752
FXUS63 KLOT 090551
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1251 AM CDT Sun Aug 9 2026

.KEY MESSAGES...

- A couple rounds of thunderstorms are expected on Sunday, the
  first round in the morning through early afternoon and the
  second in the afternoon and evening. Exact timing and coverage
  of storms with second round remains uncertain.

- Any storms on Sunday could be severe, especially during
  the afternoon and evening hours, with all severe hazards
  possible including flash flooding.

- Additional periods of storms are expected Monday through mid-
  week with the chance that a few storms could be severe
  especially on Monday.

&&

.DISCUSSION...
Issued at 227 PM CDT Sat Aug 8 2026

A surface high is settling into the northern Mississippi Valley
and western Great Lakes this afternoon in the wake of the this
morning`s cold front. Despite the northwest winds and the hazy
skies, temperatures have still managed to warm into the lower
80s and should be able to warm another 2-3 degrees prior to
sunset. Winds will turn more southeasterly overnight as the
surface high slides overhead with temperatures cooling into the
lower to mid-60s.

While we enjoy a quiet summer night tonight, a shortwave trough
will be ejecting across the northern Plains. This shortwave is
expected to lead to the development of a cluster of
thunderstorms across SD and MN overnight that will gradually
west-southwest with time. There is also a chance for some
scattered showers and storms to develop on the nose of the low-
level jet in IA and MO tonight, but guidance has been
inconsistent with coverage of storms there so confidence on that
occurring is low. Regardless of what develops in IA/MO tonight,
the northern complex is expected to ride the instability
gradient and drift towards northern IL. Initially it looked as
if the better instability would remain south and west of our
area which in turn would keep storms outside of our area through
Sunday morning. However, recent trends now show last night`s
front lifting back north as a warm front overnight into Sunday
morning which looks to bring the better instability axis into
our area prior to the complex arriving. Therefore, confidence is
increasing that a complex of thunderstorms will move across
northern IL and northwest IN during the morning hours on Sunday.
Since the storms are forecast to arrive prior to the atmosphere
fully destabilizing, the expectation is for the storms to move
through in a decaying state. As a result, confidence in storm
coverage with eastward extent is lower but the thinking is at
least some isolated to scattered storms will traverse across
most of the area between 9 AM and 12 PM CDT. Given the decaying
nature of the storms, the severe threat in the morning looks low
at this time but instances of gusty winds and hail could occur
with the strongest storms.

Heading into Sunday afternoon, the aforementioned warm front
should be stalled across northern IL and northwest IN which will
serve as the focus for a second round of thunderstorms Sunday
afternoon and evening as another shortwave pivots through. While
confidence on the second round of storms occurring is fairly
high (70-80% chance of occurrence), confidence is lower in the
exact time storms will redevelop and where the greatest coverage
will be. The reason is because the morning round of storms
could limit how quickly the atmosphere destabilizes and thus
delay the redevelopment of storms until later in the afternoon
or even the evening. Regardless of when and where storms
initialize, the environment in place Sunday afternoon and
evening will be very unstable (MLCAPE values around 3000-5000
J/kg) with 35-45 kts of deep layer shear which will support
thunderstorms organizing and becoming severe. The main severe
threat with the storms will be damaging winds upwards of 60+
mph, but instances of hail and perhaps a tornado or two cannot
be ruled out given the steep mid-level lapse rates and veering
low-level wind field respectively. Due to this threat the Storm
Prediction Center has maintained the slight risk (level 2 of 5)
across our entire forecast area.

In addition to the severe weather threat, there is also a
conditional threat for flash flooding to materialize given the
humid air mass (PWATs around 2 inches) and favorable storm
parameters. The condition, though, is whether or not storms
train over the same locations Sunday afternoon/evening or if
they remain transient enough to limit the heavy rainfall in any
one location. With the aforementioned uncertainties in storm
coverage Sunday afternoon/evening, confidence is too low to
warrant the issuance of a Flood Watch at this time but something
we will be monitoring as the event unfolds. Furthermore, there
is also the potential for storms to keep festering overnight
Sunday night as the low-level jet ramps up in the proximity of
the stalled frontal boundary. If coverage of storms Sunday night
is more expansive, then the threat for flash flooding could
quickly increase especially if coverage of afternoon/evening
storms is higher. That said, there is a chance (20-30%) for a
swath of 2-3+ inch rain amounts to occur somewhere in our
forecast area so remain on guard particularly if you live in an
urban or lower-lying area.

Another period(s) of thunderstorms are forecast for Monday as
another shortwave pivots across the region. Similar to Sunday,
the environment will be warm, humid, and unstable which should
support a fair coverage of thunderstorms especially during the
afternoon and evening hours. However, confidence on coverage and
the exact timing of storms remains low and will depend on how
storms evolve on Sunday and Sunday night. Nevertheless, any
storms that materialize on Monday will also have a chance to
become severe with damaging winds the main threat. Thus, SPC has
maintained another slight risk (level 2 of 5) for severe
weather on Monday. Thunderstorms on Monday will also have the
potential to lead to locally heavy rainfall and flooding given
the humid air mass.

The northwest flow pattern aloft is forecast to remain in place
through much of the upcoming week which will continue to
support periods of showers and thunderstorms through at least
Wednesday night. As mentioned in previous discussions, there is
the chance that as repeated rounds of storms occur for the
effective frontal zone to shift south of northern IL and
northwest IN but confidence on when or if this actually happens
remains low. Therefore, our forecast will continue to advertise
shower and thunderstorm chances through at least the middle of
next week. Outside of the rain, expect seasonable temperatures
to prevail with daily highs in the 80s and overnight lows in the
60s.

Yack

&&

.AVIATION /06Z TAFS THROUGH 12Z MONDAY/...
Issued at 1251 AM CDT Sun Aug 9 2026

Aviation forecast concerns for the 06Z TAFs:

- Patchy fog/low stratus possible through daybreak.

- At least two rounds of TSRA possible, some strong to severe,
  from late this morning through tonight. Confidence in details
  below do decrease beyond this afternoon.

Late evening surface analysis depicts low pressure in the
vicinity of northeast CO/NE panhandle region, with a warm front
extending east to near the IA/MO border and into downstate
IL/IN. This warm front is forecast to lift north across IA and
into northern IL/IN later today, as low-level southerly flow
strengthens in association with an approaching mid-level
disturbance. This will bring a warm, moist and unstable air mass
across the terminals, setting the stage for at least a couple of
rounds of strong thunderstorms from later this morning through
tonight.

Currently, a small cluster of thunderstorms has developed along
the ND/SD border ahead of the aforementioned mid-level wave.
This cluster is expected to track southeast into southern MN by
daybreak, eventually expanding on the nose of the southwesterly
low-level jet into southern WI/northern IL late this morning
into this afternoon (basically scenario 2 from our previous 00Z
discussion). Coverage is anticipated to be high enough to push
an outflow boundary through the terminals this afternoon
(potentially with gusty winds on the leading edge of the
storms), but this should be able to return north (perhaps as a
composite with the synoptic warm frontal boundary) later in the
day/evening. This boundary would then likely serve as the focus
for additional thunderstorm redevelopment as the low-level jet
impinges on the outflow/warm frontal boundary over the area this
evening, especially across the Chicago terminals. These storms
could potentially linger over/near the terminals in some fashion
past midnight before eventually sliding off to the
south/southeast as the cold pool expands and the low-level jet
veers more westerly. It`s important to note that confidence in
the details in this scenario decrease considerably beyond the
afternoon hours.

Provided storms do shift south/southeast of the terminals late
tonight, a period of light/variable winds appears likely along
with some potential for MVFR conditions to linger through
daybreak Monday.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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