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FXUS63 KLSX 282317
AFDLSX

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

.KEY MESSAGES...

- A Winter Storm Warning and Winter Weather Advisory remain in effect
  for late tonight through Saturday evening. The highest chance
  for winter weather impacts remains during the morning on
  Saturday.

- The chance for light snow has increased to 30-50 percent for
  Monday and Monday night.

- Below normal temperatures are expected to continue through next week.


&&

.SHORT TERM...  (Through Late Saturday Night)
Issued at 257 PM CST Fri Nov 28 2025

KEY MESSAGES

The primary forecast concern this afternoon continues to be the
storm system currently developing over the western Great Plains
which will begin impacting our area tonight. The low level jet
will strengthen overhead tonight as the low deepens and moves east
through the Plains into Missouri by early Saturday afternoon. All
short range guidance shows the low level jet producing moderate
to strong warm advection and moisture convergence tonight into
Saturday morning which will be the primary forcing mechanism to
produce precipitation. The atmosphere isn`t particularly cold
aloft, however, precipitation falling through low level dry air
will cause evaporational cooling which should keep temperatures
cold enough for snow or a rain/snow mix for most locations.
However, parts of central and southeast Missouri, mainly south of
the Missouri river, are expected to stay warm enough to keep the
precipitation all or mostly rain. Most short range guidance shows
the lower atmosphere becoming saturated enough for precipitation
begin reaching the ground over central and northeast Missouri
between 04-06Z. The snow then overspreads the remainder of the
area from from northwest to southeast overnight and should be
reaching the ground everywhere by 12Z Saturday. Insolation and
continued warm advection will then steadily move the rain/snow
line to the northeast through the day and current indications are
that the entire forecast area will be rain before 00Z Sunday.

Total snowfall amounts remain highly dependent upon how quickly the
snow transitions to rain.  Our deterministic snowfall forecast
ranges from less than an inch in the Eastern Ozarks to 2-3 in the
St. Louis Metro Area to 5-8 in parts of south central and west
central Illinois as well as northeast Missouri where the snow is
expected to transition slowest.  Those amounts are generally on the
higher end of the probabilistic scale, above the 75th percentile but
below the 90th.  The higher accumulations in northeast Missouri and
west central Illinois are the exception though, with our 6+ inch
deterministic forecast squarely within the IQR at Hannibal and
Quincy. However, the IQRs up there are also the widest at 5 to 7
inches. This is a prime indicator of the uncertainty in when the
snow/rain transition will occur.  Regardless...the Winter Storm
Warning and Winter Weather Advisory are where they need to be to
highlight the most probable impacts from this storm.  Once the snow
turns to rain it should melt any accumulations on roads fairly
quickly. The exception may be in the higher snowfall areas across
northeast Missouri and west central Illinois where temperatures
fall below freezing more quickly behind the cold front on Saturday
night. Snow and ice could linger on untreated roads in those
areas.

Carney

&&

.LONG TERM...  (Sunday through Next Friday)
Issued at 257 PM CST Fri Nov 28 2025

"Cold" will be the operative word for Sunday through next Friday as
a broad long wave trough dominates the pattern.  Another Arctic
airmass drops into the Mississippi Valley on Sunday behind
Saturday`s storm system.  Highs Sunday afternoon will struggle into
the mid 20s in northeast Missouri, and will likely not get above
freezing in the rest of the forecast area except in the far south
over the Eastern Ozarks.  This cold airmass remains in place Monday
as the next upstream short wave digs into the Great Plains and
brings the area another chance for precip. Forecast soundings
show that this round of precipitation will be all snow, and it
should be a much drier snow than what we see tonight and Saturday.
Snow may begin as early as mid-morning on Monday across parts of
northern Missouri, spreading slowly southeast through the day into
Monday evening. Mid-level frontogenesis appears to be the primary
forcing mechanism for snow with this system. While the intensity
of the snow should generally be lighter than tonight/Saturday`s,
temperatures will be much colder which will should allow every
flake of snow to accumulate. Additionally, frontogenetical forcing
typically leads to banded precip where snowfall rates could
exceed 1 inch per hour. Forecast soundings are also showing a
period of at least a few hours Monday night where there is strong
lift in the dendritic growth zone which would produce a very
efficient snowfall. All of this means that Monday/Monday night`s
snow could be the more impactful one particularly headed into the
Tuesday morning commute.

Another surface high drops into the Mississippi Valley on Tuesday.
This one doesn`t look as cold as the previous airmass.  While
temperatures will struggle to reach the 25-32 degree range Tuesday
afternoon, by Wednesday highs reach the upper 30s to low 40s in
return flow on the western side of the ridge.  The GFS and ECMWF
show another short wave moving through the Midwest
Wednesday/Wednesday night which drags another cold front through the
area.  The airmass ahead of it looks pretty dry, and the system will
have difficulty tapping any Gulf moisture due to the blocking ridge
axis across the deep south.  Around 90 percent of the LREF members
remain dry with this system so keeping mentionable PoPs out of the
forecast is reasonable at this time.  Models are having some trouble
resolving how far south the next cold airmass moves into the
Mississippi Valley for Thursday and Friday.  This will have a large
impact on temperatures both days.  Temperature IQRs in both the LREF
and NBM rise to 8+ degrees by Friday reflecting this uncertainty in
the forecast.

Carney

&&

.AVIATION...  (For the 00z TAFs through 00z Saturday Evening)
Issued at 509 PM CST Fri Nov 28 2025

No major changes to the forecast or to the TAFs this package. VFR
conditions will continue until precip arrives late this evening.
As it overtakes the area, ceilings and visibilities will lower to
MVFR and then to IFR and LIFR through 00z Sunday. A mix of rain
and snow is expected with this system. Most locations apart from
central Missouri will see snow from its start time until at least
dawn, with the rain/snow line moving northeast during the day and
all rain expected by late tomorrow afternoon. Central Missouri,
however, is forecast to start as a rain/snow mix before
transitioning to rain by dawn.

Jaja

&&

.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