183
FXUS63 KLOT 091455
AFDLOT

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

.KEY MESSAGES...

- Two windows for storms exist today: one from late morning
through mid to late afternoon, and the other from the early
to mid evening through tonight. The exact coverage of storms
with both rounds remains somewhat uncertain.

- Any storms in our area today will have the potential to be
severe. There is also a potential for flash flooding,
particularly with the second round of storms.

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

&&

.UPDATE...
Issued at 954 AM CDT Sun Aug 9 2026

Monitoring the organized cluster of thunderstorms across
Wisconsin this morning. While this complex is likely elevated,
the concern is as it makes east/southeastward progress that
additional destabilization will lead to a concern for surface
based/near surface based storms that would increase the damaging
wind threat. The storms are riding the MUCAPE gradient which
point to the southeast and would support some continued
southeast progression, however the gradient is also expanding
eastward this morning as warm/moist advection begins to ramp up
and we would think that would help storms to shift/redevelop
eastward. While the higher confidence that the
severe potential will be in southeast Wisconsin There will be a
short severe thunderstorm watch to cover this potential in Lake
and McHenry counties where an additional hour or so of heating
could broach the surface based convective temperatures.

As far as the rest of the day is concerned, the battle will be
between the arriving mid level shortwave progged to slide
through the area versus some capping reinforcement from lower
level warm advection and anvil shielding from ongoing
convection. Deeper forcing and colder temperatures aloft are
focused along the WI/IL border and north of I-90 where our
higher confidence for storm coverage will be. Forecast
soundings depict a window in the afternoon with the northward
moving warm front in central IL into southwest IA that any
scattered convection (like we are seeing take place ahead of the
more organized storms) may also be capable of damaging winds.

After that we will have to take a look and see where the surface
warm front will set up. The pattern tonight has some classic
looks of a hybrid frontal/mesohigh Maddox flash flood setup,
with a low amplitude shortwave, upper jet to the north, and a
substantial low level jet progged. PMM/LPMM guidance (from 0z/
6z) still have some hefty rain totals ( 4-7" in the region), so
the flash flood potential this evening is elevated.

KMD

&&

.DISCUSSION...
Issued at 423 AM CDT Sun Aug 9 2026

A prolonged, convectively-active period is expected across the
Midwest through much of this week as a muggy and unstable air
mass stubbornly hovers over the region over the next several
days. That said, confidence in forecast specifics regarding the
thunderstorm potential for any given day during this period is
inherently low given the nature of this summertime air mass, the
involvement of subtle forcing mechanisms, and the influence of
one convective cycle on the next and the subsequent domino
effects that follow.

First thing`s first, though. Clear skies and calm/light winds
have allow for some fog develop in our area early this morning,
mainly focused outside of the Chicago and Rockford metro area.
Thus far, it appears that any denser fog has remained patchy
based on area webcams, but we`ll continue to monitor the
potential need for a targeted Dense Fog Advisory as the morning
goes along.

Anyways, circling back to today`s thunderstorm potential...
There still appear to be two potential windows for thunderstorms
in our area today. The first window will be from around midday
through this afternoon and will be tied to the convection that
is ongoing to our west at this early hour in Iowa and eastern
South Dakota. There was concern earlier that the storms eastern
South Dakota could grow upscale into a mature, well-organized
MCS that would plow through our area later today. While such a
scenario isn`t quite out of the equation just yet, the
extensive, messy nature of the convection ongoing across Iowa
does lend doubt to that scenario being realized. However, some
loosely-organized conglomerate of the ongoing Iowa and South
Dakota storms and/or any new convection that sprouts between or
just north of them will still likely drift towards our area,
with the associated outflow likely to press eastward into
forecast area later today. By the time this outflow arrives here
around the early afternoon, convective inhibition to 2000-3000
J/kg of MLCAPE here will have largely been eroded, and new
updrafts will be able to go up along it uninhibited. However,
questions still remain regarding the strength of the attendant
cold pool and the degree of convergence along this boundary, as
the relative lack of synoptic-scale forcing support this
afternoon will necessitate stronger lift at the surface for
storms to develop and harness the full potential of the
thermodynamic environment in place. The storms that do end up
pulsing high enough this afternoon will be capable of producing
localized damaging winds and possibly a few instances of 1+"
diameter hail.

The second window for storms in our area is expected to come
late this afternoon into tonight as a low-level jet ramps up and
a shortwave trough passes by to our north. Note that it is
possible that there may not be much of a break between these two
potential storm windows. The outflow boundary from the earlier
convection will likely be laid out somewhere in or very near our
forecast area by this evening, and increasing isentropic ascent
atop this boundary will likely generate new convection along it
during this second thunderstorm window as long as the storms
from earlier in the day don`t end up being overly robust and
extensive in coverage. How much instability will be available
for these later storms to work with will depend on how strong
and widespread the earlier storms were, but deep-layer shear
looks to be sufficiently strong to support rotating updrafts
with any mature convection during this second thunderstorm
window. Damaging winds and hail will thus be possible with
these storms, and a threat for tornadoes also couldn`t be ruled
out with any supercells that develop near the outflow boundary
if the boundary layer on the cool side of the outflow boundary
manages to re-destabilize in time before nocturnal
stabilization starts to set in. If the outflow boundary were to
get laid out in a northwest-southeast orientation like some
CAMs have been showing, then that would increase the likelihood
of storms training over the same area. With the muggy air mass
being conducive to torrential rainfall rates, this would raise
concern for the threat for flash flooding tonight.

Regardless of how the storms in our area play out today and
tonight, today`s moist and unstable air mass will not budge much
going into tomorrow, so conditions on Monday will remain
favorable for additional thunderstorms to develop in the region
once again. Confidence in forecast specifics for Monday remains
relatively low owing to there being some dependence on tonight`s
convective footprint, but broadly speaking, most of the latest
CAM guidance keeps our area devoid of thunderstorm activity
during the daytime hours tomorrow due to a combination of
capping and a lack of large-scale forcing mechanisms outside of
a west-east-oriented corridor of fairly weak mid-level warm air
advection/isentropic ascent. This ascent may actually hinder
the potential for thunderstorms during the daytime more than it
will help as it will likely lead to extensive mid- to high-level
cloud cover (and possibly even some high-based showers) that
will filter solar insolation to some degree where it is present
and thus delay destabilization efforts.

While a storms couldn`t be ruled out to develop earlier in the
day, the most likely scenario at this time for tomorrow is that
convective initiation holds off until the late afternoon or
evening, when the low-level jet to our southwest gets going
again and a subtle mid-level disturbance is progged to arrive
from the west. Exactly where storms will initiate is not yet
clear at this time (and it`s not yet a lock that our CWA will
see storms tomorrow), but it`s likely that their point of
origin will be somewhere along the aforementioned boundary.
Wherever storms do develop, strong to locally extreme
instability will be supportive of explosive updrafts, while
largely unidirectional deep-layer shear will be robust enough to
promote some level of thunderstorm organization, including
updraft rotation and/or bowing segments.

Ogorek


Tuesday through Friday:

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 /12Z TAFS THROUGH 18Z MONDAY/...
Issued at 636 AM CDT Sun Aug 9 2026

Aviation forecast concerns for the 12Z TAFs:

- Brief period of MVFR ceilings early this morning.

- A couple of periods of TSRA potential midday/afternoon and
again this evening/overnight. Some strong to severe storms
possible.

- Potential for a period MVFR conditions Monday morning.

Forecast scenario remains complex through this TAF cycle.
Latest surface analysis indicates surface low pressure centered
near the CO/KS/NE border region, with a warm front extending
east through the IA/MO border region into south-central IL/IN.
This warm front is forecast to lift north into the area later
today, in response to a mid-level disturbance tracking east-
southeast from the Dakotas. A broad area of convection is
already ongoing across MO/IA/MN/WI within the warm advection
regime ahead of this disturbance, and should continue to spread
east across the terminals late this morning and afternoon. Some
thunderstorms may become strong to severe with gusty winds as
diurnal warming and the northward moving warm front allow for
destabilization.

While CAM guidance has struggled with the footprint and timing
of ongoing and forecast convection through today, there is a
general signal for a lull in activity across the terminals late
this afternoon into early evening. This would appear to be
somewhat short-lived however, as additional storms appear likely
to develop by mid-late evening in association with a
southwesterly low-level jet impinging on the warm front or
composite front/outflow boundary from the afternoon activity.
It`s important to reiterate that confidence in timing/extent of
the overnight storms remains of lower confidence. Eventually,
any overnight storms should weaken and slide off to the
east/southeast of the terminals prior to daybreak. There may be
a period of MVFR ceilings and light fog Monday morning in the
post-storm light wind/moist low-level environment.

Wind wise, we`ll start of light south this morning, becoming
breezy from the southwest with gusts 15-20 kts ahead of the
first round of storms. Winds may become somewhat variable in
direction for a time this afternoon in the wake of the storms
before turning southerly again tonight. Current trends suggest
winds may become light east/northeast Monday morning.

Ratzer

&&

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

&&

$$

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