textproduct: Chicago

KEY MESSAGES

- Periodic showers and storms are possible through early next week, but many hours will remain dry

- Heat and humidity ramp up next week with highest heat indices expected mid-week

DISCUSSION

Issued at 200 AM CDT Sat Aug 29 2026

Surface high pressure that has made for pleasant conditions the past couple of days is slowly departing to the east. An upper level ridge is expected to slowly slide eastward and expand across the Great Plains today. There is a fairly stout MCS in eastern South Dakota, on the east side of the ridge axis that is slowly moving into southwestern Minnesota currently. The northwest flow from the ridge that is presently overhead would normally steer this system directly to Chicago; however, models are suggesting a mid level wave to pass over the northern Plains which will help flatten the ridge. This will effectively help keep most of the better forcing to the north of the Wisconsin stateline. Regional radar trends will be monitored through the course of the day to ensure that no southern deviation occurs, but there is one other fly in the ointment which will help prevent concerns with rain/storm chances today: surface layer dry air. Event the most aggressive models that have the system making a southern shift and entering northern Illinois run into issues with the dry air that is currently over the region and shows the complex eroding away. For now, PoPs were constructed limiting the chances to north of Interstate 88 this afternoon into the early evening. There are certainly chances for sprinkles as south of Interstate 80, but confidence is low that they can make it to the surface through the dry air. With the better forcing to the north, thunder is not currently forecasted for this afternoon/evening time frame.

A slightly better short wave is expected to ride over top of the ridge tomorrow night and Sunday morning. Forcing with this wave will be slightly better over northern Illinois to provide better confidence to increase PoPs (relative to Saturday afternoon). Lapse rates look fairly poor leaving only narrow, skinny CAPE depicted in model soundings. Thunder mention was added to the forecast, but limited the probability to less than 25 percent as coverage is expected to be more isolated.

The previously mentioned upper level ridge will be responsible for the weather for much of the week next week as it sits over much of the CONUS. There is uncertainty in the exact placement of the ridge, (more flat/southerly located = more showers/storms; more amplified/centrally located = drier) to merit confidence in timing and coverage through early next week. No major changes deviated from the chance PoPs the NBM provided. While periods of showers and storms will be possible, the expectation is for far more drier times that wet.

Southwest flow will help advect warmer 850 mb temperatures under the ridge through the week. Heat and humidity is expected to slowly rise through week. Models have been consistently showing highest heat index values (in the mid to upper 90s) by the middle of the week. The GFS is notably warmer than the than the Euro, so there is still some uncertainty on the exact heat values, but there is gaining confidence in another hot stretch of temperatures as we enter September.

At the end of the forecast time period, models are trying to suggest that the ridge starts to break down and returns northern Illinois to northwest flow overhead (which would aid shower/storm development). The NBM is clearly picking up on this signal with higher end chance PoPs into next weekend. Timing at this distance is very fuzzy logic, but there may be better chances for rain/storms late Friday or Saturday.

DK

AVIATION /06Z TAFS THROUGH 12Z SUNDAY/

Issued at 1206 AM CDT Sat Aug 29 2026

A large surface high pressure system continues to serve as the dominant influence on weather locally. Nearly calm winds overnight will turn southwesterly after daybreak and increase toward 10kt (with a few gusts up to 20kt) this afternoon. Clouds will thicken first at FL200 before lowering toward FL100 this afternoon in advance of a remnant sheared-out vort max. While the vort max will pass directly overhead this afternoon and evening, a resident dry low-level airmass should render largely virga. Do think a few sprinkles may survive to the ground in the 20-00Z timeframe, but will withhold mention in the TAFs given no expected impacts to operations.

The development of a 35-40kt southwesterly low-level jet oriented into the Lower Great Lakes atop a developing broad frontal boundary should encourage the development of rain showers toward the very end of the TAF period. Will go ahead and introduce PROB30 groups at ORD/MDW from 06-12Z (tomorrow).

Borchardt

LOT WATCHES/WARNINGS/ADVISORIES

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