textproduct: Chicago

KEY MESSAGES

- Dry and mild today, turning cooler Sunday with a chance for a few afternoon showers.

- Waves of heavy rain producing thunderstorms possible in the general region early to mid next week.

DISCUSSION

Issued at 313 AM CDT Sat Sep 12 2026

A low-level thermal ridge will briefly push northeastward across the area today ahead of a cold front advancing from the Upper Midwest. This will bring warmer and slightly more humid conditions, with high temperatures reaching the mid to upper 80s and heat indices pushing into the upper 80s to near 90. The surface cold front is projected to move southeastward through the afternoon. However, dry antecedent air and a strong cap, combined with weak large-scale support, should prevent convective development. As a result, dry conditions will prevail through tonight.

Winds will shift to the north tonight into Sunday behind the front. The influx of a cooler airmass, increased cloud cover, and scattered afternoon showers driven by an approaching mid- level impulse will hold high temperatures 5 to 15 degrees cooler, generally in the 70s. Given the persistent dry air in the lower levels, rainfall amounts will remain light, providing little beneficial rain.

Primarily dry conditions are expected Sunday night into Monday as the surface cold front settles across downstate IL and IN. The best chance for rain should largely remain northwest of the area across the Upper Midwest as warm air advection increases ahead of a potent shortwave trough and deepening surface low tracking from the northern High Plains into south-central Canada. The main impact from this system for us on Monday will be increasingly breezy southeasterly winds through the day. Otherwise, Monday will be another seasonable day with temperatures in the 70s under partly to mostly cloudy skies.

The cold front to our south is expected to lift back north across the region Monday night as the strong low moves across far south-central Canada. This will bring a brief return of warm (80s) and humid conditions ahead of another approaching cold front on Tuesday. The airmass along this boundary will feature precipitable water (PWAT) values of 1.7 to around 2.0 inches, supporting showers and storms capable of heavy rainfall. We are closely monitoring forecast trends, as the front's southward progress may stall late Tuesday into Wednesday due to an expanding upper-level ridge over the southeastern CONUS. If it becomes quasi-stationary, it could focus multiple rounds of heavy rain producing strong storms across the lower Great Lakes region through midweeka pattern resembling a classic Maddox frontal heavy rain setup.

Confidence in the position of the front remains low beyond Wednesday due to large ensemble spread and potential disruption from earlier convection. However, additional waves of showers and thunderstorms will remain possible wherever the boundary settles.

KJB

AVIATION /12Z TAFS THROUGH 18Z SUNDAY/

Issued at 632 AM CDT Sat Sep 12 2026

VFR with no significant concerns through midday Sunday.

The only item of note is changing wind directions. Light south- southwest winds to start this morning will become west-southwest (occasional 15-20 kt gusts) by the mid to late morning ahead of an approaching cold front. Winds will shift to northwest behind the frontal passage in the late afternoon/early evening, and then vary to northerly at times overnight/early Sunday. It appears the best chance for a prevailing shift to a north- northeasterly direction will be after 18z Sunday, beyond the current ORD/MDW TAF period.

Can't rule out a few sprinkles prior to 18z Sunday, though made no mention in the ORD and MDW TAFs. The next issuance will likely need some precip mention for scattered -SHRA Sunday afternoon.

Castro

LOT WATCHES/WARNINGS/ADVISORIES

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