355
FXUS63 KLSX 121945
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
245 PM CDT Wed Aug 12 2026

.KEY MESSAGES...

- Dangerous heat and humidity will continue for much of the area
through Saturday, and potentially into Sunday.

- Temperatures and humidity levels are expected to drop back
  closer to normal levels early next week behind a cold front,
  providing a reprieve from the heat.

&&

.SHORT TERM /THROUGH THURSDAY/...
Issued at 244 PM CDT Wed Aug 12 2026

GOES-19 visible satellite imagery and radar imagery show a
couple of features of interest. One is an MCV near the Iowa-
Illinois border moving east/southeast. Lift downstream of this
feature (along with cooling aloft) has helped initiate
convection along a remnant outflow boundary near the Illinois-
Indiana border. Additional development farther to the southwest
along the boundary is possible, but storms generally now are
much more likely to stay to our east. Therefore, the severe
threat has lessened across the area quite a bit. That being
said, IF any storm were to develop in our area, there is plenty
of instability (SBCAPE 4000+ J/kg) and seasonably strong 0-6 km
shear (20-30 knots). There is also ongoing (elevated) showers
and a few thunderstorms across northeast Missouri and west-
central Illinois. These storms have percolated all day and have
really augmented the surface environment. Temperatures have been
quite a bit cooler than forecast, with readings mostly in the
70s to low 80s. As a result, have cancelled the heat advisory
across portions of northeast Missouri and west-central Illinois.
This activity should in theory wane and dissipate over the next
few hours as 850-hPa flow further weakens and its associated
moisture convergence.

Global guidance as well as CAMs are in pretty good agreement that
another MCS will track eastward across Iowa late tonight into
Thursday morning. This should stay north of us, though elevated
showers/thunderstorms are possible along/north of the quasi-
stationary front. Where exactly it will be is not certain, but best
chances are roughly along/east of the Mississippi River. This front
is then expected to move slowly northeastward through the day and
take the better chances of showers and storms with it on Thursday.
CAMs do have some isolated thunderstorm development within the warm
sector, which seems plausible given a weaker cap likely in place.
There will be another very unstable atmosphere in place along with
still seasonably strong deep-layer shear (20-25+ knots). Therefore,
a mix of pulse/multicellular clusters are possible with damaging
winds the main threat. There may even be a low probability for a
supercell to travel along the boundary, though this threat may be
just to our northeast, if it happens at all. As for the heat, there
remains high confidence in another very hot/humid day for most, with
highs in the 90s and peak heat index values of 105-110F. The
exception if parts of northeast Missouri and west-central Illinois.
Morning convective debris clouds along with possible outflow from
the MCS are certainly plausible, which could limit temperatures. A
slower poleward movement of the front also could happen and keep
readings a bit cooler than expected. Have left those areas out of
any heat products tomorrow as a result.

&&

.LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/...
Issued at 244 PM CDT Wed Aug 12 2026

(Thursday Night - Sunday Night)

A near repeat is expected Thursday night into Friday with yet
another MCS likely moving just to our north. This means that exact
temperatures and heat index values in parts of northeast Missouri
and west-central Illinois. Farther south however, the dangerous heat
and humidity will continue at least through Saturday. Temperatures
may rise slightly with each day as the low-level thermal ridge moves
overhead. These rising temperatures though may be offset by lower
dewpoints as topsoil continues to dry out in many locations without
any significant rain in 7+ days. That means that peak heat index
values each day are likely to stay about the same, generally in the
105-110F range. Due to the longevity of these values south of I-70,
did go ahead and convert the remainder of the advisory in southeast
Missouri over to an extreme heat warning.

Confidence decreases by Sunday as there are differences with the
timing of a cold front. In addition, there is a signal for another
potential MCS late Saturday night into Sunday morning, but this
time, it may be farther south and impact parts of the area. This
leaves much of the area very uncertain for specific high
temperatures and peak heat index values and was why heat headlines
were not extended through Sunday. However, the farther south you
travel, the better chance for another day of dangerous heat and
humidity. There may also be at least a low threat for a few strong
to severe thunderstorms along/ahead of the front Sunday afternoon-
evening given high instability and 20-25+ knots of deep-layer shear
expected, but exactly where is heavily dependent on the exact cold
frontal timing.


(Monday - Next Wednesday)

Behind the front, both temperatures and humidity levels are expected
to come back closer to normal mid August levels. However, it is
definitely a summer front where the dewpoint/humidity drop is slow.
Therefore, a slower frontal passage than expected could mean another
day with near 100 degree heat index values in southeast
Missouri/southwest Illinois. By Tuesday however, heat index values
should stay in the 90s areawide.

Forecast uncertainty increases heading toward midweek (and beyond).
Clusters of the 500-hPa height pattern show the overall variability
well, with differences in placement and strength of the midlevel
ridge as well as a trough crossing the Great Lakes/southeast Canada.
If the ridge is stronger/more to the east, we may return to
dangerous heat and humidity. However, a weaker ridge centered more
to the southwest may allow another cold front to move through and
continue the more near-normal temperatures (and bring renewed
chances of showers and thunderstorms). The overall membership of the
LREF is pretty split between these two scenarios, though I will note
that this is a bit of a change compared to yesterday, which
suggested a quick return to the dangerous heat and humidity.

&&

.AVIATION /18Z TAFS THROUGH 00Z FRIDAY/...
Issued at 1226 PM CDT Wed Aug 12 2026

An outflow boundary is currently passing through the metro. It
has weakened, though a brief wind shift to the north/northwest
may happen (~6-8 knots) before they back to the west/southwest
once again. Weak showers and thunderstorms may continue to
percolate across northeast Missouri and west-central Illinois
the next few hours. Brief reductions in visibility may occur at
KUIN early this afternoon. Farther south, additional
thunderstorms may fire along/near the aforementioned outflow
boundary in the next 1-2 hours. Have therefore bumped up the
PROB30 groups in the metro accordingly.

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 9 PM CDT Saturday for MOZ034>036-042-052.
     Extreme Heat Warning until 9 PM CDT Saturday for MOZ041-
     047>051-059>065-072>075-084-085-099.
IL...Heat Advisory until 9 PM CDT Saturday for ILZ058>060-064-065-
     070-098-099.
     Extreme Heat Warning until 9 PM CDT Saturday for ILZ069-074-
     079-100>102.

&&

$$

SHORT TERM...Gosselin
LONG TERM...Gosselin
AVIATION...Gosselin