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FXUS63 KLSX 242312
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
512 PM CST Sun Nov 24 2024

.KEY MESSAGES...

- Confidence is increasing that precipitation Wednesday into
  Thursday will be mostly rain, with a rain-snow mix possible
  (20%) at times along the northern edge of precipitation
  overnight into Thursday morning.

- Winter-like temperatures will move into the area late on Thanksgiving
  and stick around through at least the first half of the work
  week.



&&

.SHORT TERM...  (Through Late Monday Night)
Issued at 136 PM CST Sun Nov 24 2024

A clear sky and robust warm air advection amid southwesterly low-
level flow has allowed temperatures to rise into the 60s across much
of the area today. Portions of our metro areas have hit 70 today!
This will be the last nice and warm day for the foreseeable future,
we hope you liked it. A surface low over central Kansas will track
through northern Missouri and Illinois this evening into the
overnight hours, bringing a cold front through the forecast area
tonight. Warm air advection and shallow moisture will keep
precipitation limited to light rain showers overnight into early
Monday morning and probabilities of rainfall greater than 0.10" are
20% or less across the forecast area.

The cold front will usher in a pattern change and kick off the first
of many shots of cold air into the region. High temperatures on
Monday will be stunted significantly compared to today by the
passage of the cold front and to a lesser extend by thick, low cloud
cover across the region. Confidence is growing that the front will
exit the forecast area in the early afternoon, resulting in a large
spread of temperatures across the region. High temperatures across
the northern portions of the forecast area will occur overnight and
remain largely steady through the day. Across the southern portion
of the forecast area temperatures will be able to warm slightly
under the southwesterly flow before the cold front marches through
during the late morning to early afternoon and cools temperatures.

A surface high will push into the region Tuesday in the wake of the
exiting mid-level trough and will help to erode any lingering cloud
cover. Increasing sunshine will help offset some of the cooler
temperatures and highs will push into the mid 40s to low 50s across
the region.

Delia

&&

.LONG TERM...  (Tuesday through Next Sunday)
Issued at 136 PM CST Sun Nov 24 2024

During the first half of the work week a trough will strengthen over
southwestern Canada, fusing with smaller shortwaves and sliding east
into south-central Canada. Another trough that has been spinning off
the coast of the Pacific Northwest will eventually make it on shore
by mid-week, and get caught in the flow of the larger trough and
continue progressing east. Ahead of these waves making it into the
mid-Mississippi Valley, surface flow will become southerly to
southwesterly and advect warmer, slightly moister air into the area.
Despite increasing cloud cover, Wednesday`s highs will be about 5
degrees warmer than Tuesday.

A weak shortwave will slide through the upper-Mississippi Valley
Wednesday into Thursday, and will eventually phase with a stronger
trough moving eastward into the mid-Mississippi Valley. Cyclogenesis
associated with the southern trough will kick off across the Texas
and Oklahoma panhandles earlier in the week and slide through the
lower-Mississippi Valley Wednesday into Thursday. While differences
still exist between the ensemble suites, overall the low has
weakened and continued to shift southward over the most recent runs.
The weaker system, southern shift, and weak moisture return will
result in decreasing precipitation chances across our forecast area
for the holiday. Ensemble guidance has been in agreement the past
few runs that the most likely location for precipitation will be
along and south of the I-70 corridor with higher amounts to the
south of our area closer to the low. In these areas there is an 80%
chance for at least 0.01" of precipitation over 24 hours, though
this probability is inflated slightly by the Euro ensemble system,
which continues to produce a stronger system overall. Probabilities
of greater than 0.1" of precipitation peak between 50-60% largely
along and east of the Mississippi River.

In terms of precipitation type, several factors are contributing to
the reduction in probabilities of snowfall in the CWA. Most
importantly, the southward shift in the probability of precipitation
decreases our chances for snow. Secondly, the advance of the coldest
air associated with this system will be offset from the arrival of
the precipitation. Near surface temperatures will be too warm until
later on Thursday when the more robust trough pushes through the
area. The result is that while a few snowflakes could mix into the
rain on the far northern edge of the precipitation shield,
confidence is decreasing in any kind of accumulation or impact. The
total ensemble suite produces a 30-40% chance of snowfall (> 0.1"),
mainly along and north of the I-70 corridor, but this is again
skewed by the European members that are colder and produce more QPF
across our north. The remaining members indicate a 20-30% chance for
0.1" of snowfall throughout the system. Confidence continues to
remain low that any snow will fall, and should it fall it is
unlikely it would be able to accumulate given the warm ground
temperatures.

A second surge of cold air will arrive Thursday into Friday as
another, stronger mid-level shortwave pushes into the region. Long
term guidance shows evidence of reinforcing shots of cold air in the
broader trough moving through the forecast area during the end of
the period and beyond. The NAEFS and ECMWF situational awareness
tables forecast the resulting temperatures to be in the 10th
climatological percentile in the low-levels. While there remains
uncertainty in the exact temperature, the 75th percentile for NBM
high temperatures is in the mid 30s to low 40s across the forecast
area, increasing confidence in well below normal temperatures
through the holiday weekend.

Delia

&&

.AVIATION...  (For the 00z TAFs through 00z Monday Evening)
Issued at 507 PM CST Sun Nov 24 2024

Ceilings are expected to gradually lower tonight, with MVFR
conditions likely late tonight through at least early/mid Monday
morning. Low stratus is forecast to advect from northwest to
southeast out of the area, with VFR conditions then areawide by
Monday afternoon. Southerly wind gusts should quickly subside this
evening, with winds veering more to the southwest overnight ahead
of a cold front. Behind the front, look for winds to veer to the
northwest. Gusts of 20-25 knots are forecast Monday morning into
the afternoon.

Predominantly dry weather is forecast through the period. A few
very light rain showers/sprinkles may accompany the frontal
passage, but probabilities for any visibility reduction at any
terminal are very low.

Gosselin

&&

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

$$

WFO LSX