textproduct: Chicago

KEY MESSAGES

- Periodic shower and storms through the early part of the week, though there will ample dry periods.

- Hotter and more humid conditions build through midweek.

DISCUSSION

Issued at 222 AM CDT Sun Aug 30 2026

As a low level jet (LLJ) increases this morning, as sampled around 40 knots just 2000 ft AGL on the KLOT VAD, it is no surprise that shower coverage is gradually expanding over northern Illinois. PoPs were increased to around 30 to 40 percent chance to match the coverage being observed. This southwest to northeast oriented LLJ is expected to pivot and become slightly more easterly oriented through the morning and be the driving force behind an increase in shower coverage. There is modest effective shear present, but CAPE values and lapse rates are not providing confidence in overall coverage of thunder. That being said, there have already been small clusters of embedded thunderstorms develop early this morning. Most thunder is expected to be isolated, but there could be an uptick in thunder coverage right around and after daybreak. Current guidance is suggesting the LLJ will gradually diminish around midday allowing the showers to eventually erode and sink southward as it falls apart.

An upper level ridge, the center of which is currently analyzed over Oklahoma and Texas, keeps northern Illinois and northwest Indiana along its northern periphery through midweek. There will be plenty of dry times, but as embedded waves moves through the pattern, there will be additional periods of showers and storms. For example, RAP analysis is currently showing an upper level short wave from Rapid City, SD to around Dodge City, KS just behind ongoing storms in the Great Plains. There is an additional short wave in western Wyoming and Colorado. Models are suggesting that that both these waves are expected to ride along the top of the ridge and provide additional chances for precipitation overnight tonight (the Plains wave) and again overnight into Tuesday morning. If the ridge were to strengthen, it could force these waves (and subsequent shower/storm chances) farther north. Whether or not they maintain their current trajectory or strength remains to be seen. For now, chances for showers and storms remain in the forecast through the middle of the week.

As the ridge remains over the CONUS through midweek, so does southwesterly flow at the mid-levels. This wind direction will aid in advecting in a warmer air mass to the western Great Lakes with projected 850 mb temperatures over 20C, potentially pushing 25C. Winds at the surface will also remain out of the south, gradually increasing dew points through the week back into the 70s. Hotter temperatures in the upper 80s and lower 90s are expected. This combination of hot temperatures and humid conditions are expected to increase heat indices into the mid to upper 90s by midweek, with the southwestern portion of the forecast area with a chance of exceeding 100. Chances for shower/storms through the weak lowers confidence in specific temperatures values; therefore, there was no consideration at this time for any heat hazard products (advisory/watch).

Models are suggesting that the ridge could break down by the end of the work week. That will help the hottest of temperatures to cool off a bit, but it will return the upper level flow pattern to the northwest and thus bring with it additional chances for showers/storms over the weekend.

DK

AVIATION /18Z TAFS THROUGH 00Z TUESDAY/

Issued at 1238 PM CDT Sun Aug 30 2026

Forecast concerns include...

Chance of thunderstorms this afternoon and Monday morning. Gusty south/southwest winds early this afternoon.

Scattered thunderstorms are currently moving along the IL/WI state line in response to an upper level wave approaching the area. Additional showers and isolated thunderstorms will be possible across northern IL and the terminals through late this afternoon, though thunder coverage remains uncertain. Given the latest trends, prob thunder mention seems reasonable for the Chicago terminals with tempo mention at RFD. The overall trend in guidance is for this activity to slowly weaken by late afternoon/early this evening, but lingering showers may persist into this evening.

There will be two more time periods with potential isolated thunderstorms, the first around/after daybreak Monday morning and then Monday afternoon. Confidence for both time periods remains very low and opted to make no changes to the forecast, keeping current prob thunder mention for Monday morning and keeping the forecast dry for Monday afternoon.

South/southwest winds will gust into the lower/mid 20kt range for a few more hours, then gusts are expected to diminish by mid afternoon. Wind directions are expected to become south/ southeast this evening, then turn back to south/southwest overnight and continue on Monday. cms

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory until 3 PM CDT this afternoon for the IL nearshore waters.