849
FXUS63 KLSX 241306
AFDLSX

Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
806 AM CDT Tue Sep 24 2024

.KEY MESSAGES...

- Other than a few afternoon showers and storms today, largely dry
  conditions are expected through Wednesday.

- The next opportunity for rain will come Thursday through the
  upcoming weekend, but there is still large uncertainty in its
  timing and rainfall amounts.


&&

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 412 AM CDT Tue Sep 24 2024

An upper-level shortwave trough is traversing the Mid-Mississippi
River Valley this morning, while becoming increasingly negatively
tilted. In response, increasing large-scale ascent and weak low to
mid-level frontogenesis to the northwest of a developing surface low
in southern IL will continue to support showers across northeast MO
and west-central and south-central IL. With MUCAPE around 500 to 750
J/kg an isolated thunderstorm or two is also possible. Most showers
will depart to the northeast as the trough begins to depart, but
confluent low-level flow, steepening low-level lapse rates, and
large-scale ascent associated with the arrival of another, more
potent upper-level trough will support isolated to scattered showers
at times across a broad portion of the CWA through the day.  A few
stray lightning strikes could accompany these showers. Some showers
could even linger past sunset in central and northeastern MO as a
weakening cold front arrives. Widespread cloud cover and weak low-
level CAA will keep high temperatures today around 70 to the mid-70s
F.

Clouds will gradually decrease tonight from west to east,
potentially leading to patchy fog by Wednesday morning as winds
become light/calm with relaxation of the SLP gradient. The coverage
of fog will be dependent on how much of the CWA sees clearing
overnight, which varies among short-term model guidance.

Much drier conditions are expected on Wednesday as mid-level heights
rise in association with an increasingly positively tilted ridge
"breaking" and promoting formation of an upper-level cutoff low from
the aforementioned trough across the Mid-Mississippi River Valley.
Cloud cover on Wednesday will be much less than today but there will
likely still be scattered to broken diurnal cumulus and mid-level
clouds. Marginally greater insolation will permit slightly warmer
high temperatures, although again in the 70s F and seasonable.
Forcing for any precip on Wednesday will be rather nebulous, but
there could be enough residual moisture and insolation to produce an
isolated shower or two across southeastern MO and southwestern IL.

Pfahler

&&

.LONG TERM...  (Wednesday Night through Monday)
Issued at 412 AM CDT Tue Sep 24 2024

Thursday through the weekend, the forecast remains highly uncertain
due to the expected complex interaction of the upper-level closed
low near the Mid-Mississippi River Valley and Potential Tropical
Cyclone (PTC) Nine likely making landfall along the Gulf Coast as it
tracks northward. This evolution includes a non-linear, Fujiwhara-
like interaction (both features simultaneously influencing the
movement of one another) followed by absorption of PTC Nine into the
cutoff low featured in varying capacities across model guidance.
Well in advance of PTC Nine, the cutoff low is expected to interact
with a plume of anomalous, tropical moisture to produce showers and
a few thunderstorms and even steadier rain at times around its
eastern and northeastern flank somewhere across the Mid-Mississippi
River Valley and Ozarks by Thursday. Ensemble model guidance differ
on the exact placement of these showers/rain as well as their
residence time at a given location through the weekend while PTC
Nine itself possibly tracks closer to the region. To contextualize
the uncertainty, there are still members that have little to no
rainfall across much of the CWA, but the number of members that have
over 1" of total rainfall in the CWA has increased from 24 hours
ago. Regardless of precip, widespread clouds are expected through
most the period. These clouds will at least keep temperatures from
warming much and in the 70s F; however, NBM interquartile
temperature ranges span 5 to 8 F, mainly due to differences in the
influence of showers/rain across members.

Pfahler

&&

.AVIATION...  (For the 12z TAFs through 12z Wednesday Morning)
Issued at 630 AM CDT Tue Sep 24 2024

A gradual improvement in flight conditions is expected today with
MVFR and IFR flight conditions from low ceilings and patchy
fog/drizzle giving way to predominantly VFR flight conditions during
the afternoon as the lowest ceilings lift and scatter. Isolated to
scattered showers, possibly a thunderstorm, are expected at times
through this evening. Uncertain timing of showers and direct impacts
through this broad time period precludes more than VCSH, but brief
bouts of MVFR to IFR could occur if a shower moves directly a
terminal. Predominantly dry conditions will return tonight; however,
fog development will need to monitored tonight, depending on how
much clearing of clouds occurs.

Pfahler

&&

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

$$

WFO LSX