017
FXUS63 KLSX 081911
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
111 PM CST Fri Nov 8 2024

.KEY MESSAGES...

- Areawide rainfall is forecast Saturday, with most locations seeing
  0.25-0.5" of accumulation. Localized amounts of 0.75-1" are
  possible.

- Even with this rainfall, confidence is high (90%) that
  additional flooding on area rivers will not occur.

- Additional rainfall is forecast during the middle of next week,
  with a 40% chance that totals exceed 0.20".


&&

.SHORT TERM...  (Through Late Saturday Night)
Issued at 110 PM CST Fri Nov 8 2024

Afternoon water vapor imagery shows a deep upper-level trough
becoming cut off over the western CONUS as a ridge builds into the
Midwest. While mid-level temperatures have bumped a degree or two in
response compared to yesterday, the thicker upper-level cloud cover
and east-northeasterly surface flow have kept temperatures similar
to yesterday.

Cloud cover will increase through the night as a surface low
deepens across the Central Plains. This low will begin occluding
tonight as it moves into the Midwest, though forcing from the
occluded front, a low-level jet, and upper-level divergence will
support a swath of scattered to numerous showers and possibly an
isolated thunderstorm that will enter central Missouri during the
early morning hours. This rainfall will gradually move west to east
across the CWA through the day Saturday into Saturday night. The 12z
HREF supports generally 0.25-0.5" across the CWA by the time rain
winds down late Saturday night/early Sunday morning, with localized
amounts of 0.75-1" possible.

Given the recent rainfall and current flooding along the Meramec
River, there is understandably some concern that additional rainfall
will exacerbate current impacts. However, using the 95th percentile
of forecasted rainfall over the next 72 hours within the Meramec
basin delays the river falling below Minor Flood Stage at points
along the river by a few hours per ensemble guidance. Therefore,
confidence is currently high (90%) that additional impactful river
flooding is low.

Elmore

&&

.LONG TERM...  (Sunday through Next Friday)
Issued at 110 PM CST Fri Nov 8 2024

Despite subtle differences in phasing, guidance consensus is that
the upper-level trough will be moving east-northeastward through the
Upper-Midwest at the start of the extended period. At the surface,
the associated surface low will be following a similar trajectory,
with its attendant cold front and rain chances exiting the CWA to
the east in the morning. Given the deamplification of the trough as
it passes through the Midwest and quasi-zonal flow in its wake,
strong cold air advection is not expected, with the post-frontal
airmass being more Pacific in nature. Therefore, temperatures on
Sunday will see very little impact. In fact, with post-frontal
southwesterly flow providing adiabatic warming via downsloping off
the Ozarks and skies clearing early, ensemble consensus is that
temperatures for a majority of locations in the CWA will be warmer
than those expected on Saturday.

For Monday and beyond, guidance diverges notably in the phasing of
the upper-level pattern, though the general consensus is that it
will be progressive and active. The first area of uncertainty
involves a quick-moving shortwave on Monday following by a ridge
building into the Midwest and eastern CONUS. With the shortwave,
guidance can be divided into two camps: one where the shortwave
exists and sends a weak cold front through the region, and one where
the shortwave fails to materialize. Given the weak nature of the
potential cold front, lack of moisture return, and weak forcing
aloft, even if the first solution comes to fruition, guidance is dry
and very little if any impact is expected to temperatures. Ridging
building into the area toward the middle of next week will generally
keep temperatures steady relative to early week values.

Confidence is relatively higher in a high-amplitude trough moving
through the CONUS during the middle to end end of next week, though
the timing and how much moisture will be available for rainfall
remains uncertain. Currently, ensemble-based probabilities top out
around 50-60% for measurable rainfall (0.01") with this trough and
its associated surface system. Temperatures to end the week take
about a 5-degree hit per ensemble means, but remain roughly around
seasonal normals.

Elmore

&&

.AVIATION...  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1116 AM CST Fri Nov 8 2024

VFR flight conditions are expected through much of the period
ahead of an approaching cold front. As this front approaches the
local terminals late tonight and tomorrow morning, rain chances
will increase from west to east. Rain is generally expected to be
light, but there could be scattered heavier showers that cause brief
drops in visibility, possibly producing MVFR visibilities if
direct impact occurs at terminals. There is also a low chance of
isolated thunderstorms during the day tomorrow. However, direct
impacts from thunderstorms and heavier rainfall to local terminals
remain low and have been left out of the TAFs for now. Ceilings
will also begin lowering right at the end or just beyond the
current TAF period, with gradually lowering ceilings expected just
beyond the current period.

Elmore

&&

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

$$

WFO LSX