textproduct: Chicago

KEY MESSAGES

- Waves of heavy rain producing thunderstorms possible in the general region Tuesday through Thursday.

UPDATE

Issued at 953 PM CDT Sun Sep 13 2026

Relatively minor adjustments to the forecast for overnight into early Monday, with the most notable electing to extend our Beach Hazards Statement for the IN shore through 3 AM CDT.

Band of showers/clouds across the forecast area earlier had dissipated by early this evening, as frontogenetic forcing weakened rapidly with the passing of a mid-level wave to our north/northeast. Strong height rises in the wake of that short wave is inducing near 1022 mb surface high pressure building southeast across WI at this hour, with mainly clear skies and much cooler temperatures expected overnight. While surface winds were quickly diminishing across cooler inland locations, north winds of 15-20 kts persist over the south end of Lake Michigan late this evening, with the fetch maintaining waves near 4 ft especially east of Gary, IN. Streaming webcam at Michigan City continues to show decent waves along the breakwalls there. With this in mind and in collaboration with WFO IWX have extended the Beach Hazard Statement (and our Small Craft Advisory out on the lake) through 3 AM CDT.

Otherwise, with mainly clear skies and light/calm winds coupled with dew points dropping into the mid-upper 40s, a cool night is expected with lows from the mid-40s across far northern IL away from Chicago to the low 50s farther south. With the cooler air spreading over the lake, some lake-induced stratocu may develop into far northeast IL/northwest IN overnight as low- level winds eventually turn more north-northeast.

Ratzer

DISCUSSION

Issued at 230 PM CDT Sun Sep 13 2026

Modest forcing from an axis of mid-level frontogenesis within the right entrance of a 90 knot mid-level jet streak over southern Wisconsin into extreme northern Illinois is supporting a band of rain dragging across northern Illinois early this afternoon. Precip intensity aloft has been strong enough to overcome a fairly dry low-level airmass, but is slowly eroding the rain shield with southeast extent. As better forcing remains north before shifting eastward early this evening, the precip band should gradually weaken.

In the wake of this afternoon/evening's precip, high pressure will build across Wisconsin and Lower Michigan tonight. With clearing skies, light winds, and PWATs under 0.5" tonight across northern Illinois, temps are expected to fall into the mid to upper 40s across interior northern Illinois. Farther south where mid-level cloud cover and slightly stronger low-level gradient flow lingers well into the night, temps will settle in the low to mid 50s. Precip is not expected, though shallow lake effect clouds may advect across northwest Indiana tonight and northeast Illinois late tonight into Monday morning.

As a broad mid-level trough lifts northeastward across the Upper Mississippi River Valley, a weak disturbance currently over northeast New Mexico will phase into the far southeastern extent of the northern trough late tonight into Monday. Though the air mass will remain quite dry in the lowest 7-9kft, marginal lapse rates in the 10-20kft layer and modest mid-level moisture advecting northeastward ahead of a building ridge could yield a few high-based showers during the late afternoon across primarily the northwest half of the CWA.

An expansive surface low across far southern Canada will drag a slowing cold front across northern Illinois late Monday night through Tuesday, with consensus guidance stalling the front somewhere across the southern half of Illinois by Tuesday night. Ahead of the front on Tuesday, temps may push well into the 80s and even lower 90s, with increasing low-level moisture yielding heat index values nearing 100 across the far southern CWA. Strong WSW flow aloft and generally uncapped conditions south of the front in the afternoon would support organized strong to severe convection, but with no clear mid/upper level forcing aligning atop the surface front, coverage of convection may remain low. Torrential rainfall is also possible with any convection as pooling low-level moisture pushes PWATs to near or possibly over 2 inches.

With a broad subtropical high settling over the southeast CONUS mid to late week, the overall mid and upper-level flow over the western Great Lakes will remain generally zonal. The front will remain draped across Illinois during this period, though whether it drifts back north over the forecast area at any point remains unclear. Either way, periods of scattered showers and possibly some thunderstorms can be expected in the general vicinity of the low-level front through at least Thursday.

By Friday, and especially into next weekend, weak mid-level blocking will develop as a sub-tropical low over the Gulf slows/stalls south of the deamplifying ridge. A trough crossing the northern Great Plains on Friday may extend far enough south to yield showers and storms in the area on Saturday.

Kluber

AVIATION /00Z TAFS THROUGH 06Z TUESDAY/

Issued at 623 PM CDT Sun Sep 13 2026

Only aviation forecast concerns are the potential for some very patchy shallow fog mainly west/northwest of Chicago toward morning, and the potential for a few scattered showers Monday afternoon mainly along/north of the IL/WI border. Southwest winds also look to increase later Monday afternoon/evening, likely with some gusts 20-25 kts.

Band of showers/light rain was dissipating across the area as of early evening, with VFR cloud cover also clearing from north to south. Surface high pressure will build southeast across the area this evening, allowing winds to become generally light north-northwest or just light/variable overnight. Can't rule out a little shallow patchy ground fog toward morning away from the Chicago metro area, primarily to the west/northwest, though model guidance suggests coverage would be quite spotty and will just continue to monitor trends tonight in case a mention may be needed for a few hours toward sunrise mainly for RFD.

The surface high will shift east of the area on Monday, allowing winds to shift southeast and gradually increase during the day. It does appear these winds will become gusty later in the afternoon/evening (20-25 kts) in response to deepening surface low pressure tracking northeast across the Northern Plains. The pressure gradient is expected to remain fairly tight Monday night as well, with a 45-50 kt southerly low-level jet developing. Would expect our typically warmer more urban ORD/MDW terminals would see at least sporadic/occasional surface gusts so have not included LLWS mention in their 30-hour forecasts at this time.

In addition, there does look to be a brief window Monday afternoon for some VFR showers clip RFD, though HREF ensemble pops are only 10-20% at this time and have not included in the TAF. Might be worth a PROB30 mention in the 19-22Z-ish window if coverage ends up being higher. Chicago terminals, farther to the southeast, would appear to remain dry at this time.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...Beach Hazards Statement until 3 AM CDT Monday for INZ001- INZ002.

LM...Small Craft Advisory until 3 AM CDT Monday for Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.