textproduct: Chicago

KEY MESSAGES

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

DISCUSSION

Issued at 322 AM CDT Mon Sep 14 2026

The core of a 1020 mb surface high pressure system is shifting across the western Great Lakes early this morning. Light winds, clear skies, and a drier air mass beneath the high have allowed temperatures to drop into the 40s, especially west of the Fox Valley in northern Illinois. Shallow ground fog has developed in places and will likely persist through sunrise.

The surface high will move toward New England today as attention turns to a strong, early-autumn storm system tracking from the northern High Plains into south-central Canada. In response, southeasterly winds across our area will become gusty (25 to 30 mph) by late afternoon and evening. Warm air advection ahead of the deepening Canadian low will keep shower and thunderstorm chances primarily to our northwest across the Upper Midwest. However, a few isolated showers cannot be completely ruled out over northwestern Illinois this afternoon, so low (~20%) chances remain in the gridded forecast. Seasonable high temperatures in the 70s are expected under partly to mostly cloudy skies.

Tonight, the south-central Canadian low will drive a surface warm front northward across the area just in advance of a cold front sliding southeastward across the Upper Midwest. Strong theta-e advection driven by an intensifying southwesterly low- level jet will usher in a warmer, more humid airmass, pushing atmospheric column moisture (PWATs) near 2 inches. Although large-scale forcing will be modest into early Tuesday morning, strong isentropic upglide in advance of the cold front will support a blossoming region of showers and thunderstorms later tonight from IA east-northeastward across parts of northern IL into southern WI. This activity is expected to shift over areas along and mainly north of I-80 after midnight tonight, and will likely linger in the area through Tuesday morning. The primary threat with these showers and storms will be torrential downpours. Fortunately, the steering layer flow tonight will favor faster storm motions (~40 mph), so any potential flash flood threat would likely remain localized and tied to any potential areas of training storms.

Widespread cloud cover and lingering morning showers will keep temperatures cooler than previously expected on Tuesday north of I-80, where highs have been trimmed into the low 80s. South of the Kankakee River Valley, less morning cloud cover will allow stronger heating of a very humid pre-frontal air mass (dew points around 70F). Temperatures there will reach the upper 80s to near 90F, pushing heat index values into the mid-to-upper 90s. Decreasing convective inhibition in the afternoon may trigger renewed scattered thunderstorms near the front. Favorable kinematics could allow some storms to become organized, posing a risk of strong to severe wind gusts and torrential rain.

The surface cold front's southward push is expected to slow and/or stall somewhere across the southern half of IL Tuesday night. This is because its west-to-east orientation will parallel the mid and upper level flow along the northern periphery of a broad subtropical mid/upper level high centered across the southeastern CONUS. With little change in this pattern expected through at least Thursday, the surface front will remain draped across the region, and will act as the focus for multiple rounds of heavy rain producing showers and storms. This will particularly be the case near and north of its general vicinity, and near the nose of the low-level jet. While it remains a bit unclear as to which areas the heaviest rain will fall, there is some signal that storms Tuesday night could focus from southern IA and northern MO eastward into parts of central IL (south of I-80). However, the potential exists for this focus to shift back north into northern IL with the next low- level jet moisture surge over the frontal boundary late Wednesday into early Thursday.

Extended guidance indicates weak mid-level blocking as a subtropical low over the Gulf slows south of a weakening upper- level ridge. Meanwhile, a upper-level trough moving across the northern Great Plains on Friday may dig far enough south to bring showers and thunderstorms to the area on Saturday.

KJB

AVIATION /12Z TAFS THROUGH 18Z TUESDAY/

Issued at 647 AM CDT Mon Sep 14 2026

Main Concerns:

- Gusty southeast winds developing this afternoon and then LLWS outside of Chicago tonight.

- Chance of SHRA/TSRA from the pre-dawn hours of Tuesday into/through Tuesday morning.

Light/VRB to northeast winds initially today will become southeast by mid to late morning. Speeds will increase this afternoon along with some gustiness (~20-25 kt). Fairly frequent gusts should continue at ORD and MDW through the night, while the setup looks favorable for LLWS at around 2kft AGL at outlying sites, prior to 06z Tuesday at RFD and DPA. Winds will veer to south-southwest and then southwest on Tuesday morning, though there is a non-zero chance for outflow related effects on wind directions and speeds.

Spotty non-impactful VFR showers may pass by RFD this afternoon (no mention in the TAF). More notably, showers and embedded TS will likely develop farther northwest in the ZAU airspace late tonight/early Tuesday and then ooze southeast towards the terminals. Included VCSH and PROB30 mention at all sites except GYY (precip beyond current TAF period), with the PROB30 for TSRA at ORD and MDW until 16z Tuesday for now. IFR VSBY will be a decent bet if/when TS move directly across the terminals. Some model forecast soundings suggest CIGs may build down toward MVFR during Tuesday morning.

Castro

LOT WATCHES/WARNINGS/ADVISORIES

IL...Beach Hazards Statement from 5 PM CDT this afternoon through Tuesday morning for ILZ006-ILZ103.

IN...None. LM...Small Craft Advisory from 5 PM this afternoon to 9 AM CDT Tuesday for the IL nearshore waters.

Small Craft Advisory from 7 PM this evening to 7 AM CDT Tuesday for the IN nearshore waters.