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Area forecast discussion 
National Weather Service Blacksburg Virginia
644 am EST Thursday Dec 18 2014

Synopsis...
high pressure over the upper Midwest will gradually build east
across the area resulting in overall dry weather through Friday.
However passing upper level disturbances will result in periods of
middle and high cloudiness over the next couple of days. A weak area
of low pressure will eventually develop along the Gulf Coast states
later Friday and move east...perhaps bringing a period of light
wintry precipitation on Saturday.
&&

Near term /through tonight/...
as of 215 am EST Thursday...

Overall dry weather and seasonal temperatures to continue over the next 24
hours with weak surface ridging in place under a fast/confluent flow
aloft. However the degree of clouds and whether or not any
sprinkles/flurries will occur remains in question given the approach
of decent middle level energy from the west today overtop very dry air in
place. Latest guidance showing little in the way of precipitation...with this
wave now back over the Midwest...making it across the mountains but do
have quite a bit of middle/high clouds per relative humidity sections. Latest forecast
soundings also show extensive dry air beneath the middle deck so
doubting much in the way of light precipitation except possibly the far
western slopes. Thus trimmed back the going coverage of sprinkles with
only a brief mention far west this afternoon. Otherwise looking at intervals
of sun early...outside lingering upslope low clouds western slopes...
with a period of more middle deck clouds into the afternoon/early evening.
Brief flux of faint warm advection ahead of the shortwave may push
temperatures over the southeast to around 50...while most other locations
struggle in the upper 30s to middle 40s espcly if clouds are faster
to arrive which could mean more 30s for highs mountains.

Wave aloft passes east overnight leaving weak subsidence in place
as high pressure to the northwest noses back in late. This will also
intro another round of weak cold advection allowing upslope clouds to
redevelop farther east...perhaps reaching the Blue Ridge by daybreak.
Otherwise expecting slow clearing of the middle/high clouds from west to east
after midnight with infiltration of low deck across the far west late.
Went closer to the slightly warmer met MOS again tonight for temperatures per
lingering gradient given the parent high to the northwest and some clouds
persisting at times.

&&

Short term /Friday through Sunday night/...
as of 300 am EST Thursday...

Friday looks to be on the dry side as high pressure stretching from
the Great Lakes through the Ohio River valley remains the main
feature in our weather pattern. Mostly sunny skies and light
northwesterly downslope winds will allow afternoon temperatures to
range from the upper 30s along the western ridges...to around 50
over the Southside and the North Carolina Piedmont. Expect to see
high clouds increasing during the evening...with winds becoming
increasingly northeasterly after midnight...as the high shifts east
across the Great Lakes and begins to build southward against the
eastern face of the Appalachians. Cool air flowing in from the north
and radiational cooling despite the high clouds will allow overnight
lows to fall into the upper 20s to low 30s.

18/00z forecast models continue to trend weaker and slower with the
upper level trough moving east over the Great Plains and its
associated surface low developing along the Gulf Coast Friday night.
Models are in general agreement for precipitation to enter the North
Carolina Highlands shortly before dawn on Saturday. For this
morning's forecast...still pretty confident that most areas will see
measurable precipitation despite models trending toward a drier
system...so will maintain high precipitation chances. However...have
cut down on precipitation amounts...with most locations below 0.25
inches of liquid equivalent precipitation.

Precipitation type is also a challenge. The atmosphere will start
the night out dry...with the temperature profile in the lowest few
thousand feet of the atmosphere hovering within a few degrees of
either side of the freezing mark...depending on location. If heavier
precipitation moves into the area...it will cause the atmosphere to
moisten and the temperature profile to cool...making it more likely
to start as sleet or snow. Light precipitation may manage to start
as rain...especially across the slightly warmer eastern half of the
area...before changing over to sleet/snow toward dawn. Ultimately
went on the pessimistic side...trending more towards frozen
precipitation to start the event. Best bet for accumulating snowfall
will be on the higher ridges...with up to 2 inches possible along
the Blue Ridge and other higher ridgelines...and locally higher
amounts.

Precipitation will pass across the area Saturday and the early half
of Saturday night. Undercut model guidance by several degrees for
forecast highs considering precipitation moving across the area in
east northeasterly wind flow. Most precipitation will be east of the
area after midnight Sunday...with yet another high pressure wedge
building over the middle Atlantic. Another low pressure system will
develop along the Gulf Coast Sunday night...radiating moisture
northward and introducing another chance of rain...and potentially
freezing precipitation to the mountains.

&&

Long term /Monday through Wednesday/...
as of 400 am EST Thursday...

This portion of the forecast has the potential to be an active
weather pattern based upon the various models. However...these same
models offer enough differences to make the details of the sensible
weather enough of challenge that confidence in the specifics is not
high.

By Monday...latest models Show Low pressure along the North
Carolina coast with another axis of precipitation moving north associated
with increasing warm advection ahead of the developing longwave
trough to the west. Depending upon how much or little this system
deepens will correlate to how long any associated precipitation
remains over our region. The precipitation type across the area is
also a question. Each model has a different take on the magnitude
of the warm nose over our region. The only consistent element is
the existence of a warm nose. Given this...will offer rain versus
freezing rain forecast during much of the Monday portion of the
forecast. However despite quite a bit of warming aloft off the
latest 00z ec model...thermal profiles suggest some potential for
light freezing rain or sleet across the Virginia Highlands and
southern Shenandoah Valley sections espcly Monday morning.
Precipitation chances though will be on the low side...mainly
ranging in the slight chance category for now which may have to be
ramped up later today.

By Monday night...guidance is in fairly good agreement of bringing a
Canadian low pressure system into the northern Mississippi Valley.
From that point...a deep longitudinal trough develops south to the
Gulf of Mexico by Tuesday afternoon with increasing southerly low
level flow across our region...accompanied by an increasing moisture
fetch. By Tuesday night into early Wednesday...both the GFS and
ecwmf continue to deepen the upper low and associated trough and
negatively tilt the feature so that the trough axis shifts to off
the coast of the Carolinas...coincident to the position of the
associated surface occluded front....with the triple point passing
somewhere near or just south of our region. This paints a picture of
very active weather across our region late Tuesday night into
Wednesday with plenty of showers...very gusty...if not damaging...
winds...and if the triple point prognostic is accurate...severe
weather...or at least a rumble or two of thunder...over or very
close to our region of the country. However...this scenario still is
day 6 into day 7 of the forecast...and there is plenty of time for
changes in the operational models to trend toward a less aggressive
system. Given this...will offer a forecast late Monday night into
Tuesday that offers increasing chances of light rain or freezing rain
with light to moderate rain on Tuesday...with moderate rain
showers Tuesday night into early Wednesday along with gusty winds.
Timing of the push of colder air into the region as well as the
magnitude of that colder air varies between the GFS and ecwmf.
However...the forecast will allow for the potential for cold
enough air to reach western...higher parts of the area late
Tuesday night into Wednesday for accumulating snow showers.

Temperatures during this portion of the forecast will trend milder
through at least Tuesday. Wednesday may be a bit milder in the
east...and slightly cooler in the west as compared to Tuesday. All
days of this portion of the forecast are expected to be above normal
for this time of year.

&&

Aviation /11z Thursday through Monday/...
as of 620 am EST Thursday...

Most of the region will experience VFR conditions through the next
24 hours with mainly intervals of middle and high clouds passing
across. The exception will be the those areas mainly along and
west of a kblf-klwb line where lingering low level moisture will
make for lower strato-cumulus ceilings this morning. However models favor
kblf lifting to VFR after 15z/10am so kept similar timing to
previous on lifting out the low deck. Gusty northwest winds winds this
morning will also weaken with speeds mainly in the 8-15 knots range
this afternoon.

High pressure extending into the area from the northwest will maintain
mainly VFR conditions overnight into Friday. However low level
moisture may again cause upslope induced MVFR ceilings across southeast West
Virginia to spill east toward the Blue Ridge late tonight into Friday
morning. Some chance for kblf to drop into IFR late tonight otherwise
keeping MVFR levels mostly at kblf/klwb.

Extended discussion...

The Friday night-Sat system now has trended weaker but will still
need to be watched as it will likely bring a period of sub VFR
wintry weather especially from the Blue Ridge west Sat.

Aviation interests will need to monitor this situation as later
forecasts become available. This will likely have an impact on air
travel over the weekend...from the southern Appalachians and
Carolinas north to the middle Atlantic.

On the heels of this Friday/Saturday system will be another system
to potentially impact mainly the eastern portion of the region
Monday into Tuesday. Another area of low pressure is expected to
head north along the East Coast with scattered precipitation on
its western flanks entering the region. At this point a wintry mix
is possible along with localized MVFR ceilings.

&&

Rnk watches/warnings/advisories...
Virginia...none.
NC...none.
WV...none.

&&

$$
Synopsis...jh
near term...jh
short term...nf
long term...ds/jh
aviation...ds/jh/PM/wp

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