Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 240247

Area Forecast Discussion
National Weather Service Blacksburg VA
1047 PM EDT Fri Mar 23 2018

Low pressure in the Central Plains tonight will track southeast into
the Tennessee Valley by Sunday morning. This system will bring a mix
of winter weather to the region Saturday through Sunday
morning. A wedge of high pressure will remain over the Mid
Atlantic region Monday and Tuesday.


As of 1030 PM EDT Friday...

High pressure centered over eastern Canada will continue to
wedge down the east side of the Appalachians overnight...with
mainly clear skies promoting falling temperatures with readings
slipping into the upper 20s to lower 30s. Clouds will be on the
increase from the southwest per approach of a warm front which
extends from the TN Valley into an area of low pressure along
the KS/NB border. A large swath/arc of precipitation was moving
east in advance of the warm front, which is progged to reach
our southwestern CWA (Boone NC) toward daybreak. Models are
similar with onset/timing with snow and/or a wintry mix arriving
into the mountains of North Carolina up to Tazwell county
between 5-8AM Saturday.

For Saturday, we have not made any changes to the forecast or
warning/advisory headlines and will leave this part of the
forecast untouched until the overnight shift has a chance to
look at the latest model runs. Areas to take a closer look at
per current snow forecast map is whether to add a few more
counties to the warning (Patrick & Franklin) and potential need
to extend advisory headlines farther south and east into the NC
Piedmont. For the Piedmont of NC, a lot will depend on how warm
it gets Saturday as to whether the snow is able to accumulate
on anything other than the grass and elevated objects. With high
temperatures forecast near 40, it will be difficult to attain
much accumulation until either the sun goes down, or the
intesity of the snow drags the temperature down closer to
freezing. Until then, impacts will be limited.

Another assessment that will need to be made is whether to add
thunderstorms to the forecast. MUCAPE over the TN valley is
forecast between 400-600 J/KG Saturday afternoon and evening.
This may impact parts of our southwestern CWA closer to the warm


As of 420 PM EDT Friday...

Winter Storm conditions expected to affect the Blacksburg
forecast area Saturday.

Though there is still some lingering level of uncertainty
specific to the exact position of the highest QPF axis, which
influences precipitation amounts over significant distance,
there is enough evidence that a significant winter storm is
expected to affect the region Saturday into Saturday night.

The main players for this event are a weak, deamplifying
shortwave trough now coming around the northern Rockies,
spreading a elongated NW-SE warm-frontal precipitation band
across the Ohio Valley and is expected to brush a large part of
the region. The other aspect is the longwave trough in the
northeast/northern mid-Atlantic supplying a reservoir of cold
air in northerly flow. A potent shortwave trough in the longwave
feature, now over northern Quebec, will dig southward into the
Poconos late Saturday/Saturday night.

In terms of liquid/liquid-equivalent QPF, if you took a general
consensus over the last 24-36 hrs of model runs, you`d find
highest amounts across the southern and southwestern portion of
the CWA, from Bluefield to Roanoke to Danville southwest (in
excess of 1"), with about a quarter to as much as a half-inch
across the Greenbrier Valley/I-64 into the central VA Piedmont
northeast of Smith Mtn Lake.

Have made several changes to headlines. The Winter Storm Watch
was converted to Winter Storm Warnings for the New River Valley,
Roanoke, Craig Counties in VA as well as Mercer, Summers and
Monroe in WV. Precipitation in these areas should stay as
predominantly snow. Southern areas such as Carroll, Wythe and
Bland may mix with sleet at times as a warm nose tries to make
it north of I-77 and switches snow over to rain. Winter Storm
Watches were also flipped to Winter Storm Warnings for the NC
High Country and Grayson County. Here, significant accumulations
of snow are expected initially, but as the aforementioned warm
nose makes more inroads there, the potential exists for
significant amounts of freezing rain (totaling a tenth of an
inch) and cold rain. The greatest confidence in significant
hazardous wintry weather conditions is in the winter storm
warning areas.

With forecast models shifting the axis of greatest QPF further
south and east (mainly from Bluefield to Roanoke to Danville),
it leads to lower storm total snow accumulations across
Botetourt, Alleghany, Bath, Rockbridge and Greenbrier Counties.
With exception for Bath County where forecast accumulations
should prove low enough to hold off, the Winter Storm Watch for
the rest of the listed counties were replaced with Winter
Weather Advisories where accumulations project to be less. Have
also added Winter Weather Advisories to the east of the Winter
Storm Warning area across the foothills of the Ridge into part
of Southside into the central VA Piedmont. Rain is likely to
predominate through much of Saturday here, but should begin to
switch to snow and become at least briefly moderate intensity
snow especially through the first part of Saturday evening.
Further southeast expansion may be needed into the NC Piedmont
in later shifts. Finally, winter weather advisories were also
hoisted into Tazewell and Smyth Counties. Snow initially will
mix with rain or freezing rain as the warm nose advects
northeast. The combination of both some light snow accumulations
and freezing rain accumulations totaling a light glaze support
the Advisory for these two counties.

Little change overall noted to timing. Precipitation should
initially build in from southwest to northeast early Saturday AM
and as far as the New River by late Saturday AM. As
temperatures may not recover as well from overnight lows here,
precipitation should start as snow or a mix of rain/snow. The
delayed start of the wintry weather event will allow for some
modest warming to occur in the Piedmont, foothills areas. Rain
would be the initial precipitation type for these eastern areas.
The worst of the conditions should especially be felt between
about 5 PM to midnight, especially along and south of a
Bluefield to Roanoke to Danville line. Snow is likely to
accumulate on road surfaces as the sun goes down, and will be
forced by strong low-level frontogenesis that looks to run
parallel to the VA/NC line. The best potential for icing is
Saturday evening southwest of I-77 into the NC mountains. Precip
shield should expand across a large part of the Blacksburg
forecast area but further north of Rt 460 into I-64, lighter-
intensity snows are expected, deeper into the colder air but
more removed from the highest precip axis. Precip then slowly
begins to wind down from north to south late Saturday night into
the pre-dawn hours.

For accumulations: Southwest of I-77, storm total ice amounts
range from a light glaze to as much as a tenth of an inch. Snow
and sleet accumulations immediately north of I-77 to about Craig
County eastward to the Blue Ridge become particularly
substantial, with 4 to 10 inches of snow being common. Further
north, amounts begin to taper to 1-2 inches toward Bath and
Rockbridge Counties. East of the Ridge into the Piedmont,
amounts are also lower on the order of 1 to 4 inches.

Wedging should return as the low exits early Sunday. This will
keep temperatures low on Sunday. Will show lingering Chance-ish
PoPs further south into early Sunday before trending drier.


As of 420 PM EDT Friday...

Strong wedge remains in place Monday and Tuesday with below normal
temperatures as seen on the standard deviations in the ensemble
GFS. Less cloud cover is expected in northern and central
Virginia with drier air coming in from the north but deep east
to southeast winds from the surface through mid levels may hold
in cloud cover along the southern Blue Ridge and the foothills
to the east with even a chance for light stratiform

500mb height rise through the week with a warming trend over the
area, especially after the wedge recedes. Another low pressure
system in the southern stream will bring the next chance of
precipitation for the end of the week.


As of 800 PM EDT Friday...

VFR conditions through much of tonight. Swath of wintry
precipitation will enter far western VA and into the mountains
of NC after midnight, but it will take until daybreak before
reaching KBLF before spreading east of then mountains during
the day Saturday. High and mid clouds will increase from West
overnight with cloud bases lowering to MVFR south and west of
KBLF by daybreak Saturday. Models were a few hours slower with
the arrival of the precipitation on Saturday so have delayed any
start of rain/snow or MVFR ceilings at KROA, KDAN, KLYH and
KLWB until afternoon. Medium to high confidence that MVFR
conditions will develop shortly after the 18Z/2PM and then IFR
for Saturday night.

Extended Aviation Discussion...

Conditions will deteriorate into IFR and potentially LIFR for
Saturday night and into Sunday as snow and/or mixed winter
precipitation advances east. Heavy wet snow is likely to fall
within a narrow corridor vncty of BLF-DAN line...with Winter
Storm Warnings in effect for the mountains. This will likely
impact air travel late Saturday into early Sunday. Along the
southern periphery of this banding of heavy snow, there is the
potential for freezing rain and sleet. Forecast soundings also
suggest elevated instability, so cannot rule out embedded
thunderstorms within the band of heavier precipitation Saturday
evening...thunderstorms favoring KY/TN and into western VA/NC.

Precipitation decreases from north to south late Saturday
night. Models indicate some drying north of ROA/LYH during the
day Sunday with VRF to the north. Easterly upslope low level
flow will likely result in lingering low level cloudiness south
of ROA-LYH with sub-VFR conditions continuing for western

Dry weather will return on Monday and Tuesday with high
pressure to the north wedged along the mountains. But the wedge
may hold in MVFR lower clouds espcly on Monday. The wedge with
erode on Tuesday and Wednesday in advance of the next low
pressure system and rainfall.


As of 800 PM EDT Friday...

The Mount Jefferson NWR has been fixed... it should be back on
the air!


VA...Winter Storm Warning from 8 AM Saturday to noon EDT Sunday for
     Winter Weather Advisory from 8 AM Saturday to noon EDT Sunday
     for VAZ019-023-032>034-043>045.
     Winter Storm Warning from 5 AM Saturday to noon EDT Sunday for
     Winter Weather Advisory from 5 AM Saturday to noon EDT Sunday
     for VAZ007-009.
NC...Winter Weather Advisory from 8 AM Saturday to noon EDT Sunday
     for NCZ003-019.
     Winter Storm Warning from 5 AM Saturday to noon EDT Sunday for
WV...Winter Storm Warning from 8 AM Saturday to noon EDT Sunday for
     Winter Weather Advisory from 8 AM Saturday to noon EDT Sunday
     for WVZ507-508.


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