Belleville, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

448
FXUS63 KLSX 202250
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
450 PM CST Sat Dec 20 2025

.KEY MESSAGES...

- A warming trend will kick off on Monday and peak on Thursday,
  with afternoon temperatures then reaching around 30 degrees
  above seasonal normals.


&&

.SHORT TERM...  (Through Late Sunday Night)
Issued at 229 PM CST Sat Dec 20 2025

An upper-level trough is moving through the Upper Midwest this
afternoon per water vapor imagery; its surface low seen in surface
observations tracking along the US-Canadian border at the Great
Lakes. This low`s attendant cold front is roughly bisecting the CWA
from northeast to southwest as of this writing, and is expected to
clear the CWA to the southeast early this evening.

Northwesterly flow in the wake of this front is weak and short-
lived, leading to only a brief period of subtle cold air advection.
Therefore, temperatures tonight will only bottom out approximately 5
degrees colder than last night, with most locations seeing lows in
the mid to upper 20s.

The post-frontal air mass quickly shifts eastward tomorrow, with low-
level flow becoming increasingly southwesterly through the day. This
will not occur quickly enough to keep temperatures from only
topping out around climatological normals during the afternoon, with
the coolest values being across west- and south-central Illinois in
closer proximity to the heart of the air mass.

Elmore

&&

.LONG TERM...  (Monday through Next Saturday)
Issued at 229 PM CST Sat Dec 20 2025

Ensemble clusters and deterministic guidance remain strongly aligned
in building an upper-level ridge across the central CONUS Monday
into Thursday. Deep southwesterly flow across the Middle Mississippi
Valley will advect unseasonably warm air into the region, with
temperatures climbing day to day. However, there is growing
uncertainty with values on Wednesday, as guidance shows a weak cold
front dropping into the area late Tuesday before lifting northward
as a warm front on Wednesday. The timing of this front varies
greatly among guidance, leading to almost a 10-degree spread with
the IQR of global ensemble guidance for max temperatures on
Wednesday. Regardless, with even the 25th percentile of this
guidance hovering in the upper 50s to low 60s, confidence is high in
temperatures continuing to trend above normal.

Confidence also continues to be high that this period of warmth will
peak on Thursday (Christmas Day), when ensemble means show 850mb and
925mb temperatures topping out around 15 degrees C - the 99th
percentile of model climatology. These low-level temperatures
correlate with surface temperatures around 70 degrees assuming clear
skies and deep mixing. With nearly every global ensemble member
showing southwesterly surface flow, temperatures across portions of
southeastern Missouri will see a degree or two of a boost thanks to
downsloping off the Ozarks. Exact values remain uncertain, as the
GEFS and GEPS continue to have slightly cooler low-level
temperatures compared to the ENS. Additionally, there is notable
spread among guidance on the degree of cloud cover, with thicker
cloud cover hindering the warmup by a few degrees. If the ENS`
warmer low-level temperatures and mostly clear skies are realized,
max temperature records will be threatened. See the climate section
below for details.

Guidance consensus continues to show the ridge deamplifying starting
late Thursday and going into next weekend as a series of
disturbances pass through the Midwest. With the ridge weakening,
temperatures aloft will cool and take the edge off of the warmth at
the surface. One of the disturbances will send a cold front through
the region late Saturday or early Sunday that will further cool
temperatures further. While spread is still high in temperatures
behind the front, weak northwesterly flow and a majority of guidance
showing values at or above climatology lead to medium to high
confidence that this post-frontal air mass will not have much
bite.

Elmore

&&

.AVIATION...  (For the 00z TAFs through 00z Sunday Evening)
Issued at 447 PM CST Sat Dec 20 2025

Dry and VFR flight conditions will persist through the end of the
period under initially northwesterly winds. Winds will become
light and variable overnight as the center of a surface high
pushes through the terminals. Winds will pick up from the
southeast Sunday morning.

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)


&&

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

$$

WFO LSX