073
FXUS63 KLSX 251740
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
1240 PM CDT Wed Sep 25 2024

.KEY MESSAGES...

- Mostly dry conditions are expected today and Thursday, except
  for isolated showers and thunderstorms across southeastern MO
  and southwestern IL this afternoon and evening as well as
  Thursday evening.

- Rain associated with tropical moisture from Tropical Cyclone
  Helene is most likely (>60 percent chance) along and south of
  I-44 in MO and I-70 in IL early Friday through Saturday, but
  there is still uncertainty in the exact placement and timing of
  this rain along with resulting rainfall amounts.

&&

(Through Late Thursday Afternoon)
Issued at 355 AM CDT Wed Sep 25 2024

An upper-level cutoff low is positioned over the CWA this morning.
Associated mid-level clouds have gradually become more scattered
with time, allowing fog to develop where clearing has occurred due
to radiational cooling of a BL with residual moisture. This fog has
been limited in coverage and is mostly present in river valleys,
with the expectation for it to dissipate shortly after sunrise. The
cutoff low will slowly advance southward today as an upper-level
ridge "breaks" across the Upper Midwest/Mississippi River Valley.
Rising mid-level heights and temperatures in the wake of the cutoff
low will lead to dry conditions aside from isolated showers and
thunderstorms during the afternoon and evening in southeastern MO
and southwestern IL, more closely beneath the cutoff low during the
remainder of the day. There will be less cloud cover today than
Tuesday, despite scattered diurnal cumulus and mid-level clouds,
with warmer temperatures in the 70s F--close to average for late
September. With less clouds tonight into Thursday morning, stronger
radiational cooling and some residual moisture still in place will
likely allow patchy river valley fog development once again.

On Thursday, the upper-level cutoff low will continue to shift
southward into the Lower Mississippi River Valley as it begins to
interact with Tropical Cyclone (TC) Helene, forecast to become a
major hurricane this morning, moving northward and making landfall
around the Florida Panhandle. In advance of TC Helene, a plume of
increasing tropical moisture wrapping around the cutoff low will
begin to surge northwestward into the Ohio Valley and Mid-
Mississippi River Valley. This increasing moisture will mostly lead
to increasing clouds on Thursday, but isolated to scattered showers,
perhaps a thunderstorm, will become possible by the evening in
southeastern MO and southwestern IL. Ahead of thickening cloud
cover, high temperatures on Thursday should be a degree or two
warmer than today.

Pfahler

&&

(Thursday Night through Tuesday)
Issued at 355 AM CDT Wed Sep 25 2024

The forecast remains uncertain late Thursday through the weekend as
the upper-level cutoff low meanders across the Lower Mississippi
River Valley while interacting with and eventually absorbing TC
Helene as it weakens and rotates about the eastern/northeastern
flank of the cutoff low. Throughout this evolution, a broad band of
showers and steadier, heavier rain at times is expected to arc into
the region, associated with the tropical moisture and the potential
arrival of TC Helene`s remnants, but its location and movement with
time will be dictated by the aforementioned evolution. There is
still a considerable amount of variability in model guidance but the
trend over the last 24 hours has been for a slight eastward shift in
the cutoff low position and, therefore, associated showers/rain.
Ensemble model guidance rolling 6-hour probabilities of measurable
rainfall are highest (>60 percent) along and south of I-44 (MO) and
I-70 (IL) early Friday through Saturday but the 25th and 75th
percentiles of total rainfall Thursday through Sunday across those
areas are approximately 0.25 to 3", respectively. This spread is
quite large and demonstrates uncertainty with some members even
having the majority of the rainfall being southeast of the CWA
altogether. NBM high temperature distributions Friday and Saturday
are largely in the 70s F, but wherever persistent, heavier rainfall
occurs temperatures will fall in the lower end of the distributions
and in the upper 60s F.

Sunday into early next week, the upper-level cutoff low is progged
to shift east of the Mid-Mississippi River Valley, with mostly
diurnal isolated to scattered showers lingering as any large-scale
forcing also exits. Model guidance disparities are present early
next week, but there are varying signals for an upper-level trough
across the northern CONUS to force a cold front through the CWA
around Tuesday, cooling temperatures below average. However, these
solutions assume that the evolution of the preceding cutoff low and
TC Helene is captured by the current ensemble model envelope.

Pfahler

&&

.AVIATION... (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