textproduct: Chicago

KEY MESSAGES

- Periodic shower/storm chances over the weekend and into early next week

- Heat and humidity is expected to increase next week

DISCUSSION

Issued at 121 AM CDT Fri Aug 28 2026

Winds have gone calm over much of the forecast area this morning. Clearer skies west of the Fox Valley have helped drop dew point depressions down to between 2 to 4 degrees (at the time this discussion was published), whereas lingering clouds above 5,000 feet have kept much of the Chicago Metro area drier. Reduced visibility is possible this morning, but confidence is lower for the Metro. The better chances for patchy fog to develop around daybreak looks to be around and west of the Fox Valley or near and south of the Kankakee River Valley

GOES water vapor satellite imagery this morning shows a upper level trough over the northern Great Lakes slowly moving east, as a ridge spreads northward over the intermountain west (including a stout MCS moving south down the lee side of the ridge into Kansas). That ridge will be the focus for the rest of the forecast; but, for today, surface high pressure will create fairly quiet weather conditions locally. High temperatures are expected to be in the low 80s, with a weak lake breeze developing in the afternoon.

The ridge is expected to expand eastward tonight and into Saturday. This will allow slight perturbations in the height field to move eastward over the weekend. The point of uncertainty is where will the ridge axis set up and if the shape of the ridge will make it more favorable for showers and storm to move into Illinois. For now, models are suggesting a short wave trough will move through the upper level pattern Saturday afternoon through the overnight. Models are still suggesting that the EML plume should be more focused over southern Wisconsin, but there will be plenty of warm air advection and steepening lapse rates through the afternoon and evening for the chances of showers and storms to develop. Model trends will need to be monitored to get a better sense on coverage and any southern terminus for showers/storms.

That ridge is expected to continue to expand eastward Sunday and through the next week. No major changes were made to the PoPs the NBM provided as slight/chance wording certainly fits the bill for any "ridge rider" that might pass over the area. More notably is global and ensemble guidance increasing 850 mb temperatures above 20C by the middle of next week. With dew points increasing into the low 70s, there is a chance for heat indices to increase into the upper 90s. September might be approaching, but summer is far from over.

DK

AVIATION /06Z TAFS THROUGH 12Z SATURDAY/

Issued at 1202 AM CDT Fri Aug 28 2026

A benign surface pressure pattern will prevail through the early morning hours leading to nearly calm winds. A few patches of shallow fog may develop toward daybreak at outlying terminals, so will maintain the inherited TEMPO groups for modestly reduced visibility (3-4 miles) at RFD/DPA/GYY. However, will note that current temperature/dew point depressions do look rather unfavorable (a consequence of pesky stratocumulus clouds rooted near 5kft holding air temperatures a few degrees warmer than guidance). Will reevaluate at 09Z to see if the aforementioned TEMPO groups can be safely removed.

After daybreak, winds should eventually adopt a southwesterly component and increase toward 5-10 kt. The continued weak pressure gradient and a modest land/lake thermal gradient will support the development of a diurnal lake breeze by early afternoon. Am suspicious of guidance depicting the lake breeze stalling before reaching ORD/MDW, but will keep a wind shift out for the time being. Otherwise, will just go ahead and feature a transition from VRB to northeasterly winds at GYY today. Winds should resume a southwesterly direction (at or below 5kt) after sunset tonight.

Scattered cumulus rooted near 5000 feet will develop by mid- afternoon and fade after sunset.

Borchardt

LOT WATCHES/WARNINGS/ADVISORIES

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