Area Forecast Discussion

979
FXUS63 KLSX 111918
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
218 PM CDT Mon Aug 11 2025

.KEY MESSAGES...

- There is a low chance for localized heavy rainfall mainly across
  portions of northeastern Missouri this evening and tonight.

- Seasonable temperatures are expected through Wednesday before hot
  and muggy conditions return at the end of the week and this
  weekend.


&&

.SHORT TERM...  (Through Late Tuesday Night)
Issued at 216 PM CDT Mon Aug 11 2025

Regional satellite imagery and surface observations show a low
across eastern Kansas that is moving northeastward along a front
that is draped southwest to northeast across the Midwest. Water
vapor imagery shows that steering flow supports the low continuing
to move northeastward. As it does so this evening, it will skirt
portions of northeastern Missouri as a focused low-level jet
develops immediately ahead of it, enhancing lift in that part of the
CWA. As a result, isolated scattered showers and thunderstorms are
expected to blossom, but where specifically in regards to the CWA is
still uncertain. Convection may form just west of the CWA, and given
it will be moving north-northeastward, most likely remain just
beyond the CWA border. However, if convection forms further eastward
into portions of northeastern Missouri and possibly even central
Missouri and west-central Illinois, slow moving storms, warm cloud
depths of around 10kft, and PWAT approaching 2" will lead to areas
of localized heavy rainfall and isolated flooding. Rain chances
diminish late tonight/early Tuesday morning as the low continues to
track northeastward.

Through Tuesday morning, outflow from the overnight storms and/or a
cold front dropping into the CWA from the north will serve as the
focus for isolated showers, possibly a thunderstorm. However, the
better chances for rain (40-50%) will be during peak heating in the
afternoon when instability and lift from an approaching shortwave
combine. Where the front is positioned at that time is uncertain,
with one solution being that it is draped across northern portions
of our CWA, while the other primary solution being that it is
roughly along the I-44 corridor in Missouri and I-70 corridor in
Illinois. The former solution favors scattered convection across
much of the CWA, while the latter would confine rain chances mainly
along and south of the aforementioned corridor. Given the uncertainty,
PoPs have been kept broad brushed tomorrow, but given recent
guidance trends, the forecast has been tilted slightly toward the
latter solution. Deep layer shear of 10-15 kts and shear vectors
oriented parallel to the front will favor weak storms that will be
relatively short lived, mitigating a severe threat and greatly
limiting a flooding threat.

With the front moving through the area during the day, a large
spread in temperatures is expected from north to south. Where the
front is expected to clear prior to peak heating - mainly across
northeastern Missouri and west-central Illinois - temperatures are
forecast to top out in the low to mid 80s. For locations south of
the front during peak heating - mainly southeastern Missouri and
southwestern Illinois, another warm and muggy day is expected, with
temperatures rising into the low to mid 90s.

Elmore

&&

.LONG TERM...  (Wednesday through Next Monday)
Issued at 216 PM CDT Mon Aug 11 2025

The consensus among ensemble means and deterministic guidance is
that in the wake of Tuesday`s shortwave weak northwesterly flow will
be present aloft over the Middle Mississippi Valley on Wednesday.
This flow will usher in a slightly cooler airmass, bringing
temperatures down to or just below climatological normals (upper
80s) for most locations per ensemble means.

This relative cool down will be short lived, as confidence is high
that upper-level ridging will build back into much of the central
CONUS with vengeance Thursday into Friday, signaling the return of
heat. This heat is not expected to be realized until Friday, as a
majority of guidance has a surface high stubbornly positioned east
of the CWA on Thursday, keeping low-level flow east-southeasterly.
However, ensemble means support an approximately 5-degree jump in
temperatures for Friday as the ridge amplifies, the surface high
moves eastward, and low-level flow becomes south-southwesterly. This
heat peaks over the weekend, though spread in ridge positioning and
the orientation of the low-level flow making confidence low in
exactly how warm and humid. Those finer details will likely be moot,
as the 25th percentile of ensemble guidance over the weekend has
temperatures topping out in the low 90s, pointing to that barring a
drastic change in guidance, mid 90s will be realized for most
locations.

Elmore

&&

.AVIATION...  (For the 18z TAFs through 18z Tuesday Afternoon)
Issued at 1213 PM CDT Mon Aug 11 2025

Mostly VFR flight conditions are expected at the local terminals
through the TAF period, though there is a very low chance for
impacts late this evening and then near the end of period. This
evening into tonight, scattered showers and thunderstorms are
forecast to develop just west of KCOU, KJEF, and KUIN. Some of
them could briefly impact the terminals with reduced visibility,
and even if they remain west of the these terminals, gusts of wind
generally from the west would still be possible. Late tomorrow
morning into the afternoon, additional showers and thunderstorms
are forecast along a cold front. Given the lead time and
uncertainty in the timing of the front, impacts have been left out
of the TAFs for now.

Elmore

&&

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

$$

WFO LSX

NWS LSX Office Area Forecast Discussion