194
FXUS63 KLSX 260326
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
1026 PM CDT Tue Aug 25 2026

.KEY MESSAGES...

- Showers and thunderstorms are possible (20 to 40 percent
  chance) late tonight/Wednesday morning and again Wednesday
  afternoon/evening across portions of the area. A few
  thunderstorms may be strong to severe with damaging winds and
  marginally severe hail.

- Seasonable temperatures will persist through Saturday before
  summer heat returns Sunday into early next week.

&&

.SHORT TERM /THROUGH WEDNESDAY NIGHT/...
Issued at 225 PM CDT Tue Aug 25 2026

Plentiful sunshine and gradual retreat of a surface anticyclone has
resulted in warmer temperatures from 24 hours ago firmly in the 80s
F. Clouds will begin increasing tonight with approach of a cold front
alongside a remnant MCV emanating from ongoing showers and
thunderstorms across KS, resulting in noticeably warmer low
temperatures in the mid-60s to around 70 F. Additionally, the
arrival of the MCV and a downstream, weak stationary front arcing
across south-central into southeastern MO will be the catalysts for
showers and thunderstorms late tonight into Wednesday morning. With
projections of 1500 to 2500 J/kg of MUCAPE and around 30 kt of
effective wind shear, a brief supercell capable of marginally severe
hail cannot be ruled out. However, some upscale growth is possible
with time which would decrease the hail threat, but a nocturnal
inversion will likely limit the potential for damaging winds as this
process would occur.

The morning showers and thunderstorms will migrate/propagate
southeastward with time, eventually departing the CWA by midday. A
cold front will also be passing southeastward through the CWA during
the day, reaching approximately south-central IL through central MO
by the afternoon. The front will be the focus of additional shower
and thunderstorm develop during the afternoon and evening, but the
coverage and exact timing is not certain due to the combination of a
weak capping inversion, modest frontal convergence, and discernible
large-scale forcing passing both north (upper-level trough) and
south (another potential MCV) of the CWA. HREF probabilities of
measurable rainfall are highest across southwestern IL along the
front during the evening, leading to relatively higher confidence in
that portion of the CWA. With 20 to 25 kt of effective wind shear,
mainly pulse-type thunderstorms are favored, but some upscale growth
into loosely organized multicell clusters cannot be ruled out.
Approximately 2000 to 3000 J/kg MLCAPE is advertised by
deterministic model guidance along with "inverted-V" sounding
signatures suggesting a few strong to severe thunderstorms could be
capable of microbursts with damaging winds and even marginally
severe hail during the initial maturation of cells, considering much
of that CAPE is distributed in the hail growth zone. Otherwise,
additional pre-frontal WAA and southwesterly winds support high
temperatures a few degrees warmer than today, in the mid-80s to
around 90 F.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Issued at 225 PM CDT Tue Aug 25 2026

Behind the cold front, a seasonably cool and dry airmass (850-hPa
temperatures and PW below 25th climo percentile) will infiltrate the
Mid-Mississippi River Valley through Thursday night as a surface
anticyclone extends into the region. As result, dry conditions, high
temperatures in the low to mid-80s F, and low temperatures in the
upper 50s to mid-60s F are forecast through Friday night before the
anticyclone starts retreating and low-level flow becomes southerly
in response.

The anticyclone departure will be in part due to a broad, upper-
level ridge amplifying and shifting eastward across the Mississippi
River Valley over the weekend. Strong low-level southerly WAA will
quickly warm 850-hPa temperatures to above the 95th climatological
percentile by late Sunday with high temperatures reaching the 90s F
across most of the CWA into early next week. Moisture will be slower
to increase, but global model guidance indicates a connection to the
Gulf of Mexico will finally be established around Monday , leading
to rising heat index values to around 100 F in the warmest
locations. Monday, onward, model guidance is in disagreement on the
position and presence of smaller scale troughs and ridges embedded
in approximately zonal upper-level flow, but there is an underlying
signal for slowly falling heights. This evolution introduces the
concerns of a frontal boundary to shift closer to at least
northeastern MO and west-central IL at some point with increasing
potential for showers and thunderstorms to disrupt the longevity of
heat in that part of the CWA. However, the details of these types of
patterns are impossible to ascertain at this timerange.

&&

.AVIATION /00Z TAFS THROUGH 06Z THURSDAY/...
Issued at 1014 PM CDT Tue Aug 25 2026

There are three distinct low chances for impacts through the
period at the local terminals. The first is fog developing
across north-central portions of Missouri. There is a very low
chance that this spreads far enough east-southeast to impact
KUIN, KCOU, and KJEF. If this does happen, fog will dissipate
shortly after sunrise. Secondly, a very weak weather system will
pass south of the local terminals through the morning hours.
There is a low chance (20%) that this system produces scattered
showers and thunderstorms around KCOU, KJEF, KSUS, KSTL, and/or
KCPS. Lightning and reduced visibilities would be the primary
impacts from any convection. Lastly, a cold front sweeping
through the region during the afternoon hours will serve as the
focus for additional scattered showers and thunderstorms. The
coverage of this convection and the low probability of its
occurrence around the local terminals leaves confidence too low
in impacts to include in the TAFs at this time.

&&

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

&&

$$

SHORT TERM...Pfahler
LONG TERM...Pfahler
AVIATION...Elmore