textproduct: Chicago

KEY MESSAGES

- Isolated thunderstorms are possible across northern Illinois late tonight through daybreak. The strongest storms closer to the Wisconsin state line may contain small hail.

- Potential for strong to severe thunderstorms Wednesday afternoon and early evening along a cold front, with greatest severe threat (level 2 of 5) around and east of a Valparaiso to Paxton line.

DISCUSSION

Issued at 206 PM CDT Tue Aug 25 2026

An upper-level low centered over the southern Manitoba and Ontario border will track to east of Lake Superior by Wednesday evening. Weak mid-level waves rotating around the upper-level low will cross Wisconsin and clip northern Illinois and northwest Indiana overnight through Wednesday evening. Meanwhile, a cold front will cross the forecast area late Wednesday morning through the afternoon.

The first mid-level impulse will brush the area to the north overnight. A reservoir of higher 900 to 750 hPa moisture advecting in from the Missouri River Valley should arrive in time to interact with a N/S axis of low-level isentropic ascent and steeper mid-level lapse rates. Expectations are for a broken to scattered line of showers with embedded thunderstorms to cross northern Illinois overnight through daybreak Wednesday. Elevated effective shear may be marginally sufficient for a couple stronger storms with small hail.

A lull in precip is expected mid to late morning Wednesday. Guidance has come into better agreement with an incoming cold front, with latest trends indicating a much faster frontal passage than what was noted just 24 hours ago. The cold front likely will have cleared much of the northwest CWA by late morning, most of the Chicago metro early in the afternoon, and northwest Indiana mid-afternoon. With deep convection hinging on destabilization via both daytime heating and advection of higher mix-layer dew points ahead of the front, trends suggest that convective initiation may not occur until near or east of a Valparaiso to Paxton line mid to late afternoon. Conditional on these storm developing before the front clears most of the forecast area, deep-layer shear will be favorable for a mix of supercells and merging multicell clusters. Large hail will be a threat with discrete cells while damaging wind is of most concern with any storm.

With the main mid-level wave trailing the cold front by a few hours, have maintained low-end chance PoPs (under 30%) over northeast Illinois mid to late afternoon as the post-frontal environment may not fully stabilize and/or mix-out. Isolated to widely scattered showers and perhaps a couple narrow but deeper cores with lightning are possible during this window.

Mid-level ridging over the Canadian Prairies will deamplify while shifting eastward late this week into the weekend. Associated surface high pressure will track east while becoming elongated across the central Great Lakes Thursday and Friday. Cannot fully rule out sprinkles or weak showers on the lake breeze boundary across northeast Illinois late Thursday afternoon, but the subsidence inversion should be strong enough to stunt growing diurnal cumulus.

A loosely zonal pattern across North America this weekend into early next week favors a more active storm track north of the forecast area, but close enough to warrant periods of low-end precip chances for showers and some thunderstorms as early as Saturday night (and especially toward Monday and Tuesday). Summer warmth will also return Sunday and possibly Monday with increasing humidity and daytime highs pushing well into the 80s (assuming active weather remains bottled up to the north).

Kluber

AVIATION /00Z TAFS THROUGH 06Z THURSDAY/

Issued at 635 PM CDT Tue Aug 25 2026

Key Aviation Messages for the current TAF period:

- Slight chance for showers, and possibly isolated storms, to develop around or just before daybreak

- Another chance for showers and storms to develop along a front by midday and the afternoon, mainly around and south of KGYY

A stalled lake breeze is slowly retreating to the lake. Southwest winds are less than 10 knots and will gradually diminish toward midnight. Winds will increase tomorrow morning ahead of a front with gusts to around 20 knots. After the frontal passage midday/afternoon, winds are expected to turn westerly (if not occasionally northwesterly).

The previously mentioned front is still currently in central Minnesota, but will descend southeastward through Wisconsin overnight. A weak boundary out ahead of it may produce some showers after midnight and closer to daybreak. The main uncertainty with the showers is coverage so maintained a PROB30 in the TAFs. Weak instability will be present during this window, so thunderstorms are expected to be more isolated (if at all). So, thunder was kept out of the TAF presently.

There is lower confidence on the exact timing of the front's arrival, but better instability and forcing along the front increases the chances for thunder development. While the mid/late afternoon still looks like the prime window for strongest storm development well southeast of the Chicago terminals, there was enough model agreement to suggest a small window for thunder in the late morning/early afternoon near MDW/GYY. With the better chances around and south of KGYY, no -TSRA was included in the ORD TAF. However, if and unexpected slower front transpires, it may need to be reconsidered.

DK

LOT WATCHES/WARNINGS/ADVISORIES

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