textproduct: Chicago

KEY MESSAGES

- A brief break in the pattern is expected tomorrow afternoon through Friday before another period of wet weather materializes this weekend. Localized flash flooding will be possible where the heaviest rain sets up.

DISCUSSION

Issued at 158 PM CDT Thu Sep 17 2026

A diffuse front/differential heating zone in the vicinity of I-90/88 continues to slowly work its way southward early this afternoon. To the north of the boundary, persistent low stratus has held temperatures in the upper 60s and lower 70s, while to the south temperatures have warmed into the upper 80s. This boundary will continue to ooze southward through the rest of the afternoon and evening. As it does so, low-level saturation depths are forecast to increase a bit along and north of I-80 as a modest trailing mid-level shortwave passes by overhead. This may be sufficient to crank out some showers or drizzle at times. Ahead of the front, some showers or storms may develop late this afternoon and into the evening as lingering convective inhibition erodes following peak heating. Low-level convergence along the boundary doesn't look particularly robust however, and this may end up being a limiting factor in terms of any convective coverage. As modest low-level isentropic upglide continues atop the sagging frontal zone, additional showers and a few storms may continue to fester through the evening south of I-80.

Conditions look to remain largely dry within the post-frontal airmass tomorrow. However, some guidance does suggest the surface baroclinic zone may slosh back northward just far enough to push the elevated instability layer back across our far south and west locales during the afternoon. Have included a slight chance of storms to account for this potential, but most guidance suggests this boundary should remain south of the CWA. Some additional spotty showers can't be ruled out across northern Illinois ahead of the next developing storm system.

Attention then shifts to what looks like another period of occasionally stormy weather in the region over the weekend, with an increasing threat for heavy rainfall and at least a localized flash flood threat. Several mid-level impulses will slingshot around the eastern periphery of a broad and persistent upper-level trough across the western CONUS Friday night into Saturday morning. This in turn will induce cyclogenesis across the Central Plains. As this occurs, intensifying southwesterly flow will overtop the surface baroclinic zone, resulting in a significant expansion of rain and embedded thunderstorms across the Upper Midwest. Increasing upper divergence within the right entrance region of a fast anticyclonic jet streak, coupled with broad and fairly intense WAA/upglide within a richly-moist airmass characterized by essentially record PWATs north of 2 inches will facilitate the potential for embedded heavy training convective elements with torrential rainfall. While some uncertainty in the exact placement of the salient baroclinic zone and low track remains, extended CAM and coarse- resolution ensemble guidance continues to pain the greatest heavy rain probs roughly near/north of I-88 late Friday night through Saturday morning. Conceptually, the set up is a concerning one from a flash flood potential, although the most persistent jet divergence/deepest moisture may set up just north and west of our CWA--still to be determined though.

With AM convection decidedly elevated, the severe threat appears low. CAMs hint at the potential for some elevated Gravity Wave Associated Convection (GWAC), but meager MUCAPE/DCAPE should curtail the threat for severe gusts in the area.

It's a bit unclear where the surface boundary will reside Saturday afternoon. With widespread morning convection, the boundary may have a difficult time making it meaningfully back into our forecast area (suspect the latest ECMWF remains a bit too far north). In this scenario, any additional afternoon activity should generally be of the light-moderate shower variety. However, a second wave is forecast to shift overhead Saturday night into Sunday, and may result in another round of locally heavy rainfall nearly across the same areas.

Have issued a Hydrologic Outlook for areas roughly along and north of I-80 given the potential for two rounds of heavy rainfall. May need consider Flood Watch(es) for parts of the area if confidence in training convection continues to increase. Some shower activity may linger into Monday, but high pressure is forecast to build into the region towards the middle and end of next week.

Carlaw

AVIATION /18Z TAFS THROUGH 00Z SATURDAY/

Issued at 1236 PM CDT Thu Sep 17 2026

MVFR/borderline IFR cigs remain in place from RPJ - DPA - ORD and northward early this afternoon, with VFR to the south. A diffuse cold front will continue to shift southward this afternoon. As it does so, anticipate additional low-MVFR to near IFR cigs to spread southward. While some holes in this low stratus may develop later this afternoon, it looks like these should fill back in through the evening. Confidence in IFR remains a bit too low to warrant inclusion, but may become necessary, especially near ORD, DPA and possibly GYY this evening pending observation and guidance trends.

Additionally, a deeper layer of low-level moisture is forecast to develop north of about I-88 this afternoon and into the evening as a secondary disturbance pivots overhead. This may result in the development of some drizzle. The better parameter space is forecast to remain just north of ORD, so have refrained from a mention in the outgoing 18z TAFs but will continue to monitor trends. Chances are a bit greater for BR and/or some DZ at RFD, and may need to consider TEMPO groups as trends become a bit more clear.

Drier air will eventually shift across the terminals late tonight/early Friday morning resulting in a return to VFR. NE to E breezes will develop and persist through the day.

Carlaw

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Dense Fog Advisory until 3 PM CDT this afternoon for Winthrop Harbor to Wilmette Harbor IL.