828
FXUS63 KLSX 161924
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
224 PM CDT Mon Sep 16 2024

.KEY MESSAGES...

- Most of the area will remain dry through this week.

- Above normal temperatures will persist through this week, with
  temperatures gradually warming each day.


&&

.SHORT TERM...  (Through Late Tuesday Night)
Issued at 222 PM CDT Mon Sep 16 2024

Water vapor imagery currently shows the remnants of Francine being
sheared apart over the Midwest as another tropical system comes
ashore at the Carolinas and a deep trough digs over the western
CONUS. Lift from Francine`s remnants combined with about 1,500 J/kg
of SBCAPE per recent SPC Mesoanalysis has led to isolated to
scattered showers and a brief thunderstorm or two this afternoon.
Due to the placement of lift and deeper moisture, this convection is
expected to remain confined mainly to central and southeastern
Missouri through early this evening when instability decreases as
the sun sets.

Tonight, Francine`s remnants will continue to be sheared apart and
dissipate. Moisture pooling along the periphery of high pressure
over the Midwest will lead to patchy fog mainly across central and
southeastern Missouri as temperatures cool into the low 60s area
wide.

Despite Francine`s remnants dissipating by tomorrow, the
aforementioned tropical system coming ashore at the Carolinas will
drift into the Ohio Valley as a cutoff and keep the Rex Block going
across much of the eastern CONUS. Thusly, the CWA will once again be
between the Rex Block just to the east and the upper-level trough to
the west, keeping our temperatures during Tuesday afternoon similar
to those today. The exception is a slight boost over portions of
central and southeastern Missouri thanks to less cloud cover. A lack
of upper-level lift, sufficient boundary layer moisture, and
instability is expected to lead to a convection-free day.

Elmore

&&

.LONG TERM...  (Wednesday through Next Monday)
Issued at 222 PM CDT Mon Sep 16 2024

At the start of the long term period, guidance consensus is that the
Middle Mississippi Valley will still be situated between an upper-
level trough to the west and a cutoff to the east. This pattern will
keep a broad, nebulous surface high situated over the Great Lakes.
Given its positioning, the CWA will be on the edge of a surface wind
direction transition, with easterly winds to the east and southerly
winds to the west. Given the lead time and current spread in
guidance, confidence is low in where exactly this transition will
take place. If winds remain more easterly, the current temperature
forecast may be a few degrees on the warmer end of possibilities. If
winds are more southerly, then more wide-spread values in the upper
80s to near 90 degrees can be expected.

On Thursday, the eastern cutoff will weaken as it slowly drifts
eastward and begins to phase with a deepening trough over the
northeastern U.S. as a shortwave within the western trough moves
into Canada. The consensus among ensembles is as this occurs, a
portion of the trough will become cut off over the southwestern
CONUS late Thursday into Friday. However, solutions diverge notably
beyond this point, as ensembles vary on the phasing of the cutoff.
In general, an upper-level ridge building into the Middle
Mississippi Valley ahead of the cutoff will allow for a thermal
ridge to shift eastward over the region, causing mid-level
temperatures to warm through the end of the week. This will
correlate with warming surface temperatures, with the ensemble
median hovering around 90 degrees.

How the cutoff phases as we get into the weekend is uncertain and
will dictate our temperatures and rain chances. A portion of
guidance has the cutoff only slowly progressing eastward and
stalling west of the CWA for the weekend, keeping temperatures
similar to those experienced late in the week and conditions mainly
dry. The other camp that guidance falls into is that the cutoff
begins to phase quickly with southwesterly flow over the Northern
Plains and passes north of the region, causing temperatures to drop
and rain chances to increase. Confidence in either solution at this
lead time is low, and the going forecast represents the middle
ground. Even with the second solution, moisture return ahead of the
cutoff will be meager, leading to low rain chances at best and
continuing drought conditions for much of the area.

Elmore

&&

.AVIATION...  (For the 18z TAFs through 18z Tuesday Afternoon)
Issued at 1154 AM CDT Mon Sep 16 2024

VFR flight conditions are expected through much of the TAF period
at all local terminals. Isolated showers and possibly a
thunderstorm are expected this afternoon mainly south of the local
terminals. If any of the local terminals were to be impacted by
these, KCOU and KJEF have the best chances; though, the chances
are still quite low. Another round of fog is expected tonight,
though impacts to terminals are uncertain currently, so reductions
in visibility have been left of the TAFs for now.

Elmore

&&

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

$$

WFO LSX