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 /12Z TAFS THROUGH 18Z MONDAY/

Issued at 630 AM CDT Sun Aug 30 2026

Showers and thunderstorms continue to expand across the area this morning within a region of broad isentropic ascent being forced by a southwesterly 30-40kt low-level jet. The peak in coverage over the terminals should be from roughly 12-16Z with a downward trend in coverage thereafter as the low-level jet weakens. Winds this morning should remain southwesterly with an increase in gusts toward 20-25kt along the back edge of the showers.

Am currently anticipating a relative minima in coverage of activity through early afternoon before showers attempt to redevelop by 19-20Z as a hybrid gravity wave/MCV currently moving into Iowa lifts toward the Illinois and Wisconsin border. Do think there is a very realistic chance that a few thunderstorms develop this afternoon with the MCV given little to no change in the integrity of the lapse rate plume currently supporting thunder across the area, but will advertise -SHRA from early afternoon through the evening for now owing to somewhat low confidence and to avoid having continuous thunder in the TAF. Southwesterly winds may swing southeasterly for a time this evening (23-04Z or so) on the backside of the gravity wave before settling back on a south-southwest direction before midnight.

Toward the end of the TAF period, another low-level jet will develop over the middle Mississippi River Valley and terminate over the Lower Great Lakes. With little change in the overall low- to mid-level moisture and lapse rates profiles, suspect we may have a repeat performance of this morning with scattered showers and thunderstorms in the general region by daybreak. So, will introduce PROB30 groups for -TSRA at RFD/ORD/MDW and consider inclusion at DPA/GYY in the next TAF set.

Borchardt

LOT WATCHES/WARNINGS/ADVISORIES

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