textproduct: Chicago

KEY MESSAGES

- Scattered thunderstorms are possible this evening south of I-80, greatest coverage likely south of the Kankakee and Illinois Rivers.

- Storms this evening will be capable of locally torrential downpours which could result in localized flash flood threat. In addition, isolated severe storms capable of large hail and damaging winds are possible too.

MESOSCALE

Issued at 1208 PM CDT Wed Aug 19 2026

Diffuse cold front roughly parallels I-80 across northern Illinois and Indiana midday. Large area of light/variable winds near the front is resulting in little if any meaningful convergence. Despite the lack of low level forcing, water vapor imagery depicts a shortwave trough over northern IA and southern MN. This feature is moving east-southeastward and should result in increased mid-upper level ascent across northern IL later this afternoon into this evening, likely leading to increasing coverage of showers/storms.

Dewpoints in the low-mid 70s are pooling near and just south of the boundary. ACCAS has been festering across the region this morning, indicative of mid-level instability. Continued heating of the very moist boundary layer should erode any remaining boundary layer CINH. An axis of 1500-2000 J/kg of MLCAPE with little if CINH is expected this afternoon near and south of the I-80 corridor. Relatively strong mid level flow (35-40kt at 500 mb) is resulting in effective shear around 40kt, which certainly would be supportive of supercells which could pose some risk for either large hail and/or localized damaging winds. Weak low level shear and flow should limit any tornado threat.

Precipitable water values around 1.75" and deep warm cloud depth should result in storms this afternoon and evening being very efficient heavy rain producers. Soil moisture is above normal from recent heavy rainfall with 1 hour FFG generally around 1.5". Given the threat of locally torrential downpours with isolated rainfall totals of 2-3" in just an hour or so, there is concern for a localized flash flooding developing later this afternoon and early evening. Given the farther north placement of the diffuse boundary, opted to expect the flood watch to include Jasper and Newton Counties.

Biggest uncertainty at this point is when convective initiation will take place. Given the weakening convective inhibition, should see convective initiation take place as early 18 or 19z, but the greater coverage of convection is more likely toward and after 21z as we begin to feel the effects of the approaching shortwave.

- Izzi

DISCUSSION

Issued at 233 PM CDT Wed Aug 19 2026

Through Tonight...

Showers and thunderstorms should tend to gradually weaken by mid evening as the area of showers/storms pushes gradually south of the area as diffuse front pushes slowly southward. Behind the front, skies will clear some tonight with light winds and still a fairly moist boundary would suggest that there could be some fog again late tonight into early Thursday morning. Best chance of clearing and light winds is over western CWA, so hit the fog the hardest in this area in the grids.

- Izzi

Thursday through Wednesday...

By early tomorrow, the stationary front and associated broad low pressure center will be moving away to the east making room for relative high pressure to slide over the region. The dry airmass will bring mostly sunny skies to tomorrow while afternoon temperatures look to peak in the upper 70s to near 80 degrees. The quiet pattern will be short-lived, however, as another trough axis is progged to swing across the Midwest late tomorrow night through Friday. A surface cold front will drop across the area on Friday bringing shower and thunderstorm chances.

Some pre-frontal activity will be possible in our northwest during the morning hours. Greatest chances will be during the afternoon and evening as the front works across. There is considerable uncertainty in the exact timing of the front and especially in the amount of instability that can build out ahead of it. Both will play a critical role in the potential strength of thunderstorms. The highly unstable solution from the NAM (4000+ J/kg of MLCAPE) seems unlikely as it's notably warmer at the surface and cooler aloft than other camps. But a couple to a few thousand Joules by the afternoon appear to be on the table. Shear is marginal at best, but that sort of instability would be supportive of potential severe weather with damaging winds and marginally severe hail being the primary concerns. Would like to think models should have a much better handle on the pre-frontal environment by tomorrow. The convection should largely be out of our area by the end of the evening.

On Saturday, the upper trough will strongly dig into the Great Lakes region and quickly spin up a center of surface low pressure to our east. It's possible that a few widely scattered showers develop on the backside of the system and fall on parts of the area during the day on Saturday, although the day should be largely dry with highs again forecast in the upper 70s to near 80. High pressure will then meander about the region early next week as seasonably warm conditions are favored to continue.

Doom

AVIATION /00Z TAFS THROUGH 06Z FRIDAY/

Issued at 602 PM CDT Wed Aug 19 2026

Showers and storms are expected south of the Chicago Metro through midnight and thus, no weather hazards in the 00Z TAF.

Northeast winds around 10 knots are expected to prevail through sunset before weakening. With clearing skies and light winds to the northwest, there is a chance for localized BR or MIFG around KRFD tomorrow morning, but dense fog is not expected and confidence was too low for mention in the TAF. Winds will return to around 10 knots out of the northeast tomorrow afternoon before weakening and becoming easterly after tomorrow's sunset.

DK

LOT WATCHES/WARNINGS/ADVISORIES

IL...Flood Watch until 1 AM CDT Thursday for ILZ032-ILZ033-ILZ039.

IN...Flood Watch until 1 AM CDT /2 AM EDT/ Thursday for INZ010- INZ011-INZ019.

LM...None.