textproduct: Chicago

KEY MESSAGES

- Hot and humid conditions are expected much of the upcoming work week.

- Daily lake breezes should provide at least some relief near the lake during the afternoons Sunday through at least Wednesday.

DISCUSSION

Issued at 145 PM CDT Sat Jul 11 2026

Low amplitude mid-upper level ridging over the western half of the country is progged to amplify significantly while expanding eastward. There is strong model agreement in developing an anomalously strong mid-upper level ridge from the northern High Plains east into the upper Great Lakes the first half of the upcoming work week. By Tuesday-Wednesday, 500mb heights near the center of this massive ridge could exceed 600dam. The core of the potentially record breaking heat with this upper ridge is expected to largely remain just to our north and northwest, however, our area should be in close enough proximity to this massive heat dome to see our own share of heat and humidity this week.

Temperatures should be several degrees warmer Sunday, particularly southern CWA where cloud cover today is holding temps back a bit. Full or nearly full sunshine is expected tomorrow through the middle of the upcoming work week. As the upper ridge strengthens and expands eastward, look for gradually building heat. By Monday afternoon, 925mb temps are progged to warm to 23-27C, highest northern CWA. This should support high temps ranging from the mid-upper 80s toward central IL to the lower 90s north of I-80.

The hottest temperatures of the upcoming heat look to be Tuesday into Wednesday when the strength of the ridge is expected to peak. Both afternoons should feature high temps in the lower to middle 90s. Worth noting that the GFS and ECMWF both seem to be struggling with over-mixing and overly warm temperatures over the urban areas, in some cases 5-10F warmer than nearby less urbanized areas. While it will likely be a bit warmer in Chicago urban heat island, the raw 2m temps in the GFS and ECMWF appear unrealistically hot and have largely been discounted.

Synoptic flow will allow for afternoon lake breezes to form tomorrow through Wednesday, which should provide some heat relief for areas closer to Lake Michigan. Lake Michigan water temperatures are running much above average and which should limit magnitude and inland extent of lake cooling.

Finally, probably one of the bigger wildcards for this upcoming heat will be the dewpoints. The strong upper ridge and subsidence should result in a noteworthy inversion which could limit the depth of mixing and ability for dewpoints to mix out. We are entering the climatological peak of evapotranspiration (ET), so even though low level flow will not be originating from the Gulf, nearly full sunshine and relatively weak low level flow should result in substantial evapotranspiration. The NAM model tended to perform best with temps and dewpoints with the late June heat wave and have trended our temp/dewpoint forecast closer to the NAM Monday and Tuesday. Certainly plausible that given the lack of added Gulf moisture and the more shallow depth of the ET driven low moisture that dewpoints may indeed mix out. Confidence in the forecast dewpoints and resultant heat indices is lower than average for the upcoming heat wave.

Upper ridge is expected to begin to gradually weaken toward the of the upcoming work week and into next weekend, likely being replaced with more of a northwest flow. Weaker capping and the potential for subtle low amplitude disturbances riding the northwest flow into our region certainly opens the door to at least some threat for showers/storms by Friday into next weekend.

- Izzi

AVIATION /18Z TAFS THROUGH 00Z MONDAY/

Issued at 1220 PM CDT Sat Jul 11 2026

There are no key concerns to highlight for the 18Z TAF period.

NE winds around 12 kt may occasionally gust as high as 20 kt this afternoon before subsiding this evening. Direction will hold at NE into Sunday with near or below 10 kt expected through the day. Expect VFR throughout the period.

Doom

LOT WATCHES/WARNINGS/ADVISORIES

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