textproduct: Chicago

KEY MESSAGES

- Showers and isolated thunderstorms are expected Friday into early Saturday morning, with the highest coverage of showers expected south of Interstate 80.

- Another chance for showers and storms Saturday night into Sunday morning.

- First heat wave is expected next week for temperatures in the 90s and peak afternoon heat index values over 100 degrees.

DISCUSSION

Issued at 207 PM CDT Thu Jun 25 2026

The Rest of This Afternoon and Early Evening:

Clearer skies today have allowed temperatures to warm up quickly into the 70s to around 80F this afternoon after the cool start to the day. While lingering mid level moisture and weak instability from diurnal heating has allowed for widespread Cumulus to bubble up, prominent lower to mid-level dry air supports a dry forecast for the vast majority of the forecast area. Winds remain out of the northwest with decent mixing providing occasional gusts to 20 mph this afternoon. However, winds at Northerly Island have already turned to the northeast, due to a lake breeze, pretty clearly defined on KLOT and TORD radars, slowly meandering inland. The exception to the previous shower forecast would be areas along the lake breeze this afternoon, with the best chance for an isolated shower in Lake County Illinois and farther north into southeastern Wisconsin.

Tonight Onward:

Models are still showing decent spread in the track of the next upper level impulse with a subsequent surface low that will pass over central Illinois late tonight through tomorrow. However, the consensus was showing a northeastward movement of the low just south of Ford, Iroquois and Benton counties, which justified dragging slight chance PoPs as far north as the Wisconsin stateline. That being said, the higher confidence in shower coverage remains closer to the low, and therefore along and south of Interstate 80. Lapse rates do not look particularly steep. Perhaps there will be isolated embedded storms, but confidence is lower on the thunder threat. Model soundings are showing a fairly moist air mass with precipitable water amounts approaching two inches for areas along the US-24 corridor. While the higher concentration of total QPF is still expected in central Illinois, there is a chance that efficient rain processes could create instantaneous rates in excess of an inch per hour tomorrow afternoon and evening. Downpours are possible south of Interstate 80, with better chances of any localized flooding concerns farther south around US-24, which is an area that has fairly saturated soils from recent rains.

Guidance has also slowed the progression of the wave eastward. Showers may linger into Saturday morning, but overall drier conditions are expected through the rest of the day. An upper level high centered over Louisiana will quickly amplify northward through the weekend. As an upper level low spins over the Rockies, a mid level wave is projected to eject out and follow a northeastward trajectory between the high and low Saturday night into Sunday morning. If a weaker solution for the ridging (and associated capping) materializes, there could be an increase in PoPs needed. However, there is currently decent agreement that the higher PoPs look to remain west of Interstate 55 for another chance of showers and thunderstorms overnight and slowly diminishing Sunday morning.

As conditions dry out on Sunday, winds will turn to the southwest and advect in a warmer air mass. The upper level high is expected to deepen through the early part of the week and sit near the Ohio River Valley temperatures are expected to climb into the 80s on Sunday and 90s through the middle of the week. Dew points are forecasted into the 70s which will create a humid air mass that will feature heat index values into the 100s. The main uncertainty with temperatures right now will depend on where the core of the upper level high actually is located. If it were to establish itself a little farther south, there may be a disturbance or two that passes through the upper level pattern, yielding a few periods of some showers and storms to break the grip on the heat. But for now, confidence is low and will continue to be watched. Aside from the lower end/lower confidence convective potential, next week's heat and humidity looks to be the first formidable stretch of potentially dangerous heat this summer.

DK

AVIATION /18Z TAFS THROUGH 00Z SATURDAY/

Issued at 1257 PM CDT Thu Jun 25 2026

- Wind shift to northeasterly expected later this afternoon at ORD and MDW.

- Rain and low end MVFR condition are possible Friday morning at GYY and Friday afternoon at ORD and MDW.

Lake breeze will move across GYY shortly after the start of this TAF cycle, then continue west and likely moving across MDW and ORD later this afternoon. As is often the case with the lake breeze, there is some uncertainty with respect to timing of the lake breeze passage at ORD and MDW. There has been a trend toward an earlier lake breeze wind shift in guidance, so adjusted timing up slightly at ORD and MDW. Behind the lake breeze, the initial push will likely result in northeast winds around 10kt for the first hour or two, before winds gradually ease this evening.

A storm system is expected to move across the general region tomorrow, but there is still considerable spread in guidance with respect to how far north the rain shield will get tomorrow. If the farther north guidance were to verify, then moderate rain and low end MVFR or even IFR conditions would be possible as far north as MDW and ORD Friday afternoon. Other guidance keeps the rain farther south, which would keep Chicago area terminals dry with VFR conditions. Given the uncertainty, introduced a PROB30 for now for moderate RA and MVFR conditions. If farther north guidance were to verify then later TAF issuances would need to trend conditions considerably more pessimistic.

- Izzi

LOT WATCHES/WARNINGS/ADVISORIES

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