textproduct: Chicago

KEY MESSAGES

- Generally above normal temperatures (highs in the 80s) and dry conditions are expected through early next week. Onshore winds will keep temperatures cooler near Lake Michigan each day (60s-70s).

- Backdoor cold fronts will lead to cooler conditions (60s and 70s) extending farther inland late Wednesday into Thursday and potentially again over the weekend.

DISCUSSION

Issued at 334 AM CDT Tue May 26 2026

An expansive area of thin cirrus remains overhead early this morning associated with a mid-level cut-off low and associated surface reflection currently over the Lower Mississippi Valley. This cloud layer will likely remain overhead through the day (perhaps with varying coverage at times). It is unclear how much of an influence this will have on temperatures today but opted to cap high temperatures in the low to mid 80s with some of the warmer guidance (upper 80s to near 90) potentially too warm unless skies fully clear out. Either way, dewpoints in the upper 40s and 50s should help keep heat indices in check. Winds will be light out of the south and southeast today with an afternoon lake breeze dropping temperatures back into the 60s and lower 70s near the lakeshore.

Thunderstorms are expected to develop to our northwest over portions of Minnesota and Wisconsin today associated with a stalled frontal boundary intersecting with a residual unstable airmass. Model guidance continues to depict these storms dissipating with southward extent as they propagate toward Illinois overnight into early Wednesday, with not much more than a sprinkle possible here locally. There is the potential that outflow boundaries associated with these storms manage to hold together into Illinois, turning winds northerly in their wake late tonight. This could allow for an earlier arrival of an expected backdoor cold front which is currently forecast to move inland late Wednesday afternoon through the evening. There is a narrow window where additional isolated storms from Wisconsin manage to persist into far northern/northwest Illinois late Wednesday afternoon ahead of the front. The potential remains very low (20% chance) given storms will be encountering a much drier airmass into Illinois.

We will also have to keep an eye on the northern periphery of any showers associated with the aforementioned cut-off low to our south which is expected to gradually lift northeast through tonight into Wednesday as it elongates and merges into the broader upper level flow. There remains a low (20%) chance that showers graze the far southern forecast area (south of a Gibson City, IL to Fowler, IN line). There will likely also be a very sharp gradient in PWATs (from 0.7" to 1.8"+) very near the area. Any showers that reside within the deeper moist airmass would be capable of efficient rainfall rates. However, dry conditions are still favored this far north.

With all that said, dry conditions are expected for most of, if not the entire, area on Wednesday with highs in the low to mid 80s inland and lower 70s near the lakeshore. Temperatures then steadily fall late afternoon/evening in the wake of the backdoor front with continued onshore flow keeping temperatures cooler into Thursday, potentially much farther inland (highs in the mid 60s to upper 70s).

An expansive 1020+ mb Hudson Bay surface high is then expected to translate south and stall out across the western Great Lakes region mid to late week as a pseudo-Omega blocking pattern becomes established across the broader CONUS and Canada. Accordingly, above normal temperatures are expected for inland areas most days with highs in the upper 70s to lower 80s. Onshore winds and daily lake breezes will lead to continued cooler conditions near the Lake Michigan shore (highs in the 60s to near 70). There is lower confidence on temperature trends over the weekend into early next week owing to differences in timing for any potential additional backdoor cold fronts. Suspect that high temperatures could end up trending cooler (potentially by as much as 10+ degrees!) as the timing of these features gets refined.

Petr

AVIATION /18Z TAFS THROUGH 00Z THURSDAY/

Issued at 1224 PM CDT Tue May 26 2026

No significant aviation forecast concerns for the 18Z TAFs.

Weak surface high pressure ridging lingers from western Lake Erie into northern IN/IL. Surface winds have become light/variable across the terminals late this morning in the vicinity of the ridge axis, but are expected to become southeasterly early this afternoon as the ridge drifts away to the east. A lake breeze developing off of Lake Michigan should further enhance the southeasterly winds for ORD/MDW, while turning GYY east-northeast. Within an otherwise weak surface pressure gradient, winds are expected to become light/variable again by late evening and remain so overnight. Winds are then forecast to turn northeasterly after sunrise Wednesday, as a stronger area of high pressure builds across Ontario and the northern Great Lakes.

Otherwise, VFR conditions are expected to persist through the period, with mainly a high-level cirrus overcast through tonight.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

IL...Air Quality Alert until 6 AM CDT Thursday for ILZ005-ILZ006- ILZ012-ILZ013-ILZ020-ILZ021-ILZ103-ILZ104-ILZ105-ILZ106- ILZ107-ILZ108.

IN...Air Quality Alert from midnight tonight to midnight CDT Wednesday night for INZ001-INZ002-INZ010-INZ011.

LM...None.