422
FXUS63 KLSX 042035
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
235 PM CST Thu Dec 4 2025

.KEY MESSAGES...

- Arctic air will result in near-record lows (single digits) over
  parts of the area Friday morning.

- Dry conditions are expected through Saturday with light snow
  possible (30-50%) Saturday night into Sunday. Minor
  accumulations are possible over parts of the area.

&&

.SHORT TERM...  (Through Late Friday Night)
Issued at 232 PM CST Thu Dec 4 2025

High pressure continues to slide west to east across the region this
afternoon with cold air advection ushering in colder, drier air
behind a southbound cold front south of the Missouri/Arkansas
border. The magnitude of the dry air is well-depicted in the surface
dewpoints with dewpoints in the 20s along the front and single
digits to the north. Gradual clearing will continue this evening
with a few high clouds hanging on to the far southern section of
Missouri and Illinois tonight.

Tonight is setting up to be another frigid one, especially north and
northeast of the St. Louis Metro. Mainly clear skies, light wind,
and remnant snowpack will result in efficient radiational cooling
for all locations. The only exception is central and southeast
Missouri, where return flow commences sooner and limits cooling
potential with respect to locations to the north and northeast.
HRRR/RAP are the coldest among the deterministic solutions with sub-
zero temperatures roughly from Alton to Litchfield, IL. Despite
being a conducive environment for anomalous cold, hi-res guidance
may giving too much weight to snowpack. Hi-res model temperatures
fall below the 5th percentile of HREF ensembles, which seems
unlikely, but it will still be frigid. Gridded forecasts were
manually blended tonight to lean between the 10th-25th percentiles
for the coldest locations and just below the 50th percentile for
warmer locations further south and west. With that said, some
locations will approach record territory, primarily at KUIN, where
tonight`s forecast low of 5 degrees would break the December 5th
record set in 2005.

There was some concern that fog development could lead to freezing
fog in the coldest areas (IL), but this is likely a result of cold
bias from the hi-res guidance. Surface dewpoints fall well below
zero and freezing fog impacts usually need 1/4 mile visibility to
produce impacts. Additionally, warm air advection doesn`t strengthen
in this area until after 12z, lacking the overrunning conditions
that might lead to better fog potential over snowpack. For now, it`s
a monitoring situation, but unlikely.

The remainder of Friday welcomes milder temperatures under mostly
clear skies, return flow, and strengthening warm air advection. High
temperatures will almost feel warm in comparison. Friday`s highs
range from the low-30s over northeast Missouri to west-central
Illinois, to the mid-40s over central and southeast Missouri.
Another cold front passes through the area late Friday, but not soon
enough to greatly impact low temperatures (low/mid-20s). The front
is also moisture-starved and passes through quietly otherwise.


Maples

&&

.LONG TERM...  (Saturday through Next Thursday)
Issued at 232 PM CST Thu Dec 4 2025

Friday night`s cold front will have more of an impact to
temperatures Saturday, as it reinforces the arctic airmass
southward. The difference with this front is lack of southward
progress, which then creates a large disparity in temperatures
through the day Saturday. The greatest spread (6-8 degrees) among
the HREF ensembles extends from northwest Missouri through central
and southeast Missouri, where partial clearing and frontal position
will be more of a factor on temperature variability. If the front
begins to stall sooner, temperatures will favor the upper quartile
with highs in the 40s and possibly a localized reading near 50
degrees. Otherwise, temperatures move about 10 degrees from their
morning lows with highs in the 30s.

An Alberta Clipper quickly drops down the east side of the Rockies,
through the Plains, and into the St. Louis Region Saturday night
into Sunday. Another weak surface low tracks due west to east across
northern Texas into Louisiana. The southern system pulls moisture
northward into the Gulf states, assisting the clipper system with
moisture access. Unfortunately, we`re in no-mans land in
reference to model guidance as it falls on the cusp of medium and
hi-res solutions. This makes it even more difficult to decipher
exactly where the snowfall potential is highest. Clipper tracks
are notoriously difficult to pinpoint for this reason, but the
stage is set for at least some minor accumulations with a few
inches possible along and north of the surface low track. 6-hourly
LREF QPF shows a smearing of measurable QPF (0.01 or greater)
extending from north- central Missouri through St. Louis, eastward
through Effingham. Amounts trend upward toward KUIN with 0.15".
Both GFS/ECM multi-run snowfall ensembles show a fairly consistent
beat on accumulations, but I`m concerned in the fact they`re as
low as they are with less than 1" of snowfall even near KUIN. This
tells me it`s not likely keying in on the finer details and the
fact that some individual solutions pump out as much as 0.25" of
QPF along the wing of warm air advection, higher snow ratios with
these systems could easily produce advisory criteria snowfall with
enough moisture. 1-3" doesn`t look unreasonable for our
north/northeast counties, trailing off to less than an inch and
maybe even some light, cold rain in our southwest counties.

Guidance shows weak mid-level forcing closely follows the clipper as
the cold front pushes south through the day Sunday. Most guidance
picks up on lingering light snow or flurries through much of the day
Sunday. Much of this registered below measurable QPF with the 75th
percentile suggesting 0.01-0.02" could fall. If so, whatever impacts
result from the clipper may be extending into Sunday afternoon with
a few slick spots on untreated surfaces. Again, much of this will be
determined with the system track - stay tuned to subsequent updates
over the next couple of days. By-and-large, it does not look like a
big event, but it could toss some surprises with a localized
corridor of higher totals if all comes together right.

High pressure builds in behind the clipper, starting off next week
cool and dry. Beyond Monday, things become a little less certain
with specifics. Overall, milder conditions return for Tuesday with
southwest flow behind the departing high. A couple of additions
clipper systems cross the northern half of the U.S., but there is a
poor handle on track/timing through midweek.


Maples

&&

.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1130 AM CST Thu Dec 4 2025

Scattered/broken VFR clouds are gradually clearing from north to
south as high pressure crosses the region. High pressure will
result in light, northerly surface winds this afternoon, becoming
light/variable tonight, then shifting out of the south as the
surface high moves east Friday. VFR persists through the end of
the period.

Maples

&&

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

$$

WFO LSX