textproduct: Chicago

KEY MESSAGES

- Areas of fog, some locally dense, may linger through mid morning, mainly west and northwest of Chicago.

- Isolated to scattered thunderstorms may develop along a cold front this afternoon and evening, mainly east of I-55.

- Drier weather is in store Monday into Tuesday before storm chances return midweek.

DISCUSSION

Issued at 144 AM CDT Sun Aug 16 2026

A shortwave trough continues to pivot across northern IL and has lead to the development of a cluster of thunderstorms. This cluster of thunderstorms will continue to push east across northwest IN through the early overnight hours before exiting our area. The main threat with this storm cluster will be torrential downpours, but some localized pockets of gusty winds (upwards of 45-50 mph) have been occurring and that will likely continue as the cluster moves across our northwest IN counties. Given that this cluster is moving rather quickly, the flooding threat with it appears to be low.

In the wake of this cluster, light to moderate stratiform showers will persist through the predawn hours. However, additional showers and thunderstorms are developing in northern MO and west-central IL along the remnant outflow boundary from last night's storms. Latest radar trends show that these developing showers/storms may stay just south of our forecast area, however, if any development can occur further north then portions of Livingston, Ford, and Iroquois Counties would be in play. Given that the steering flow and forcing are oriented parallel to the outflow, showers/storms in central and north- central IL will likely train over the same areas. Couple this with the 2.0 inch PWATs and the modest instability and conditions are primed for another period of torrential rainfall. If the heaviest rainfall does occur over our southern areas, then a threat for significant flash flooding would quickly materialize as these areas have already seen nearly 6-10 inches of rain over the past few days. Due to this threat a Flash Flood Watch was issued earlier this evening and remains in effect until 10 AM CDT this morning for areas near and south of I-80 where the flooding threat is greatest.

The showers and storms tonight are expected to gradually wane around daybreak as the low-level jet weakens and the aforementioned shortwave pushes east of the area. However, a cold front (which is currently analyzed along a Sioux City, IA to Minneapolis, MN line) will be moving across northern IL and northwest IN today. Initially, it looks as if the atmosphere should be stable enough to keep dry conditions around through much of the morning, but another shortwave is expected to pivot across southern IA and north-central IL this afternoon. As this wave approaches the front is forecast to be near the I-55 corridor which is where the focus for additional showers and storms this afternoon should reside. That said, forecast soundings do continue to show some weak capping in place and with the bulk of the wave's forcing forecast to be aimed into central IL it seems shower and storm coverage locally will be more hit and miss. For now have opted to maintain some 20-30% chances for showers and thunderstorms in the forecast, mainly for areas southeast of I- 55, but suspect many locations should remain dry especially those to the north of I-55. Regardless, the threat for severe weather with any storms today is low due to the lack of wind shear though heavy downpours and locally gusty winds will still be possible.

The cold front should move through our area this evening allowing a surface high to start building in from the north. This building high will finally break us out of this stormy pattern for Monday and most of the day on Tuesday. Therefore, expect mostly to partly sunny skies both days with highs in the low to mid-80s and overnight lows in the 60s. Unfortunately, the nice weather won't last long as another shortwave and its associated cold front are forecast to dive through the Midwest Tuesday night into the day on Wednesday. This system will bring more chances for showers and thunderstorms to the region as early as Tuesday evening, but the bulk of the stormy weather should occur overnight Tuesday night into Wednesday. While confidence is high on showers/storms occurring during this period, there continues to be some uncertainty in exact timing and intensity so make sure to stay updated as we get closer.

On the bright side, long range ensemble guidance continues to hint that high pressure should move in behind Wednesday's storm system which looks to keep the bulk of any additional storms south and west of us for the later half of the week. Though if this does occur it looks as if temperatures may be more on the seasonably cool side with highs in the 70s and lows in the 50s to 60s.

Yack

AVIATION /12Z TAFS THROUGH 18Z MONDAY/

Issued at 705 AM CDT Sun Aug 16 2026

Key aviation forecast messages:

- LIFR to IFR CIGs early this morning with gradual improvement through early afternoon.

- Patchy BR/FG west and northwest of Chicago through mid- morning.

- Steady WNW to NW winds around 10 kt with gusts to 15-20 kt expected today.

- Chance of -SHRA mid to late afternoon, greatest potential at GYY (30% chance).

LIFR to near VLIFR CIGs have developed in the wake of the showers early this morning. Expect them to remain in place for at least the next couple of hours with very gradual improvement expected thereafter to MVFR by mid-late morning and VFR toward early to mid afternoon. Confidence in timing of improvement remains on the lower side.

Areas of fog, some locally dense, have also developed west and northwest of Chicago early this morning. Lowest VSBYs are at DPA (3/4 SM) with 4 SM at ORD. Expect similar VSBYs to persist for the next hour or two with a gradual return to VFR by mid-late morning.

A cold front will sweep across the area during the afternoon and early evening. Can't fully rule out a shower or isolated storm along the front as it moves through. However, the potential remains too low for a formal TAF mention at the Chicago area terminals. Did introduce a PROB30 at GYY for SHRA given closer proximity to where they may initiate.

Winds will be light SW to W initially this morning around 5-10 kt. Winds then turn WNW by mid morning with sustained speeds increasing to around 10-11 kt. Gusts up to around 15-20 kt are then possible at times through the afternoon, becoming NNW late afternoon/early evening behind the cold front. Winds then trend northeasterly toward daybreak on Monday.

Petr

LOT WATCHES/WARNINGS/ADVISORIES

IL...Flood Watch until 10 AM CDT this morning for ILZ032-ILZ033- ILZ039.

IN...Flood Watch until 10 AM CDT /11 AM EDT/ this morning for INZ010-INZ011-INZ019.

LM...None.