Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 170345

945 PM CST Tue Dec 16 2014

Issued at 926 PM CST Tue Dec 16 2014

Stratus clouds were slowly clearing out of portions of western MO
this evening, and this clearing should finally make it into at
least the western portions of our forecast area late tonight. Mid-
high level clouds will be advecting eastward from the Plains into
the forecast area late tonight. The surface winds should continue
to gradually diminish as the surface ridge over the northern
Plains continues to build southeastward into our area. Temperatures
tonight will be colder than last night, but still close to
seasonal normals for mid December. Latest NAM model run continues
to show the potential for light accumulating snow mainly late
Wednesday night and Thursday morning.



.SHORT TERM: (Through Friday Afternoon)
Issued at 320 PM CST Tue Dec 16 2014


Deep cyclonic flow and cold air advection in the wake of yesterday`s
storm system kept diurnal temperature rises minimal today. Solid
shield of stratus yielded an occasional period of drizzle or
flurry, but nothing measurable was observed.

Cloud trends for tonight are uncertain. An extensive area of stratus
was located upstream, and northwest winds near 925mb remain
favorable for continued advection of these clouds into our area. On
the other hand, a hole was located in the cloud deck over western
IA, and if this area does not fill in, then skies should start to
clear over the Columbia and St. Louis areas between 23-02z. RAP
plots of increasing condensation pressure deficits between 975-900mb
tonight along with BUFKIT soundings which depict drying aloft both
support the idea of dissipating stratus/stratocu this evening,
although mid/high clouds will be increasing later tonight regardless
of what happens with the stratus/stratocu. However, BUFKIT profiles
also show a fair amount of subsidence above the low cloud layer,
which may act to trap moisture below the inversion and keep the
clouds in place. Partial clearing appears more likely over central
MO than elsewhere. Winds should remain around 10 kts for most of the


(Wednesday - Friday)

Deep trof of low pressure currently coming onshore along the west
coast with our eyes tracking a pair of circulations that will bring
two chances of precipitation to the area the next seven days.

The first circulation will come ashore along the Baja tonight and
lift northeast across the southwest CONUS and southern Plains on
Wednesday. Warm advection will ramp up across the area Wednesday
night with light snow expected to rapidly develop and move into the
area from the southwest. Models continue to "moisten" the forecast
with higher QPF noted. Have increased POP and expected snowfall
amounts with up to two inches now possible across central Missouri
and the eastern Ozarks and an inch into the western suburbs of the
St. Louis Metro Area. Unfortunately it appears that the snow will be
hitting the St. Louis area either right before or at rush hour
Thursday morning. Luckily, ground temperatures are relatively warm
and air temperatures should be in the upper 20s or lower 30s. The
hope is that impact on travel will be low with this event. Snow will
taper off during the morning hours on Thursday as the shortwave
lifts east of the area. Lingering low level moisture and rising
surface temperatures suggest drizzle will be possible in the
afternoon hours.

The second and more potent circulation of the two will dig even
further south across the Baja before lifting east/northeast across
the deep south on Friday. Meanwhile, a northern stream shortwave
will also be diving southeast across the Plains. Unfortunately for
the snow lovers these two systems don`t appear to fully phase until
they are east of the area. Some snow flurries or light snow will be
possible across the far southern CWA Friday afternoon.


.LONG TERM: (Friday Night through Next Tuesday)
Issued at 320 PM CST Tue Dec 16 2014

Continuing with the late week system, models push the southern
system, which will be the main driver of pcpn, well to our south
with some interaction with a northern system still anticipated to
help bring perhaps some light pcpn well to the north into a good
part of our region for Friday night. PoPs remain low because of
concerns of measurability, with many locations perhaps tracing out
instead. Pcpn-types favor snow.

The northern system itself will not quite get thru the forecast area
until later on Saturday and maintained slight chances for that

Southerly flow at the low levels will then be seen for Sunday and
Monday, helping temps to at or above normal levels once again.

Heading towards the middle of next week, a longwave RIDGE will
attempt to re-establish itself over western North America in
imitation of the dominant pattern in November, with a strong
shortwave disturbance dropping down in NW flow and a developing
upper level TROF over the central CONUS.  Look for increasing PoPs
for this period with temps beginning to drop back to below normal
once again.

Christmas could be pretty quiet across our region, taking a sneak
peak at the long range models.  But it looks to only be a timing
issue, with one system leaving and another lurking about a day away,
so this could change very quickly given all of the typical timing
issues the models have this far out.



.AVIATION:  (For the 00z TAFs through 00z Wednesday Evening)
Issued at 615 PM CST Tue Dec 16 2014

MVFR ceilings are expected to move out of the TAF sites between
06-08Z based on latest satellite trends. Then dry and VFR
conditions are expected the rest of the TAF period. Northwest
winds will become light and variable by late afternoon Wednesday.

Specifics for KSTL: Ceilings around 2000 feet are expected to
clear out around 08Z. Then dry and VFR conditions are expected the
rest of the TAF period.





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