Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 231421

Area Forecast Discussion
National Weather Service Blacksburg VA
1021 AM EDT Fri Mar 23 2018

High pressure extending south across the region will linger to
the north of the area into Saturday. Low pressure will track
east into the Tennessee Valley Saturday before redeveloping
off the Carolinas by Sunday. This system looks to result in
widespread wintry weather during the weekend before a wedge of
high pressure brings drier weather later Sunday into Monday.


As of 1020 AM EDT Friday...

Have bumped up highs a few degrees based on current obs along
with gridded LAMP guidance. No other changes at this time.

Quiet weather to continue today under weak high pressure that
along with persistent northwest flow will keep very dry air in
place. Still likely to see some mountain wave clouds at times
given pieces of energy rotating along the back of the upper
trough, otherwise mostly sunny. Given some slight warming at
85h this afternoon along with eastern downsloping, appears highs
to bounce back well into the 50s east and into the 40s west
despite a gusty northwest wind.

Ridging weakens overnight with weak wedging developing from the
north late under strong high pressure north of the Great Lakes.
Cross sections continue to show very dry air in place ahead of
the upstream developing wave and subsequent warm front that will
help spill overrunning east toward the far western sections
late. Latest GFS again the fastest to bring light precip into
the I-77 corridor while most others show mainly increasing
clouds through 12z. Since already running with low pops across
the southwest only opted to slow down precip chances with mainly
light snow pushing into the southwest third late with light
accumulations toward daybreak. Lows mainly 20s to around 30 with
coldest lows north and east where clouds will be less early on.


As of 420 AM EDT Friday...

There is moderate chance of another impactful winter storm to
affect a large part of the western half of the RNK CWA Saturday
into the early morning hours of Sunday. Light to moderate snowfall
amounts are also expected across eastern areas during the
weekend. Significant accumulations of snow are likely along/west
of the Blue Ridge.

Models continue to key on a strong short wave embedded within
the back side/base of a persistent northeastern U.S. trough.
This short wave will track quickly from the Northern Plains
southeast into the central/southern Appalachians today through
Saturday. This is a part of the same system that brought heavy
rainfall to southern California yesterday. Strong dynamics will
combined with PWATS of 0.7 to 1.0 to bring general qpf to the
region of 0.25 to 0.75 inch. With 850mb temperatures largely at
or below 0C, most of this precipitation will fall in the form of
snow. Model soundings show most if not all of the atmospheric
column, except perhaps in some warmer areas right near or at
the surface, below 0C throughout the duration of the event. With
an average 10/1 ratio for the rain to snow ratio, this
potentially could bring 2-8 inches of snow to much of the CWA.

The main thing that changed from yesterday`s forecast for this
event is not so much the amounts, but more the exact track of
the storm system and where the heaviest snow will likely fall.
Given the tight, compact nature of this Alberta Clipper type
system, significant variations in snow amounts will occur over a
short distance. Yesterday the model consensus track was from
central WV through west central VA then toward north
central/northeast NC. Latest models, including even now the 06Z
NAM, have shifted the track about 100 miles further southwest,
now through the New River Valley and more northwest NC then
north central NC. As a result, the heaviest snow has shifted
from yesterday`s thinking closer to the I64 corridor and the
Greenbrier Valley into the southern Shenandoah Valley more now
to the New River Valley and the mountains of southwest VA and
northwest NC. Again, there will be a sharp gradient in snow
amounts from potentially 8+ inches in the core of the heaviest
snowfall, to less than 1 inch across the eastern and
northeastern parts of the CWA.

Following a conference all with WPC and neighboring offices,
have decided to leave the Winter Storm Watch in place for the
original set of counties and added, because of the new projected
track of the storm system, Grayson, Ashe, and Watauga counties.
Confidence is still not high enough to upgrade to a third period
warning or issue any advisories, although most likely both will
be coming with the afternoon forecast package. Time of year,
time of day, and relatively warm ground temperatures will have a
significant impact on how much snow accumulates, when, and
where. Across the Piedmont of VA/NC, much of this may translate
to a "white rain".

Once the storm system moves out of the area Sunday, a high
amplitude yet progressive ridge will progress from the central
U.S. into the eastern U.S. reaching the forecast area by
Wednesday. In advance of this ridge, northwest flow aloft
bringing cold air from Canada and the northeast will remain in
place with much below normal temperatures and mostly dry
conditions. Temperatures will run around 15-20 degrees below
normal through Monday with highs in the 30s mountains to 40s
Piedmont and lows in the 20s mountains to the 30s Piedmont.


As of 520 AM EDT Friday...

A high amplitude upper ridge will reach the CWA midweek and take
until Friday to shift to the east. Once it does shift to the
east, a conveyor build of southwest flow and deep moisture will
slowly shift from the Tennessee Valley into our region,
promising to bring several days of wet weather but with mild
well above freezing temperatures (hence, no winter weather!). In
general the GFS appears to be on the fast end of bringing this
moisture into the CWA toward Thursday/Friday. Until the upper
ridge reaches the area, conditions should be mostly dry, at
least Mon-Wed with just increasing mid/high clouds and a slow
moderate trend in temperatures. However, a classic wedge setup
is also expected to develop underneath the upper ridge, likely
abating the warmup much more quickly than most would like. Will
plan on highs in the 40s and 50s during the first part of the
week warming to the 50s west and lower to mid 60s east by the
end of the week. Too early to say whether or not rainfall will
become an issue later in the week as timing is not certain at
this point. Hydro issues, if any, should be just beyond this
forecast period.


As of 715 AM EDT Friday...

High pressure should maintain overall VFR conditions through
much of the valid TAF period under mainly clear skies today.
High and mid clouds will increase from southwest to northeast
overnight with ceilings lowering to MVFR south/west of KBLF by
daybreak Saturday.

Northwest winds should continue gusty at times through this
afternoon before becoming light overnight. This lingering
pressure gradient may again result in winds gusting to 20-25
kts during much of Friday.

Conditions will deteriorate from west to east Saturday as snow
and/or mixed precip advances east during the day. Should see
most drop to sub-VFR by/during the afternoon with areas of IFR
or worse in heavier snow/sleet.

Extended Aviation Discussion...

Poor flying conditions will continue Saturday evening before
precipitation decreases from north to south overnight. Models
indicate a bit faster drying as well by Sunday when spots across
the north could return to VFR Sunday afternoon with lingering
sub-VFR to the south for much of the day.

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.


As of 1050 AM EDT Thursday...

The Mount Jefferson NWR remains off the air. The earliest this
system will again be operational will be Friday afternoon.


VA...Winter Storm Watch from Saturday morning through Sunday
     morning for VAZ010>020-022-023.
NC...Winter Storm Watch from Saturday morning through Sunday
     morning for NCZ001-002-018.
WV...Winter Storm Watch from Saturday morning through Sunday
     morning for WVZ042>044-507-508.


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