385
FXUS63 KLSX 102017
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
217 PM CST Wed Dec 10 2025

.KEY MESSAGES...

- Dangerous cold is expected late this weekend. Wind chill values below
  -10F are likely (60-80+%) along/north of I-70 late Saturday
  night/Sunday morning.

- Mostly dry weather is expected through the period, though we
  will be watching for the possibility of light snow clipping
  northeast Missouri and western Illinois Thursday afternoon/night
  (30-60% chance) and Saturday (20-40%) over a slightly larger
  area. The best chances of accumulating snow with roadway impacts
  are to our north and east however with each event.


&&

.SHORT TERM...  (Through Late Thursday Night)
Issued at 216 PM CST Wed Dec 10 2025

Strong and gusty northwest winds will continue to subside this
afternoon/evening as the pressure gradient weakens. The surface
ridge of high pressure will gradually move across the area from
west to east late tonight into Thursday morning. While dry weather
should prevail, it does look like a "dirty" ridge with plenty of
cloud cover expected. This will help keep temperatures in check
and from dropping too far. Lows in the low to mid 20s are
expected, or about 5 degrees below normal for the date.

The focus for Thursday afternoon/night will be on the track of yet
another clipper/hybrid clipper. Trends over the past 24 hours have
shifted this further and further southwest. Like traditional
clippers, the axis of accumulating snow should be just northeast of
the track of the surface low. The consensus track of the low is now
from far southeast Nebraska through east-central Missouri into
southern Illinois. This would put far northeast Missouri into west
central and south-central Illinois in play for accumulating snow.
Even these areas however will be on the southwest gradient, subject
of course to further shifts in the track. Admittedly, this is not a
high confidence forecast. While model trends have been to the
southwest, the concern is that there may have been an over-
correction. Climatology strongly argues for accumulating snow to
stay to our northeast. The air mass ahead of this clipper is also
very marginal (i.e., not bitterly cold), which argues for a track a
bit further to the northeast. Finally, in a similar setup last
Saturday, guidance trended quite a bit to the north/northeast within
24-36 hours. That is a long way of saying that there may be
subsequent shifts to come, with a shift to the northeast more likely
than further southwest.

In terms of snowfall, an axis of 2-4" is possible, though this axis
should stay to our north and east. The gradient on the southwest
flank should be very tight, but 1-2" may occur in parts of west
central and south-central Illinois (roughly from KUIN>>K3LF). This
is where the LREF has 30-50% probabilities for at least 1" of snow.
Like most clippers, this axis of accumulating snowfall will be
driven in part by low/mid level frontogenesis, as well as low-level
warm/moist advection. There may be a brief period of snow mixing
with or changing over to sleet/freezing rain before ending as rain,
but this should be inconsequential/non impactful.


Gosselin

&&

.LONG TERM...  (Friday through Next Wednesday)
Issued at 216 PM CST Wed Dec 10 2025

(Friday - Friday Night)

A period of dry weather with seasonably cold temperatures is
forecast to end the work week behind the departing clipper. Highs on
Friday are forecast to be in the 30s/40s with lows Friday night
in the 10s/20s.


(Saturday - Sunday Night)

There are two concerns this weekend: bitter cold centered on
Saturday night/Sunday and the potential for light snow on Saturday.
First, let`s start with the cold. Confidence continues to increase
on the strength of this incoming air mass. Ensemble guidance has
trended colder and colder over the past 48 hours, particularly with
that Saturday night/Sunday time frame. This air mass is coming from
the arctic circle southward over a wide expanse of snow pack.
Therefore, there should not be much modification of this incoming
arctic air mass. The strength of the incoming air mass is very
impressive, especially this early in the winter season. Temperatures
at 850 hPa on the GEFS/EPS are in the -12 to -18C range, or near the
5th percentile of climatology. The surface high (1042+ hPa) on the
NAEFS is also near the 99th percentile as it slides into the
Mississippi Valley. Given the trends and the aforementioned strength
of this air mass, high and low temperatures have been lowered on
average by about 2-4 degrees. Lows in the -5 to 10 degree range are
expected Saturday night with highs in the 10s and low 20s on Sunday.
These values are closer to the 50th percentile of the NBM, but
largely still above. Therefore, I would not be surprised to see
these come down another degree or two over the next 24-48 hours.
Regardless, temperatures near or below zero combined with northwest
winds around 10 mph late Saturday night and early Sunday morning are
expected to produce dangerously cold wind chills. The latest LREF
now shows a 50-80+% chance of -10F or colder wind chill values
during this time period along/north of the I-70 corridor. The
coldest locations should be across parts of northeast Missouri and
west-central Illinois where minimum wind chills below -20F are
possible (20-40%).

Before the heart of the bitter cold arrives, we also are watching
the potential for some light snow accumulations. Chances of
accumulating snow in our area, mostly in northeast Missouri
east/southeast into south-central Illinois have also increased over
the past 24-48 hours. Given the trends on the incoming air mass,
this likely has some merit. Unlike the snow events over the past
week or so, this light snow looks to be not from a traditional
clipper but due to low/mid level frontogenesis behind the surge of
arctic air. Some weak upper-level jet dynamics along with lift
downstream of a midlevel disturbance also are aiding in this area of
snowfall, but it is more mesoscale focused than the clippers.
Therefore, we could see this area of light accumulating snowfall
shift in time to cover a larger portion of the forecast area. The
forcing for ascent should be strong and persistent enough to
overcome the dry low-level air in place ahead of it, but this will
also be something for us to monitor. In terms of potential snow, 1-
2" is possible across northeastern sections of the area at this
time. That is where the LREF has probabilities for at least 1" of
snow (assuming high ratios) of 30-60%. Speaking of ratios, this snow
likely will be drier and quite efficient with lift expected to be
centered within an anomalously deep dendritic growth zone.


(Monday Night - Next Wednesday)

Not much has changed in the expected pattern for early/mid next
week. We are still looking at a fairly rapid warmup as mid/upper
level ridging begins to head eastward and strong low-level warm air
advection commences in the wake of the departing arctic surface
high. Dry weather along with temperatures getting back closer to
normal is forecast as we head toward midweek next week.


Gosselin

&&

.AVIATION...  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1107 AM CST Wed Dec 10 2025

Northwest winds/gusts will gradually slacken off over the next
several hours, with gusts diminishing entirely by early this
evening. Winds will go light/variable tonight as a surface ridge
of high pressure moves across the region.

MVFR stratus will continue to impact KUIN through the
afternoon/evening, with KSTL also likely seeing bases staying
below 3 kft AGL. KSUS/KCPS will be close to 3 kft AGL, but should
stay just above.

Gosselin

&&

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

$$

WFO LSX