510
FXUS63 KLSX 072355
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
555 PM CST Fri Feb 7 2025

.KEY MESSAGES...

- Expect intermittent light rain tonight. There may also be areas
  of drizzle. There is a <20% chance for freezing rain/drizzle
  before sunrise across far northeast Missouri and west central
  Illinois, though little if any impact is expected. Rain will end
  from west to east on Saturday afternoon.

- Accumulating snow continues to look likely (70-90%) along and
  south of the I-70 corridor on Monday night. Snow may mix with or
  switch over to freezing rain across parts of the eastern Ozarks
  before sunrise Tuesday morning. Accumulations and impacts remain
  uncertain at this time.

- A second storm is expected Wednesday into Wednesday night. This
  second storm has the potential to be stronger and produce
  greater snowfall accumulations, however the track of the storm
  is very uncertain at this time.


&&

.SHORT TERM...  (Through Late Saturday Night)
Issued at 247 PM CST Fri Feb 7 2025

A short wave trough moving off the Rockies into the western Plains
is forcing low level cyclogenesis over the plains this afternoon.
This is turning the flow between 850-700mb around to the south and
southwest and producing weak to moderate moisture convergence and
light showers along and south of I-70 this afternoon.  Much of this
precip is not reaching the ground due dry air near the surface with
10-15 degree dew point depressions.  This very light precip will
continue through tonight, though the low levels will moisten up so
eventually more and more will reach the ground this evening and
tonight.  In addition to the light rain showers, there is some
signal for drizzle, across northeast Missouri into west central
Illinois mainly after midnight as low level RH increases to nearly
100% and low level lift increases ahead of the approaching wave.
There`s some concern for freezing drizzle in far northeast
Missouri/west central Illinois.  However, I think the chance is very
low (<20%) that there will be any impacts if there is freezing
drizzle as temperatures should be in the 32-34 degree range, and
southeast flow is not conducive to cold advection needed to offset
the release of latent heat from freezing.

The wave will move through the Plains into the Midwest on Saturday,
with the low level trough just ahead of it producing another round
of moisture convergence over eastern Missouri and western Illinois.
There won`t be a layer of dry air to contend with Saturday morning,
so 60-80% PoPs are in order on the nose of the low level jet from
east central Missouri into southwest and south central Illinois
where the strongest moisture convergence is indicated.  Rain will
end from west to east as the the wave exits into the Ohio Valley
Saturday afternoon and drier air moves in behind the cold front.
Cold high pressure builds into the Iowa and eastern Nebraska
Saturday night, and northerly flow across the mid Mississippi Valley
will push temperatures down into the 20s.


Carney

&&

.LONG TERM...  (Sunday through Next Friday)
Issued at 247 PM CST Fri Feb 7 2025

The primary concern in the medium range is the chance for wintry
precip Monday night into Tuesday morning, and again Wednesday into
Wednesday night.  After a relatively cool and dry Sunday under the
influence of high pressure, low level flow will swing around to a
southerly direction again in response to the next upstream short
wave as it enters the Plains.  The deterministic GFS and ECMWF shows
the potential for precip starting as early as Monday afternoon,
however when digging into the ensembles the 50-75th percentiles are
dry.  Only the 90th and up show any precip over our area which lends
higher confidence to the dry NBM forecast for our area Monday
afternoon.  Better forcing and deeper moisture moves into the area
on Monday night as the low level wave moves from the east central
Plains into the Mississippi Valley.  The primary forcing mechanism
for this precip appears to be low level warm advection/isentropic
lift overnight on Monday night.  Forecast soundings show a limited
amount of time with a saturated dendritic growth zone, so
snow/liquid ratios are not expected to be particularly high, but the
entire column where lift is occurring is cold enough to support snow
for most of the forecast area.  The exception is our far southern
counties across the eastern Ozarks where we loose cloud ice.
Therefore there could be periods of mixed precipitation
(snow/freezing rain) or just freezing rain.  Luckily that mixed
precip potential looks highest during the overnight period when
precipitation should be lighter.  There`s still a lot of uncertainty
on snow accumulations, but this event looks capable of around 1-3
inches of wet snow, with perhaps a light glaze of ice across the
eastern Ozarks.

Attention turns to Wednesday as the next trough shifts east across
the Plains.  This trough will be deeper which will produce stronger
low level forcing.  As the midnight shift noted, the limited
sounding data we have available at that range is showing a deeper
saturated dendritic growth zone which gives this the potential for
higher snow/liquid ratios.  However the track of the storm is still
very much in question and this will affect the temperatures in the
column and therefore have a large impact on snow amounts.  As an
example, the LREF interquartile range at St. Louis is 9 degrees for
high temperatures next Wednesday, and the low temperature range is
14 degrees.  Obviously this does not lend a lot of confidence to the
the forecast that far out.  However, this second storm bears
watching as guidance is hinting our area may be in the right
entrance region of an upper level jet streak which would produce
enhanced divergence aloft in addition to the stronger low level
forcing.  Add all this up, and the Wednesday system looks like it
will be the more impactful of the week.

The end of the week looks quieter as another high pressure system
builds into the Mississippi Valley on Thursday.  Temperature spreads
in the ensembles continue around 10 degrees, but current indications
are there won`t be any precip to worry about.  After a cold Thursday
night with lows in the teens under the influence of the high,
there`s the potential for return flow on Friday to warm temperatures
into the upper 30s to mid 40s.


Carney

&&

.AVIATION...  (For the 00z TAFs through 00z Saturday Evening)
Issued at 554 PM CST Fri Feb 7 2025

Flight conditions are expected to deteriorate as ceilings lower late
this evening and overnight, eventually reaching IFR as showers and
drizzle increase in coverage Saturday morning also reducing
visibilities. The overall probability of precipitation is lowest at
KUIN, but a period of IFR flight conditions is still expected at the
terminal. As precipitation ends and a low pressure center/cold front
advance eastward through the region, winds will become northwesterly
and occasionally gust 18 to 23 kt, shortly followed by ceilings
rising/scattering. As a result, flight conditions are forecast to
improve at least to MVFR, if not VFR, during the afternoon and
evening.

Pfahler

&&

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

$$

WFO LSX