textproduct: Chicago

KEY MESSAGES

- Dry and more seasonable conditions today. Threat for scattered showers and perhaps a few thunderstorms in the region on Friday.

- Unseasonable warmth on Saturday followed by a period of showers Saturday night and Sunday morning mainly near and southeast of I-55.

DISCUSSION

Issued at 108 AM CDT Thu Oct 8 2026

An elongated 1020 mb surface high will build across the Upper Mississippi Valley today yielding another pleasant early fall day. Northwesterly breezes will develop towards midday and into this afternoon, and have inched gusts up a bit towards 20 mph along the northeastern flanks of the high where progged deeper mixing is forecast. Have continued to generally undercut NBM dewpoints given the anticipated mixing today which yields afternoon RH values into the 25 to 35 percent range. Winds will then trend light/variable this evening as the surface high translates overhead.

A broad region of low-level warm advection will develop and subsequently intensify across Iowa late tonight before spilling eastward across northern Illinois through Friday morning. Moist upglide impinging upon the northeastern terminus of a zone of modestly steep mid-level lapse rates will likely yield an expansion of elevated showers and some thunderstorms. As is typical with these types of set ups, the location of the main moist/unstable axis remains a bit unclear. That said, global guidance has remained fairly consistent, depicting the main zone of mid-level saturation lifting across northern Illinois through the afternoon. Will show scattered showers and isolated thunderstorms in the weather grids during this period, and suspect PoPs may have to be boosted further as hires guidance better latches onto the salient forcing mechanisms. While the severe weather threat is low given a dearth of instability, a corridor of 30-40 kt effective bulk shear is forecast to develop as a 40-50 kt 500 mb jet max scoots overhead. Some recent CAMs have an interesting presentation to a convective conglomeration pushing east of the Mississippi River towards midday, although this would likely weaken with time as it outpaces the main instability reservoir.

Any lingering activity will depart the region late Friday afternoon/early evening. Saturday continues to look dry, somewhat breezy, and warm, with initial high cirrus streaming northward through the afternoon. The remnants of Isaias interacting with a northward-advancing trough will result in a chance for showers across parts of our area Saturday night into Sunday morning. With such a dry antecedent airmass in place, combined with northern Illinois remaining on the western extent of the advancing jet stream, suggests that precipitation coverage and amounts may end up being fairly limited in the region with max upper jet support focusing across the Ohio River Valley (barring any westward jog to the main synoptic scale features).

Conditions currently look dry to start next week, but an additional cold front will bring the next chance for a brief period of showers to the region Monday night into Tuesday before high pressure builds back across the area mid week.

Carlaw

AVIATION /18Z TAFS THROUGH 00Z SATURDAY/

Issued at 1236 PM CDT Thu Oct 8 2026

Key aviation forecast messages:

- Weak lake breeze wind shift likely at GYY/MDW later this afternoon/early evening.

- SHRA likely Friday, with a chance of TSRA late morning into afternoon.

Surface high pressure ridge extends from southern MN into the lower OH Valley at midday, just west of the terminals. This will continue to favor northwest winds across the TAF sites, and there may be some sporadic gusts in the 15-20 kt range this afternoon during peak diurnal mixing. A lake breeze is expected to affect GYY by 20-21Z, and as winds diminish quickly with the approach of sunset the lake breeze may get as far west as MDW in the 23Z hour. Otherwise, winds are expected to become light/variable early this evening as the surface high pressure ridge drifts east. Southeast surface winds are expected after sunrise Friday.

Warm advection will develop west of the ridge axis on Friday, in response to a mid/upper level jet streak transiting the upper Midwest. Guidance is in good agreement in depicting showers developing across the area from the west during the morning and midday hours with at least some elevated TSRA potential within a plume of steep mid-level lapse rates aloft. Have gone with prevailing VFR SHRA as early as 16Z at RFD and 18Z for the Chicago terminals, with a corresponding PROB30 TS mention. Activity should move east of the Chicago terminals by 00Z Friday evening.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

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