Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 132336

Area Forecast Discussion
National Weather Service Blacksburg VA
636 PM EST Wed Dec 13 2017

Increasing clouds expected tonight ahead of an Alberta Clipper
system. This storm system moves across the region overnight,
producing a period of light accumulating snow and strong gusty
northwest winds overnight into early Thursday. High pressure
then returns Thursday afternoon through the weekend with dry
conditions and temperatures trending above normal.


As of 630 PM EST Wednesday...

Raised pops in the west for this evening into tonight closer to
a blend of the HRRR, ECMWF and NAM to capture the light chances
in the northwest portion of forecast area and to add scattered
snow flurries further south highlighted on WSR-88d images.
Adjusted temperatures with last surface obs, their trends and
shaped towards GLAMP. More changes later this evening...

As of 324 PM EST Wednesday...Potent 500 mb shortwave trough and
associated 992 mb Alberta Clipper low centered near the Chicago
area is trailing a surface warm front southeastward from it
into the western Appalachians. Currently the central
Appalachians to the mid-Atlantic region is in a southwesterly
warm advection pattern, in the warm sector of the cyclone.
This clipper will be the primary weather maker for the region`s
weather over the next 24 hours, producing another round of light
accumulating upslope snow showers in southeast WV and into the
mountains of NC late tonight, and another burst of strong
northwesterly winds overnight into Thursday.

Due to the potential for gusty to strong northwest winds, I`ve
issued a Wind Advisory with this forecast, taking effect at 7 PM
tonight and lasting through noon on Thursday for areas along and
adjacent to the southern Blue Ridge and the Interstate 81 corridor
near/south of Roanoke.

Will be looking at a general deteriorating trend through tonight,
with clouds on the increase and pre-frontal southwesterly
winds/gusts increasing in lockstep. Southwest wind gusts project to
be as much as 40 mph at elevations above 3000`, but it will be
relatively breezy in most areas through the first part of the night.
Would expect temperatures to fall slowly if much at all through
midnight given warm advection, blanket of clouds and breezy
conditions. Moisture into the clipper system will be very slow to
recover, as dewpoints in the warm sector air mass in the Deep South
are only in the low 20s. Thus expect any precip with the cold
frontal passage to probably evaporate. Despite being well-developed,
the clipper system is really fast-moving, and the entire forecast
area should be post-cold frontal by the pre-dawn hours. Look for
light snows to once again develop into the favored northwesterly
upslope areas in southeast West Virginia into Tazewell County after
midnight tonight. There may also be periods of light snow showers or
flurries into the Blue Ridge and NW North Carolina mountains into
the early morning hours, as indicated by the past several cycles of
the HRRR and 3-km NAM. But bear in mind this is not the same
situation that we saw unfold yesterday. Though low level instability
is present given sharp cold advection, we simply don`t have the
Great Lakes moisture connection, the progressive nature of the
system limits storm total QPF/snow amounts, and poor dendritic snow
growth microphysical conditions to produce the near-whiteout
conditions we saw at times Tuesday. Snow should come to an end in
the early-morning Thursday hours. So from an accumulations
perspective, showed an coating to a inch from Lewisburg to Boone
northwest, 1-2" in Summers and Mercer Counties to 2-3" in the
favored western Greenbrier mountains.

Northwesterly wind gusts are the more notable aspect of the clipper.
850 mb northwest winds increase to around 55 kts behind the cold
front during the after-midnight/pre-dawn hours, while the strongest
6-hourly pressure rises occur essentially during the Thursday
morning commute. Local wind guidance research indicated marginal
Advisory criteria conditions for the southern Blue Ridge given those
conditions. Given the timing of the strongest winds being around the
morning commute with some potential travel impact, and in
collaboration with surrounding WFOs who already have wind headlines
in effect, opted to issue for areas along/adjacent to the Blue Ridge
and along I-81. I-77 in the Carroll County/Fancy Gap area and along
the Parkway may encounter some difficult travel as well. Peak gusts
should top out around 55 mph, with a better likelihood of achieving
gusts this high being at elevations above 3000`. As low-level jet
eases, should see northwesterly winds begin to ease up into the
afternoon hours.

Lows tonight in the upper 20s to near 30, while forecast highs
Thursday are in the low 30s to mid/upper 40s.


As of 324 PM EST Wednesday...

More in the way of zonal flow will develop Thursday night in advance
of the next complex of both northern/southern stream energy that
will approach and pass across the region on Friday. These continue
to remain out of phase with deeper moisture remaining to the east
Friday with a surface wave offshore, while just ahead of the
northern shortwave trough. Guidance showing lots of mid/high clouds
from later Thursday night through much of Friday but with little
precip. Thus leaving it dry for now Thursday night and Friday
followed by spotty upslope snow showers far northwest sections
Friday night, but only 20ish pop for now given lack of moisture with
this system. Otherwise expecting another cold Thursday night under
lighter winds with lows mostly upper teens to mid 20s pending how
fast clouds arrive. Went below Mos on Friday given progged clouds
for much of the day which should hold the mountains in the 30s and
low 40s east.

Fast west/northwest flow to continue behind this system into the
weekend with slowly building heights by Saturday as the 850 mb cold
pool starts to lift out. This warm advection wont deepen enough to
be realized at the surface until Saturday with sunshine helping to
push highs into the 40s to perhaps around 50 southeast. Some
increase in high clouds possible Saturday night but likely thin
enough to still allow lows to reach the 20s to around 30.


As of 324 PM EST Wednesday...

Warming trend will continue late in the weekend into early next week
as weak southeast ridging keeps a more zonal trajectory in place
until Wednesday when a passing trough aloft digs back into the
region. However with the warm advection will see moisture return
perhaps by late Sunday but moreso Sunday night into early Monday
with a residual upper low ejecting northeast across the Gulf states.
Appears much of this precip would be light and mostly liquid given
westerly flow and well above freezing 850 mb temps. Models then
diverge on whether or not this feature helps develop a weak low
offshore, aided by yet another shortwave headed out of the
southwest states, or things remain more progressive and drier
into Tuesday. Given somewhat wetter trends will continue chance
pops Sunday afternoon into Monday while leaving in low pops
northwest Monday night/Tuesday for showers ahead of the next
front and over the south per lingering shortwave energy. Drier
and colder air should follow the front for Wednesday as deeper
northwest flow kicks in. Otherwise looking at highs rebounding
to above normal Sunday-Tuesday and well into the 50s east for
early next week.


As of 636 PM EST Wednesday...

VFR conditions will prevail this evening into tonight. After
04z, aviation conditions begin to deteriorate particularly for
the western mountain terminals, due to northwesterly upslope
snow showers and snow flurries, low ceilings and strong post-
frontal northwest winds. Though VFR conditions should
predominate Lynchburg, Danville and Roanoke through the TAF
period, should see ceilings drop to MVFR at Bluefield and
potentially at Lewisburg and Blacksburg. Lower confidence in
overnight ceiling forecast for Lewisburg and Blacksburg. Will
see most persistent snow showers with highest confidence mainly
on the West Virginia side and in the southern Blue Ridge in
North Carolina, but there may be periods tonight especially
between 09-14z with potential snow showers/flurries as far east
at Lewisburg, Blacksburg and Roanoke. Snow showers and snow
flurries look to abate by noon Thursday, with ceilings also
trending VFR. Northwesterly winds become gusty post-frontal
overnight lasting into midday Thursday, though the core of
strongest winds should be between around 07-13z offering
potential for low- level wind shear and turbulence along the
spine of the Blue Ridge.

Medium confidence in ceilings, visibilities and winds through

Extended Discussion...

Looking at VFR conditions with abating wind speeds/gusts
Thursday afternoon and evening. Other than an outside shot of
MVFR/VFR ceilings and possible upslope snow showers at Bluefield
Friday night, VFR conditions should prevail until Sunday night.
A better shot exists for sub-VFR conditions from a system system
forecast to emanate from the mid-Mississippi Valley Sunday
night into Monday.


VA...Wind Advisory until noon EST Thursday for VAZ009-012>017-022.
NC...Wind Advisory until noon EST Thursday for NCZ001-002-018.

&&to high

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