255
FXUS63 KLSX 030916
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
316 AM CST Tue Mar 3 2026

.KEY MESSAGES...

- Showers and thunderstorms are expected today through Thursday
  morning, then again Friday night into Saturday. It is still
  uncertain if thunderstorms can become severe or produce flash
  flooding, but at least a soaking, beneficial rainfall will
  result.

- Above average temperatures will prevail through the next 7
  days, with the warmest day forecast to be Friday.


&&

.SHORT TERM...  (Through Late Wednesday Night)
Issued at 314 AM CST Tue Mar 3 2026

Showers and a few thunderstorms will continue to spread
northeastward this morning, in response to increasing low to mid-
level isentropic ascent and moisture at the nose of a LLJ
overrunning a warm front across southern MO. In addition to showers
and thunderstorms, areas of fog and drizzle are being observed with
localized occurrences of visibilities as low as 1/4 mi at times.
However, drizzle should become less common through sunrise as
boundary layer lapse rates decrease and even some of the fog could
be perturbed by the showers and thunderstorms, improving
visibilities.

As the aforementioned warm front lifts to near or just south of the
I-70 corridor by the afternoon, most showers should also shift
northward and even decrease in coverage as mid-level heights rise
slightly. There will be a contrast in high temperatures across this
front with upper 40s to 50s F to the north with abundant clouds and
lingering showers, and upper 60s to low-70s F to the south among
some breaks in clouds in the warm sector. The exact position of the
front through the day still varies in short-term model guidance,
leading to the greatest NBM interquartile spread near the I-70
corridor ranging from mid-50s to mid-60s F.

During the evening, showers and thunderstorms are expected to
increase in coverage once again across the area as the LLJ and
associated forcing restrengthens. CAMs generally agree that the
greatest coverage of showers and thunderstorms will be along and
north of I-70, numerous and continuing through the overnight hours.
The general consensus in model guidance is that 750 to 1250 J/kg of
MUCAPE will be available during this time and there cloud be a few
stronger, weakly rotating updrafts, suggesting small to marginally
severe hail is possible. However, CAPE in the hail growth zone is
expected to be rather modest and decreases confidence in any hail
reaching severe thresholds.

There could be a brief lull Wednesday morning, but an upper-level
shortwave trough and associated open wave cyclone will track through
the region and be the catalyst for more showers and thunderstorms
developing/tracking into the CWA Wednesday afternoon through night.
Along/south of the cyclone`s track, the threat of severe
thunderstorms will need to be monitored with the potential for
surface-based instability to develop in an environment characterized
by strong wind shear, including low-level directional wind shear.
However, there remains quite a bit of uncertainty on how far north
into the CWA the warm sector can reach, especially considering
antecedent precipitation and clouds potentially counteracting any
northward advancement. The latest HREF has 30 to 50 percent
probabilities of SBCAPE of 750+ J/kg only reaching far southeastern
MO as this point, which tempers confidence sufficient to message
severe thunderstorms on Wednesday.

The ingredients are at least in place for a beneficial, soaking
rainfall this evening through Wednesday night from repeated and
training showers and thunderstorms including PW around the 99th
climatological percentile, sufficient instability, and deep-layer
flow nearly aligned with both the warm front tonight and the cold
front Wednesday night. Accordingly, the NBM interquartile QPF range
is 1 to 3" across south-central IL into east-central MO from this
evening through Wednesday night. Although the pattern bears watching
for heavy rainfall and flash flooding, there are a couple of factors
that appear to marginalize the environment including modest warm
cloud depths and the LLJ, albeit broad, only forecast to be around
30 to 35 kt, which is not particularly strong for this time of year.
Therefore, confidence is lowered that we will get the sustained,
high rainfall rates needed to produce flash flooding, unless
significant pre-conditioning (moistening) of initially dry soils
takes place beforehand, lowering flash flood guidance.

Pfahler

&&

.LONG TERM...  (Thursday through Monday)
Issued at 314 AM CST Tue Mar 3 2026

The aforementioned cold front is finally expected to advance
southeast of the CWA on Thursday, but showers and a few
thunderstorms could linger through at least the morning across
southeastern MO and southwestern IL. There will likely be a break in
precipitation altogether during the afternoon and evening behind the
front and with slightly drier air arriving. Since there will be less
clouds, high temperatures are also forecast to be warmer on Thursday
and in the 60s and 70s F, with a mild, modified Pacific post-frontal
airmass and low-level WAA actually returning by the evening.

Friday into Saturday, the upper-level flow pattern will become more
amplified as a ridge builds across the eastern CONUS. This pattern
will help Wednesday/Thursday`s cold front lift quickly back
northward as a warm front Friday morning. Around 30 to 60 percent of
ensemble membership have showers and thunderstorms during the
morning with the passage of this front, but dry conditions are
favored the rest of the day due to a capping inversion across the
warm sector and a scarcity of forcing. Friday will easily be the
warmest day of the week, with 850-hPa temperatures around the 99th
climatological percentile, and a southwesterly component to low-
level flow promoting downslope warming of the Ozark Plateau. As
such, high temperatures will reach the 70s and even 80s F
along/south of the Missouri River. Probabilities of showers and
thunderstorms do not increase again until Friday night into Saturday
as an upper-level trough and associated cold front finally reach the
area. Currently, the most likely area for severe thunderstorms is
across the Central Plains, with instability decreasing by the time
showers and thunderstorms reach the CWA. There are some difference
in the speed of the cold front, with slower solutions depicting the
front still across southeastern MO and southwestern IL on Saturday,
accompanied by continued showers and thunderstorms.

It is not until Sunday, that ensemble based probabilities of
measurable rainfall finally fall below 20 percent with greater
confidence that the front will finally clear the CWA. The airmass
behind the cold front will be mild with the entire NBM temperature
distribution still well-above average.

Pfahler

&&

.AVIATION...  (For the 06z TAFs through 06z Tuesday Night)
Issued at 1054 PM CST Mon Mar 2 2026

IFR flight conditions will continue to prevail across the majority
of the area through the period. Flight conditions across the
eastern Ozarks may improve to MVFR and possibly up to VFR Tuesday
afternoon as a warm front moves north through that region and
stalls near the I-70 corridor.  Widespread showers and a few
thunderstorms will develop after 06Z and overspread areas
generally along and north of I-70. Showers should diminish and
become widely scattered between 14-18Z. Another round of showers
and thunderstorms is expected to develop during the afternoon into
early evening, once again generally along and north of I-70.

Carney

&&

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

$$

WFO LSX