Area Forecast Discussion

592
FXUS63 KLSX 122338
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
538 PM CST Mon Jan 12 2026

.KEY MESSAGES...

- Anomalous warmth will continue through Tuesday.

- Normal to below normal temperatures are expected through the
  remainder of the week, with Arctic air making a return to the
  area this weekend.


&&

.SHORT TERM...  (Through Late Tuesday Night)
Issued at 237 PM CST Mon Jan 12 2026

Light southwesterly flow will continue to weaken tonight due to a
weakening pressure gradient as surface high pressure slides away to
the southeast. Calm southerly surface winds, coupled with a mostly
clear sky, will allow for a brief period of efficient radiational
cooling tonight before southerly winds begin to pick back up shortly
after midnight. This will initially lead to a quick drop in
temperatures before midnight, especially for river valleys, before
areawide temperatures begin to flatten or even moderate slightly by
sunrise tomorrow.

Northwest flow aloft will continue to prevail through the short term
period with upper-level ridging over the western CONUS and a robust
trough in place over the Great Lakes region. Our next weather
system, a shortwave trough currently traversing the western ridge
over Alberta, will begin to affect our area by tomorrow afternoon as
it progresses into the Upper Midwest. Ahead of this system, west-
southwesterly flow will strengthen with gusty (20-25kts) surface
winds peaking around noon tomorrow. This wind direction supports
downsloping off the Ozark plateau that will help lead to another
quick warmup tomorrow, with a spread of highs from the mid 50s to
mid 60s north to south across the area. Some uncertainty with highs
exists tomorrow, particularly for the northern half of the CWA as
increasing mid to upper-level clouds ahead of the shortwave may
limit the amount of diurnal heating, potentially leading to
slightly cooler highs than currently forecasted.

The aforementioned shortwave trough is forecasted to bring our
next low chance (20-30%) for light rain Tuesday night. Light rain
is the key word here as the HREF 50th percentile for accumulated
precipitation is only around 0.05" with the 90th percentile
barely reaching 0.10" for some locations. These light amounts may
still be slightly high considering the deep westerly-northwesterly
flow ahead of the clipper-like system is not advecting sufficient
moisture into the region, leading to a relatively dry vertical
profile when the best forcing moves through. The most likely
scenario seems to be scattered stratiform showers, meaning that
some locations may not see any rain with this system. Something
more notable will be the switch from well above average
temperatures to slightly below average temperatures behind the
cold frontal passage sometime Tuesday night. There are still
slight discrepancies in the exact timing of the front with most
model guidance keying in on a 03-12Z timeframe Wednesday morning.
Low-level CAA ramps up right behind the front leading to
widespread lows in the 30s by Wednesday morning across the region.

Peine/Elmore

&&

.LONG TERM...  (Wednesday through Next Monday)
Issued at 237 PM CST Mon Jan 12 2026

During the day on Wednesday, confidence is high that the upper-level
trough will continue digging across the eastern two-thirds of the
CONUS, with northwesterly flow deepening over the Middle Mississippi
Valley. This will push Tuesday night`s front well south of the
region, with a cold air mass settling into the region in its wake.
Given the degree of cold air advection, a weak diurnal curve with
temperatures is expected at best, with temperatures either stalling
or dropping through the day. Ensemble means support lows Wednesday
night in the teens and highs on Thursday failing to break freezing
for most locations.

For Thursday night into Friday, guidance consensus is that a brief
period of upper-level ridging will build into the Middle Mississippi
Valley ahead of the next trough. This will help temperatures through
the lower atmosphere rebound somewhat, and despite narrow ensemble
spread on this day, the degree of relative warmth on Friday shown is
suspect. Through the day friday, a potent shortwave and surface
system will swing through the Midwest, with many deterministic
guidance sources showing a FROPA early in the day on Friday. The
post-frontal air mass will be Arctic in nature, though the more
potent cold air will lag behind the front due to the phasing of the
shortwave and trough. If the front is any quicker than what is
currently showing in a majority of guidance, Friday`s high
temperatures may need to be lowered. The bitterly cold air will push
into the area Friday night and Saturday as flow through the
atmosphere over the CWA becomes increasingly northwesterly. Guidance
diverges notably on the phasing of the trough Sunday onward, with a
majority of the GEFS favoring continued troughing and bitter cold,
while the majority of ENS members favor troughing pushing quickly
eastward and temperatures moderating by the end of the weekend.

As for precipitation chances, the best chances (albeit low) are
Friday with the FROPA and Saturday as the axis of the upper-level
trough passes overhead. Moisture return will be lacking, so any
precipitation that occurs will be light. The characteristics of the
post-frontal air mass supports that this precipitation would be snow
flurries and/or light snow, with a majority of ensemble members that
do produce snow showing less than an inch of accumulation across
the region.

Elmore

&&

.AVIATION...  (For the 00z TAFs through 00z Tuesday Evening)
Issued at 532 PM CST Mon Jan 12 2026

VFR flight conditions and southwest to westerly flow is expected
to prevail through the period. Low level wind shear continues to
look very likely across the area tonight as a low level jet
increases in speed ahead of the next weather system. Wind shear
conditions will end Tuesday morning as the night time inversion
mixes out with daytime heating. Gusty winds up to around 25kts
will then prevail into the afternoon.

Carney

&&

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

$$

WFO LSX

NWS LSX Office Area Forecast Discussion