textproduct: Chicago

KEY MESSAGES

- Low 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.

UPDATE

Issued at 944 PM CDT Mon Aug 31 2026

Weak warm advection regime looks to support widely isolated showers through the night and into early Tuesday morning, though the threat for any lightning appears quite low (and thus not included in the grids).

Looking ahead to tomorrow (Tuesday afternoon), some of the most recent guidance suggests that our overnight shift may need to strongly consider expanding the Heat Advisory northward to at least the I-88 corridor counties. This is due to the possible thunderstorms (and associated severe threat) for parts of far northern Illinois looking more likely to focus safely north of the IL/WI state line. In turn, this would enable the 100-105+ degree heat indices to also include areas farther north in Illinois. One of the biggest concerns with the heat and humidity tomorrow and Wednesday is impacts to afternoon outdoor activities, such as youth sports.

Castro

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 chance 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 /00Z TAFS THROUGH 06Z WEDNESDAY/

Issued at 623 PM CDT Mon Aug 31 2026

Key messages for the 00Z TAF period:

* Low potential for thunderstorms Tuesday afternoon

The scattered thunderstorms are finally winding down early this evening and lingering shower activity will continue over Chicagoland through around 01Z. Winds below 10 kt will favor a SW direction through the night, but may be variable in the vicinity of lingering showers. Uncertainty in magnitude of SW winds on Tuesday. Looking to gust to around 20 kt, but possibly higher at times.

A low (around 20%) chance for thunderstorms exists Tuesday afternoon. Highest potential exists along the WI state line, including up near RFD. Should thunderstorms develop, the environment will be supportive of deep convective cores with heavy rain and strong thunderstorm winds possible. Any mention was omitted from this TAF cycle owing to the low potential, but will closely monitor the need for a mention in future issuances.

Doom

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.