221
FXUS63 KLOT 090112
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
812 PM CDT Fri Aug 8 2025

.KEY MESSAGES...

- Expect uncomfortable heat and humidity again on Saturday with
  afternoon heat indices near or in excess of 100F. Peak heat
  indices on Sunday may approach 100F in portions of northeast
  IL, central IL, and northwest Indiana.

- A more active weather pattern is expected later Saturday
  afternoon through midweek with periodic bouts of showers and
  thunderstorms producing localized torrential rainfall.

- The strongest thunderstorms on Saturday afternoon and early
  evening may be capable of producing isolated to widely
  scattered damaging downburst wind gusts.

&&

.UPDATE...
Issued at 812 PM CDT Fri Aug 8 2025

An early-evening surface analysis reveals a broad surface
trough extending from the central Plains northeastward into
central Minnesota. A broad outflow-reinforced warm front
stretches across Minnesota toward Lake Superior, ahead of a cold
front stretching southwestward from eastern North Dakota and
through central Nebraska. Meanwhile, visible and water vapor
satellite imagery depicts broad upper-level cyclonic flow across
the northern Rockies to northern Plains, with implied ascent
(via arcing cirrus clouds) approaching the Dakotas/Minnesota
border regions. As the synoptic-scale ascent moves over the
frontal boundaries over the next 2-4 hours, one or more clusters
of storms should develop and propagate eastward through the
night.

While the "big picture" items are right where they are supposed
to be this evening, clues on exactly how, where, and at what
coverage convection develops along the aforementioned frontal
boundaries remain elusive. Incoming 00Z RAOB observations from
Omaha, NE and Minneapolis, MN ahead of the frontal boundaries
depict an explosively unstable airmass with mid-level lapse
rates near 9 K/km and warm/moist surface observations affording
5000 to 6000 J/kg of MUCAPE and PWATs near 1.9". With that
said, strong low-level capping remains over the expansive EML.
While clear signs of synoptic-scale ascent are evident along
the western side of the instability reservoir (the cap cooled by
about 7C while rising 75mb between 18 and 00Z at OAX), the last
30 minutes or so of the final 1-min GOES-19 visible satellite
images of the day depict very limited growth of cumulus clouds
along the frontal boundaries.

Given the degree of incoming ascent, would have to think that
at least one cluster of storms will develop overnight, perhaps
with highest confidence in southwestern to central Minnesota
near the intersection of the broad warm frontal zone and cold
front. However, continued mid-level cooling and ascent across
southeastern South Dakota may allow for a separate area of
thunderstorms to develop overnight, which would be poised to
track eastward into Iowa. With the very high MUCAPE/PWAT
envionrment and availability of deep-layer shear along the
outskirts of the lagging cyclonic flow to the west (~50kt at
500mb near the Red River of the North), any storm that develops
would be prone to producing expansive and/merging cold pools
leading to upscale growth into forward-propagating clusters.
Depending on how many, and where, storms develop, forecast
scenarios tonight into tomorrow morning range from isolated
coverage limiting storms to Wisconsin by daybreak to others with
higher coverage leading to a large southward-diving MCS toward
the Mississippi River. We`ll have to simply wait and see what
develops upstream before being able to react and steer the
forecast.

This is all to say, the inherited forecast remains on track.
The only change made this evening was to introduce a 20% chance
for thunderstorms west of I-39 and north of I-80 tomorrow
morning to cover the forecast scenario where a large MCS dives
southward toward the Mississippi River. Otherwise, felt no need
to alter the forecast in favor of letting the midnight shift
having the benefit of watching upstream trends.

Updated products will be sent soon.

Borchardt

&&

.DISCUSSION...
Issued at 306 PM CDT Fri Aug 8 2025

Through Saturday Night:

Hot, humid and breezy conditions persist today under mostly
sunny skies. Heat index values are currently in the mid to upper
90s, with several locations likely reaching around 100 over the
next couple hours. Little to no forcing over the area combined
with dry mid-level air and modest surface-based capping should
maintain precip-free conditions over the area through tonight.

An amplified mid-level pattern with a closed low over southern
Saskatchewan and a strong ridge centered over the western Great
Lakes this evening will begin to break down overnight, with
both features deamplifying while drifting eastward through
Saturday night. This will put our area in a complex setup where
periods of convection to the west struggle to survive with
eastward extent into the ridge, with a notable difference in the
thermodynamic set-up spanning across the CWA.

Initially, a mid-level disturbance over southeast South Dakota
is expected to initiate an MCS across southwest Minnesota this
evening, with the parent wave shifting northeast and trailing
convection along an effective cold front/outflow turning
southeastward into a more favorable thermodynamic environment. A
40-50 knot LLJ over Iowa will support continued convective
development over northern Iowa and into southwest Wisconsin
overnight. As the LLJ veers and the deeper effective shear
decreases farther east, the complex should quickly become
outflow dominate and decay before reaching the northwest CWA
after daybreak. This would favor a dry forecast for the area,
including the northwest CWA, through at least early afternoon.
In this case, expect a hot, humid and breezy day similar to
today.

As Saturday afternoon progresses, weak ascent ahead of the main
trough and daytime heating will erode existing capping across
at least the northwest CWA. Another wave currently crossing
Colorado will track across Iowa late morning through the
afternoon, providing a focus for a new round of convection on
the effective front across central Iowa and west-central
Wisconsin. Modest deep-layer shear west of the Mississippi River
could foster an organized line of convection that tracks into
an increasingly hostile environment with rapidly decreasing
shear and much drier air aloft. Ultimately, an outflow-laden
broken line of convection with locally damaging winds may near
the western CWA, with a potential for stronger convection to
survive into the western CWA early to mid-evening.

A developing LLJ will likely impinge on any existing cold pool
from convection by mid-evening. As can be implied from the chain
of conditional scenarios above, it is very difficult to
pinpoint where this would be. But, with little mid-level flow
supporting weak/turning propagation vectors, PWATs nearing 2",
and a continuous feed of unstable air, there is a notable risk
for substantial rainfall totals in the region Saturday night.
This could include part of our western CWA, so future forecasts
may require the inclusion of a Flood Watch for areas around and
west of the I-39 corridor during this time.

Kluber


Sunday through Friday:

In the wake of whatever plays out convectively speaking Saturday
night into the pre-dawn hours of Sunday, this certainly further
lowers forecast confidence into the day on Sunday. The potential
for decaying or even renewed shower and storm development along
with associated convective debris cloud cover into the morning
will dictate temperature, dewpoint, and subsequent storm trends.
Can`t offer much in the way of specifics other than that highest
forecast PoPs in the morning (40-60%) are focused northwest of
I-55. A greater chance for at least partial clearing into Sunday
afternoon exists across the eastern or southeastern half or so of
the CWA, which would allow temperatures to climb back into the
upper 80s to lower 90s. The temperature climb looks to be more
muted farther west/northwest.

Regarding potential Sunday afternoon into Sunday evening convective
evolution, the main synoptic scale forcing mechanisms for additional
storms currently remain west of the region, but outflow from any
Saturday night/Sunday morning complex could serve as a renewed
focus for additional storm development. Our highest forecast PoPs
(40-50% range) during this timeframe are once again focused
primarily northwest of I-55. Deep layer shear continues to look
unimpressive, but MUCAPE to 2000-2500 J/kg or more may yield a
threat for localized strong downburst wind gusts. Mid-level lapse
rates will decrease notably from Saturday PM, which likely entails
a reduced threat for any severe criteria downbursts.

The story into the middle of the week will continue to revolve
around additional/periodic chances for showers and thunderstorms
as the persistent eastern/southeastern CONUS mid-upper ridging
becomes flattened southward. Amidst a plume of deep atmospheric
moisture streaming overhead and only very slowly progressing east
and southward, this will be a pseudo "ring of fire" type pattern
albeit with lacking deep layer shear to yield more organized
severe threats. No changes were made to the blended offering of
PoPs peaking in the 30-60% range Sunday night through Wednesday.
It certainly won`t be storming the entire time, but waves of
thunderstorms look like a good bet during this timeframe. PWATs
near and just over 2 inches spell a continued threat for localized
drenching downpours and a localized flash flooding threat. Once
currently unknown and unknowable mesoscale details become more
clear, a more focused flooding threat may unfold Sunday night into
portions of northwest and north central Illinois.

There is some signal for a possible brief break in the action
towards the end of next week, especially if sufficiently strong
high pressure over the Great Lakes forces the effective front
far enough south. This would also bring a couple days of modest
Lake Michigan shoreline cooling. Even that possible break (if it
occurs) might end up being short-lived, as deeper moisture along
with heat and humidity may slosh back into the region next weekend
with the return of 590+ DaM 500 mb heights.

Castro/Carlaw

&&

.AVIATION /00Z TAFS THROUGH 06Z SATURDAY/...
Issued at 625 PM CDT Fri Aug 8 2025

Forecast concerns include...

Chance of thunderstorms Saturday afternoon/evening.
Gusty south/southwest winds Saturday.

Southerly winds at or below 10kts will continue tonight with
some gusts into the 15-20kt range possible through sunset.
Directions may turn south/southeast this evening and then back
to the south/southwest overnight. Speeds will increase into the
12-15kt range Saturday morning with gusts into the lower/mid 20
kt range, which will diminish with sunset Saturday evening.

There may be a decaying line of thunderstorms and then showers
moving across northwest IL around/just after daybreak Saturday
morning. Confidence is very low that this will occur but if it
does, these showers may persist all the way to the Chicago
terminals. There is a better chance for thunderstorms across
northwest IL later Saturday afternoon into Saturday evening,
with some uncertainty for timing, which may be centered around
00z. Prob mention seems reasonable for RFD with this forecast.
Models currently show decreasing shear/instability across the
Chicago terminals Saturday evening and any thunderstorms that
do form may struggle to reach the Chicago terminals. While some
thunder mention may be needed with later forecasts, confidence
is too low for any mention with this forecast. cms

&&

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

&&

$$

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