textproduct: Chicago

KEY MESSAGES

- There is a threat for significant flash flooding tonight primarily near and south of Interstate 80.

- There is a trend toward less coverage of showers and storms Saturday night into Sunday morning, but no guarantees yet.

- A brief break in the stormy pattern is expected Monday into Tuesday before another series of storm systems move through the region during the middle of the week.

..THREAT FOR SIGNIFICANT FLASH FLOODING TONIGHT

Early afternoon surface charts depict an elongated stationary front extending from central Virginia along the Ohio River northwestward toward Kansas City, Missouri. A series of mid- level perturbations are riding along and north as the front as evident by festering areas of showers and thunderstorms from Omaha, Nebraska to Indianapolis, Indiana. The western-most perturbation (near Omaha) is the strongest in the region, as demonstrated by broad anticyclonic upper-level venting depicted in mid-level water vapor imagery as well as a clear cyclonic low- level circulation (a "warm-core" low) being sampled by both KOAX/KDMX WSR-88D radars.

This evening, the Omaha warm-core low pressure system will shift eastward and encounter an increasingly unstable airmass (MUCAPE > 3000 J/kg) across southern Iowa. Rapid thunderstorm development and associated regions of latent heat release will be poised to intensify the warm core low west of the Mississippi River by midnight, leading to augmented low-level moisture transport north of the stalled front and into the circulation. Intense moisture convergence will lead to unusually high tropical-like moisture values with mean PWATs forecasted to approach 2.5" (near all-time records per sounding climatology at ILX/DVN) ahead of the low. Taken altogether, a cluster of thunderstorms with intense rain rates (3 to 5"/hr) is expected to develop and move into northern Illinois during the pre-dawn hours of Saturday morning. (This process is not unlike how tropical systems intensify and produce intense rain rates).

While the center of the warm core low is expected to track more or less along the I-88 or I-80 corridors during the morning hours, the west-to-east orientation axis of the instability axis south of the core should promote the highest coverage of thunderstorms near and south of I-80. The thunderstorms are expected to reach the I-39 corridor sometime in the 1 to 3 AM timeframe, and then the I-55 and I-57 corridors sometime in the 2 to 5 AM timeframe, and then northwestern Indiana after 5 AM. The back edge of thunderstorms would then work eastward through the area after daybreak, perhaps not clearing northwestern Indiana until the 10 AM to 1 PM timeframe.

Owing to the repeated rounds of thunderstorms this past week, soils along and south of I-88 are saturated and prone to cause widespread flooding with as little as one half to one inch of rain. Even with the expectation for the warm-core low and associated region of heavy rain to be somewhat progressive tonight (moving quickly rather than staying over the same exact area), the intensity of rain rates should still nevertheless lead to a swath of 3 to 6 inches of rain (locally higher), as supported by 12Z HREF and REFS 24-hour LPMM QPF. Such rainfall over saturated soils would lead to widespread significant flash flooding including widespread road closures and even structure flooding. Accordingly, a rare Level 3 out of 4 flash flood threat has been issued by WPC for areas along and south of I- 80 (primarily along and south of a line from Mendota, Illinois to Fowler, Indiana).

A Flood Watch for flash flooding has been issued across much of northern Illinois in two segments. The first segment is along and south of I-80 and features heightened wording for significant flash flooding. The second segment is along the I-88 corridor for somewhat lower confidence flash flooding should the track of the warm core low and associated region of thunderstorms track further north than currently expected. Both segments will be in effect from 10 PM this evening through 10 AM tomorrow.

Not to be forgotten, cannot rule out locally damaging winds along the leading edge of the thunderstorm cluster tonight. Even if winds do not technically meet severe criteria (58 mph), the saturated soils may allow for trees to fall easier than in typical conditions. At this point, currently see the threat for severe weather as secondary to the threat of flash flooding.

Saturday Afternoon onward:

In the wake of the warm-core low, a few showers and areas of drizzle may linger as late as the afternoon. However, current thinking is that coverage should markedly decrease during the late morning and early afternoon hours. Attention then turns toward the next upper-level wave and an associated cold front due to sweep across the area late Saturday night into Sunday. Am noting a trend toward less instability ahead of the front which may limit the coverage of showers and storms across our area, but it remains to be seen if such a trend will continue. So, will maintain mid-range chance PoPs (30% or so) across the area through Sunday morning.

A surface high pressure system will move into the Great Lakes in the wake of the front and facilitate a much-needed break from this incredibly stormy pattern. Monday and at least the first half of Tuesday look completely dry before a series of storm systems move through the general region by midweek.

Borchardt

AVIATION /18Z TAFS THROUGH 00Z SUNDAY/

Issued at 1236 PM CDT Fri Aug 14 2026

Key Aviation Messages for the current TAF period:

- Spotty showers expected this afternoon

- Increasing chances for showers after 00Z, ahead of the next system

- SHRA/TSRA expected after 06Z, highest coverage of TSRA around and south of ORD. IFR vis possible in strongest downpours.

- Lingering MVFR cigs and VCSH tomorrow morning before improvement midday.

A cluster of showers has developed south of MDW and headed to GYY presently. Much of the region will be dry this afternoon, but spotty showers are possible. Winds will remain out of the east and up to 10 knots. FEW/SCT cigs 1500 to 2000 ft AGL will linger, but trends are for more VFR.

An area of low pressure near Omaha, Nebraska is continuing to track east through the afternoon. A more cohesive area of showers out ahead of the low near KDVN and the Quad Cities is expected to increase shower coverage after 00Z over the region. Winds will drift to the southeast and MVFR cigs are expected with these showers, with a chance for cigs down to IFR levels. As the low moves in after midnight, there is a threat for moderate to heavier rain rates, lower cigs, gustier winds around 25 knots and TSRA through daybreak tomorrow.

The low is expected to move east of Chicago around 15Z tomorrow, taking the thunderstorm chances with it. Lingering showers behind the low are possible into midday with MVFR cigs. Gradual improvement to VFR conditions is expected, though lower confidence on timing. Winds will slowly become southerly and eventually southwesterly after 18Z.

DK

LOT WATCHES/WARNINGS/ADVISORIES

IL...Flood Watch from 10 PM CDT this evening through Saturday morning for ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019- ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ103-ILZ104- ILZ105-ILZ106-ILZ107-ILZ108.

IN...Flood Watch from 10 PM CDT /11 PM EDT/ this evening through Saturday morning for INZ001-INZ002-INZ010-INZ011-INZ019.

LM...None.