Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 101516

Area Forecast Discussion
National Weather Service Blacksburg VA
1016 AM EST Sat Dec 10 2016

High pressure will build east across the Mid-Atlantic region
today before sliding to the coast tonight and then offshore on
Sunday. The associated cold airmass will remain over the area
through early Sunday before moderating. A cold front will approach
the region late Sunday night before working east through the area
later Monday into Monday night.


As of 245 AM EST Saturday...

Latest visible satellite imagery shows shallow upslope clouds
west of the Blue Ridge through Wv and VA eroding with diurnal
heating and mixing. There are also some light flurries under the
clouds that are not showing up on radar. The trend for dissipation
in upslope clouds and flurries will continue through this morning,
with clouds being most stubborn in WV from western Greenbrier
southward. Temperatures will be quite chilly with highs around 40
east to generally mid 20s/lower 30s west.

Previous discussion...

Warm advection aloft will develop behind the exiting ridge
overnight with the main axis of lift taking shape to the northwest
along a weak warm front aloft and within strong upper zonal flow.
Appears most of this deep moisture will stay to the north with the
upper jet helping to shear an axis of mid/high clouds eastward
across mainly the northern half of the region through daybreak per
most model humidity fields. However southward extent and thickness
of the canopy to likely play into how low temps could go given
residual very dry dewpoints in place. Think also likely to see
some spots plummet early on before steadying out under the clouds
and at elevation where return southwest flow will start to mix
down late. Therefore running on the colder side of Mos valleys and
southern sections per better radiational cooling while leaning a
bit warmer north but still teens to low/mid 20s under intervals of
clouds at this point.


As of 433 AM EST Saturday...

Broad surface anticyclone across the eastern seaboard, with
ridging extending back westward to the Alleghany Mountains early
Sunday, in a low-amplitude quasi-zonal mid-level flow regime.
Ridging gradually shifts eastward into early Sunday evening. The
daytime hours stand to be mostly sunny with veered/southerly
return allowing for 850 mb temps to rise to -1 to +2C, supporting
highs in the upper 30s to mid/upper 40s.

Our weather then turns more active Sunday night and into a good
part of Monday as a cold front moves across the region, associated
with a developing winter storm affecting the northeast US. Strength
of warm advection increases markedly Sunday night ahead of the
cold front as low- level southwest flow increases to near 50 kts.
Strongly non-diurnal temperature trend with a brief period of
radiational cooling where lows in the 30s are more likely to
occur. Surface temperatures then stand to steadily rise through
most of the overnight hours. Most of the global models and the NAM
bring in some light QPF overnight Sunday; further inspection of
BUFKIT soundings off the GFS/NAM reveal an unsaturated cloud- ice
layer with all the RH in the -3 or warmer isothermal level. Via
the top-down methodology, feel that any precip into the overnight
is largely drizzle. A brief period of patchy freezing drizzle and
potential for slippery roads/sidewalks are possible where surface
temperatures are initially near or below freezing, particularly in
the Alleghany Highlands and in the elevated terrain in the
southern New River Valley and the VA/NC border on I-77. But
temperatures should warm above freezing rather quickly so the
period of freezing drizzle is limited. As the column saturates we
should see steady light to at times moderate rain associated with
the front`s passage Monday with PoPs in the Categorical range
before PoPs diminish down to slight chance behind the front Monday
night. QPF amounts range from about a quarter to a half inch.
Mondays highs should be well into the 40s to mid/upper 50s highest
out in the Virginia piedmont and southside areas.

For Tuesday, there`s some sizable differences apparent between
the GFS and ECMWF as far as how far south the baroclinic zone
associated with the cold front`s trailing edge makes it into
North Carolina. The ECMWF pushes the front barely into the central
NC piedmont and western mountains and would allow for some precip
to overrun the frontal zone later Tuesday; while the GFS advances
the front southward further enough to keep our area dry. Since the
east-west front will essentially parallel the mid-level flow
aloft, thinking closer to the slower ECMWF idea and maintained
idea of Chance-level PoPs for mainly light rain Tuesday (mixing
with wet snow in the Alleghany Highlands into Tuesday night). I
also chose to keep highs cooler Tuesday than otherwise with more
cloud cover - in the 40s with lows in the 30s. If the GFS proves
correct, highs could be a few degrees warmer and lows cooler.


As of 433 AM EST Saturday...

Strong isentropic lift will develop over an in-situ wedge as a complex
low approaches from the southwest. There may be some p-type issues at
the higher elevations with the initial surge of warm air advection
into the first part of Wednesday. However, a significant surge of
cold air will start moving in during the day Wednesday behind the
departing low and transition precipitation to snow, especially in
upslope areas west of the Blue Ridge. There may also be some
freezing issues as the cold air arrives on blustery winds and
interacts with wet surfaces Wednesday night. Lingering snow
showers west of the Ridge will dissipate as cold high pressure
builds in through the end of the workweek.

Temperatures will generally exhibit a cooling trend for most of next
week with highs on Tuesday well above normal, falling to well below
normal by Thursday.


As of 1132 PM EST Friday...

Upslope cloudiness should keep KBLF/KLWB and perhaps KBCB in MVFR
to ocnl IFR at times through about mid morning before clouds
diminish. Otherwise once the clouds fade, expecting widespread VFR
through the period with high pressure building in from the west
into this evening. Swath of mid/high clouds likely to skirt across
central and northern sections overnight but still should see cigs
remain above 8-10K feet into Sunday.

Extended aviation discussion...

Next weather system to follow will be a cold front, models
suggesting a Sunday Night arrival which will likely produce sub-
VFR conditions at times, and possibly wintry mix in the mountains.
The cold front crosses the area Monday with continued threat of
showers and sub-VFR at times, then somewhat of a break Monday
night and maybe Tuesday, with another system arriving midweek
with more precip, and more than likely sub-VFR at times.




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