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 /18Z TAFS THROUGH 00Z THURSDAY/

Issued at 115 PM CDT Tue Aug 25 2026

Key Messages:

- There is around a 30-35% chance for showers and possibly a few isolated storms to affect the terminals late tonight into early tomorrow morning.

- Another chance for showers and storms may come late tomorrow morning into the afternoon, though confidence in this round affecting the terminals is lower.

Increasing isentropic ascent tonight may support a line of convective cells developing within an encroaching unstable layer late tonight into tomorrow morning. Between the modest forcing for ascent and somewhat limited instability, it appears that most of this activity will struggle to grow tall enough to support lightning, so have maintained the -SHRA mention in the going PROB30 groups for tonight. However, a few isolated storms may still nevertheless develop somewhere in or near northern Illinois late tonight, so couldn't rule out needing to introduce a TSRA or VCTS mention for one or more of our TAF sites at some point tonight. Flight conditions may also temporarily be reduced to MVFR levels if these showers and storms do end up materializing and affecting the terminals.

A cold front will track southeastward across the area after daybreak tomorrow. Another batch of showers or storms will likely develop along or just ahead of this front by the mid- afternoon, but most of the latest high-resolution model guidance favors this to occur southeast of the terminals, so have kept the TAFs dry during the late morning into afternoon time period for now. Still, at this time, the chances for a shower or storm to affect GYY and MDW during this later time period are right around 25-30% (closer to 20-25% for ORD and DPA, and less than 15% for RFD), so forecast trends will need to be monitored closely.

Lastly, winds will gradually veer from a south-southwesterly to a west-northwesterly direction tomorrow as the front approaches and eventually crosses the terminals. Gusts to around 20 kts may be observed at times tomorrow afternoon.

Ogorek

LOT WATCHES/WARNINGS/ADVISORIES

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