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 /00Z TAFS THROUGH 06Z SATURDAY/

Issued at 658 PM CDT Thu Jun 25 2026

Key aviation forecast messages:

- NE wind shift behind lake breeze/backdoor front this evening

- Potential for a period of rain on Friday with greatest coverage near/south of I-80

A lake breeze is beginning to make inroads toward ORD after having stalled east of the terminal all afternoon with a slightly slower westward progression occurring near MDW. Have adjusted the northeast wind shift to 00:15 to reach ORD and 00:30 at MDW based on the latest trends. Meanwhile a backdoor front diving down Lake Michigan recently reached UGN where the initial wind shift was around 11-12kt before easing closer to 8kt after 30 mins or so. Expect something similar at area terminals as the combined lake breeze/front move through. Have added more detail for the wind shifts at DPA and RFD as well before winds eventually ease late in the evening and overnight. Winds then remain generally east to northeast through the end of the period, increasing back to around 10 kt after daybreak on Friday.

A disturbance is expected to drift across central Illinois and Indiana on Friday bringing rain to at least portions of the area mid-morning through early evening. At this time confidence in rainfall is highest near and south of Interstate 80, with lower confidence on the northward extent. In addition to shifting the PROB30s to begin a bit earlier (closer to 16Z), have accordingly trended them slightly less pessimistic for MDW (high-end MVFR) and even moreso for ORD (remaining VFR). The potential for prevailing showers is highest at GYY (50% chance).

Petr

LOT WATCHES/WARNINGS/ADVISORIES

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