textproduct: Chicago

KEY MESSAGES

- Hot and humid conditions are expected for much of the upcoming week; cooler near Lake Michigan.

DISCUSSION

Issued at 200 AM CDT Sun Jul 12 2026

Cirrus clouds seen on nighttime microphysics satellite imagery over the southeastern part of the CWA are slowly drifting southward and diminishing as an upper level trough continues to sink toward the Gulf. Clearer skies and sunny conditions are expected today as an upper level ridge over the Plains slides east. High temperatures are expected to nudge into the mid to upper 80s for most of the area this afternoon. Slightly cooler conditions are expected near the lake with a lake breeze. But given models projecting fairly weak lake breeze, temperatures could still climb into the low 80s along the shore.

The upper level ridge is expected to amplify and park itself over the Twin Cities tomorrow. 850 mb temperatures are expected to exceed 20C for the early part of the week leading to temperatures in the upper 80s and mid 90s at the surface through at least Wednesday. There are two bits of uncertainty with the upcoming heat wave: 1.) the longevity (more on that in the next paragraph) and 2.) dew point temperatures. Models have been struggling to agree on how much dew points will increase and/or how well they will mix out during the afternoon. Given the model spread, there is lower confidence with how things will shake out, but as noted in yesterday's afternoon discussion, it is getting into that time of year where better evapotranspiration is observed across the region. This forecast keeps the higher dew point trend into the mid 70s during the early part of the week. With highs in the 90s and dew points in the mid 70s, heat index values can be expected in the upper 90s to at times around 100, especially on Tuesday which is currently projected to be the hottest day of the week. Given the uncertainty and that it is still days away, no consideration for a Heat Advisory was given.

The weather pattern beyond Wednesday shows the ridge eventually breaking down, but there is a lot of model spread in how and when. An upper level low, currently over British Columbia, is projected to be shunted over the dominating Plains upper level ridge, but then descend down the east side of it mid week, deepening as it crosses over Hudson Bay and into northern Quebec. There is a chance that that low sends a reflective surface cold front southward overnight Wednesday into Thursday which may help mute temperatures ever so slightly. Beyond that, the GFS and the Canadian models show the ridge weakening, but generally remaining in place, whereas the Euro shows another upper level trough developing and passing farther south over the Great Lakes. Regardless, as the ridge breaks down any northwest flow opens the door to a potential wave to descend near the area and provide some chance for showers/storms over the weekend.

DK

AVIATION /06Z TAFS THROUGH 12Z MONDAY/

Issued at 1217 AM CDT Sun Jul 12 2026

Light winds will become northeasterly around 10 kts through Sunday morning and afternoon before becoming light and variable in the evening. VFR is expected through the period. There is a small chance for some shallow 5-6 SM BR towards daybreak, but chances are too low for any mention in the TAFs.

Carlaw

LOT WATCHES/WARNINGS/ADVISORIES

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