textproduct: Chicago

KEY MESSAGES

- Widely scattered showers are expected tonight.

- Generally slightly below to below average temperatures expected into at least the first part of the upcoming weekend.

DISCUSSION

Issued at 210 PM CDT Tue Sep 22 2026

Water vapor imagery early this afternoon shows a compact, but well defined closed mid-upper level low over eastern IA and northwest IL. This system has been a bit slower to weaken and devolve into an open wave than guidance progged yesterday. The 18z sounding that just came in from DVN depicts a deeply nearly saturated to saturated column with a PWAT of 1.31", which is notably more moisture than depicted in most guidance yesterday. The likely stronger forcing with the more defined mid-upper low combined with greater moisture has resulted in a greater coverage of showers and even a couple of isolated thunderstorms this afternoon into the western half our CWA.

Tricky forecast for the next 12-18 hours with guidance continuing to largely keep rain chances minimal and quickly weakening and opening up this system. Certainly not seeing any evidence in water vapor imagery of imminent weakening, and guidance's the poor handling of this feature over the past 24 hours there is reason to be skeptical that the showers out there will imminently dry up. Recent ACARs soundings from MDW do depict a pretty stout dry layer between 900 and 700 mb, so it is plausible there will be a decrease in showers as it moves east. Given the pocket of moisture evident in low and mid level water vapor imagery with the core of the upper low, think it is prudent to maintain respectable shower chances into tonight near the core of this upper low as it moves across the area.

Once this upper low pushes east of the area tomorrow morning, an extended period of dry weather is expected for the remainder of the week and likely through the upcoming weekend. A very blocky upper level pattern is progged to set up across central and eastern North America, so very little day to day change in our weather is expected. By later in the weekend into early next week, some breakdown in the blocking pattern is possible with upper ridging developing eastward. Assuming this plays out as progged (lower than avg confidence given the tendency for guidance to break down blocking patterns too quickly), then a warning trend would take place late in the weekend into early next week.

- Izzi

AVIATION /18Z TAFS THROUGH 00Z THURSDAY/

Issued at 1214 PM CDT Tue Sep 22 2026

Slow moving scattered SHRA and isolated TS associated with upper level low pressure are the main weather item of note through tonight. Sporadic isolated lightning with this activity should remain confined to far western portions of C90 this afternoon. While precip chances are relatively higher at DPA and RFD vs. elsewhere this afternoon, opted to monitor trends vs. formal inclusion in the TAF. Overall SHRA coverage should diminish as the upper low pivots southeastward tonight.

Patchy MVFR CIGs through tonight should expand in coverage on Wednesday, and included TEMPO mention in the TAFs for now. Steady to at times gusty northeast winds will prevail.

Castro

LOT WATCHES/WARNINGS/ADVISORIES

IL...Beach Hazards Statement through Wednesday evening for ILZ006- ILZ103-ILZ104.

IN...Beach Hazards Statement through Wednesday afternoon for INZ001- INZ002.

LM...Small Craft Advisory until 7 PM CDT Wednesday for the IL nearshore waters.

Small Craft Advisory until 10 AM CDT Wednesday for Calumet Harbor IL to Burns Harbor IN.