textproduct: Chicago

KEY MESSAGES

- Heat and humidity will return to the region Sunday and continue into Monday. The magnitude of heat may require heat headlines at some point.

- There is currently a 20-40% chance for thunderstorms on both Sunday and Monday. Severe thunderstorms with destructive winds and flash flooding may occur on both days in the Great Lakes region, but whether such a threat will materialize here locally is uncertain at this time.

DISCUSSION

Issued at 252 PM CDT Thu Jul 23 2026

The September-like air mass that has settled into the region will stick around for yet another day tomorrow. A third consecutive day of air mass warming paired with modest return flow on the backside of a departing ~1022 mb surface high will likely allow for at least a few low 80s air temperature readings to be observed tomorrow afternoon amidst a mix of sunshine and clouds, with upper 70s high temperatures expected otherwise. Dew points will also increase from what they were today, but humidity levels should nevertheless still remain in a comfortable range tomorrow.

A couple of shortwave troughs will track east-southeastward tonight into tomorrow from the Great Plains towards the Mississippi and Ohio river valleys and will be accompanied by rain in some areas. The northernmost of these waves will pass over or track very near our CWA and will likely induce enough saturation in the middle and upper levels of the troposphere to support precipitation in the associated cloud layer over or very near our southern counties. However, descending hydrometeors will have to work through a robust dry sub-cloud layer (initially over 10000 ft deep), and this may prove to be difficult to overcome in the absence of stronger forcing aloft. Thus, while some sprinkles could be observed in some of our southernmost locales during the daytime tomorrow, measurable precipitation appears unlikely to occur.

After this shortwave disturbance departs, a plume of meager instability will lift northeastward into the region Friday night into Saturday morning. While lapse rates will remain unimpressive during this time frame, increasing low-level moisture may nevertheless allow for some shower activity to sprout within the associated wing of warm air advection/isentropic ascent -- primarily across our southern counties and points farther to the south. Opted to introduce slight chance PoPs to the gridded forecast database across the southern half or so of our forecast area for the overnight Friday into Saturday morning time frame to account for this possibility. If any cells associated with this activity manage to grow tall enough, then a few isolated lightning strikes could also occur. Otherwise, conditions should dry out for Saturday afternoon as the associated forcing progresses eastward and strengthening capping quashes any attempts from surface-based air parcels to freely convect within an increasingly moist/humid air mass.

By Sunday, a much more July-like air mass will become established across the region with air temperatures building into the upper 80s and 90s and evapotranspirationally-supplemented dew points climbing solidly into the 70s (and possibly even near 80F in spots) as an upper-level ridge strengthens and expands across the central third of the CONUS. Heat indices in our area are thus likely to eclipse the century mark and could potentially rise high enough to warrant heat headlines for at least part of our forecast area on Sunday and possibly again on Monday as well.

This muggy air mass will also be primed for thunderstorm activity. Particularly, the consistent model signal for multiple shortwaves navigating around the top of the aforementioned ridge and diving into the moist and strongly unstable air mass present across the Upper Midwest/Great Lakes amidst strong northwesterly flow does raise concern for one or more severe to potentially destructive convective complexes tracking through the region on Sunday and/or Monday. Additionally, with precipitable water values progged to approach and even exceed 2", torrential rainfall will accompany any storms that develop and could result in flash flooding wherever storms persist for a sufficiently long period of time. With this potential thunderstorm time window still being 3-4 days out, it remains practically impossible to pinpoint precisely where and when such threats may materialize in the region, and it's entirely within the realm of possibility at this point for this thunderstorm activity to avoid our area altogether (though this isn't a scenario that we'd bet on based on the look of this forthcoming weather pattern). The main message at this point in time is that conditions will be favorable for multiple rounds of severe thunderstorms somewhere in the general region on Sunday and on Monday, but specific details regarding this severe thunderstorm potential remain highly uncertain at this time.

Forecast confidence from Tuesday onward also remains relatively low, in part due to the uncertainties with what will happen in the preceding days. Ensemble guidance generally favors midweek conditions that aren't as hot and humid compared to Sunday and Monday, and this outcome would be likely to come to fruition if extensive thunderstorm activity materializes on Sunday and/or Monday and shunts the muggiest air well to our south and/or east.

Ogorek

AVIATION /18Z TAFS THROUGH 00Z SATURDAY/

Issued at 1158 AM CDT Thu Jul 23 2026

There are no significant aviation weather concerns. VFR cloud cover, with bases around 6 kft AGL will diminish this afternoon and evening. A weak lake breeze will turn winds out of the ESE at ORD/MDW this afternoon although exact timing remains a bit uncertain. Wind speeds should generally remain under 10 kts. Winds will then become light SSW to variable tonight.

Wind trends Friday are a bit uncertain. Largely expecting winds to shift out of the southeast at the Chicago-area terminals, but there is a chance they switch back out of the southwest during the afternoon. Confidence is too low to warrant an additional line with a SW wind shift at this time, however.

Carlaw

LOT WATCHES/WARNINGS/ADVISORIES

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