Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 090456

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1056 PM CST Thu Dec 8 2016

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 308 PM CST Thu Dec 8 2016

Deep cyclonic flow persists across the region with low pressure in
southeastern Canada and expansive high pressure in the Plains the
controlling surface features. The resultant gusty west-northwest
surface winds will diminish this evening and especially overnight as
high pressure gradually shifts eastward from the Plains. The only
clouds we will need to keep an eye on is the stratocu skirting
locations along/east of the MS River, otherwise clear skies look to
be the rule. Tonight looks to be the coldest night thus far this
meteorological winter.

The surface high pressure system will be one of the dominate
features as it shifts across the region on Friday, while WAA aloft
and a weak migratory disturbance will contribute to increasing high
and mid level clouds. The main thrust of clouds from the west-
northwest will be in the afternoon, and in combination with the
surface high, will result in a chilly day with highs roughly 15
degrees below average.


.LONG TERM...  (Friday Night through Next Thursday)
Issued at 308 PM CST Thu Dec 8 2016

(Friday night through Sunday)

Mdls remain in good agreement thru this period. At the sfc, ridge
builds ewd overnight allowing winds to become sely to sly by 12z
Sat. Given the very cold airmass in place with max temps on Fri not
expected to reach freezing, have still trended aob the coldest
guidance for Fri night. With sly flow at the sfc and good WAA thru
the low levels, expect temps to warm thru the weekend.

Looking aloft, the NW flow quickly becomes zonal as the trof over
the Great Lakes region pulls ewd. The low over the ern Pacific south
of AK will reach the wrn U.S. coast early this weekend and be the
next system to watch. Uncertainty increases rapidly on Sun as mdl
solns diverge with this approaching system. The NAM/local WRF are
more amplified solns with a stronger sfc ridge in place. Therefore,
these solns keep the sfc system further south with colder air in
place. Meanwhile, the GFS/ECMWF/GEM continue with a more zonal
pattern with the sfc wave developing sooner and further north.

Believe the NAM solns is influenced by the snow pack that is not
only currently too much, but also likely retains too long into the
forecast. This is a common bias within this mdl. Have tried to trend
twd a compromise, but more so twd the ECMWF/GFS solns. Best chances
for snow will remain across nrn portions of the CWA. Can not rule
out brief periods of sleet during transitions in p-type.

(Monday through Thursday)

Mdl solns diverge somewhat thru the extd period with the GEM
becoming an outlier. Have kept low PoPs on Tues with a weak s/w
system. However, with little run to run consistency, have low
confidence on this system.

Mdls also differ regarding how cold the airmass will be behind the
Tues system with the GFS a much more amplified system compared to
the ECMWF. For now, have trended twd a compromise, but will need to
closely watch how this system evolves over the coming days.



.AVIATION...  (For the 06z TAFs through 06z Friday Night)
Issued at 1055 PM CST Thu Dec 8 2016

Main concern continues to be area of low clouds from eastern IA
into IL, or in our northeastern forecast area. Cloud bases
continue to remain in the low-VFR and hi-MVFR range (2500-4000
feet). Cloud level flow shifted more northerly earlier this
evening but now we are expecting this to back heading towards dawn
to a more W-NW direction which will encourage this cloud area to
finally clear out, but probably will not be entirely realized
until mid to late Friday morning, but effectively from TAF sites
near the MS river by sunrise. Should be able to get away with SCT
for coverage as the edge remains near the MS river and the cloud
bases are VFR anyway. Later on Friday, look for mid and high
level cloudiness to increase from the west and largely continue
into Friday night. Otherwise, surface winds from the NW will
persist overnight and become light and variable on Friday.





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