textproduct: Chicago

KEY MESSAGES

- An outbreak of severe thunderstorms is expected across the region this afternoon and evening. Storms, those well south of I-80 (particularly south of the Kankakee River), will have the potential to produce damaging tornadoes, destructive straight line winds, and large hail.

- A flood watch remains in effect. Locally heavy rainfall is expected to result in an enhanced threat of localized flash flooding.

- Tranquil weather is expected to close out the work week in the wake of this system.

- Another strong storm system is expected to bring a return of showers and thunderstorms later this weekend, with some potential for heavy rainfall and/or severe weather somewhere in the general region.

DISCUSSION

Issued at 211 PM CDT Wed Jun 17 2026

Early this afternoon, we find a widespread mess of showers and a few embedded thunderstorms falling on the area east of a double-barreled surface low centered over IA and MN. These showers have been riding along the MUCAPE gradient as it spread east across northern IL on the nose of the low level warm conveyor. A surface warm front is extending across central IL south of the CWA with higher thunder coverage near and south of the front into the instability reservoir. The front continues to sharpen up with time as strong theta-e advection hits it from south. A lack of elevated instability extending north of the front owing to poor lapse rates aloft accounts for the lack of thunder coverage into northern IL. Nonetheless, strong kinematic and low level upglide in a moisture-rich environment allowed for widespread moderate to heavy rainfall across northern IL through much of the morning. A wake low also developed north of I-80 behind the departing heavier rain. Strong southeast winds with gusts periodically over 50 mph prompted us to issue a Wind Advisory for our northern CWA, but they continue to gradually ease from west to east.

Still a somewhat tricky setup for this afternoon with unresolvable mesoscale uncertainties looming through the next several hours. The warm front, while sharpening with time, has made little to no headway north or southward through about 1 PM. All eyes will be on the evolution of this boundary over the next several hours as it will dictate the northern extent of the much higher severe threat, including the appreciable tornado and destructive wind threat, as additional storms are slated to develop along and ahead of the approaching cold front. We're already seeing some pre-frontal development over SE IA which could evolve into severe convection in the near term, especially cells that stay along and south of the warm front. Latest CAM guidance favors this front inching just northward into mid- afternoon and opening up the far southern CWA to this higher severe threat. However, enough of a signal remains to be ignored, including a consistent one from the RAP, for the front to surge farther north, possibly closer to I-80. At this time, this does not look very likely, aside from perhaps a brief window toward the late afternoon/early evening as the low center works across and perhaps brings a brief window of ML instability farther north later today.

The more impressive kinematic field will be found out ahead of the front this afternoon. In fact, SPC mesoanalysis paints eye- catching long, looping hodographs over northern and central IL at the moment, we just don't have the instability to make any good use of that right now. Meanwhile, hodos straighten out upstream toward the Mississippi thanks to a veering, weakening low level wind field closer to the cold front. Any convection that can initiate ahead of the front, such as along the pre- frontal trough axis, will be exposed to a much more concerning shear profile than activity along the front itself. Storms currently developing over SE IA fit this mark and will need to be watched closely as they progress across IL.

Put together, the threat for strong tornadoes and destructive wind gusts is still alive and well, but now may be confined to only the far southern portions of our CWA through the afternoon. Namely, areas south of the Kankakee River are most favored to reside along/south of the warm front and offer the highest ceiling for severe weather over the next few to several hours. Quickly stabilizing conditions should put a halt on the tornado threat and highest wind threat north of the front, but damaging winds will certainly be possible across northern IL with elevated convection through the afternoon. The main window for the severe potential in our CWA will be roughly 3 PM through 6 PM with additional, likely non-severe storms possible through mid-evening along the cold front.

Additionally, the moisture-rich environment with PWATs just under 2" across northern IL will lead to torrential rainfall in thunderstorms and a threat for flash flooding. Strong dynamic and kinetic forcing and efficient upglide along the elevated face of the warm front will spread the heavy rain and flooding threat north of the surface front. Guidance is consistent in resolving over a widespread 1 inch+ of rain with areas of higher totals very attainable. The highest QPF is favored near and south of I-80. A Flood Watch is currently in effect outside of our northern-most counties.

Gusty gradient winds will develop tonight behind the departing low with wind 25 to 30 mph wind gusts forecasts for much of the night. After this evening, rain-free conditions are forecast into the weekend. Cooler-than-normal temperatures will begin trending warmer over the coming days with lower 70s forecast tomorrow with upper 70s/near 80 again by Saturday. Another synoptic system is progged to move across the region in the Sunday to Monday period. Previously, had been looking like another decent opportunity for heavy rain and severe weather, although latest medium range guidance has been trending south of the area. Look for a more detailed analysis of this system in future discussions once today is behind us.

Doom

AVIATION /18Z TAFS THROUGH 00Z FRIDAY/

Issued at 1244 PM CDT Wed Jun 17 2026

Key Messages:

- Strong SE winds lingering through about 19z near MDW/GYY

- Showers and embedded thunderstorms expanding in coverage across the Chicago-area terminals this afternoon.

- SW wind shift this afternoon and then a NW wind shift overnight into Thursday morning.

- Chances for showers/thunderstorms this evening.

- Periods of MVFR cigs through Thursday morning.

A lingering burst of strong southeasterly winds near MDW/GYY will gradually subside through 19z. The threat for embedded thunderstorms will then increase after 19-20z. The storm threat should then shift south of the c90 TRACON by 23-00z. Winds will then shift out of the southwest and ease late this afternoon.

Later this evening, winds will increase once again and MVFR cigs are expected to re-develop across the region late tonight into Thursday morning. There is a small chance for showers and a few thunderstorms this evening as a front moves across the region, but chances remain a bit too low for TEMPO groups. Rough timing for this would be about 23-01z at RFD and 01-03z at the Chicago- area sites.

MVFR cigs should lift to VFR through late Thursday morning.

Carlaw

LOT WATCHES/WARNINGS/ADVISORIES

IL...Beach Hazards Statement through late tonight for ILZ006-ILZ103- ILZ104.

Flood Watch until 9 PM CDT this evening for ILZ006-ILZ012- ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039- ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.

Flood Watch until 5 PM CDT this afternoon for ILZ008-ILZ010- ILZ011.

IN...Flood Watch until 9 PM CDT /10 PM EDT/ this evening for INZ001- INZ002-INZ010-INZ011-INZ019.

Beach Hazards Statement from 1 AM CDT Thursday through Thursday afternoon for INZ002.

LM...Gale Warning until 7 PM CDT this evening for the IL and IN nearshore waters.

Small Craft Advisory from 7 PM this evening to 9 AM CDT Thursday for the IL and IN nearshore waters.