503
FXUS63 KLSX 011134
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
534 AM CST Mon Dec 1 2025

.KEY MESSAGES...

- Snowfall is expected (80-100% chance) today across the entire
  area. Where confidence is greatest in moderate accumulations
  (up to 4-5") impacting travel, particularly during the evening
  commute, a Winter Weather Advisory is in effect.

- Seasonably cold temperatures will continue through the weekend,
  with another Arctic blast expected late Wednesday through
  Thursday.


&&

.SHORT TERM... (Through Tuesday Night)
Issued at 328 AM CST Mon Dec 1 2025

Early morning water vapor imagery shows a short wave digging into
the Four Corners region, with flow becoming increasingly
southwesterly aloft over the Middle Mississippi Valley. This flow
will deepen moisture over the region ahead of the wave and upper-
level divergence via a jet streak. As this divergence enters the
CWA, frontogenetical forcing will increase, leading to snowfall
nosing into northeastern Missouri mid to late morning. As these
forcing mechanisms pivot eastward through the day, the band of
associated snowfall will move east-southeastward, exiting the CWA
late this evening.

Across northeastern Missouri and west-central Illinois, guidance
consensus is for QPF to top out around 0.25". Here, deterministic
soundings show lift maximized at times with the dendritic growth
zone (DGZ). This also coincides with the vertical thermal profile
being occasionally isothermal within the DGZ. These factors paired
with weak winds in the low-levels, support SLRs at or just above
climatology, with the latter favored and represented in the forecast
(around 16:1). This SLR paired with the aforementioned QPF will yield
widespread amounts in this part of the CWA around 4", supporting the
continuation of the Winter Weather Advisory. Guidance consensus has
also shifted slightly southward with this system, and the Advisory
has been expanded southward by tier of counties to account for this.
A minority of guidance points to QPF being as high as 0.3" within
the advisory, and if this is realized, localized amounts as high as
6"+ will be realized. Confidence in this occurring is low at this
point.

Further south toward east-central Missouri (including the St. Louis
metro) and the I-70 corridor in IL, lower QPF and SLR thanks to a
slight weakening in the frontogenetical forcing will yield
relatively lower amounts of roughly 1-2". Given the relatively
lighter nature of this snowfall and that it is expected to occur
just after the evening rush hour in metro St. Louis, only minor
impacts to travel are expected. If the band of snowfall progresses
quicker, then impacts to the metro during rush hour will occur. We
will continue monitoring guidance trends and observations for this
possibility.

Similarly light snowfall accumulation is forecast across
southeastern Missouri and southwestern Illinois late this evening.
However, there is potential for a narrow strip of higher totals, as
several guidance sources show QPF and frontogenesis increasing as
the snow band begins to depart the area. If this scenario occurs, a
strip of up to 4" would occur across this portion of the CWA. The
forecast has been nudged in this direction, and we will also monitor
guidance and observation trends through the day for this potential
scenario. There is also a low chance for sleet to mix in with the
snow across southeastern Missouri, but this is not expected to cause
any more of an impact than the snow itself.

The end of the snow this evening will signal the arrival of a
reinforcing shot of cold air, with most locations tonight seeing
lows bottom out in the teens. This air mass will traverse the region
quickly, as a surface ridge turns low-level winds southwesterly
through Tuesday. The result is temperatures over much of the area
being similar to those expected on Monday, and even warmer over
portions of central and southeastern Missouri thanks to the earlier
onset of the southwesterly flow.

Elmore

&&

.LONG TERM... (Wednesday through Sunday)
Issued at 328 AM CST Mon Dec 1 2025

At the start of the extended forecast period on Wednesday, guidance
consensus is that the Middle Mississippi Valley will be beneath
zonal flow aloft. Despite this benign pattern, our weather locally
will still be active. A trough passing well to the north across
Canada and nosing into the northern Great Lakes will send a cold
front through the Midwest late Wednesday, dealing yet another Arctic
blast to the area. The spread within the IQR of global ensemble
guidance is still a little too high (5-10 degrees) to be confident
in specific temperatures Wednesday night through Thursday night. The
current forecast favors the 75th percentile of ensemble guidance,
with lows both nights in the teens south of I-70, single digits
north of I-70 and highs on Thursday in the 20s and teens for most
locations. The fact that this is around the 75th percentile of
ensemble guidance points to that there is plenty of guidance showing
even colder temperatures. While winds remain light enough to keep us
above cold weather headline criteria, this will still be a biting
air mass.

Our attention then turns to a series of shortwaves ejecting out into
the Plains into Friday, deamplifying and shearing out as they do so.
The leading wave will still provide enough forcing to spawn a weak
surface reflection along a baroclinic zone across the Gulf coast. As
a result, precipitation will blossom across the Mid-South and
Southeast, and up through about 24 hours ago, ensembles were
trending more aggressively with this precipitation making its way
into our CWA. This trend among guidance has stalled and reversed
course slightly, leading to low confidence in this happening - NBM
currently gives us about 30% chance of seeing precipitation. If this
scenario plays out, the precipitation would be fed via warm air
advection occurring over a retreating Arctic air mass. This would
support a variety of precipitation types, with rain and snow being
the primary types, though there is a low signal for freezing rain.

Guidance diverges for the weekend on the phasing of the upper
levels, though the primary signal is for troughing across the
eastern 2/3rds of the CONUS. Potential embedded shortwaves would
lead to precipitation chances, but there is no consensus for this
among guidance at the moment. At the very least, the troughing
supports temperatures continuing to run below normal through the
weekend.

Elmore

&&

.AVIATION...  (For the 12z TAFs through 12z Tuesday Morning)
Issued at 525 AM CST Mon Dec 1 2025

Impacts are expected through the period as a winter weather system
slides through the region. As moisture deepens this morning,
ceilings will decrease to lower-end MVFR. Snow will overspread
the region from the northwest slowly to the southeast through the
late morning into the evening. KUIN has the greatest chance of
seeing heavy snowfall, reducing visibilites at times down to a
quarter of a mile. Brief bouts of similar snowfall rates are
possible at the other local terminals, including KSTL, though
confidence in this occurring is low at this time. With the onset
of the relatively heavier snow at each of the terminals, ceilings
are expected to drop below 1kft, with little if any improvement
expected with ceilings through the end of the period.

Elmore

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Winter Weather Advisory from 9 AM this morning to 9 PM CST this
     evening for Audrain MO-Boone MO-Knox MO-Lewis MO-Marion MO-
     Monroe MO-Pike MO-Ralls MO-Shelby MO.

IL...Winter Weather Advisory from 9 AM this morning to 9 PM CST this
     evening for Adams IL-Brown IL-Pike IL.

&&

$$

WFO LSX