textproduct: Chicago

KEY MESSAGES

- Widespread soaking rainfall is expected across the area through tonight. A corridor of locally higher rainfall rates may occur near/south of I-80 leading to localized flooding.

- There is a low chance (20-30%) for isolated non-severe thunderstorms south of a Pontiac to Rensselaer line this late afternoon through early evening.

- A heightened risk for rip currents will exist at southern Lake Michigan beaches from this evening through Monday night.

DISCUSSION

Issued at 211 PM CDT Sun Jun 21 2026

A pair of shortwave troughs continue to phase over the Upper Midwest with a broad area of showers spreading across eastern IA, IL, and northwest IN ahead of these features. The earlier MCV that was traversing central IL seems to have weakened with now only a broad area of lower pressure being analyzed over northern MO. Extending from this low is a warm front which stretches from the IA-MO border southeast across central IL into central IN. Given the ongoing showers and the dissipating MCV, the warm front is expected to stay near and south of a Bloomington to Lafayette line which should keep the better surface instability south of our area. As a result the threat for strong to severe storms into northeast IL and northwest IN has greatly decreased. In fact the Storm Prediction Center has pulled the marginal (level 1 of 5 risk) from our area for today. However, there is still a chance (20-30%) for an isolated embedded thunderstorm or two later this afternoon and evening as the approaching troughs aid in steepening the mid-level lapse rates. If any storms occur the main area for them looks to be along and south of a Pontiac, IL to Rensselaer, IN line.

Regardless of the thunderstorm coverage, the main forecast concern through this evening will be the threat for locally heavy rainfall. The reason is because as the aforementioned surface low approaches a mid-level baroclinic zone is expected to establish overhead and interact with the broad forcing from the shortwaves and the modest instability. Couple these forcing mechanisms with the relatively high moisture content (PWATs around 1.3 to 1.5 inches) and conditions are prime for periods of heavier rainfall. Latest trends continue to support that the greatest potential for heavy rainfall will be near and south of I-80 closer to the baroclinic zone and slightly better instability. As a result, total rainfall in south of I-80 may get into the 1-2 inch range with locally higher totals possible which may result in pockets of flooding especially in lower- lying and poor drainage areas. Furthermore, there is also the chance for an additional axis of heavier rainfall to develop along the system's deformation band this evening. While confidence on the placement of this deformation band is still low, trends continue to suggest it may set up between the I-88 and I-80 corridors. If this does set up here, the large urbanized area may allow the 1-1.5 inch rain amounts to result in more instances of flooding and ponding of lower-lying and poor drainage areas. Due to the long duration of the rainfall (12-14 hours) and the modest rain rates (around 0.5 to 0.75 inches per hour at worst), confidence remains too low to warrant the issuance of a Flood Watch at this time. That said, areas south of I- 88 remain in the level 1 and 2 of 4 risk areas for flooding from the Weather Prediction Center.

Heading into tonight, the surface low and the associated shortwave wave will pivot east across the area which will allow rain to gradually taper after midnight and through early Monday morning. Though, depending on how fast the system exits a few showers may linger across portions of eastern IL and northwest IN through mid-morning Monday. Otherwise, expect mostly dry conditions for Monday with skies gradually clearing through the day. Winds, however, will become breezy behind the surface low this evening and will remain as such through Monday with gusts around 20-30 mph. Given the northeast wind directions, these winds will result in building waves (upwards of 4 to 7 feet) and dangerous rip currents on Lake Michigan. Therefore, a Beach Hazard Statement remains in effect for this evening through Monday night so be sure to stay dry when waves are high. Temperatures on Monday will also be on the seasonably cool side with highs in the mid-70s, but cooler conditions (mid to upper 60s) are forecast for areas near the lake due to onshore winds.

Winds will subside Monday night into Tuesday as a surface high pivots into the Great Lakes. While wind directions look to maintain a northerly component on Tuesday, temperatures are expected to be warmer with highs in the upper 70s inland and lower 70s near the lake.

As we enjoy the break in the weather Tuesday, another shortwave trough is forecast to be traversing the US-Canada border and eventually pivot into the northern Great Lakes Tuesday night. Associated with this wave will be a cold front which is progged to move through northern IL and northwest IN on Wednesday. While this front does look to bring some scattered showers and thunderstorms with it, the limited moisture return ahead of it and the northward trajectory of the shortwave may limit storm coverage locally. For now saw no reason to adjust the offered POPs from the NBM which keeps 30-50% chances across our area, but suspect the better storm coverage may be further north than shown in the current forecast. That said, if sufficient instability can be realized Wednesday afternoon the 30-35 kts of shear may allow for a few storms to become strong to severe.

In the wake of the front Wednesday night, guidance has trended towards a drier end to the work week as modest mid-level height rises move overhead. However, the signal remains for a pattern shift towards more west-southwest flow aloft next weekend and beyond which looks to support the return of more shower and thunderstorm chances but details at this range remain fluid. Regardless, the pattern shift also looks to allow temperatures to warm towards more typical summer conditions this weekend into next week as well.

Yack

AVIATION /18Z TAFS THROUGH 00Z TUESDAY/

Issued at 1252 PM CDT Sun Jun 21 2026

Key aviation forecast messages:

- Widespread rain to continue into tonight with periods of MVFR to IFR CIGs and VSBYs expected.

- Gusty east to northeast winds may develop late afternoon through the evening, with gusts to 25-30 kt near and south of I-80.

Steady rain will continue through the rest of the day and into the early overnight hours across the area. Expect a gradual lowering of CIGs and VSBYs through mid evening to MVFR and potentially IFR. An axis of higher rain rates and associated IFR VSBYs may set up somewhere within the vicinity of the I-80 corridor by early evening. The favored timing of this feature has shifted from late afternoon to early evening and have adjusted the inherited TEMPO groups accordingly, with lower VSBYs (2SM) at MDW and GYY.

Winds may also become increasingly gusty late this afternoon through the evening out of the east and northeast. Depending on the strength of the surface low, a narrow corridor of gusts up to 25-30 kt may materialize, currently favoring areas south of I-80. Have nudged up gusts slightly for the Chicago area terminals and included a TEMPO for higher gusts at GYY.

A rogue lightning strike can't be ruled out within the broader stratiform rain area but confidence and coverage continues to be too low for a formal TAF mention (<15% chance), with the better potential remaining well south of I-80.

Expect the rain to gradually come to an end late tonight between roughly 7-9Z with lingering IFR to MVFR CIGs through daybreak that gradually scatter out by mid Monday morning.

Petr

LOT WATCHES/WARNINGS/ADVISORIES

IL...Beach Hazards Statement from 10 PM CDT this evening through late Monday night for ILZ006-ILZ103.

Beach Hazards Statement from 7 PM CDT this evening through late Monday night for ILZ104.

IN...Beach Hazards Statement from 7 PM CDT this evening through late Monday night for INZ001-INZ002.

LM...Small Craft Advisory until 1 AM CDT Tuesday for the IL nearshore waters.

Small Craft Advisory until 4 AM CDT Tuesday for the IN nearshore waters.