textproduct: Chicago

KEY MESSAGES

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

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 /18Z TAFS THROUGH 00Z TUESDAY/

Issued at 1254 PM CDT Sun Sep 13 2026

Key Messages:

- Rain showers will occur at times through this afternoon.

A passing weather disturbance will bring periods of rain showers to the terminals this afternoon. Based on upstream observations, brief visibility reductions to MVFR may occur beneath the pockets of steadiest rainfall, but VFR conditions will otherwise prevail as these showers pass through. Another wave of showers is expected to pass through portions of northern Illinois tomorrow afternoon. The best shower chances will be at RFD, though they will come just after the 18Z end time of the current TAF there. ORD and MDW may get grazed by this shower activity sometime after 19Z tomorrow as well, but confidence in this occurring was too low to warrant a formal precipitation mention in the ORD and MDW TAFs at this time.

Otherwise, winds may a bit squirrelly at MDW and GYY this afternoon, wavering between a north-northwesterly and north- northeasterly direction with a lake breeze close by. This lake breeze resulted in a temporary north-northeasterly wind shift at ORD earlier, but it has since retreated back eastward, which should favor north-northwesterly winds prevailing at ORD through this afternoon. Winds will become lighter tonight as surface high pressure settles into the region before turning southeasterly after sunrise and gradually increasing through the daytime hours tomorrow.

Ogorek

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...Beach Hazards Statement until 10 PM CDT this evening for INZ001-INZ002.

LM...Small Craft Advisory until 7 PM CDT this evening for Northerly Is. to Gary IN.

Small Craft Advisory until 10 PM CDT this evening for Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.