578
FXUS63 KLSX 251955
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
255 PM CDT Wed Sep 25 2024

.KEY MESSAGES...

- Generally dry conditions are expected through Thursday
  evening/night.

- Rain associated with tropical moisture from Tropical Cyclone
  Helene is likely to move into the region Friday morning,
  continuing through Saturday morning.

- An axis of 1-3" of rain is expected. Recent model trends have shifted
  this axis further south southeast of the STL metro. However,
  uncertainty remains in the exact placement of this heavy rain
  axis.

&&

.SHORT TERM... (Through Late Thursday Night)
Issued at 145 PM CDT Wed Sep 25 2024

The cut off low continues to spin over southern Illinois/SE
Missouri this afternoon, with the vast majority of our region
on the subsidence side of the cut off low and associated trough.
That being said, a scattered cumulus field has developed this
afternoon, with a few isolated showers ongoing south of the STL
Metro toward the St. Francois Mountains. Showers (and the
scattered Cu field) should dissipate by late afternoon/early
evening with the loss of daytime heating. Generally clear skies
tonight paired with light winds should allow for decent
radiational cooling, with overnight/morning lows ranging from
the low to mid 50s (upper 50s over the STL urban heat island).
Fog is not anticipated to be as widespread tomorrow morning, but
patchy fog will be possible once again in the river valleys.

Tomorrow, the aforementioned cut off low slowly slides a bit
further south toward the Missouri Bootheel as mid level ridging
noses its way into the Upper Midwest. Meanwhile, Hurricane
Helene will move northward across the eastern Gulf of Mexico,
making landfall across the panhandle/big bend region of Florida
around 7 pm tomorrow. The result for our region will be high
temperatures generally in the upper 70s under partly cloudy
skies for the daytime hours, with increasing high level clouds
overspreading the region from the east/southeast tomorrow
evening/night as a plume of tropical moisture wraps around the
cut off low and nudges its way into our area. Some drizzle/light
showers may be possible as early as late Thursday evening/night
across our far southeastern areas, but otherwise rain should off
until after midnight Friday morning.

BMW

&&

.LONG TERM... (Friday through Next Wednesday)
Issued at 145 PM CDT Wed Sep 25 2024

The cut off low continues its slow meander southward early
Friday morning, where it will begin to interact with Helene and
likely slowly absorb it into one primary center of low
pressure. Increased PWATs, potentially as high as 2", and
associated showers, is progged to rotate around the tropical low
and into portions of our CWA, especially areas south/southeast
of STL, during the daytime hours of Friday. There is still a
decent amount of uncertainty in the exact evolution of these
features and just how far into our CWA the axis of heavier
rainfall may progress. The 12z suite of models has continued
with a slight southeastward trend for the axis of heaviest
rainfall, with a fairly large rainfall accumulation gradient
likely setting up across our region. The most recent WPC
guidance suggests up to 3" of rain is possible from Friday
morning through Saturday morning across the far south
southeastern reaches of our CWA (Ellington to Fredericktown to
Ste. Genevieve), with only a half inch or so across the STL
metro. NBM probabilistic guidance shows roughly a 25% chance for
rainfall exceeding an inch, and around a 10% chance for
rainfall exceeding 2" at STL (Lambert). For the Ste. Genevieve
area, NBM probabilities for rainfall exceeding 1", 2" and 3" are
55%, 33%, and 17%, respectively. High temperatures for Friday
into Saturday should remain generally into the mid 70s, with
breezy northerly winds (especially on Friday afternoon).

Sunday into early next week, the upper-level cutoff low is progged
to shift eastward into the Ohio Valley, with mostly diurnal
isolated to scattered showers lingering as larger scale forcing
exits. Model guidance continues to suggest an upper level trough
moving across the Northern Plains into the Great Lakes region
early next week, bringing a cold front through the region from
northwest to southeast on Tuesday, bringing a much cooler and
drier air mass to the region in its wake.

BMW

&&

(For the 18z TAFs through 18z Thursday afternoon)
Issued at 1229 PM CDT Wed Sep 25 2024

Currently VFR at all terminals, although a scattered cumulus
field around 4000 feet has developed this afternoon. A few
isolated showers may be possible this afternoon in the vicinity
of the St. Louis metro terminals, but coverage/confidence will
be low enough to preclude any precip mention in the TAF. The
cumulus field should dissipate with the loss of daytime heating
this evening, with generally clear skies persisting through the
remainder of the period. Patchy fog may be possible once again
toward sunrise tomorrow morning, primarily in the river valleys,
but have kept any lowered VSBYs out of the TAF for now. Winds
should remain light and out of the north (fluctuating between
roughly 320 and 030 degrees) before establishing themselves out
of the NNE by mid to late tomorrow morning, remaining below 10
knots.

BMW

&&

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

&&

$$

WFO LSX