336
FXUS63 KLSX 030825
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
325 AM CDT Wed Sep 3 2025

.KEY MESSAGES...

-  A strong cold front will move through the region this
   afternoon/evening with showers and a few thunderstorms likely
   (50-70%).

-  Behind the front, much-below normal temperatures are forecast
   into early next week. Mostly dry weather is also forecast,
   with the only rain chances (30-60%) confined to Friday/Friday
   night mainly across southeast Missouri and southwest Illinois.



&&

.SHORT TERM...  (Through Late Tonight)
Issued at 318 AM CDT Wed Sep 3 2025

A surface low is currently located just north of Lake Superior
with an attendant cold front draped to its southwest. The front
has moved through northwestern Iowa as of 0800 UTC and will
continue to progress to the southeast today. There is pretty good
agreement on the timing of this front, with it reaching parts of
northeast Missouri around 1800 UTC. By 0000 UTC Thursday, the
boundary should be nearly bisecting the CWA from southwest to
northeast. The front should then exit southeastern portions of the
CWA near or just after 0300 UTC. There is uncertainty with how
much convection is actually able to initiate along the front, with
most CAMs showing predominantly post-frontal convection. This
likely has merit with weak surface convergence in our area along
the boundary and the stronger forcing for ascent via low-level
moisture convergence and midlevel vorticity advection behind the
front. Given the expectation that most, if not all, of the
convection will be post-frontal, it also should be elevated in
nature. The amount of CAPE within the hail-growth zone is pretty
low, mainly due to modest midlevel lapse rates (~6 C/km).
Therefore, while some small hail is possible with any stronger
cores, the threat for severe hail looks low. The environment
along the front itself, in case there is any surface-based
convection, also does not look supportive of organized strong to
severe convection. The stronger shear lags behind the front, and
instability continues to look to be fairly modest. Probabilities
for at least 1000 J/kg of SBCAPE peak out only at about 40% on the
latest HREF in northeast Missouri. The more favorable environment
continues to look like eastern Kansas. That is where low-level
convergence is stronger in an environment characterized by SBCAPE
of 1500-2000 J/kg and 30-40+ knots of deep-layer shear.

We still are expecting fairly widespread showers and scattered
thunderstorms late this afternoon through the early overnight hours.
The best chances (50-70%) are tied closer to the 850-hPa cold
front, which progresses southeast through the bi-state area.
While most areas should see at least some rainfall, there is
expected to be quite the variety in rainfall totals. Most
locations will not see significant rainfall, with probabilities
for at least 0.50" of rain in the 10-30% range from the HREF. That
being said, a few spots may see amounts around 1" similar to the
LPMM of the HREF where any elevated convection manages to train
over the same areas.

Temperatures today ahead of the front will warm nicely with plenty
of sunshine and surface winds veering more to the west/southwest.
This is a very favorable direction for downslope flow off of the
Ozark Plateau, and mid to potentially upper 80s are expected across
central and east-central Missouri as well as southwest Illinois as
a result. Slightly cooler temperatures are expected in northeast
Missouri and west-central Illinois where the front will pass
through by mid/late afternoon and mid/high level clouds increase.

Strong low-level cold air advection will allow temperatures to drop
fairly rapidly behind the cold front tonight. Lows ranging from the
upper 40s (northeast Missouri and west-central Illinois) to mid 50s
are expected, or about 10 degrees below normal for the date.


Gosselin

&&

.LONG TERM...  (Thursday through Tuesday)
Issued at 318 AM CDT Wed Sep 3 2025

(Thursday - Sunday Night)

Ensemble guidance is in very good agreement with the overall
mid/upper level pattern across the CONUS. A very deep mid/upper
level low is expected to remain quasi-stationary between Lake
Superior and Hudson Bay. Multiple vortices will rotate cyclonically
around this low, each of which bringing a secondary frontal passage
through the area. This will help reinforce the well-below normal
temperatures through the weekend. Highs in the 70s are expected each
day with lows in the 40s to low 50s for the most part. The
exception is Thursday night due to strong low-level warm air
advection ahead of the initial secondary cold front. Lows are only
expected to drop back into the mid 50s to near 60 degrees. There
remains some uncertainty with respect to high temperatures on
Friday, mainly across southeast Missouri and southwest Illinois.
While differences in frontal timing have decreased slightly from
yesterday, there is a few hours of spread. That is why the
differences between the 25th/75th percentile from the NBM for high
temperatures is still high (5-7 degrees) in those areas.

Mostly dry weather is also forecast through the weekend, though
there remains a chance (30-60%) for light rain mainly across
southeast Missouri and southwest Illinois. This area of light rain
is primarily forced by increasing low/mid level frontogenesis
behind the secondary cold front. The key word is light though,
with probabilities for even 0.10" or more of rain from the LREF
only in the 30-40% range.


(Monday - Tuesday)

Forecast uncertainty begins to increase as we head into next week
with the overall pattern across the CONUS. Most members of the grand
ensemble finally break down the anomalous mid/upper level troughing
across the Great Lakes/southeast Canada, though almost 20-25% keep
it through Tuesday. Remaining members show more zonal flow moving
into the mid-Mississippi Valley with weak troughing moving toward
our region. Rising mid/upper level heights and low-level warm air
advection commencing behind the departing surface ridge suggests at
least some moderation in temperatures. The daytime warmup however
may be muted slightly as troughing to the west suggests at least
increasing cloud cover and maybe even some low (~20%) rain chances.

Gosselin

&&

.AVIATION...  (For the 06z TAFs through 06z Wednesday Night)
Issued at 1042 PM CDT Tue Sep 2 2025

VFR conditions are expected through at least early Wednesday
afternoon. Light and variable winds will become westerly and
slightly breezy tomorrow morning and early afternoon. A cold front
will move through the region between late afternoon and the end of
the TAF period, switching winds to the north and bringing a round
of scattered to broken showers and a few thunderstorms. There is
some uncertainty in the timing of showers, and thus they have
been represented with a PROB30 group, although it is more likely
than not (50 to 70%) that all terminals will see at least some
rain during this period. A few thunderstorms are also possible,
with best chances in central Missouri (COU/JEF), although other
areas may see a few lightning strikes as well. Visibilities within
stronger showers may reduce flight categories, with a lower
chance of ceiling category reductions. However, flight conditions
are expected to remain VFR for a large majority of the period.

BRC

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX