204
FXUS63 KLSX 181908
AFDLSX

Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
108 PM CST Tue Nov 18 2025

.MESOSCALE DISCUSSION...
Issued at 1258 PM CST Tue Nov 18 2025

Technical Discussion...

As of this writing (1pm), a shortwave is zipping through the
Midwest, with an associated surface low over eastern Missouri. As a
result, the southern 2/3rds of the CWA is in the warm sector of the
low and is a rough approximation of where there is sufficient
moisture and instability to support thunderstorms this afternoon. We
also need some lift to see thunderstorms develop, and while we`ll
see some mid-level lift over the region on the southern flank of the
above shortwave, we`ll also need some assistance from the cold front
to initiate thunderstorms in the mid-MS Valley.

The front, currently extending from just NW of St. Louis south-
southwestward toward the Lake of the Ozarks, is expected to slide
quickly eastward through the afternoon. However, winds in the
warm sector are largely parallel to the front, limiting convergence
and lift. As such, the vast majority of high res model guidance
shows at most isolated thunderstorm coverage in our forecast area
before storms become more widespread in southern IL. Further, there
are several models that don`t develop storms in our CWA at all
and that`s a real possibility for us.

All that being said, we are seeing cumulus develop across the warm
sector, which is an indication that at least weak convection is
trying to develop. Further, visible satellite reveals a
combination of SW to NE oriented roll clouds and S to N oriented
billow clouds, which is indicative of at least moderate wind
shear. This lends credence to the ~50kts of deep layer wind shear
and ~20 kts of low level wind shear that we`re seeing in model
data. Therefore if a thunderstorm is able to develop (~20% chance)
it may quickly become severe before moving out of the forecast
area, with all hazards possible.


BSH

&&

.KEY MESSAGES...

- Showers and thunderstorms will affect 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.

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 warm air advection.

Everything moves east tonight into Tuesday, allowing precipitation
to taper off early this evening. Surface high pressure crosses
the MS Valley from west to east Tuesday as an upper ridge amplifies
over the Midwest. It`ll be one out of a couple of 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 18z TAFs through 18z Wednesday Afternoon)
Issued at 1127 AM CST Tue Nov 18 2025

Areas of broken stratus are accompanying a slow-moving frontal
system this afternoon. Ceilings have generally oscillated between
MVFR and VFR; however, upstream observations support the stratus
scattering out for a brief time, namely between 21z-03z. It is
after that time that an area of low stratus will creep into the
regional terminals from the north. Confidence is reasonably high
(greater than 70% chance) that MVFR ceilings will return by 06z,
if not shortly before such time. There is even some potential for
IFR ceilings to develop between 09z-12z, but confidence is not
high enough to introduce it as the dominant category. Low ceilings
(MVFR/IFR) will then persist through the remainder of this TAF
cycle.

SPECIFICS FOR KSTL:

There is a low probability (less than 30% chance) for scattered
thunderstorms to develop early on in the TAF period, particularly
between 18z-20z, as a cold front slumps into the regional
terminals. Confidence and coverage are both too low to include in
the TAF at this time.

WFO LSX

&&

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

$$

WFO LSX