064
FXUS63 KLSX 281145
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
545 AM CST Fri Nov 28 2025

.KEY MESSAGES...

- A Winter Storm Warning and Winter Weather Advisory is in effect
  for portions of the area, including the St. Louis metro area,
  through Saturday evening. The chance for impacts peaks Saturday
  morning.

- There is a medium chance (30-40%) for light snowfall Monday
  night.

- Arctic air will push temperatures below climatological normals
  Sunday through the end of the upcoming workweek.


&&

.SHORT TERM... (Through Saturday Night)
Issued at 353 AM CST Fri Nov 28 2025

The main focus for the short-term period is the winter storm
forecast to begin impacting the region tonight, continuing through
Saturday evening. The shortwave driving this system can be seen in
water vapor imagery this morning digging through the Pacific
Northwest. Through today into tonight, it will eject out in the
Great Plains and deepen. It`s surface reflection will deepen over
Southern High Plains tonight and track toward the Great Lakes
through Saturday.

As the low approaches the Middle Mississippi Valley this evening and
tonight, isentropic ascent thanks to an increasingly strong low-
level jet will yield precipitation spreading generally west to east,
encompassing the entire CWA by around sunrise Saturday morning. At
onset of precipitation, surface temperatures will be around
freezing, and thanks to dry air in place, wetbulbing will keep
temperatures at or just below freezing for much of the morning
across the area. With the atmosphere being below freezing up through
the dendritic growth zone, this supports snow as the initial
precipitation type. Seeing as this initial round of precipitation
will be fed via warm air advection, temperatures will gradually warm
above freezing at the surface from the southwest to the northeast
late morning into the afternoon, causing snow to change over to rain
for much of the area. When this changeover happens, there is a low
chance (20%) for sleet to briefly mix in.

The portion of the area taking the longest to changeover is still
expected to be portions of northeastern Missouri, west-central
Illinois, and south-central Illinois. Here, temperatures hold around
freezing longest, leading to an extended duration of snowfall. This
is where the Winter Storm Watch has been upgraded to a Winter Storm
Warning. Accumulations here have the greatest chance (70-80% per the
NBM) of reaching and exceeding warning criteria (5"+). Within the
newly issued Winter Weather Advisory area, the changeover to rain is
expected to occur sooner (late morning to early afternoon), leading
to relatively lower totals up to around 4". Forecasted ranges
between the 75th-90th percentiles (high end) and 25th-10th
percentiles (low end) still varies drastically across the warning and
advisory areas - as much as about 10 inches. This is due to subtle
variability in snow-rain change over times and snowfall rates
(detailed below), with snowfall being very sensitive to precipitation
rates and dynamic cooling. A very tight gradient in snowfall totals
is expected across north-northeastern portions of the forecast area.

It is important to note that while SLR will be climatologically low
thanks to the rising temperatures and strong low-level winds
fracturing dendrites and crystals, impressive lift and dynamic
forcing will allow for brief bouts of higher snowfall rates, with
the totals forecast to occur potentially over the course of a few
hours. The 00z HREF shows probabilities for 1"+/hr rates topping out
at 75% probability during the early to mid morning across the
advisory and warning area. For areas that experience these
magnitudes of rates, travel conditions will rapidly deteriorate. As
temperatures warm and precipitation changes over to rain and
lightens during the late morning and afternoon, impacts will lessen.
A brief change over to light snow is expected during the late
afternoon and early evening as a cold front swings through the
region. Guidance has been trending quicker with the exit of this
system, with the current end time of the advisory and warning being
generous. If this trend continues, the headlines` expiration will be
able to be bumped up to sometime late Saturday afternoon or evening.

Elmore

&&

.LONG TERM... (Sunday through Thursday)
Issued at 353 AM CST Fri Nov 28 2025

Sunday morning, guidance consensus is that the axis of the upper-
level trough responsible for Saturday`s system will be just east of
the region over the Ohio Valley pivoting toward the Northeast. With
Saturday`s surface low departing northeastward across the Great
Lakes and attendant cold front well southeastward of the CWA, deep
northwesterly flow will be advecting an Arctic air mass into the
region. This will be the coldest air of the season thus far. NBM
probabilities for high temperatures on Sunday topping 32 degrees are
15% or less for much of the CWA, with high confidence (80-90%) in
most locations seeing lows Sunday night in the teens and single
digits forecast for the Highway 36/I-72 corridor and northward.
While a majority of deterministic models and ensemble members show a
brief period of upper-level ridging late Sunday into Monday ahead of
another wave, the post-frontal high will remain stubbornly in place
through Tuesday with the temperature forecast modulating little
through then.

The axis of the aforementioned wave is clustered around the
Southwest Monday morning within guidance, pivoting quickly through
the center of the CONUS through the day. While ensembles favor the
wave`s surface reflection passing through the Southeast, upper-level
divergence thanks to a passing jet streak and forcing provided by
the upper/mid-level trough axis will utilize what little moisture is
available over the region to produce a band of light snowfall over
the Middle Mississippi Valley. The spread in placement of this band
of snowfall varies greatly among guidance, leading to a smattering
of snowfall chances (30-40%) across our entire CWA. In reality, the
chance is much greater, but spread in solutions prevents confidence
from being higher at the moment.

For the remainder of the week, the exactly phasing of the upper-
levels diverges among ensemble clusters and deterministic guidance;
though a majority have quasi-zonal flow aloft over the CWA. This
opens the door to additional disturbances impacting the region with
ensemble means supporting portions of the CWA below freezing through
the end of the workweek. That being said, a majority of global
ensemble members (70-80%) keep us dry through this time period, so
while confidence remains low in specifics of the forecast, there is
a lack of a signal for additional high-impact winter weather events.

Elmore

&&

.AVIATION...  (For the 12z TAFs through 12z Saturday Morning)
Issued at 531 AM CST Fri Nov 28 2025

Ahead of a winter storm later in the period, VFR flight conditions
are expected through the day. This evening and into tonight, the
system will approach from the west, with precipitation expected to
start as snow before transitioning to rain later in the morning.
This changeover is in the KCOU, KJEF, and KSTL TAFs given the
current time range, but will happen later tomorrow morning and
afternoon at KSUS, KCPS, and KUIN. Snowfall rates during the
morning may be more intense than indicated in the current TAFs,
with confidence low to medium in IFR visibilities. Impacts will
linger beyond the current TAF period into Saturday afternoon and
evening.

Elmore

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Winter Storm Warning from midnight tonight to midnight CST
     Saturday night for Knox MO-Lewis MO.

     Winter Weather Advisory from midnight tonight to midnight CST
     Saturday night for Audrain MO-Lincoln MO-Marion MO-Monroe
     MO-Montgomery MO-Pike MO-Ralls MO-Saint Charles MO-Saint
     Louis City MO-Saint Louis MO-Shelby MO-Warren MO.

IL...Winter Storm Warning from midnight tonight to midnight CST
     Saturday night for Adams IL-Brown IL-Greene IL-Macoupin IL-
     Montgomery IL-Pike IL.

     Winter Weather Advisory from midnight tonight to midnight CST
     Saturday night for Bond IL-Calhoun IL-Clinton IL-Fayette IL-
     Jersey IL-Madison IL-Marion IL-Saint Clair IL-Washington IL.

&&

$$

WFO LSX