Area Forecast Discussion

961
FXUS63 KLSX 220934
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
334 AM CST Mon Dec 22 2025

.KEY MESSAGES...

- Above normal temperatures are expected for the next several
  days, including the potential for record highs Christmas Eve
  and Christmas Day.


&&

.SHORT TERM...  (Through Late Tuesday Afternoon)
Issued at 255 AM CST Mon Dec 22 2025

An above-normal, potentially record-challenging (Christmas Eve/Day)
warming trend begins today, driven by southerly return flow behind a
departing surface ridge; 850mb temperatures (15-20C) over the
central Plains exceed the 95th percentile of climatological means,
showing the potential in coming days. For today, however, that
magnitude of warm remains west. Local temperature will be about 10
degrees warmer than Sunday.

One thing that will be watched is the thickening cloud cover along a
warm front to our southwest. This will be the primary limiting
factor to today`s warmth with HREF ensemble guidance showing the
greatest spread (10-12 degrees) in highs coinciding with the highest
probabilities (greater than 70%) for obscured sunshine. These areas
include locations from KCOU, northeast through KUIN, and bending
back southeast through interior sections of Illinois. This could be
the difference between 40s vs. 50s. Forecasts highs in the upper
40s/low-50s are more of a compromise, hovering close to the 50th
percentile of NBM IQR.

Clouds shift east of the Mississippi River tonight with partial
clearing over central Missouri. A weak, stalling cold front drifts
into the region tonight, acting on winds more than a having a
sensible impact on temperatures. This boundary runs under warm air
advection through the mid-levels as winds lighten under the strong
inversion. Narrowing dewpoint depressions favors at least patchy
fog. This will have to be watched as the HRRR is most bullish with
indication of patchy dense fog, mainly in valleys and low-lying
areas across the north/northwest portion of the CWA.

The boundary returns northward Tuesday, adding another 10 degrees or
so to Tuesday`s high temperatures. The warmest air remains
along/south of I-70 with increasing uncertainty to the north. This
is where ensemble spread increases from around 10 degrees just north
of I-70 to 18 degrees over northern Missouri and west-central
Illinois. Despite HREF spreads, NBM IQR remains tightly clustered
around the low/mid-50s. This may be a simple product of data run
time as trends solidify the warm push and grab onto a more reasonable
solution with time. Confidence is high that the warming trend will
continue, bringing many areas into the 60s, sans northern Missouri
and west-central Illinois.


Maples

&&

.LONG TERM...  (Tuesday Night through Sunday)
Issued at 255 AM CST Mon Dec 22 2025

Moisture will continue to increase from late Tuesday through the
middle of the week. Dewpoints are expected to climb into the 50s
across most of the forecast area, with some southern locations
flirting with dewpoints near 60 degrees at times.

Anomalous warmth from the central Plains will be directed northward
into the central U.S. as the Gulf ridge builds westward, maintaining
a consistent feed of above-normal temperatures and moisture through
the week. The fog potential to the west from Monday night/Tuesday
morning shifts east with the increase in moisture and advancing warm
airmass. Another night of patchy fog is possible Tuesday night into
early Wednesday, most favorable along and behind the boundary,
deeper into the warm sector. This would favor areas west of the
Mississippi River south of I-70. This was not included in the
gridded forecast just yet. Monitoring tonight`s trends could
provide better evidence on what translates eastward for Tuesday
night and early Wednesday.

A weak cold front will move into the region Wednesday, though it
will have a marginal (almost unnoticeable) impact on temperatures.
The main influence will be slightly cooler temperatures over
northern Missouri and west-central Illinois. The strong Gulf ridge
wins out with warmth continuing to build into the region. 500mb
height anomalies show a fairly broad area of height that area 2-3
standard deviations above normal. Considering this, along with the
high percentile ranking of mid-level air, it boosts confidence in
near-record warmth with widespread 60s and 70s Christmas Eve and
Christmas Day. One thing to think on is how guidance has poor
handling of warmth in early spring, when vegetation is dormant and
southwesterly flow becomes more prevalent. Though forecast dewpoints
are well above normal, it isn`t a far break from what is seen in
spring. All that being said, the argument is strengthened for near-
record temperatures.

To add context, I referenced climate extremes for the period-of-
record at KCOU, KUIN, and KSTL, for the dates spanning 12/22-12/27.
If current forecasts verify as-is, this would be the warmest stretch
in this period on record. The warmest means for this timeframe
reflect mean temperatures ranges from the low-50s (KUIN) to the mid-
50s (KSTL and KCOU). To reiterate, these are mean (average of highs
and lows) temperatures, which makes it even more significant that
means from the current forecast data would surpass the past warmest
period by 5 degrees! That`s impressive.

Warm temperatures will extend through Christmas and into the
beginning of next weekend. A strong cold front is expected to
approach late next weekend, returning temperature to near-normal
territory by Sunday. Timing and positioning of the front remain
somewhat dispersed across guidance. This results in a widening
spread in data with less confidence at the end of the period. The
Gulf ridge does begin to show signs of flattening, as quasi-zonal
flow introduces a system into the Midwest and brings the potential
for light rain along the cold front.


Maples

&&

.AVIATION...  (For the 12z TAFs through 12z Tuesday Morning)
Issued at 330 AM CST Mon Dec 22 2025

LLWS potential will drop off through early this morning with VFR
conditions expected through much of the period. A warm front is
expected to lift northeastward through the region today, initially
thickening high clouds that eventually build through the mid-
levels (4-5k feet) with frontal passage. Considering that bases
remain above VFR thresholds, prevailing groups in the 12z update
will not reflect the mid-level layer in favor of TAF
simplification. There is some potential for patchy fog tonight
into early Tuesday. Fog potential is favored around KCOU/KJEF with
the remainder of the terminals far enough east of the front to
avoid fog until Tuesday night/early Wednesday morning.

Maples

&&

.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