935
FXUS63 KLSX 230919
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
419 AM CDT Mon Sep 23 2024

.KEY MESSAGES...

- A severe thunderstorm or two is possible across southeastern MO
  and southwestern IL this afternoon and evening with damaging
  winds and marginally severe hail.

- Multiple rounds of showers and thunderstorms are expected to
  persist into Tuesday with additional beneficial rainfall across
  the region.

- Additional showers are possible Thursday through the upcoming
  weekend, but the forecast is highly uncertain and dependent on
  the evolution of a tropical disturbance currently in the
  Caribbean Gulf.


&&

.SHORT TERM...  (Through Late Tuesday Afternoon)
Issued at 418 AM CDT Mon Sep 23 2024

Multiple rounds of numerous showers and thunderstorms are expected
through Tuesday morning as at least two mid-level shortwave troughs
pass over the Mid-Mississippi River Valley, increasing large-scale
ascent and enhancing prevailing, broad low to mid-level
WAA/isentropic ascent across the region. Showers and thunderstorms
associated with the first of these rounds are already developing
across southern and southwestern MO and are expected to increase in
coverage and spread northeastward into a broad portion of the CWA,
continuing through the morning. By afternoon, CAMs start to disagree
on the coverage of showers and thunderstorms and whether or not
there will be any breaks in cloud cover. At least modest elevated
instability (MUCAPE up to 750 J/kg) sufficient for thunderstorms is
indicated by short-term model guidance through tonight up to the I-
70 corridor, but the development of surface-based instability this
afternoon across southeastern MO and southwestern IL is much more
uncertain and contingent on breaks in cloud cover. The latest HREF
interquartile range of SBCAPE across southeastern MO and far
southwestern IL spans 100 to 1500 J/kg. With 25 to 35 kt of deep-
layer shear forecast, the higher end of this SBCAPE distribution
would be sufficient for a severe thunderstorm or two with damaging
winds this afternoon and evening, but even an elevated thunderstorm
could be capable of marginally severe hail. Similarly, high
temperatures today will be limited by the cloud cover and precip to
the mid-60s to mid-70s F.

Short-term model guidance and CAMs agree that there will be at least
one additional round of numerous showers and thunderstorms this
evening and/or early Tuesday morning as the main mid-level shortwave
trough begins to approach and then cross the Mid-Mississippi River
Valley. This trough is also expected to facilitate gradual low-level
cyclogenesis across the region, providing additional low-level
moisture convergence that could further augment shower and
thunderstorms coverage. Most showers and thunderstorms will exit the
CWA during the morning on Tuesday as the shortwave trough departs,
but the combination of another upper-level trough quickly arriving
in its wake and steepening low-level lapse rates from weak low-level
CAA in tandem with modest insolation, will be catalysts for
scattered showers and developing during the afternoon. With precip
less prevalent on Tuesday, temperatures will be slightly warmer in
most locations than today and around 70 to the mid-70s F.

The showers and thunderstorms through Tuesday are expected to
provide additional widespread beneficial rainfall, with the latest
HREF QPF LPMM indicating some swaths of 1 to 3" somewhere in the
general vicinity of I-44 in MO and I-70 in IL. Since this rainfall
is likely to fall in multiple waves, prolific training of
thunderstorms is not expected, and soils are moistening but still
relatively dry, the threat of any flooding is low.

Pfahler

&&

.LONG TERM...  (Tuesday Night through Sunday)
Issued at 418 AM CDT Mon Sep 23 2024

On Wednesday, drier conditions are expected across the Mid-
Mississippi River Valley as mid-level heights rise in association
with an upper-level ridge "breaking" and leading to cutoff
cyclogenesis from the trough across the Mid to Lower Mississippi
River Valley. Less cloud cover and subsiding low-level CAA support
temperatures similar or slightly warmer than Tuesday.

Thursday through the upcoming weekend, sensible weather across the
Mid-Mississippi River Valley will be greatly influenced by the
position of the upper-level cutoff low and its interaction with a
tropical disturbance, currently in the northwestern Caribbean Gulf
(AL97), that will likely form into a tropical cyclone in the next
48 hours and track northward across the Gulf of Mexico and then Gulf
Coast. Global model guidance are coming into better agreement that a
plume of moisture ahead of the to-be tropical cyclone will begin to
wrap northward around the cutoff low into the Mid-Mississippi River
Valley by Thursday, resulting in periods of showers. Friday through
the weekend, ensemble model guidance has the tropical cyclone itself
tracking into the Ohio River Valley and/or the Mid-Mississippi River
Valley as a result of a complex interaction with the cutoff low. The
complexity of this evolution and the fact that the tropical cyclone
has yet to develop leads to a highly variable and low-confidence
forecast in terms of precip through the weekend. As a result,
ensemble model membership QPF ranges from little to no rainfall
across much of the CWA to over 1" of rainfall. There will at least
be increasing cloud cover limiting high temperatures to the 70s F,
but NBM interquartile temperature ranges span 5 to 8 F from the
differences in precip amounts.

Pfahler

&&

.AVIATION...  (For the 06z TAFs through 06z Monday Night)
Issued at 1123 PM CDT Sun Sep 22 2024

A broad area of MVFR and IFR flight conditions over central and
northeast Missouri is expected to overspread most of the area
through the rest of the night. Widely scattered showers have
developed over central Missouri and this development will continue
to spread eastward through the early morning hours. The strength
and coverage of this convection is still somewhat uncertain,
though most guidance agrees that it will continue at least
through mid-morning, with diminishing activity by later in the
morning or early afternoon and then another uptick in activity
Monday evening.

Carney

&&

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

$$

WFO LSX