Area Forecast Discussion

238
FXUS63 KLSX 121758
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1158 AM CST Fri Dec 12 2025

.KEY MESSAGES...

- Dangerous cold is expected late Saturday night into Sunday
  morning. 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.

- Snow is expected on Saturday, mainly across northeast Missouri
  and west central into south central Illinois. Temperatures will
  be cold enough that any snow that falls will accumulate and
  cause impacts to travel.


&&

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 333 AM CST Fri Dec 12 2025

.KEY MESSAGES...

Low pressure over south central Missouri will move southeast through
Tennessee today.  Northerly flow behind this system along with a
deepening mid-upper level trough over the Upper Midwest/south central
Canada will push cooler air into the region today and tonight.  A
broad band of low clouds is also spreading south behind the low
pressure system, and these clouds along with the cold air advection
will limit the diurnal temperature rise to less than 10 degrees in
most locations today.  Temperatures will struggle to reach 30-32
degrees across northeast Missouri and west central Illinois, an the
mid 30s to near 40 along and south of I-70 this afternoon.

Cold advection continues tonight, setting up the first real weather
concern for this forecast: snow on Saturday. Light snow will
overspread northeast Missouri into west central Illinois very late
tonight into Saturday morning as the leading edge of a very cold
Arctic airmass noses into the Mid Mississippi Valley.  Most short
range guidance indicates the snow will hold off until after 12Z in
northeast Missouri, however a few CAMs do show some precip between
10-12Z, so have ramped up to 20-30 PoPs in that time frame.  Main
threat for accumulating snow begins 12-15Z, spreading into south
central Illinois by 17-19Z.  Deterministic GFS and ECMWF agree that
the primary forcing for this round of snow will be low to mid level
frontogenesis behind the Arctic front and ahead of the sharpening
trough which is moving from the Upper Midwest into the Great Lakes
region through the day Saturday.  Strongest frontogenesis stays
north of our forecast area across Iowa into central Illinois, but
some periods of moderate snow in far northeast Missouri into west
central Illinois are possible.  Liquid QPF is low, raging from only
a few hundredths to about 0.20 inches.  However with the Arctic air
moving in, SLR values rise from about 12:1 Saturday morning up to as
high as 18:1 during the afternoon.  Forecast soundings suggest there
will be solid lift in the DGZ, so another efficient snowfall in
northern and eastern portions of the area looks likely.  Even with
the light QPF, 2-3 inches of light fluffy snow is attainable, and
this agrees well with the HREF 24hr LPMM snow forecast.  This also
looks like another system which will have a sharp gradient from
north to south 2-3 inches and less than an inch as the strongest
frontogenetical forcing fails to coincide with sufficient moisture
farther to the south and southwest.  Snow will end from west to east
Saturday afternoon into the early evening as the very dry Arctic air
moves into the Mid Mississippi Valley.

Carney

&&

.LONG TERM...  (Saturday Night through Thursday)
Issued at 333 AM CST Fri Dec 12 2025

Attention turns to dangerous cold potential late Saturday night into
Sunday morning.  Guidance shows the Arctic airmass moving from the
Central Plains into the Midwest Saturday night into Sunday.  The
latest NBM shows a very tight IQR for both lows Sunday morning and
highs Sunday afternoon of only 2-4 degrees in most locations.  The
latest LREF has similar values for temperatures, so I feel confident
in the temperature forecast on Sunday.  With that in mind, NBM wind
forecasts can have a low bias at times, and the current wind
forecast may not be high enough on Sunday morning with the 1040+ mb
high moving into central Iowa.  At least advisory criteria wind
chill values look very likely (70-90%) before sunrise Sunday morning
and extending until mid to late morning generally along and north of
I-70, particularly in west central and south central Illinois.
Warning criteria is possible if the wind is only a few knots higher
than current forecast.

The Arctic high continues moving southeast into the lower
Mississippi and Tennessee Valleys.  Winds turn back to the south on
Sunday night, and current thinking is that Sunday night lows will
occur around 06Z.  Steady or slowly rising temperatures are expected
thereafter.  Wind chill values may approach advisory criteria in
south central Illinois early Monday morning where temperatures will
remain the coldest through the night, though confidence is not high
given the potential for warming and less wind than currently
forecast.  Southerly flow continues into midweek with temperatures
rising into the 50s to perhaps 60 degrees by Wednesday.  Medium
range guidance shows a potential frontal passage on Thursday with a
chance for precip. Current mild temperatures in the forecast would
preclude any wintry precip, although the GFS and ECMWF do not agree
on the strength of the short wave and cold front moving through, so
confidence in Thursday`s forecast is fairly low at this time.

Carney

&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1155 AM CST Fri Dec 12 2025

The most pressing item of concern during this TAf period is the
persistence of low clouds. MVFR to IFR conditions are currently in
place due to low stratus, and will likely persist to varying
degrees throughout the region. Some improvement is likely at UIN
later this afternoon, but this will be more muted at terminals
farther south where flight conditions may not improve beyond MVFR
levels. Late in the period, light accumulating snow is likely to
begin impacting UIN, with some visibility and ceiling reductions
expected.

BRC

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Cold Weather Advisory from 9 PM Saturday to noon CST Sunday for
     Knox MO-Lewis MO-Lincoln MO-Marion MO-Pike MO-Ralls MO-
     Saint Charles MO-Saint Louis City MO-Saint Louis MO-Shelby
     MO.

IL...Cold Weather Advisory from 9 PM Saturday to noon CST Sunday for
     Adams IL-Bond IL-Brown IL-Calhoun IL-Clinton IL-Fayette IL-
     Greene IL-Jersey IL-Macoupin IL-Madison IL-Marion IL-Monroe
     IL-Montgomery IL-Pike IL-Randolph IL-Saint Clair IL-
     Washington IL.

&&

$$

WFO LSX

NWS LSX Office Area Forecast Discussion