textproduct: Chicago

KEY MESSAGES

- Heat and humidity will return to the region Sunday and continue into Monday, potentially requiring headlines for at least some areas.

- Some potential for thunderstorms late Sunday through Monday night, but predictability in timing, location, and coverage of storms is very low this far out.

DISCUSSION

Issued at 244 AM CDT Fri Jul 24 2026

Our preview of September-like weather will continue today, though temps will likely be a couple degrees warmer than Thursday, but dewpoints are likely to remain unusually low for late July. One difference for today is the expectation of more cloudiness this morning as a fairly pronounced shortwave trough moves across the region. Majority of the associated ascent with this feature is going toward saturating the parched atmosphere, though rather strong and deep f-gen circulation has been effective at allowing a band of some sprinkles or very light rain showers to actually reach the ground across Iowa. This f-gen band is progged to translate eastward into northern Illinois this morning and certainly can't rule out some sprinkles making it eastward into our area. The best chance of sprinkles looks to be roughly near and just south of the I-80 corridor.

In the wake of this shortwave, skies should clear out from northwest to southeast this afternoon, though some high level smoke noted upstream on satellite imagery Thursday could make it into the area. The HRRR smoke guidance does show a bit of an increase in vertically integrated smoke this afternoon, though smoke looks to remain aloft and much less thick than last week, so only nudged sky cover up a bit to account for the smoke.

Another, convectively enhanced, shortwave trough is likely to pass south of our area tonight into Saturday morning. Fairly deep (300-315K surfaces) but modest isentropic ascent could result in a few showers across mainly our southern CWA late tonight into Saturday morning.

Upper ridge is progged to amplify over the central U.S. Sunday into Monday, likely allowing for a quick shot of heat and humidity. Evapotranspirationally-supplemented dewpoints appear likely to surge well into the 70s and probably lower 80s in the densest agricultural areas Sunday into Monday while nudging temperatures down a bit from the NBM. Net result is Sunday heat indices peaking well into the 90s to slightly over 100 degrees in some areas, with more widespread 100 to locally over 105 degree heat indices possible Monday.

The caveat to the heat Sunday and Monday is the potential for convection. Conceptually, a northwest flow pattern like the one coming Sunday into Monday can produce robust MCS activity. However, the seedling low amplitude shortwaves that ultimately go on to spawn strong MCSs and convectively enhanced vorts tend to be much too low amplitude to resolve at this distance. Often, though not always, guidance at this distance would have a more robust convective signal, but that certainly doesn't preclude something more coherent coming into sight in later runs. Barring any convectively enhanced shortwaves/MCS activity, forecast soundings Sunday and Monday exhibit a strong capping inversion, which would likely keep things dry. Opted to maintain slight chances for thunderstorms Sunday afternoon through Monday, to cover the potential for an upstream MCS developing and ripping southeast across our area. If a well timed MCS or MCS debris move across the area during this time frame, then that would obviously have big implications on temperatures and heat potential.

Cold front and more of synoptic shortwave trough could result in better chances for thunderstorms Monday night followed by somewhat cooler and less humid weather heading into the middle of next week.

- Izzi

AVIATION /12Z TAFS THROUGH 18Z SATURDAY/

Issued at 614 AM CDT Fri Jul 24 2026

Main Item of Note:

- Somewhat uncertain wind directions at ORD and MDW, especially this afternoon.

While it will be another VFR period with no significant concerns, the wind forecast remains a bit tricky today at the near lake terminals. Initial light south-southwest winds at ORD and MDW will likely become southeast by the mid to late morning. As a lake breeze boundary sharpens up this afternoon, winds to the west of the boundary should veer to south-southwest. Given typically weak lake breezes in late July, current thinking is that the boundary will remain east of ORD. At MDW, if the lake breeze makes it through, winds will become east-southeast near 10 kt at times, then shift back to south-southwest late this afternoon as the boundary retreats. Confidence is low regarding whether the lake breeze will be able to make it across GYY.

Aside from the uncertainty near the lake, expect light to moderate south-southwest winds inland, diminishing with sunset this evening. A bit stronger southwest winds can be expected on Saturday. Finally, can't rule out a few sprinkles this morning and again on Saturday morning.

Castro

LOT WATCHES/WARNINGS/ADVISORIES

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