Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 170533

Area Forecast Discussion
National Weather Service Blacksburg VA
133 AM EDT Sat Mar 17 2018

High pressure across the region tonight will weaken and give
way to a warm front working into the area from the southwest
toward morning. A low pressure system will move along this front
Saturday into Saturday night, with the front sinking south into
the South Carolina and Georgia by Sunday morning. Another storm
system works in from the west early next week.


As of 940 PM EDT Friday...

Due to the dry air in place and slackening winds, temperatures
are falling a little faster than previously anticipated as
boundary layer decoupling is taking place. Lows were pushed
downward a few degrees to account for this latest trend.
However, the lows may occur around 3-4 AM instead of toward
sunrise as a canopy of cirrus should arrive overhead from the
west by 3 AM. Thus, temperatures should begin to rise in the
several hours leading up to sunrise, which results in a slight
reduction of the area that could experience freezing rain. Trace
amounts of ice will still be possible between 7-9 AM for parts
of Greenbrier County, West Virginia and Bath County, Virginia.

As of 230 PM EDT Friday...

Weak high pressure across the region will maintain mostly clear skies
this evening with some increase in mid/high clouds through midnight.
Moisture should then begin to ramp up as the flow aloft backs more
southwest ahead of low pressure heading east out of the central states
and north of an approaching warm front. However any precipitation
associated with this lead swath of lift will encounter very dry air
that remains in place from this afternoon. Latest guidance also shows
much of this precipitation slower to reach the region with the majority
holding off arrival until around 12z Saturday. This timing critical to
possible light icing across the far northern valleys where cold air
likely to be trapped from rapid cooling this evening. Current forecast
soundings also suggest a quick jump in temps once clouds thicken, and
if the rain is slower then little threat of icing as expect all to
slide above freezing by mid morning Saturday if not sooner.

Therefore since the window for mainly elevated icing appears small,
wont hoist any headlines at this point, while maintaining some light
ice mention around daybreak from the Greenbrier Valley east across
Bath/Rockbridge counties. Otherwise mainly chance pops far west late
with increasing clouds elsewhere including lows in the 20s/low 30s
north to low/mid 30s south before all rise late.

Warm front ahead of a trailing strong upper disturbance will lift
northeast across parts of the south and west Saturday before getting
shunted back south by late in the day. Guidance showing a rather
disorganized area of rain working through the region into the afternoon
with best coverage west and less east per increasing westerly flow
aloft. This suggests high pops west and mainly chance east for a tenth
to a quarter inch QPF. Models attempt to break southwest sections into
the warm sector late in the day ahead of the main upper impulse with
some instability along the nose of a strong 85h jet to the west.
However the degree of leftover clouds and stability key to just how
much at least elevated convection could develop with best chances of
stronger storms along the southwest periphery where closer to the
better upper support. Thus left in some afternoon thunder mention
southwest third while trimming a bit southeast where should be more
stable from earlier rainfall. Could have a large range in highs pending
where the warm front ends up with readings running from 40s extreme
north to low/mid 60s south.


As of 330 PM EDT Friday...

Shallow deamplifying ridge over the central part of the country will
slowly slide eastward over the region through the weekend. This will
push the cold front through the region Saturday night along with any
associated shower/thunderstorm activity. As mentioned, convective
parameters are marginal but any convection that does manage to form
along the boundary Saturday afternoon will likely ride the boundary
through the region early Saturday night with a continued threat for
large hail/wind.

Once the boundary is finally clear of the region high pressure will
build over the mid Atlantic region with generally fair weather for
Sunday, though weak southeast flow may be able to generate some
sprinkles in the mountains.

A short wave will undercut the ridge Sunday night with a surface low
brushing by to our south, possibly close enough to bring some showers
to the NC/VA border by daybreak. As the upper ridge axis crosses the
region a warm front and short wave energy ahead of an approaching
closed low will provide good forcing over a developing wedge to help
spread the chance of showers further north through the region on

By Monday night a full latitude trof will begin pushing in from the
west with a good wedge signature at the surface. This will generate
abundant isentropic lift over the wedge as a low moves through the Ohio
valley, creating widespread rainfall across the Appalachians and
central mid Atlantic.

Temperatures will start the period near seasonal normals, but start
trending cooler as wedge conditions become established.


As of 330 PM EDT Friday...

A closed low will develop in a broad full latitude trof and move
through the eastern US through late next week. This will develop
another strong low along the mid Atlantic coast and keep temperatures
well below normal through Friday.

Tuesday looks to be rather wet especially in the east with good dynamic
forcing over the wedge, and there may be an opportunity for some
thunder outside the western periphery of the wedge on Tuesday
afternoon but at this point confidence in convection is low so will
not mention in the grids.

Things get more interesting Tuesday night into Wednesday as the
primary area of low pressure strengthens along the coast and pulls
colder air down into the region. While confidence is not high this
far out in the forecast, the scenario currently advertised by
guidance brings the potential for a transition to wet snow Tuesday
night through Wednesday with accumulations possible. As the low
moves up the New England coast we will transition to upslope snow
showers west of the Blue Ridge with additional snow accumulations
possible into Friday morning. Definitely worth keeping an eye on
later model runs to see how solutions evolve over the next few days.


As of 130 AM EDT Saturday...

Clouds will gradually thicken and lower as the morning progresses,
with patchy light rain arriving across far western sections
towards daybreak with the approach of a warm front. However, VFR
conditions are expected into at least the mid-morning. By mid-
day, many area in the west will be experiencing a generous
coverage of light rain with MVFR ceilings. Precipitation east of
the mountains will still be limited with VFR conditions still
prevailing. By the time we get to the afternoon, the
precipitation will take on more of a convective nature, with
showers likely across the mountains with scattered to isolated
showers east of the crest of the Blue Ridge. Ceilings across the
mountains will be IFR/MVFR with MVFR likely east of the
mountains. Isolated to scattered thunderstorms will be possible
by the mid to late afternoon mainly within an area south of a
KBLF-KBCB line, and west of a KBCB-KMWK line.

Extended Aviation Discussion...

Should see an improvement Sunday as the front shifts south. A
more organized low pressure system will track into the area from
the central and southern Plains for Sunday night and Monday.
This is expected to bring sub-VFR ceilings and visibilities and
widespread rain. MVFR conditions are expected Monday night into
Tuesday. As the low deepens near the coast, wrap around
precipitation including mountain upslope snow showers will
likely keep sub-VFR in place for much of Wednesday including
strong north to northwest winds.


As of 940 AM EDT Friday...

The Mount Jefferson NWR remains off the air and will likely be
sometime next week before it is operational again.




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