094
FXUS63 KLSX 180939
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
339 AM CST Tue Nov 18 2025

.KEY MESSAGES...

- Showers and thunderstorms to parts of the region this afternoon.
  There is a chance that a thunderstorm or two becomes strong,
  but conditions are highly conditional for severe thunderstorms.

- Widespread rainfall is expected from late Wednesday through late
  Friday.

- Temperatures will warm 10-15 degrees above normal today,
  especially along and south of I-70, where highs will reach the
  low to mid-70s. Temperatures cool the rest of the week with
  highs/low near to slightly above normal.


&&

.SHORT TERM... (Through Late Tonight)
Issued at 337 AM CST Tue Nov 18 2025

An upper level low is entering western IA early this morning with a
surface low over northwest MO, both tracking southeast through the
region today. Showers and thunderstorms follow the wing of warm air
advection as they continue to lift northeast and out of the CWA. A
second area of showers and thunderstorms has been developing over
SGF`s CWA to the southwest. Development is focused over the remnant
surface boundary that originally brought cooler air into the region
yesterday. This is being aided by mid/upper level vorticity that is
rotating around the south side of the upper system, over top the
boundary and along a CAPE gradient with MUCAPE of near 2000 J/kg
along the MO/AR border.

The upper level system and associated surface low will pass through
the area today, bringing chances (30-50%) for showers and a few
thunderstorms. Dewpoints in the 60s are being drawn northward out of
OK/AR into southern sections of MO/IL. As the system rotates over
the region this morning into this afternoon, additional showers and
a few thunderstorms will continue to develop over the southern
sections of MO, where upper vorticity is rounding the base of trough
and over top a remnant boundary and CAPE gradient over southern MO
with MUCAPE near 2000 J/kg to the south.

The main concern this afternoon will be whether additional
development has any chance to organize and become strong or perhaps
severe. Hi-res guidance shows this morning`s activity over southern
MO tracking eastward through southeast MO. Additional development is
shown over the eastern CWA as the upper low arrives from the
northwest, but activity gets going just before much of it begins to
shift east of the area. While the marginal outlook covers the
eastern half of the CWA, clearing will be slow for some to the east,
limiting instability. Lapse rates don`t look particularly exciting
with higher values (7-8C) displaced to the north of I-70 and low
level LRs and CAPE of 1500-2000 J/kg running just ahead of the
approaching low. While jet dynamics good and moisture is certainly
improving, the setup looks highly conditional. If strong
thunderstorms happen to materialized, it`s likely it will be over a
very limited area within the CWA (southeast) between 20z-00z.
Otherwise, the inversion holds between 850-800MB through the day,
preventing thunderstorms from being rooted in the BL and taking
advantage of backing winds that extend east of the system in the
region of advection warmth and moisture.

Everything move east tonight into Tuesday, allowing precipitation to
taper off this early this evening. Surface high pressure crosses the
MS Valley from west to east Tuesday as an upper ridge amplified over
the area. It`ll be one of a couple dry days this week.


Maples

&&

.LONG TERM... (Wednesday through Monday)
Issued at 337 AM CST Tue Nov 18 2025

Dry conditions are favored heading into Wednesday as surface high
pressure begins to move to the east with return flow behind the
departing surface high. A closed low over the southwest U.S. will be
the next system that will approach the region, bringing widespread
rain from late Wednesday through Thursday and into Friday.

An upper level ridge that helps keep conditions dry Tuesday into
Wednesday will begin to shift east with southwesterly flow
exhibiting greater influence over the region from midweek onward.
The upper ridge flattens some as vorticity is ejected eastward from
the southwestern low/trough and around the northern side of the
upper ridge. Global guidance shows a large pool of surface moisture
in terms of dewpoints (60-65 degrees) stretched across the Gulf
States. Vorticity passes over a stalled surface boundary with ascent
necessary to result in a rather broad area of precipitation as the
upper low spokes through the southwest U.S. and sends several pieces
of energy into the southwest flow aloft. Ensembles have shown wide
spread until recently. Latest LREF ensembles indicate that all
locations will receive measurable rain, while the heaviest will fall
along and south of I-70. Probabilistic data indicates much of the
region will see 1 inch of total rainfall by Friday evening with 20-
30% of the members at or above 2 inches across the same region.
Amount drop off toward KUIN, where the lower quartile (25th
percentile) still supports 0.25" total through Friday. IQR are about
1" almost anywhere in the CWA with 1-2" (locally higher) along and
south of I-70 with tapering to the north.

Another system drops into the western side of the trough over the
west coast last in the week, broadening the troughing pattern over
the western U.S. This pattern seems to stick heading into the next
week with a gradual cooling trend later in the period. This could
lead to additional rain chances heading into next week.


Maples

&&

.AVIATION...  (For the 06z TAFs through 06z Tuesday Night)
Issued at 1052 PM CST Mon Nov 17 2025

Additional scattered showers and thunderstorms are expected to
develop in the next few hours affecting all but the UIN terminal,
so have added PROB30 groups between 06-10Z. Any showers and a few
thunderstorms will have the potential to produce MVFR (possible
IFR) visibilities and ceilings. There is also a small window for
shower and thunderstorm redevelopment on 15-18Z on Tuesday, but
the chances are too low (20%) to include in the terminals at this
time. MVFR ceilings above 2000ft AGL are also expected tomorrow
morning and early afternoon ahead of a cold front at all of the
terminals. There will be enough of a low level jet to continue
LLWS at all but the UIN terminal through 12-14Z. South to
southeasterly winds tonight will turn out of the west to northwest
behind a front on Tuesday afternoon.

Britt

&&

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

$$

WFO LSX