Area Forecast Discussion

816
FXUS63 KLSX 271126
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
526 AM CST Thu Nov 27 2025

.KEY MESSAGES...

- Seasonably cold and dry conditions will continue through Friday.

- Our first winter storm of the season will impact the region
  Friday night through Saturday. At least minor snow accumulations
  are forecast areawide, but generally increase from southwest to
  northeast.

- A winter storm watch has been issued for parts of northeast and east-
  central Missouri as well as west central and south-central
  Illinois where there is a 50-80% chance of 5+ inches of snow.


&&

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 326 AM CST Thu Nov 27 2025

A surface ridge of high pressure will gradually move eastward
over the next 36 hours from the central Plains across the
Mississippi Valley. Seasonably cold and dry conditions are
expected through Friday near this ridge along with slackening
winds. Highs today are expected to be a few degrees colder than
yesterday, mainly in the upper 30s to low 40s. Increasing mid/high
level clouds on Friday will help keep temperatures another couple
of degrees colder with highs topping out in the mid 30s to low
40s from north to south. A clear sky, light winds, and low
dewpoints (mid to upper teens; ~10th percentile of climatology)
all point to the coldest night so far this winter season. Lows
are forecast to range from the mid teens to low 20s.


Gosselin

&&

.LONG TERM...  (Friday Night through Wednesday)
Issued at 326 AM CST Thu Nov 27 2025

(Friday Night - Saturday Night)

Latest GOES-19 water vapor imagery shows our system of interest
about 150-200 miles offshore the Pacific northwest coast. As has
been said the past couple of days, it still appears that it will
come ashore late this evening, which means there will be a partial
sample of the disturbance and a full sample Friday morning from the
North American upper air network.

Overall, this midlevel shortwave trough has trended a bit
weaker/further south as it ejects out of the Rockies and quickly
transverses the Mississippi Valley on Saturday. This has some
ramifications, namely that there is a weaker and further south
surface reflection as well. A bit more of a delayed warming is the
result for surface temperatures, which leads to a longer duration of
snowfall. Very deep dry air in the lowest 10 kft AGL is still
expected, with max dewpoint depressions around 25C. This very dry
air is critical as sublimation cooling allows for snow to be the
primary precipitation type areawide after the onset of
precipitation.

High precipitation rates/dynamic cooling is also expected
to play a large factor. Probabilities on the LREF for at least 0.25"
of precipitation in a 6-hour period are now in the 50-80% range.
These probabilities are about 20% higher than yesterday.
Deterministic guidance has corridors of 0.50"+/6 hours, and given
the very strong warm/moist advection (850-hPa low level jet
speeds>50 knots), there seems to be quite a bit of merit to the
high precipitation rates. As was stated yesterday, it is very
difficult for moderate to heavy snow to changeover to rain. Any
lulls in the heavier rates could however allow for the
changeover, even if temporary. Locations furthest west and south
are still expected to changeover to rain first as even the
colder GFS solution begins to introduce some rain by late
morning/midday. A bit further north and east, from parts of
central and east-central Missouri into southwest Illinois, at
least a 3-4 hour period of snow is expected. However, there is
still a lot of spread with how long it will stay snow. The GFS is
coldest longer, and keeps this area snow through the morning hours
and even into early afternoon. That is plausible IF precipitation
rates are high enough. Meanwhile, the ECMWF surges low-level warm
air faster and further north and east. Not coincidentally, it
also has lower precipitation rates. These strong warm air
advection-driven snow cases often ride the fine line between a
cold rain and moderate-heavy snow and this looks no different.
Where it does stay snow longest though should see some heavy
totals as rates of 0.5-1.5"/hr are expected. In some ways, this
event may be somewhat reminiscent of 1/11/2019 where areas saw as
much as 6" in just a 6 hour period. Those type of hourly rates
for several hours of course will pile up the snow quickly and make
any travel very hazardous. A winter storm watch has been issued
with this forecast package for parts of east-central Missouri
(including metro St. Louis) and all of northeast Missouri over
into Illinois along/north of I-70. This is where probabilities
for at least 5" of snow are in the 50-80% range from the LREF. The
highest confidence in warning criteria snow is in west-central
Illinois where the precipitation may stay all snow for the entire
duration. In this part of the CWA, there may be some isolated
totals of 8+".

In terms of snowfall efficiency, this certainly will be a wet snow.
Average SLR values of 6-8:1 are expected given surface temperatures
near freezing. In addition, soundings generally show the max lift
below the DGZ. This isn`t surprising given the strongest ascent is
centered right around 850-hPa. This means that the predominant
snowflake type is likely to be columns, not dendrites. Other
negative factors include strong winds (fracturing snowflakes),
heavy riming, and compaction/settling of the snowpack itself.

Overall, there remains several uncertain factors that could lead to
more or less snow than currently forecast. The battle between
dynamic cooling and strong low-level warm air advection is definitely
a challenge. There typically is a very tight southwest to northeast
gradient in snowfall totals with these types of setups, and that
should lay out somewhere right through our CWA unfortunately with
this event.


(Sunday - Monday)

Arctic air will infiltrate the bi-state area behind this system,
with well-below normal temperatures and dry weather anticipated.
High temperatures each day are only expected to be in the 20s to
low 30s with lows Sunday night in the upper single digits to upper
teens. These temperatures very well may not be cold enough across
northeast Missouri and west-central Illinois which are likely to
have a snowpack.


(Monday Night - Tuesday)

There is high confidence that the well-below normal temperatures
associated with the arctic air mass will continue into early next
week. What is more uncertain is if there will be any light snow
that will impact the region. Models differ still with the track
and strength of a midlevel shortwave trough moving across the
south-central Plains on Monday. The pattern still looks favorable
for a lot of mid/upper level confluence across the Upper
Mississippi Valley/Great Lakes. This means that the system should
weaken/get sheared out as it heads northeast. However, this
shortwave is not the only mechanism that could produce light
accumulating snow. Deterministic guidance suggests some moderately
strong low/mid level frontogenesis Monday afternoon/night as
well. That is why probabilities for measurable snowfall are in
the 50-60% range ending early Tuesday morning. Chances for at
least 0.10" of liquid equivalent are more modest, between 20-35%.
Given the cold air mass in place, SLRs though should be at least
slightly above normal and any snow that does fall will stick on
any untreated surfaces so this period also bears watching.


(Tuesday Night - Wednesday)

Ensemble guidance is in good agreement that mid/upper level
shortwave ridging will move across the area with increasing low-
level warm air advection. This should lead to at least a temporary
warmup along with dry weather. The moderation though is relative as
highs are expected to still be in the mid 30s to low 40s from north
to south. Those readings would still be almost 10 degrees below
normal for early December.


Gosselin

&&

.AVIATION...  (For the 12z TAFs through 12z Friday Morning)
Issued at 522 AM CST Thu Nov 27 2025

An area of SCT-BKN stratus with bases just below 3 kft AGL will
continue to impact KUIN over the next 2-3 hours. Some of this
stratus may clip the metro St. Louis terminals, but it may lift
slightly by the time it gets down here. By late morning, this
stratus is expected to dissipate. Dry/VFR conditions with
northwest winds around 10 knots are forecast today. Winds will
slacken off this evening as a surface ridge of high pressure
begins to approach from the west.

Gosselin

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Winter Storm Watch from late Friday night through Saturday
     evening for Knox MO-Lewis MO-Lincoln MO-Marion MO-Monroe MO-
     Pike MO-Ralls MO-Saint Charles MO-Saint Louis City MO-Saint
     Louis MO-Shelby MO.

IL...Winter Storm Watch from late Friday night through Saturday
     evening for Adams IL-Bond IL-Brown IL-Calhoun IL-Fayette IL-
     Greene IL-Jersey IL-Macoupin IL-Madison IL-Montgomery IL-
     Pike IL.

&&

$$

WFO LSX

NWS LSX Office Area Forecast Discussion