textproduct: Chicago

KEY MESSAGES

- Increasing heat and humidity is expected next week with periodic shower and storm chances.

DISCUSSION

Issued at 306 AM CDT Thu Aug 27 2026

Surface high pressure continues to expand across the region early this morning in the wake of yesterday's cold front. This will lead to a period of largely quiescent weather through the end of the week. A lake breeze is expected to push inland today which may provide enough lift for a few spotty showers or sprinkles as a weak disturbance moves across the area (15% chance). Coverage should remain very isolated if they even develop given limited moisture availability and shallow cloud depths. Otherwise rain-free weather is in store across the area through at least Saturday afternoon with highs in the upper 70s to lower 80s and comfortable humidity.

Mostly clear skies early this morning and again tonight may allow for sufficient radiational cooling for shallow patchy fog development, particularly along river valleys and typical fog prone low spots outside of Chicago.

Looking ahead, the upper level pattern begins to transition toward upper ridging building across the central CONUS with multiple waves expected to traverse the Northern Plains and Upper Midwest this weekend through much of the upcoming week. The first of these waves may arrive as early as Saturday night in the form of a decaying convective complex. Latest guidance suggests that if this convection forms upstream it would likely be outrunning the unstable airmass with southeast extent and residual low-level dry air across the area would likely limit the coverage of lightning or any potential severe weather here locally.

The degree of heating as well as our shower and thunderstorm chances for much of the upcoming week will be dependent upon the strength of the aforementioned upper ridge. A stronger ridge would result in warmer (potentially highs in the 90s) and more humid conditions with the focus for greater thunderstorm coverage ending up north of the area. In contrast, a weaker upper ridge would support warmer conditions but to a lesser degree (mid-upper 80s) paired with frequent rounds of showers and thunderstorms. Suspect we may end up somewhere in between these two scenarios and will continue to refine the details as it gets closer. Stay tuned.

Petr

AVIATION /12Z TAFS THROUGH 18Z FRIDAY/

Issued at 619 AM CDT Thu Aug 27 2026

Forecast concerns for the terminals...

- Lake breeze to result in easterly winds at the Chicago terminals and GYY this afternoon.

- Period of patchy fog expected tonight away from Chicago, IFR visibilities possible.

A surface high continues to build into the region this morning which will keep dry and generally tranquil weather around for this TAF period. While there remains a low chance (10-15%) for an isolated shower or sprinkle along the lake breeze this afternoon, confidence on coverage remains too low for a formal TAF mention but did keep a BKN050 mention going at ORD, MDW, and GYY to tease this potential. Expect winds to remain northwesterly through the afternoon at DPA and RFD with speeds around 5-7 kts. A lake breeze will move through the Chicago terminals and GYY, however, which will result in an easterly wind shift and uptick in speeds to around 10 kts. Overall lake breeze timing continues to favor around 19-20z for MDW and ORD respectively and 16-17z at GYY, but will obviously keep a close eye on trends in case that timing shifts.

Winds will gradually become light and variable overnight as the center of the surface high passes overhead before settling into a southwesterly direction Friday morning. Given the light winds and clear skies forecast tonight, conditions look favorable for patchy fog to develop after midnight and persist through daybreak Friday morning away from the urban core of Chicago. With such a strong signal in guidance and the fact that shallow fog has developed in spots the past couple nights, have opted to introduce TEMPO groups at DPA, RFD, and GYY for MVFR visibilities and fog. Though, there is a chance that visibilities could dip into the IFR range in spots. Aside from the fog, VFR conditions are expected through the TAF period.

Yack

LOT WATCHES/WARNINGS/ADVISORIES

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