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FXUS63 KLSX 120514
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1114 PM CST Thu Dec 11 2025

.KEY MESSAGES...

- Dangerous cold is expected late this weekend. Wind chill values
  at or below -10F are likely (60-90+%) early Sunday morning
  along/north of I-70 in Missouri and I-64 in Illinois.

- Light snow is forecast for parts of west central and south- central
  Illinois through late this evening. Total amounts of 1-2" are
  expected, with some isolated totals near 3" possible within the
  winter weather advisory.

- Another round of light snow is likely (50-90%) along/north of
  I-70 in Missouri and I-64 in Illinois on Saturday. Any snow that
  does fall may cause travel impacts given the very cold air
  temperatures.


&&

.SHORT TERM...  (Through Late Friday Night)
Issued at 238 PM CST Thu Dec 11 2025

(Tonight)

A clipper system will move from northwest to southeast across
central Missouri to near the confluence of the Mississippi-Ohio
Rivers overnight tonight. Strong low/mid level frontogenesis along
with increasing low-level moisture advection is expected to yield a
narrow axis of accumulating snow this afternoon into the evening
hours. The majority of the area will not see any snow, with the
accumulating snow almost exclusively confined to parts of west
central and south-central Illinois. A general 1-2" of snow is
forecast in these areas, with higher snow amounts the further north
and east you travel. The gradient on the southwest flank is
typically very sharp with clippers, and this looks to be another
example. I would not be surprised to see some totals of near 3"
in/just northeast of a Mount Sterling to Ramsey, IL line. In areas
that do see amounts closer to 2" (or higher), at least some minor
travel impacts are expected. Therefore, a winter weather advisory
has been issued for Brown, Montgomery, and Fayette counties in
Illinois until midnight.


(Friday - Friday Night)

Dry weather is forecast behind the departing clipper. Deterministic
guidance also shows quite a bit of low stratus hanging around during
the day, particularly along/east of the Mississippi River. These low
clouds along with moderately strong low-level cold air advection is
expected to yield another seasonably cold day across the region.
High temperatures are forecast to range from near 30 degrees to the
low 40s from north to south. The influences from the incoming arctic
air mass will begin to be felt a bit more Friday night, as lows drop
back into the teens to low 20s. Coldest locations are forecast to be
across parts of northeast Missouri and west-central Illinois with
lows closer to 10 degrees.


Gosselin

&&

.LONG TERM...  (Saturday through Next Thursday)
Issued at 238 PM CST Thu Dec 11 2025

(Saturday - Sunday Night)

Light accumulating snow is the initial concern this weekend, mainly
from about mid morning on Saturday through the late afternoon.
Accumulating snow likely will be focused across northeast Missouri
east/southeast into west central and south-central Illinois. This is
where the low/mid level frontogenesis is collocated with enhanced
upper-level divergence beneath the right-entrance region of a jet
streak across the Upper Midwest. The deep and more persistent ascent
is necessary to overcome the dry low/mid levels of the atmosphere.
These forcing mechanisms shift ESE during the day, but weaken
slightly with time. In addition, only northern sections of the area
will really feel the impacts of a midlevel disturbance moving across
the Upper Midwest early Saturday afternoon. This means that light
snow will struggle to reach the surface Saturday afternoon/evening
along/south of I-70 in Missouri and I-64 in Illinois. Accumulating
snow is more likely (again) the further north/northeast you travel.
Amounts of 1-3" are possible in parts of northeast Missouri and west-
central Illinois, with totals gradually tapering off with southern
extent. One major limiting factor for this event will be the short
duration of accumulating snowfall. Any given location likely will
only see up to about 6 hours of light to moderate snow. A factor
that will help with accumulations though is higher than
climatologically average snow-to-liquid ratios (SLRs). Deterministic
model soundings show an anomalously deep dendritic growth zone (DGZ)
with lift centered within the DGZ for at least a few hours. This
means that dendrites should be the primary ice crystal type. These
dendrites are then expected to fall through a column with light
winds below the cloud base (limits fracturing of snowflakes) to a
very cold ground. These factors all point toward high SLRs. Mean
SLRs for this event are likely to be right around 15:1, but there
may be room to be a bit higher than currently forecast.

The bitter cold will impact everyone Saturday night through Sunday
night. There has not been much in the way of change over the past 24
hours. The incoming arctic air mass continues to look very
impressive for the middle of December, characterized by 850-hPa
temperatures of -12 to -18C (~5th percentile of climatology)
overhead with a 1042+ hPa surface anticyclone (>99th percentile).
There may also be just enough snow cover to impact temperatures in
parts of northeast Missouri over into south-central Illinois as
well, albeit only slightly. Lows Saturday night are forecast to
range from -5 to +10F from north to south. These temperatures
combined with northwest winds staying close to 10 mph are expected
to yield wind chill values below zero areawide. Probabilities for
wind chill values below -10F on the latest LREF range from 60-90+%
along/north of I-70 in Missouri and I-64 in Illinois. The coldest
wind chills of around -20F are expected in far northern sections of
the area where LREF probabilities peak out in the 30-60% range for
sub -20F values. Sunday will be about as cold of a day as you see
around here in mid December, especially with very little/no snow on
the ground for a majority of the CWA. Highs in the 5-20F range are
forecast from northeast to southwest, or some 30 degrees below
normal. These values may threaten the daily record low maximum
temperature at both KUIN (7F-1989) and KSTL (13F-1901). Sunday night
is expected to be the second (and last) bitterly cold night with
lows ranging from -5 to +10F. What is different Sunday night
compared to Monday night is the lighter winds. The surface high is
expected to move across the Mississippi River overnight. Very light
winds are forecast near this feature. By late night, some southerly
return flow will begin in central/northeast Missouri, but this will
help warm up temperatures. We will have to watch wind chill values
Sunday night as well, but they should not be as cold as Saturday
night. Chances for wind chill values below -10F on the LREF are only
in the 20-30% range along/east of the Mississippi River for
reference.


(Monday Night - Next Thursday)

The bitter cold will quickly depart as we head into the middle of
next week. Ensemble guidance remains in very good agreement with the
general pattern across the CONUS. Guidance shows rising mid/upper
level heights across the central US along with increasing low-level
warm air advection. High temperatures are forecast to get near to
slightly above normal by Tuesday, and above normal readings by
Wednesday. Dry weather is also forecast through Wednesday, with the
next chance (20-30%) of precipitation coming Wednesday night into
Thursday in the form of liquid rain.


Gosselin

&&

.AVIATION...  (For the 06z TAFs through 06z Friday Night)
Issued at 1114 PM CST Thu Dec 11 2025

IFR stratus continues to race south from northern Missouri amidst
cyclonic flow aloft, and some flavor of MVFR/IFR conditions will
take hold at all terminals through the morning into the early
afternoon. While there are some indications that CIGs will lift,
this TAF issuance leans pessimistic as there is little evidence to
support drier low-level air advecting in to scatter the low
stratus. There is a reasonable worst-case scenario that the
reduced flight conditions last through the entire day areawide.
The best chances for improvements exist in central Missouri,
closer to the edge of the stratus and more removed from the
cyclonic flow aloft.

MRB

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...Winter Weather Advisory until midnight CST tonight for Brown IL-
     Fayette IL-Macoupin IL-Montgomery IL.

&&

$$

WFO LSX