868
FXUS63 KLSX 311120
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
620 AM 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.

&&

.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 THURSDAY/...
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 /12Z TAFS THROUGH 18Z SATURDAY/...
Issued at 620 AM CDT Fri Jul 31 2026

A weakening convective complex will continue to push east across
the region this morning. All terminals are likely to be affected
with at least some rain, though it`s unclear how long it will
maintain its thunder and lightning as it approaches the St Louis
metro. As this activity exits to the east our eyes turn to a
cold front approaching form the northwest during the afternoon.
Another round of thunderstorms is expected to develop ahead of
this front, though confidence on impacts at each terminal
remains low enough for a PROB30 covering the most likely periods
for now. As the front slides southeast it ends the thunderstorms
by this evening, but guidance is indicating some lower ceilings
developing behind the front. This is most likely to affect UIN,
but could affect all terminals with conditions as low as IFR
through Saturday morning.

&&

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

&&

$$

SHORT TERM...Kimble
LONG TERM...Kimble
AVIATION...Kimble