260
FXUS63 KLOT 142341
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
641 PM CDT Fri Aug 14 2026

.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.

&&

.DISCUSSION...
Issued at 130 PM CDT Fri Aug 14 2026

     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 /00Z TAFS THROUGH 06Z SUNDAY/...
Issued at 641 PM CDT Fri Aug 14 2026

Some spotty light showers will develop across the region through
this evening. Have continued VCSH and PROB30 mentions for -SHRA.
Any activity that develops should be light and largely VFR.

Late this evening and overnight, a compact but robust
disturbance currently spinning across Iowa will shift into the
region. This will drive a rapid increase in widespread rain and
embedded thunderstorm development. Guidance continues to wobble
with regard to the exact placement of the initiating
disturbance, and a slight southward and later trend has been
noted in recent guidance. Have slightly pushed back the timing
of VCTS at the Chicago-area terminals to account for this. It`s
possible that the main TS potential shifts south of the
terminals, but at this time, we are not confident enough in this
solution to pull TEMPO TSRA groups at this time. RFD will
remain on the northern periphery of the stronger forcing, but a
period of TS remains possible towards daybreak Saturday. Any
thunderstorms (and even general showers) will be capable of
sharply knocking vsbys down to IFR or LIFR given a tropical-like
airmass.

The bulk of precipitation should diminish through the late
morning and towards midday, but showers will likely continue
intermittently into the afternoon as MVFR/IFR cigs slowly lift.
Expecting southwesterly winds to return to the afternoon. VFR
conditions may not return until Saturday evening.

Carlaw

&&

.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.

&&

$$

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