textproduct: Chicago

KEY MESSAGES

- Scattered showers and isolated thunderstorms through early this evening north of I-80.

- Chance of thunderstorms again Tuesday afternoon, mainly north of I-88/290.

- Hot and humid conditions expected Tuesday and Wednesday, especially near/south of I-80.

DISCUSSION

Issued at 247 PM CDT Mon Aug 31 2026

While we're about to turn the page on the calendar to meteorological fall, the weather pattern is much more reminiscent of mid-summer with a strong mid-upper level ridge of high pressure from the central plains east to the mid-Atlantic. Low amplitude shortwave troughs continue to ride along the flanks of this upper ridge, with one such wave responsible for the elevated showers and t-storms over northern Illinois this afternoon. Short range guidance has largely been performing poorly handling these small scale features and their associated convection, so the short term forecast was driven largely by extrapolating the shortwave eastward using water vapor imagery. The trough axis and associated forcing should be moving east to Lake Michigan early this evening, so would anticipate a decrease in coverage of precip late this afternoon into early this evening from west to east.

The next shortwave trough of interest is located over the 4 corners region and should turn northeast and east around the western periphery of the ridge tonight into Tuesday. Guidance that has initialized this wave generally tracks it north of our CWA Tuesday afternoon as the upper ridge strengthens a bit over the mid-Mississippi Valley. An axis of very strong instability with minimal convective inhibition is expected from Iowa east into southern Wisconsin and far northern Illinois. CAM guidance, which hasn't performed well lately, keeps convection with this feature focused mostly north of the IL/WI border. Despite what looks to be only a glancing shot of forcing with this shortwave, we have opted to carry chance PoPs tomorrow afternoon near the WI border and Slight chace PoPs south to nearly I-80. If any mature convection can become established in this environment Tuesday afternoon, severe thunderstorms would be likely given the very strong instability and 30-40 kt of 0-6 km shear. Confidence remains low given the potential for the stronger forcing to remain north of the uncapped/strongly unstable air mass over our region. If it becomes clear that forcing will be sufficient for storms into our CWA (most likely northern CWA), then a substantially greater risk of severe thunderstorms would be likely.

The other big story is the late season heat and humidity. Pretty good agreement among the various models that 925 mb temps in the mid to upper 20s Celsius will advect into the area Tuesday and Wednesday. This should support high temperatures in the lower to potentially mid 90s both afternoons, though the potential for convection/convective debris cloudiness leads to lower confidence in temperatures north of I-80.

In addition to the heat, very humid conditions are expected with 70 degree dewpoints pooling from Iowa east across northern and central Illinois into Indiana. While we are past peak evapotranspiration (ET) season, climatologically ET in early September is not negligible and tends to be similar to what we typically see in mid-late June. Evidence of this ET was quite apparent today with dewpoints jumping 4-8F late this morning into early this afternoon from southern IA east into northern/central IL. Morning soundings suggest this jump in surface dewpoints wasn't the result of mixing down high moisture and with lower dewpoints upstream, this jump wasn't from horizontal advection, strongly suggesting it was largely driven by ET. There is reason to be concerned that this ET could result in dewpoints locally reaching the mid to even locally upper 70s Tuesday/Wednesday afternoons, especially in the more heavily agricultural areas.

The combination of high temperatures in the low to potentially mid 90s and dewpoints in the 70s should result in peak afternoon heat indices of 100 to locally over 105 degrees. Opted to hoist a heat advisory for both Tuesday and Wednesday afternoon/early evenings for areas near/south of I-80 where confidence is highest in temps. If it becomes more apparent that cloud cover and/or convection won't hinder heating north of I-80, it is quite plausible that the heat advisory will need to be expanded north, especially Wednesday. While our currently forecasted heat indices are only approaching heat advisory criteria, we opted to issue an advisory anyway due to the potential for dewpoints/heat indices ending higher than forecast. Also, with schools back in session with outdoor activities and it having been weeks since we've seen heat indices over 100F, erring on the side of caution seemed warranted.

As it looks now, the parade of ridge-rider shortwaves may push safely north of the area Wednesday, but by Thursday and Friday the upper ridge will begin to weaken and slowly shift westward. This could put our area back into the favorable track area for shortwaves bringing occasional showers/thunderstorm chances. This should also result in an easing of the more intense heat and humidity. Medium range guidance supports a continuation of an unsettled northwest flow pattern could support occasional chances for showers and thunderstorms into the upcoming weekend.

- Izzi

AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/

Issued at 1233 PM CDT Mon Aug 31 2026

Forecast concerns for the terminals...

- Isolated showers (and maybe thunderstorms) this afternoon.

- Breezy southwest winds this afternoon and again Tuesday afternoon.

A shortwave disturbance is advancing across eastern IA this afternoon which is starting to develop isolated showers in eastern IA and northwest IL. The expectation is for isolated to widely scattered showers to keep developing across northern IL and portions of northwest IN through early evening. While forecast soundings do show some decent instability aloft, low- level instability continues to look meager and with the weaker forcing think the coverage of thunder will be rather sparse especially with eastward extent. Therefore, have opted to remove thunder from the Chicago area terminals where confidence in lightning actually occurring is low (15-20% chance), but did maintain a TSRA mention at RFD where confidence is a bit higher. Any showers/storms will diminish after sunset as instability wanes and the disturbance pushes east.

Otherwise, expect VFR conditions to prevail through the TAF period. Winds this afternoon will remain breezy out of the south-southwest with gusts around 20 kts before winds ease this evening. Though, southwesterly gusts will return Tuesday afternoon.

Finally, there is a non-zero (10-20% chance) for another period of showers and storms on Tuesday mainly during the late morning and afternoon hours as another disturbance pivots through. Right now the trend is for this disturbance and its associated rain to track north of our area so have maintained a dry forecast. Though, will continue to watch trends in case things change as we get closer.

Yack

LOT WATCHES/WARNINGS/ADVISORIES

IL...Heat Advisory from noon to 9 PM CDT Tuesday for ILZ019-ILZ020- ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ105-ILZ106-ILZ107- ILZ108.

Heat Advisory from noon to 9 PM CDT Wednesday for ILZ019- ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ105-ILZ106- ILZ107-ILZ108.

IN...Heat Advisory from noon CDT /1 PM EDT/ to 9 PM CDT /10 PM EDT/ Tuesday for INZ001-INZ002-INZ010-INZ011-INZ019.

Heat Advisory from noon CDT /1 PM EDT/ to 9 PM CDT /10 PM EDT/ Wednesday for INZ001-INZ002-INZ010-INZ011-INZ019.

LM...None.