combined with increasing PWATs into the 1.5-2.0" range supports an increasing damaging wind threat from precip loaded downbursts with any storms that develop. This activity may then drift southeast into northern and northeast Illinois this evening (~5-10 PM timeframe). Given the higher PWATs, locally heavy rainfall will likely accompany any storms that develop and could result in a localized flash flood threat, particularly if multiple cells move over the same areas. The slow southward progress of the cold front may lead to showers and thunderstorms lingering into the early overnight hours south of I-80 before they fully exit the area toward daybreak Wednesday.

Another break in this active pattern is in store midweek as a cooler and less humid airmass settles into the area. Temperatures will be slightly cooler for this time of year (typically lower 80s in mid August) with highs in the mid to upper 70s each day amidst cooler onshore flow. Confidence in the details then decreases late week into the upcoming weekend as additional waves move within the upper level northwest flow pattern which will bring periodic shower and storm chances into the weekend.

Petr

AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/

Issued at 615 AM CDT Tue Aug 18 2026

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

Multiple residual convective waves stretching from southern Minnesota to northern Missouri this morning will drift eastward today. SHRA originating from the associated mid-level cloud deck may survive across northern Illinois (particularly RFD) late morning into mid 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, coverage of TS remains low enough to maintain PROB30 (roughly 23-03Z at ORD/MDW). This window may need to be narrowed and transitioned to a TEMPO, but a more concrete time window likely will not be apparent until much later today. Any TS will be capable of strong gusty winds and torrential rainfall rates.

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.