632
FXUS63 KLSX 311740
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
1240 PM CDT Fri Jul 31 2026

.KEY MESSAGES...

- Showers and thunderstorms are expected today, with some severe
  storms possible during the afternoon and evening. Damaging
  winds will be the primary threat.

- Much cooler air moves in behind a cold front this weekend,
  with temperatures gradually warming back closer to normal
  through the upcoming week.

&&

.UPDATE...
Issued at 1012 AM CDT Fri Jul 31 2026

Trends continued to be monitored with respect to the afternoon
thunderstorm potential. Morning showers and embedded
thunderstorms are running ahead of an MCV currently centered
just east/northeast of KCOU. This activity has impeded the
typical diurnal trends with temperatures hovering in the low to
mid-70s. Regional visible satellite shows broken/overcast skies
extending from Illinois westward through central Missouri before
breaking to scattered cu in the western sections of the state.

SPC mesoanalysis continues to show 25-35kts of 0-6km shear over
the region this afternoon, along with SRH values of 200-300
m/s2. Instability reestablishes as the clearing works from west
to east. However, this provides a very tight window for eastern
sections of the state as the upper trough dips southward through
the core of typical diurnal peak (3PM-6PM). Unfortunately, this
complicates the threat with a highly conditional atmospheric
setup with a strong early-day capping inversion that may only
briefly erode. The window for rotating updrafts may only be in a
narrow spatial (north-south) and temporal window with organized
multi-clusters developing at the eastern fringes of instability.
Meso analysis shows SBCAPE resurging to around 4000 J/kg over
southwestern Missouri, but quickly tapering to 1500 J/kg over
eastern Missouri by 4PM this afternoon with mid-level lapse
rates of 6-7C. If we can get thunderstorms to develop and
organize, tornado potential is there.

Therefore, while the severe threat is plausible, the window for
the potential may be pretty short and somewhat limited in
coverage, if recovery remains sluggish with better SBCAPE offset
to the west. The threat remains, but the conditionality
provides a lot of uncertainty.

&&

.SHORT TERM /THROUGH SATURDAY/...
Issued at 233 AM CDT Fri Jul 31 2026

High pressure which brought us the last few days of low
summertime humidity is moving east into the Appalachians today.
To our west, a shortwave trough is moving southeast out of the
Northern Plains around the periphery of a western US ridge. This
shortwave trough is rather potent for this time of year, and
moist advection through the Plains ahead of the trough has led
to the development of a convective complex currently entering
northwest Missouri this morning. This is expected to continue
east southeast over the next several hours, decreasing in
intensity as it moves east potentially outrunning the advancing
moisture and instability. However, we`re still likely to see it
track through much of our area during the morning hours.

Behind this convection, dewpoints rise 10 degrees or more in a
SSW flow ahead of an approaching cold front. This greater
moisture combined with daytime heating should lead to surface
based instability in the 2-3000 range just as the cold front
moves into our region this afternoon. Just how unstable it gets
will depend on how quickly the morning clouds and rain move
out and how much sun we get locally. But we do expect enough
destabilization for another round of thunderstorms to develop
this afternoon. With the potent mid level wave we`ve got
seasonably strong wind shear available, enough to provide some
organization to the storms that develop this afternoon. A mix of
supercells and convective clusters is likely. With a warm and
humid air mass, long skinny CAPE, and very high precipitable
water values near 2 inches, heavy rain and damaging downburst
winds will be the primary threats. Relatively quick storm
motions should limit total rainfall amounts, but some localized
areas where convective scale elements favor longer duration
rainfall could see higher totals up to 3+ inches. There`s also
enough low level shear and curvature in the wind fields to be
alert for a few tornadoes as well. This may be with initial
supercells or with QLCS clusters which align with the low level
shear vectors.

All of this convection gets swept southeastward with a cold
front this evening, ending the threat locally and ushering in a
cooler, drier air mass for Saturday. Although it will be drier
in an absolute sense, with dewpoints falling back into the 60s
after briefly rising into the 70s ahead of the front, we aren`t
quite done with the rain just yet. The shortwave trough closes
off into a closed low over Illinois with wrap around showers
reaching back into Missouri. These should be relatively light
with limited instability available, but the combination of
clouds, showers, and cold advection will hold temperatures in
the 70s for highs.

&&

.LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/...
Issued at 233 AM CDT Fri Jul 31 2026

The lingering low will gradually move east on Sunday with
surface high pressure arriving in its wake. We`ll gradually
start to see a warming trend as the August sun modifies the air
mass and modest ridging replaces the departing trough. By
midweek temperatures will be warming back up closer to normal,
in the upper 80s to low 90s.

Considering the position and trajectory of the incoming surface
high, moisture return from the Gulf will be shut off through
midweek. There`s some discrepancy in guidance on how high
dewpoints get with a skewed distribution in ensemble guidance.
Most raw model guidance is relatively modest in the 60s, while
NBM is at the top of the available guidance in the low to mid
70s. This seems to be driven largely by the bias corrections in
NBM. If these higher dewpoints do occur, then we could be
looking at another period of 100+ heat index values developing.
However, our confidence in this is lowered due to the surface
high blocking moisture from the Gulf, limiting moisture
increases locally to evapotranspiration and moisture pooling
ahead of a potential front later in the week. It`s that next
front in the Thursday-Friday time period that represents our
next best chance for rainfall.

&&

.AVIATION /18Z TAFS THROUGH 00Z SUNDAY/...
Issued at 1233 PM CDT Fri Jul 31 2026

Morning rainfall is moving east of the Mississippi River with
very low thunder potential as activity weakens. MVFR and
isolated IFR will gradually erode with clearing from west to
east this afternoon. The primary concern is thunderstorm
redevelopment this afternoon through this evening. Coverage
looks to be isolated/scattered with the potential that a few
thunderstorms could become strong. This potential largely favors
sections of central and southeast Missouri, where the airmass is
more likely to destabilize with more sunshine. Confidence in
thunderstorm development is high, but intensity is far less
certain due to the stabilizing influence of this morning`s
round. Any thunderstorm will be capable of producing locally
heavy rainfall, gusty winds, and MVFR/IFR visibility
restrictions. If thunderstorms become strong, localized IFR
conditions are expected, which will be handle via amendments.

Conditions remain active through the period as the system rotate
southeastward through the region Saturday. Additional showers
are expected to develop during the afternoon period (after 18z).
MVFR ceilings are expected with IFR possible from KUIN through
the metro terminals late tonight into Saturday morning.

&&

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

&&

$$

UPDATE...Maples
SHORT TERM...Kimble
LONG TERM...Kimble
AVIATION...Maples