textproduct: Chicago

KEY MESSAGES

- Below average temperatures expected through the weekend before temperatures turn more seasonable early next week.

- No significant widespread precipitation chances are expected at this time until the middle of next week.

DISCUSSION

Issued at 220 AM CDT Thu Sep 24 2026

Forecast area remains on the northwestern periphery of an upper-level low which is progged to deepen across the mid- Atlantic region over the next couple of days. GOES water vapor imagery currently indicates a smaller-amplitude short wave wrapping west-southwest across lower MI/LM and IN around the back side of this system. The main impact of this disturbance on our forecast area is to continue to spread an expansive area of low clouds to the west across the cwa early this morning (though some large breaks have developed toward the IL/WI border), with some patchy drizzle reported earlier south of the I-80 corridor into central IL which should end with the passage of the disturbance this morning. Mostly cloudy skies are expected today especially for our south/southeastern cwa. While some significant breaks are likely farther north, increasing high clouds are expected later this afternoon as another short wave tracks into the upper Midwest from the Northern Plains. Otherwise, less-breezy conditions are expected today with high temps in the upper 60s to lower 70s (mid-60s along the Lake Michigan shore).

Various model guidance indicates the potential for some spotty showers to spread into far north/northwest IL toward daybreak Friday, in association with the aforementioned short wave propagating into the upper Midwest. This wave will be running into short wave ridging aloft on the northwestern fringe of the deepening east coast low however, which along with easterly low-level dry air advection is expected to limit the eastward development of precipitation. Have included some slight chance (<=20%) pops for these decaying showers mainly west of the Fox River Valley across northern and northwest IL between about 4AM-10AM Friday morning.

Behind Friday's wave, the narrow upper-level ridge is then progged to briefly redevelop into the upper-Midwest Friday night into Saturday before another short wave approaches the upper- Midwest Saturday evening. Similar to Friday morning's scenario, this wave will also be fighting the ridge and dry easterly low- level flow. Some decaying showers will again be possible Saturday night across our far western counties of the forecast area, where again some slight chance (<20%) pops are in place west of I-39. Daytime hours look to be dry over the weekend, though due to the blocking of the pattern of the deep east coast upper low, our anticipated gradual warming trend looks to be on hold. Partly to mostly cloudy skies and continued modest east- northeast surface winds will likely keep our temperatures slightly below average in the mid-60s to lower 70s both Saturday and Sunday.

A slower evolution is not unusual in such highly-amplified upper air patterns, and the latest global model/ensemble solutions have delayed kicking out the eastern low until early- mid next week. As this occurs, there are indications that the pattern will become more active with an eastward-moving frontal zone and surface low developing into the region toward mid- week. NBM initialization depicts this with 40-60% pops later Tuesday into Wednesday and made no changes at this distance, though there is quite a bit of ensemble spread in the current 00Z EPS/GEFS/CMCE solutions with the timing/strength and placement of features at this time.

Ratzer

AVIATION /12Z TAFS THROUGH 18Z FRIDAY/

Issued at 630 AM CDT Thu Sep 24 2026

Mvfr cigs have become more widespread this morning, extending from north central IL through east central IL and northwest IN. These mvfr cigs may persist through much of today and possibly spread back across the terminals this afternoon. Overall confidence for prevailing mvfr cigs is low and opted for scattered mvfr mention and trends will need to be monitored.

There may also be a few sprinkles or isolated showers south of the terminals today and then across northwest IL and near RFD early Friday morning. Confidence is also low and maintained a dry forecast for RFD.

Easterly winds around 10kt today will become east/southeast this evening and diminish under 10kt. cms

LOT WATCHES/WARNINGS/ADVISORIES

IL...Beach Hazards Statement until noon CDT today for ILZ006.

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