Area Forecast Discussion

863
FXUS63 KLSX 212304
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
504 PM CST Sun Dec 21 2025

.KEY MESSAGES...

-  Warming temperatures are expected this week, peaking with near
   record highs on Christmas Day.


&&

.SHORT TERM...  (Through Late Monday Night)
Issued at 240 PM CST Sun Dec 21 2025

Cool surface high pressure is centered over Illinois this afternoon,
headed east toward the Appalachians tonight. This has left us under
a clear sky with only some passing high level cirrus in a west
northwest flow aloft. As the high continues to shift east tonight
into Monday, we`ll see a warming and moistening southerly flow
develop in the low levels just as ridging aloft spills east out of
the Rockies. Highs tomorrow warm some 10 degrees or more over
today`s values, generally in the 50s. Increasing moisture on the
southerly flow will also tend to bring more clouds especially late
Monday into Monday night. The greater moist advection, and light
rain chances, will be more centered over the Ohio Valley where the
southwesterly low level jet is strongest. However our area will also
likely see clouds and increasing moisture which will tend to buoy
nighttime low temperatures beginning Monday night but also through
much of the week.

Kimble

&&

.LONG TERM...  (Tuesday through Next Sunday)
Issued at 240 PM CST Sun Dec 21 2025

Although ridging aloft continues to push eastward into the center of
the country on Tuesday, a trough moving through southern Canada and
the Great Lakes sends a cold front southward behind it. This front
dips into the northern half of our area during the day on Tuesday.
While winds shift to the north behind the front, there`s not a lot
of cool air arriving with it. 925MB temperatures are similar on both
sides of the boundary. So this front may just stall or dampen the
warming trend a bit in the north, but overall temperatures continue
to rise relative to the day before. 60s become more widespread,
especially south of that front.

The front begins to lift back northward as a warm front late Tuesday
night into Wednesday morning. With anomalous moisture moving
northward overnight there`s an even greater potential for low clouds
and fog, though confidence in impactful fog is too low to include in
the going forecast just yet. With the northward push of the front,
even warmer temperatures are expected to spread through the area on
Wednesday. However, the morning clouds and fog could delay that warm
up especially in northern parts of the area where the surface front
will be slowest to push through. South of the front, and where
clouds disperse, we begin to see the full extent of this warm up
with temperatures rising into the 70s. NBM interquartile range for
high temperatures is fairly low for this time range, indicating high
confidence in temperatures in the 70s south of I-70. Further north,
though, that range rises to 10 degrees or more, indicating that
uncertainty closer to the frontal boundary.

By Thursday, Christmas Day, confidence is even stronger in those
near record warm temperatures. Low pressure moving out into the
Northern Plains causes an increased pressure gradient bringing a
stiff southwesterly flow across our area. That has the effect of
maximizing the warm up just ahead of an incoming cold front. With
expected full mixing enhanced by downsloping from the Ozarks, our
confidence in near record warmth in the 70s is very high. NBM
probability of 70 degrees is at or above 80 percent from I-70
southward. Nighttime lows remain similarly balmy as dewpoints in the
50s to near 60s, also near record values for this time of year,
prevent significant cooling.

Confidence in the forecast decreases beyond Thursday, however.
Guidance is in generally good agreement on the low pressure in the
Northern Plains on Thursday, but guidance diverges on how quickly to
push that low eastward which affects the timing of the trailing cold
front that brings our first cool down. Some guidance, notably the
GFS, are quicker with the front, bringing the cooler air in for
Friday. Other guidance, notably the ECMWF and CMC, keep the record
warm temperatures around for another day. The NBM interquartile
range for high temperatures again illustrates this uncertainty well.
On the warm end (75th percentile) St Louis again makes a run at 75
degrees. On the cooler end (25th percentile), it "only" reaches the
low 60s. So while this front may cool things down, it will still be
well above normal for this time of year, but there`s some question
as to whether it will arrive to cool things down at all by Friday.
More uncertainty is added this weekend as yet another wave sends yet
another front toward our area Saturday. Guidance is in fairly good
agreement that this front moves through on Saturday, but varies on
how strong the parent trough will be. Guidance sources with a
stronger trough bring considerably cooler air with it, along with
some rain chances with the frontal passage. Guidance with a weaker
and more progressive trough are not as cold and keep the frontal
passage dry. Although confidence in the precise temperature forecast
is low, confidence is high in the general trend of an eventual cool
down closer to normal for late December.

Kimble

&&

.AVIATION...  (For the 00z TAFs through 00z Monday Evening)
Issued at 501 PM CST Sun Dec 21 2025

LLWS will develop overnight as surface winds are near 5-7 kts and
winds within the 2000 ft near surface layer increase to 35-40 kts
with the developing southwesterly low-level jet. This will
continue to impact all terminals into the mid-morning when daytime
mixing will reduce its effects.

Otherwise VFR conditions are expected to largely prevail through
the end of the period. The exception is the low potential (20-30%
chance) for MVFR ceilings to develop midday with increasing
moisture. Confidence is low in this occurrence, and no mention
other than a scattered deck at the St. Louis metro terminals is
included in the TAFs.

Delia

&&

.CLIMATE...
Issued at 200 AM CST Thu Dec 19 2025

Well above normal temperatures are forecast around Christmas,
with record temperatures within reach. Records for our three
climate sites and the years that they occurred are listed below.

   KSTL  KCOU  KUIN
12/2473(2021)74(2021)69(2021)
12/2571(1889)74(1889)66(2019)
12/26   70(1942)    69(1942)    63(1942)


&&

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

$$

WFO LSX

NWS LSX Office Area Forecast Discussion