792
FXUS63 KLOT 100021 CCA
AFDLOT

Area Forecast Discussion...CORRECTED
National Weather Service Chicago/Romeoville, IL
721 PM CDT Sun Aug 9 2026

.KEY MESSAGES...

- Renewed thunderstorm development is expected over portions of
the area late this evening and overnight, some of which could
be strong to severe.

- Training of heavy thunderstorms (localized 2"/hr+ rain rates)
over the same areas late this evening and overnight may lead to
flash flooding.

- Additional periods of storms are expected Monday through mid-
week. Some of these storms could be severe, especially late
Monday afternoon or Monday night and lead to additional
flooding.

&&

.UPDATE...
Issued at 700 PM CDT Sun Aug 9 2026

- Flash Flood Watch issued for much of northeast IL and
northwest IN late this evening through early Monday AM.

Concerns are increasing that a corridor of training heavy
thunderstorms (capable of rainfall rates in excess of 2 inches
per hour) will set up somewhere over northeast Illinois into
northwest Indiana late this evening through the overnight hours.

The combination of several factors have led us to issue a Flash
Flood Watch for portions of northeast Illinois and northwest
Indiana, including: high PWATs upwards of 2", the nose of a
strengthening low-level jet oriented into the area atop a stalled
surface boundary, and saturated soils from the first round of
storms over portions of the Chicago metro. This watch is in
effect from 9 PM CDT this evening through 7 AM CDT Monday.

Those that live in especially flood prone areas should monitor
trends closely through this evening and have multiple methods to
receive warning information that will wake you up.

Not to be overshadowed, the severe threat will continue
overnight, with the primary threat being damaging wind gusts.
Large hail will be possible with the strongest storms. We will
also be monitoring a non-zero tornado threat with any storms that
develop along and south of the stalled surface front.

Petr

&&

.DISCUSSION...
Issued at 132 PM CDT Sun Aug 9 2026

We have been able to destabilize ahead of the morning line of
convection as a northward moving warm front has made steady
northward progress. 3000 J/Kg of MLCAPE is mostly uncapped at
this point, and there is a narrow ribbon of enhanced DCAPE
nosing into the Chicago area. The stronger lead shortwave is
largely moving through here over the next few hours, and this
will keep a marginal severe thunderstorm risk in place through
mid afternoon. The main concern would be these transient
downburst signatures in the more mature developed line of
storms along with a few instances of hail. The orientation these
storms are taking will also lead to varying water levels near
the shore in wake of these storms, so the sea will remain
hazardous even after the storms pass this afternoon.

As far as the rest of the day is concerned farther south, the
battle will be between the arriving mid level shortwave sliding
through the area versus some capping reinforcement from lower
level warm advection and anvil shielding from ongoing
convection. Colder temperatures aloft are focused north of
I-80. Therefore, with an uncapped environment, expect some
isolated to widely scattered storms farther south given that
forcing is more muted. The signal then continues for a bit of a
lull in activity as shortwave forcing temporarily eases, and
capping will get reinforced via an uptick in warm air advection

After that we will have to take a look and see where the surface
warm front will set up this evening. The pattern tonight has
some similarity to a Maddox flash flood setup, with a low
amplitude shortwave, upper jet to the north, and a substantial
low level jet progged. How much of mesohigh influence is a bit
more vague in the latest guidance. PMM/LPMM guidance (from 0z/
6z) had some hefty rain totals ( 4-7" in the region). The signal
is a little more muted in latest guidance, at least on the
footprint of the heavy rainfall, but the flash flood potential
this evening is still elevated. The warm front has moved into
the Chicago metro area, and it seems like that is where it will
remain into the evening hours. Forecast soundings do still
suggest, in spite of some disagreements in low level jet
strength and orientation with respect to the warm front, a
period this evening into the early overnight where slow storm
motions would be favored very strong lift (lower level moisture
convergence), even if not quite as long lived as considered
before. The most concerning time frame still looks to be in the
mid evening through early overnight across the Chicago area
before the low level jet veers more to westerly. As this occurs
the area of concern would shift to east Central IL (Kankakee
area) and into NW Indiana. There is still considerable
uncertainty here on coverage, but the signs are there to
continue this message tonight.

The forcing during the day on Monday is not as clear and
confidence on storm evolution on Monday is somewhat low. The
warm front will briefly shift south (or to southern portions of
our area) during the morning hours, however the low level flow
will turn back around to the southwest. Any weak ripples in the
flow aloft though could a trigger a storm or two during the
afternoon. Deeper forcing may not arrive until the evening with
a very warm. moist and unstable air mass in place coupled with
strong deep layer shear. This environment would support all
hazards initially and also eventually bowing/line structures.
Another flash flood threat would be in place, especially with
another uptick in the low level jet Monday evening. We will have
to watch where the warm/stationary front lays out tomorrow, with
high uncertainty on that.

Unfortunately the pattern looks fairly active with a warm/moist
airmass in place and a endless stream of shortwaves moving through
in northwest flow upon a lingering frontal boundary Tuesday and
Wednesday. A more dynamic system will move through eastern Canada
later in the week, and this will help to move the frontal boundary
farther south. This will take the storm track with it, and also lead
to cooler onshore flow in this system`s wake Thursday and Friday.

KMD

&&

.AVIATION /00Z TAFS THROUGH 06Z TUESDAY/...
Issued at 700 PM CDT Sun Aug 9 2026

Key aviation forecast messages:

- Waves of TSRA likely late this evening through tonight capable
of gusty winds and rainfall rates >2"/hr

- Low confidence in SHRA/TSRA trends on Monday

A narrow axis of showers with isolated embedded TS has pushed
out over Lake Michigan with a lull in precipitation expected
for the next 2-3 hours before additional SHRA/TSRA development
occurs. It remains uncertain exactly where this corridor will
set up but the location of a stalled surface boundary near the
I-80 corridor suggests that this may occur very near or
directly over the terminals.

Have maintained TEMPOs for TS from 3-7Z to account for this
potential and extended the duration of the VCTS mention through
10Z for the Chicago metro terminals. Once trends become more
clear prevailing TS will likely be needed for portions of the
overnight hours. Gusty winds may accompany the strongest storms
in addition to torrential rainfall (rates >2"/hr possible).

Looking ahead to the day on Monday, confidence in SHRA/TSRA
trends remain very low and will likely be dependent up on how
things unfold overnight and whether the surface boundary shifts
farther south or remains in place near the terminals. For now
have left the TAFs dry. There could be several hours of SHRA/TS
during this period but identifying when and where this were to
occur remains too low confidence to include with this update.

Winds will be light southeast to southerly this evening (outside
of any gusty thunderstorm winds) becoming light and variable
overnight. Winds then trend toward a prevailing light east to
northeast during the day on Monday.

Petr

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Flood Watch through Monday morning for ILZ005-ILZ006-ILZ011-
ILZ012-ILZ013-ILZ020-ILZ021-ILZ023-ILZ103-ILZ104-ILZ105-
ILZ106-ILZ107-ILZ108.

IN...Flood Watch through Monday morning for INZ001-INZ002-INZ010-
INZ011.

LM...None.

&&

$$

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