Belleville, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

524
FXUS63 KLSX 210932
AFDLSX

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

.KEY MESSAGES...

- Temperatures will steadily rise Monday through Thursday, with
  near daily record afternoon temperatures in the low to mid 70s
  Christmas Day.


&&

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 220 AM CST Sun Dec 21 2025

While we still anticipate a significant warmup over the next several
days, we have one more chilly day to contend with before our warming
begins in earnest. Yesterday`s cold front has now settled to our
south, and surface high pressure is in place for the time being.
This will keep temperatures on the cool side today, with highs
reaching into the 40s area-wide by mid afternoon. While we may see
some wispy high clouds through the day, the combination of ample sun
and light winds should help to take the edge off of those
temperatures a bit, with little in the way of wind chill to contend
with. These temperatures are very close to seasonal averages, which
is likely to be the last time we are able to say that for the
rest of the week.

By tonight, the steady buildup of a broad upper level ridge will
begin, which will kick off our much-anticipated warming trend. While
this begins, a ridge-running shortwave will drive a substantial
southwesterly low level jet into our region overnight, which will
bring the first round of strong low level warm-air advection. This
feature will move through the area over the course of the day
tomorrow, and will begin the process of eroding the ambient cool,
stable surface layer.

This process will take a bit of time, and this warmup will not all
happen in one day. In fact, tomorrow may see some areas remain in
the mid to upper 40s, particularly in southeast Missouri and south-
central Illinois. Model soundings suggest that in these areas the
aforementioned stable layer at the surface will be slower to erode,
and the overrunning low level jet may even produce a bit of drizzle
or very light rain in addition to substantial stratus cover (all
underneath a layer of more widespread cirrus) during the day Monday.
While there is some uncertainty as to both how far north this
stratus (and drizzle) will reach, it will likely keep surface
temperatures from reaching the values that temperatures aloft
would otherwise suggest. Still, tomorrow will be several degrees
warmer than today in spite of the clouds, with some areas in
central Missouri potentially climbing to near 60 degrees.

BRC

&&

.LONG TERM...  (Monday Night through Saturday)
Issued at 220 AM CST Sun Dec 21 2025

At this point, there is not a whole lot left to elaborate on
regarding this week`s warmup, as it remains a near certainty that
temperatures will continue an upward climb before peaking near daily
record highs Christmas Day. There are a few minor points of
uncertainty before we get there, but for the most part, the forecast
remains on track.

Tuesday remains likely to see another bump in temperatures compared
to Monday simply due to the continually building ridge and also
relatively lower cloud cover. However, the strong low level jet that
will move through the area Monday will move out of the area, and as
a result, there will be a bit of a pause in low level warm air
advection. Meanwhile, another ridge-running shortwave will drive a
weak cold front into our area during the afternoon, which may stunt
our warming slightly in northern areas, but not enough to prevent
all areas from reaching 10 to 20 degrees above normal by the
afternoon.

Similarly to Tuesday, a strong southwesterly low level jet will once
again move into the area Wednesday, putting us back within a regime
of substantial warm air advection. While it is a bit less certain
than Monday, there are again hints that isentropic lift may produce
more stratus in some areas throughout the day Wednesday, along with
another round of high clouds as well. There are even hints of more
patchy drizzle or light rain, although this is very uncertain and
amounts would be largely insignificant. This seems to be reflected
in increasing ensemble temperature forecast spreads, which remain
higher on Wednesday than any other day during our warmup. While the
persistent warm advection aloft and building ridge will ensure that
temperatures will be well above normal, afternoon highs Wednesday
are a bit less certain than the remainder of the Monday-Thursday
timeframe.

However, Thursday (Christmas Day) remains very likely to be the
warmest day of the week, as the building upper ridge will reach it`s
maximum strength (5800-5820m 500mb heights, 99th+ percentile) and
will be centered almost directly overhead. Meanwhile, ensemble mean
850 mb temperatures remain in the 15 to 17 degree C range, which is
near climatological maximums. Meanwhile, persistent southwesterly
surface winds are also likely to add a couple of degrees due to
downsloping off the Ozarks, adding more confidence to the idea that
Thursday is clearly the favorite for our warmest temperatures of the
week.

The only noticeable fly in the ointment for Thursday will be
cloud cover, as there are some suggestions that persistent cirrus
and even some mid-level stratus may linger through the day, which
may shave a couple of degrees off the final readings. Even when you
consider that, though, ensemble temperature spreads remain quite
narrow Thursday, and we`re really only talking about a difference of
a few degrees at non-hazardous temperature ranges (70 to 75
degrees). Clouds or not, temperatures Thursday afternoon will reach
25 to 30 degrees above average, and will get very close (if not
exceed) daily record highs. Like last night, the operational NBM
(and official forecast) remains on the higher end of the ensemble
envelope (greater than the 75th percentile), which represents the
scenario where persistent downsloping overwhelms the mitigating
effect of afternoon cloud cover.

From Friday through the weekend, the upper ridge will finally
flatten out and allow a series of cold fronts to move through the
area. While this will gradually moderate temperatures, these cold
fronts appear to be relatively weak overall, and it will take
several days before temperatures return even to seasonal averages.
Meanwhile, this also opens the door for some modest precipitation
chances over the weekend, although probabilities remain low overall
(20%), and the prospects of meaningful amounts remain minimal for
the foreseeable future.

BRC

&&

.AVIATION...  (For the 12z TAFs through 12z Monday Morning)
Issued at 329 AM CST Sun Dec 21 2025

VFR flight conditions are expected throughout the 12Z TAF period.
Light southeast winds will turn to the south and increase slightly
overnight. Meanwhile, some marginal low level wind shear will be
possible near the end of the period at all terminals. Some low
stratus may approach I-70 corridor terminals (all but UIN) just
beyond the end of this cycle, but is not included in this
iteration of the TAFs just yet.

BRC

&&

.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)


&&

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

$$

WFO LSX