614
FXUS63 KLSX 292328
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
628 PM CDT Tue Oct 29 2024

.KEY MESSAGES...

- Well-above normal temperatures and gusty winds will persist through
  Wednesday. Highs in the 80s will be accompanied by southerly
  gusts of up to 30-40 mph.

- A cold front will bring widespread showers and thunderstorms to
  the region Wednesday night into early Thursday morning. A few of
  these storms may be strong to severe with wind gusts to 60 mph
  and a low chance for an isolated tornado (mainly in central MO).

- A cooler and more active pattern could bring additional chances
  for beneficial rain from late weekend into next week.


&&

.SHORT TERM...  (Through Late Wednesday Night)
Issued at 240 PM CDT Tue Oct 29 2024

The morning began incredibly warm with temperatures in the mid-60s
to low-70s. This was already about 5 degrees above the normal highs
(upper 50s/mid-60s) for today. Mainly clear skies have allowed for
deeper mixing, leaning into the warmer side of the guidance as
temperatures sit in the low-80s just prior to 20z. This environment
also supports stronger winds sustained at 15-25 mph and gusts
that have reached 30-40 mph. Consequently, relative humidity has
ranged from the mid-30s to mid-40s (percent). It is conceivable
that RH values could drop a little more as we continue to heat
into the mid-80s, continuing the concern for elevated fire danger
through the next few hours.

Strong surface ridging over the east coast and the developing
surface low at the front range of the Colorado Rockies will extend
the warmth through Wednesday. The primary difference will be the
slight increase in moisture with dewpoint subtly climbing through
the 50s with some locations over central/southeast MO and southwest
IL approaching 60 degrees. Minimum RH values jump by approximately
10 percent, which should be enough to mitigate another day of
elevated fire danger. That being said, southerly winds remain strong
with gusts not much weaker than today. Caution should still be
practiced with the persistent dry conditions we`ve experienced in
recent weeks.

Attention turns to the approaching cold front that will return
temperatures to normal (60s) and bring our first, near-certain (70-
100 percent) shot at measurable precipitation in weeks. Latest
guidance has trended slightly downward in QPF amounts between
Wednesday night and early Thursday. GFS/ECM ensembles show much of
the area in the 0.25-0.50 inch range. This isn`t too surprising
given how the upper trough and bulk of the vorticity remains north
of the CWA, pulling the surface low through Iowa into western
Wisconsin. Much of the precipitation potential over the mid-
Mississippi Valley will rely on the anomalous moisture (PWATs 1.5-
1.75 inches/3-4 standard deviations above normal), strong surface
convergence along the front and limited instability (MLCAPE 200-600
J/kg) to maintain remnant showers and thunderstorms into the diurnal
lull of the nighttime period. Provided the convective nature, while
QPF average will generally fall in the 0.25-0.50 range, locally
higher amounts are possible under the stronger cells.

There are some minor timing differences between deterministic
guidance with NAM/RAP showing a later arrival than most other
guidance. The run-to-run ensembles also point to this trend with the
last 3 update cycles of the 6-hourly precipitation amounts delaying
precipitation accumulation past 06z Wednesday night/Thursday
morning. Fortunately, we may keep severe weather isolated and
limited to the western sections of the CWA. Shear is certainly not
the limiting factor with speeds at 45-50+ knots. MLCAPE values are
marginal and highest (200-600 J/kg) over central Missouri. Values
trail off quickly through the night as the front approaches the
Mississippi River in the same manner than mid-level lapse rates
erode from 7-8C to our west, to essentially nothing as the front
closes in on central Missouri. Ongoing thunderstorms, especially any
that show bowing, may produce isolated damaging wind gusts near 60
mph and a low chance for an isolated tornado over the far western
CWA. This threat is expected to quickly diminish to the east
through early Thursday morning.

Maples
&&

.LONG TERM...  (Thursday through Next Tuesday)
Issued at 240 PM CDT Tue Oct 29 2024

Thursday will see improving conditions as the cold front progresses
to the southeast. Aside from a remnant shower over far southeast
Missouri and southwest Illinois late Thursday morning, precipitation
chances will diminish by afternoon. Much cooler, near-normal air
then surges into the region with an 850mb airmass that fall from the
mid-teens to the single digits (C). Halloween events will feel cool
in comparison to previous days with temperatures falling through the
60s and into the 50s at most locations. The coolest conditions will
be found over central/northeast Missouri and west-central Illinois,
where we cool into the 40s for the tail end of trick-or-treating
(8pm/9pm Thursday evening).

Quasi-zonal upper level flow briefly sets up over the central
sections of the CONUS Friday as surface high pressure extends from
the north-central U.S. southward through the mid-Mississippi Valley.
The surface high continues to build south into the Great Lakes
Region Friday as mid/upper level ridging amplifies over the east
third of the country. This keeps conditions dry and more like fall
with highs in the low to mid-60s.

The next system comes ashore over the Pacific Northwest late Friday
into Saturday, when deterministic guidance diverges on more specific
details in the synoptic pattern. The differences are related to the
orientation of the strong surface ridge over the eastern U.S. and
eastward extent of the western trough. This pattern eventually
introduces additional shortwaves in the upper level flow, which
eject out ahead of the deepening western trough. NBM guidance
spreads low chances (15-40 percent) from west to east through the
day Saturday, but this a product of the diverging solutions.
Realistically, if the surface high holds on long enough, Saturday
may largely end up dry. All global ensemble members show beneficial
rain arriving as soon as Saturday evening with accumulating rainfall
continuing into next week. One thing to note is that not as many
members are producing some of the higher numbers (3-4 inches) as far
east as they were yesterday. Suffice to say, it is only one forecast
cycle and it still to far out to rely on specifics. The outlook
remains positive with much-needed rainfall favored for late weekend
into next week.

Maples
&&

.AVIATION...  (For the 00z TAFs through 00z Wednesday Evening)
Issued at 628 PM CDT Tue Oct 29 2024

VFR flight conditions expected through the forecast area.
Otherwise, the main issue continues to be the gusty south winds.
Will see winds diminish slightly at onset of forecast period, then
winds will pickup by 15z-16z Wednesday with gusts up to 30 to
35kts at times. As for any LLWS tonight, with decent mixing from
the strong winds aloft, confidence is low for any LLWS. Thus did
not add mention to TAFs at this time.

Byrd
&&

.CLIMATE...
Issued at 241 PM CDT Sat Oct 26 2024

Daily High Temperature Records:
     |---Oct 29---|
KSTL   86 F (2016)
KCOU   86 F (1937)
KUIN   85 F (1937)

Daily MAX Low Temperature Records:
     |---Oct 29---|---Oct 30---|
KSTL   69 F (2004)  64 F (1876)
KCOU   67 F (2004)  65 F (1950)
KUIN   67 F (1946)  66 F (1927)


&&

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

$$
WFO LSX