Area Forecast Discussion

024
FXUS63 KLSX 150912
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
312 AM CST Mon Dec 15 2025

.KEY MESSAGES...

- After a cold start to the day, southwest winds and full sunshine
  will rapidly push temperatures to near seasonal normals in the
  upper 30s to mid 40s in most locations. Above normal temperatures
  are expected Tuesday through Thursday

- Showers and possibly an isolated thunderstorm are likely
  (40-80%) ahead of a cold front on Thursday, especially in
  eastern Missouri and southern IL.

- Temperatures cool down to near normal on Friday behind the cold
  front.


&&

.SHORT TERM...  (Through Late Tuesday Afternoon)
Issued at 301 AM CST Mon Dec 15 2025

The strong Arctic high will continue to move east southeast today,
and is forecast to be centered near the South Carolina coast by 00Z
this evening.  Southwest flow on the western side of the high
combined with full sunshine will produce rapidly warming
temperatures today.  In fact, temperatures have already risen a few
degrees in many locations since bottoming out during the evening.
Guidance has been consistently showing a warm up into 40s generally
along and west of the Mississippi today.  Temperatures will be lower
across parts of west central and south central Illinois where the
snow pack will hamper the warming, but should still reach 32-35
degrees.  Moisture return will be slower than the warm up though, so
low relative humidity is expected, particularly across the Ozarks.
Elevated fire danger is still a possibility, though winds will be
marginal where relative humidity is lowest.  Additionally, fuels may
not warm rapidly enough for an elevated fire danger threat to be
realized.  Will continue to mention this in fire weather products
but not message beyond that.

Warm advection in southwest flow continues tonight and Tuesday.  The
wind does get light enough tonight that a mostly clear sky can
produce some modest radiational cooling.  Traditional MOS guidance
drops temps back below freezing again tonight, and this jives well
with the NBM and LREF temperature forecasts.  I wouldn`t be
surprised if some areas remain above freezing though given
persistent southwest flow.  Strong insolation again on Tuesday and
the southwest flow will produce another 25+ degree rise in
temperatures into the upper 40s to mid 50s.  A little more moisture
finally returns to the region Tuesday which should mitigate any
further threat for elevated fire danger.


Carney

&&

.LONG TERM...  (Tuesday Night through Sunday)
Issued at 301 AM CST Mon Dec 15 2025

Models have been showing a weak short wave trough moving through the
Midwest Tuesday night into Wednesday for the past few runs.  The
surface cold front associated with this trough digs through Iowa and
the latest runs push the wind shift all the way to the I-70
corridor.  However, winds swing almost immediately back to the south
driving the front back and temperatures warm well into the 50s along
and south of I-70 with mid to upper 40s farther north.  Another much
stronger trough moves into the eastern Plains Wednesday night which
will produce our next significant chance for precipitation.
Temperatures Wednesday night only dip into the low to mid 40s ahead
of the front, and temperatures aloft are supportive of only liquid
precip.  There`s still some instability on Thursday, however latest
guidance has shifted the steeper mid-level lapse rates farther to
the southeast which would support removing thunder.  The NBM kept a
slight chance (~20%) for thunderstorms in the morning ahead of the
cold front, and given the presence of weak instability have decided
to let it ride. Models continue to show the precipitation ending
Wednesday afternoon before temperatures are supportive of wintry
precip.  Even if the cold air is faster and there are a few flurries
during the late afternoon it will be very light and ground
temperatures will be warm from the previous few days.

Temperatures cool back down into the mid 30s to low 40s in most
locations on Friday behind the front.  However, southerly flow is
quickly reestablished Saturday ahead of another wave.  The
deterministic NBM guidance pushes highs Saturday back up into the
50s, however confidence in these temperatures is low at this time.
The IQR temperature range is around 10 degrees on Saturday, most
likely due to differences in the position of the cold front
associated with the aforementioned short wave.  The deterministic
GFS and ECMWF both show that front drifting south through Missouri
and Illinois on Saturday night to put our area well on the cold side
on Sunday.  It does look likely that Sunday will be around 10
degrees colder as both the LREF and NBM show this drop in
temperatures. However the high temperature IQR on Sunday remains
about 10 degrees. Interestingly, the 90th percentile temperatures on
Sunday are about as high, or in some cases even higher than the 90th
percentile on Saturday which hints that the front doesn`t make it
through the area at all in some solutions. Needless to say this
doesn`t build much confidence in the temperature forecast for this
weekend.


Carney

&&

.AVIATION...  (For the 06z TAFs through 06z Monday Night)
Issued at 1034 PM CST Sun Dec 14 2025

Dry and VFR conditions are expected through the period with
surface winds increasing after 12Z with gusts between 20-25 knots
during the morning and afternoon. A 45-50 knot low level jet on
Monday morning will set up LLWS conditions between 12-17Z at
UIN/COU/JEF.

Britt

&&

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

$$

WFO LSX

NWS LSX Office Area Forecast Discussion