textproduct: Chicago

KEY MESSAGES

- The potential for showers and potentially strong to severe thunderstorms returns Wednesday afternoon and evening along a cold front.

DISCUSSION

Issued at 207 PM CDT Mon Aug 24 2026

High pressure centered over the area this afternoon will drift southeastward through Tuesday, supporting a dry and relatively quite period of weather. Patchy shallow fog will be possible late tonight primarily for interior areas south of the Chicago metro and across northwest Indiana. Otherwise, expect another cool night tonight with lows in the 50s and mild day Tuesday with highs around 80.

An upper-level low centered just north of the Boundary Waters of Minnesota Tuesday night will track to east of Lake Superior by Wednesday evening. A series of weak mid-level impulses will pinwheel around the low across Wisconsin and Lower Michigan during this time. Meanwhile, mid-level moisture originating from the southwestern CONUS will advect northeast across the mid- Mississippi River Valley as modestly steeper mid to upper-level lapse rates shift over the area. Conditions appear favorable for at least some convection over the forecast area during the afternoon and early evening, but there are several factors that complicate both the coverage and severity of that convection. The initial weak waves late Tuesday night through early afternoon Wednesday will bring mid-level clouds and possibly decaying high-based showers and embedded thunderstorms, resulting in questions with sufficient destabilization even as a narrow ribbon of higher low-level moisture advects in from the southwest. Forcing may also remain mis-aligned, with the upper- low to the north, tail-end of the mid-level troughs overhead, relatively weak upper-level jet to the south, and trailing cold front to the west. Conditional on erosion of the low-level cap and better alignment of the forcing noted above, deep-layer shear will be favorable for multicell clusters with embedded weak supercell structures, with locally damaging winds the primary threat, mid-afternoon into early evening.

Mid-level ridging over the Canadian Prairies will deamplify while shifting eastward late this week into the weekend. Associated surface high pressure will track east while becoming elongated across the central Great Lakes Thursday and Friday. Cannot fully rule out sprinkles or weak showers on the lake breeze boundary across northeast Illinois late Thursday afternoon, but the subsidence inversion should be strong enough to stunt any growing diurnal cumulus.

A loosely zonal pattern across North America this weekend into early next week favors a more active storm track north of the forecast area, but close enough to warrant periods of low-end precip chances for showers and some thunderstorms as early as Saturday night. Summer warmth will also return as we close out August with increasing humidity and daytime highs pushing well into the 80s (assuming active weather remains bottled up to the north).

Kluber

AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/

Issued at 1237 AM CDT Tue Aug 25 2026

No major forecast concerns are expected through this TAF period as high pressure will remain in control over the region. Therefore, expect light and variable winds tonight to become southwesterly Tuesday morning with speeds increasing into the 5-10 kt range. While there will be some increasing VFR cirrus clouds tonight into Tuesday morning, there remains a chance for some patchy shallow (5-6 SM) ground fog tonight at the outlying terminals. Confidence on fog coverage at the terminals is low, however, given the 3-4 degree temperature-dewpoint depressions and aforementioned cloud cover so have held off on any formal fog mention in the TAFs. Similarly to last night, any fog that does materialize tonight will erode shortly after sunrise leaving us with tranquil weather conditions for the rest of our Tuesday.

Yack

LOT WATCHES/WARNINGS/ADVISORIES

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