Area Forecast Discussion

356
FXUS63 KLSX 130917
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
317 AM CST Sat Dec 13 2025

.KEY MESSAGES...

- Snow is very likely (60 to 80%) to cause minor impacts to travel
  in parts of northeast Missouri and central/south-central
  Illinois today. A Winter Weather Advisory remains in effect for
  these areas.

- There is a slight chance (20%) for patchy freezing drizzle today
  which could cause slippery conditions on untreated roads south
  of I-70 in Missouri and along and south of I-64 in Illinois.

- Wind chill values will drop to dangerous levels tonight into
  Sunday morning across much of eastern Missouri and Illinois. A
  Cold Weather Advisory has been issued for these areas.
  Temperatures will remain very cold through Monday morning.
  Temperatures warm rapidly Monday and continue warming through
  midweek.

&&

.SHORT TERM...  (Through Late This Afternoon)
Issued at 309 AM CST Sat Dec 13 2025

Going forecast looks on track for a period of snow across northeast
Missouri into west central and southwest Illinois today.  The
leading edge of an Arctic airmass is nosing into northeast Missouri
this morning.  Short range guidance continues to show strong low to
mid level frontogenesis back in the cold air across Iowa and
Illinois, stretching into northeast Missouri.  The HREF LPMM shows
the heaviest accumulations in a band stretching from Ottumwa IA to
Lincoln IL where the strongest frontogenesis is forecast. However,
it also shows 1-3 inches in far northeast Missouri into west central
and south central Illinois.  HREF Snowfall probabilities continue to
suggest there will be a tight gradient across these areas.  For
example, Quincy IL is showing a 70-80% chance of 1 inch accumulation
or more, but Hannibal MO drops to around 30%.  Increase to 2 inches
or more and both Hannibal and Quincy drop to virtually 0%.  With
that in mind, f-gen bands are tough to pin down and small shifts in
the position of the snow bands can make a big difference in how much
snow ultimately falls.  Regardless, the current Winter Weather
Advisory looks good, so will continue it without changes at this
time.

New to the forecast this morning is a low chance (~20%) for some
mixed precip, generally south of I-70/I-64 in Missouri and Illinois
respectively.  RAP soundings are showing a moist layer near the
surface with weak lift in the layer and no cloud ice above it.
Drizzle is the preferred precip type with this setup.  The RAP and
NAM both show this signal, but the GFS is drier in the low levels
and doesn`t show as much lift.  The NAM is the most bullish with
simulated reflectivity developing very light returns around 15Z and
continuing until 22-23Z, presumably when the cold dry Arctic air
builds far enough south to dry the low levels out.  Other CAMs are
dry except for a brief, fast moving band of very light precip moving
southeast in the afternoon.  The RAP can be good at forecasting
drizzle, and with temperatures at or below freezing there is the
potential for a light glaze of ice on untreated surfaces. Have
therefore added mention of freezing drizzle to the forecast, but
limited to 20% due to lack of model consensus.  This will need to be
monitored closely this morning for a potential Winter Weather
Advisory if the freezing drizzle does indeed develop.  Whether it`s
snow to the north, or potential for drizzle/freezing drizzle to the
south, precip should end by 00Z or shortly thereafter as the
cold/dry Arctic airmass pushes farther south and dries out the low
levels.


Carney

&&

.LONG TERM...  (Tonight through Friday)
Issued at 309 AM CST Sat Dec 13 2025

Cold Weather Advisories also look on track for Saturday night/Sunday
morning.  The 1040+mb Arctic high continues building southeast and
temperatures dip into the single digits across most of the area to 1
to 3 degrees below zero in northeast Missouri and west central
Illinois.  The pressure gradient remains fairly tight due to the
strength of the high, and the wind remains in the 10-14kt range
until almost 18Z Sunday.  It will be bitterly cold no matter where
someone walks outside on Sunday morning with wind chill values
ranging from -5 in southeast Missouri to -20 in west central
Illinois.  With the coldest values of -10 to -20 in eastern Missouri
and Illinois, the Cold Weather Advisories will continue unchanged.
Temperatures Sunday afternoon will struggle into the teens in most
locations despite what looks like a full-sunshine day.

After another cold night Sunday night, temperatures warm up rapidly
on the western side of the Arctic high which will move into the
Southeast U.S. by Monday afternoon.  Highs rise into the upper 30s
to low 40s in most locations Monday, the upper 40s to low and mid
50s on Tuesday, and mid 50s to near 60 on Wednesday in persistent
south to southwest flow.  The next significant chance for precip is
still thursday as a cold front moves through the Mississippi Valley.
 Deterministic GFS and ECMWF are still differing on the amplitude of
the trough driving this cold front through the region.  While
temperatures should cool off behind the front in either case, the
magnitude of the cool down is uncertain.  To illustrate, LREF
temperature IQRs increase from around 8 degrees on Thursday to 12+
degrees behind the front on Friday.  One way or another, the precip
associated with the front looks to be all liquid at this time. Even
the colder solutions end the precip on Thursday/Thursday night
before temperatures can cool enough for more wintry weather.


Carney

&&

.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1022 PM CST Fri Dec 12 2025

Ceilings will remain below 2000 feet AGL through 16-18Z at all of
the terminals except UIN, before climbing above 2000 feet.  At
UIN, snow will likely move into the terminal by 13Z with the best
chance (>90%) for accumulating snow between 16-20Z. IFR
conditions are likely during this time with potential for 1-3" of
snowfall during the day. Snow will move out of UIN by 00Z leaving
ceilings above 2000 feet. VFR conditions will be possible at the
terminals after 06Z. West to northwest winds will slight shift to
the north by the end of the period.

Britt

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Cold Weather Advisory from 9 PM this evening 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.

     Winter Weather Advisory until 4 PM CST this afternoon for Knox
     MO-Lewis MO.

IL...Winter Weather Advisory from 10 AM this morning to 6 PM CST this
     evening for Fayette IL-Greene IL-Macoupin IL-Montgomery IL.

     Cold Weather Advisory from 9 PM this evening 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.

     Winter Weather Advisory until 4 PM CST this afternoon for Adams
     IL-Brown IL-Pike IL.

&&

$$

WFO LSX

NWS LSX Office Area Forecast Discussion