textproduct: Chicago

KEY MESSAGES

- Cooler today with a chance for afternoon showers.

- Waves of heavy rain producing thunderstorms possible in the general region early to mid next week.

DISCUSSION

Issued at 304 AM CDT Sun Sep 13 2026

Northerly surface winds are ushering in cooler, less humid air early this morning following yesterday's cold frontal passage. This post-frontal airmass will bring fall-like temperatures today, with highs remaining in the lower 70s north and the mid- to-upper 70s south. Skies will turn mostly cloudy this afternoon as a mid-level shortwave trough advances from the Upper Midwest into the western Great Lakes. Model guidance consistently shows a band of frontogenesis-induced high-based showers developing this afternoon along a lingering 850-700 mb baroclinic zone across northern Illinois. A few rumbles of thunder are also possible as mid-level lapse rates steepen. Rainfall amounts will remain light (<0.10") due to a deep (8,000-10,000 ft) sub-cloud layer causing significant evaporation and virga. Activity should wane early this evening.

Skies will clear tonight as the core of a 1020 mb surface high settles across the western Great Lakes. The combination of light winds, a dry low-level airmass, and clear skies will promote efficient radiational cooling, leading to a chilly early autumn night outside the Chicago urban area. Typical cool spots along and west of the Fox Valley could see temperatures drop into the mid-40s by daybreak Monday.

On Monday, models and ensembles remain in good agreement in a potent shortwave trough and deepening surface low tracking from the northern High Plains into south-central Canada as the Great Lakes surface high shifts towards New England. These two pressure features will result in winds turning southeasterly and increasing during the day on Monday. Shower and thunderstorm chances should largely remain northwest of the area across the Upper Midwest during the day as warm air advection ramps up in advance of the deepening south- central Canada surface low. Otherwise, Monday will be another seasonable day with temperatures in the 70s under partly to mostly cloudy skies.

Monday night, the Canadian surface cyclone will push a warm front northward across the area ahead of a cold front moving southeastward across the Upper Midwest. Strong theta-e advection driven by an intensifying southwesterly low-level jet will usher in a much warmer, more humid airmass, pushing atmospheric moisture (PWATs) near 2 inches. Although large-scale forcing will be modest into early Tuesday morning, strong isentropic upglide will support increasing shower and thunderstorm chances late Monday night, particularly north of I-80.

The cold front will sag southward into northern Illinois Tuesday afternoon, reaching areas along and south of I-80 during peak diurnal heating. Weakening convective inhibition should support scattered afternoon and evening thunderstorm redevelopment near the boundary. Given favorable atmospheric kinematics, storms along and south of I-80 could produce torrential downpours and become strong to severe. Additional storms are likely Tuesday night as the low- level jet strengthens over the frontal boundary. The highest rainfall totals may remain just southwest of the area near the nose of the low-level jet in southern Iowa and northern Missouri, but heavy rain potential will continue to be monitored for Tuesday night into Wednesday morning.

The surface boundary is expected to stall and become quasi- stationary across the region Wednesday into Thursday. Exactly where it stalls remains uncertain, as Tuesday night convection could push the effective boundary further south. Wherever the front settles, additional rounds of heavy rain and thunderstorms will be favored as the low-level jet re-strengthens late Wednesday into early Thursday. Some guidance suggests this second moisture surge could push the heavy rainfall threat further north into our area.

Extended guidance suggests a brief surge of drier air around Friday, followed by another robust disturbance approaching next weekend that will bring additional rain chances to the local area.

KJB

AVIATION /12Z TAFS THROUGH 18Z MONDAY/

Issued at 639 AM CDT Sun Sep 13 2026

Main Items of Note:

- Scattered VFR showers this afternoon

- Temporary wind shift to NNE/NE near the lake this afternoon

Expect VFR conditions through the period. Winds will be north- northwest to at times north today, becoming light northerly tonight (VRB outside of Chicago), and east-southeast on Monday morning. Light rain/showers with no VSBY restrictions will likely occasionally reach the ground this afternoon as an upper disturbance moves over the region.

There's been a sufficiently consistent signal for a wind shift to north-northeast at MDW and GYY to maintain it in the TEMPO for -RA this afternoon. Confidence is lower at ORD (only ~20-30% chance of a wind shift), which appears more likely to hold at 320-350 deg winds while the wind shift occurs just off to the east.

Castro

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...Beach Hazards Statement from 10 AM CDT this morning through this evening for INZ001-INZ002.

LM...Small Craft Advisory from 1 PM this afternoon to 10 PM CDT this evening for Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.