Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 250600

Area Forecast Discussion
National Weather Service Blacksburg VA
200 AM EDT Tue Apr 25 2017

Wet and unsettled weather will continue tonight into Tuesday, as a
slow moving area of Low pressure center moves from the southern
Appalachians to along the South Carolina coast by Tuesday morning
and northeast along the coast Tuesday afternoon. High pressure
builds into the region late Tuesday night and Wednesday.


As of 1153 PM EDT Monday...Refreshed the PoPs/Wx grids to
account for current radar and observational trends, and adjusted
QPF amounts slightly, increasing them in the Southside and NC
Piedmont by up to an additional tenth of an inch, while reducing
them west of the Blue Ridge.

Pronounced stream of moisture/rainfall currently on radar across
much of the eastern Carolinas then trailing back westward into
the Piedmont Triad/Triangle areas in North Carolina and into our
southern and eastern counties. Rainfall in our area has been
comparatively lighter to that taking place in the Carolinas,
with recent 3-hourly amounts of around 0.25-0.35" in the
Southside/northern NC Piedmont areas. Feel that this area still
has the best chance at seeing brief periods of at times heavier
rains as the band begins to pivot north-northwest. More light
to moderate rainfall to extend from the Piedmont to the Blue
Ridge foothills. Along and west of the Blue Ridge, only very
light rain is expected being more further removed from the
deeper moisture shield.

No other changes made with this update.

Previous near-term discussion issued at 405 PM follows...

Flood watch has been expanded east of the Blue Ridge Mountains into
Tuesday morning.

An easterly upslope flow will enhance the rain tonight especially
east of the Blue ridge mountains. The rain will be heavy at times
into tonight. This area has the greatest chance for the
redevelopment of heavy showers and could aggravation and/or
maintenance of existing flooding. Upper divergence to the northeast
of the mid level low will help generate efficient rain. Current, WSR-
88d images showed an area of convection across central North
carolina lifting northward into our region. Additional rainfall of 1
to 2 inches is possible mostly in the east, on top of what has
already fallen or saturated ground. Rainfall totals will be lighter
in the western mountains.

Vertically-stacked cyclone will slowly move east tonight into
Tuesday. Moisture plume on the eastern periphery of the cyclone,
associated with PWAT values of up to 1.3 inches will continue to be
drawn back westward, supporting a continued period of rain
particularly for the central Virginia Piedmont and into the
Southside of Virginia. Weak forcing elsewhere across the CWA
supports periods of light showers into Tuesday morning. As the low
center pulls away, rain chances will decrease from west to east on

CAD wedge will continue tonight holding temperatures generally from
around 40 degrees in the mountains to the lower 50s in the Piedmont.
A slow erosion of the wedge will allow temperatures to rebound into
the lower 50s in the mountains to near 60 degrees in the Piedmont.


As of 406 pm EDT Monday...

Closed low continues to slowly fill and lift northeastward up the
Mid Atlantic coast Tues night into Wednesday, as modest upper ridge
builds over the central Appalachians through Wednesday. This
evolution may tend to be a bit slower than model forecasts and thus
some uncertainty about how long to leave some scattered showers
across the northeast portion of the forecast area Tues evening. Held
on to some low chance to slight chance pops through midnight for now
but for the early evening time frame may have to increase this if
the low indeed proves stubborn to rotate on out. More confident that
at some point over night any showers, as well as clouds, will begin
to break up and move east of the area. Clearing skies across all but
the far northeast and light winds, combined with saturated ground,
will lead to fog development in many areas overnight into Wednesday
morning. Possibility of some dense areas but for now included
generic patchy and areas of fog until we get a better handle on
clearing potential.

Wednesday under the ridge it is much drier and we finally warm up
significantly under mainly clear skies. Guidance giving some lower
80s from Roanoke Valley into the Piedmont but thinking is that a
percentage of the solar energy will go into evaporation of the
saturated ground so high temps likely to be a few degrees cooler
than this. Went with mid to upper 70s most locations...except for
highest ridges in the upper 60s.

Upper trough and weakening surface cold front approach late in the
day Thursday and there appears to be enough lower level moisture
with increasing southerly low-level winds, which will add some weak
upslope along ridges, and plenty of moisture still in the ground, to
warrant at least a slight chance for showers and thunderstorms as
far east as the Blue Ridge, even though it is fairly dry aloft. With
scattered convection closer to front possibly sneaking in to far
west by late in day will include this area with chance pops,
lingering into the evening and overnight as this weak boundary tries
to move through Appalachians, but dissipates at the same time.
Thursday should be slightly warmer ahead of front under warm
advection and in part due to the ground not being quite as saturated
as the previous day, so went pretty close to guidance in the low to
mid 80s in many locations.

Friday may end up being a dry day with airmass behind the
dissipating boundary somewhat drier and still very warm, and no
obvious signs of any lingering convergent zones, but there is some
limited instability and some of the guidance suggests a shower or
two is possible over the higher terrain. Kept a slight chance pop
then from the Blue Ridge west, but we may eventually be able to
lower this.


As of 355 pm EDT Monday...

Upper ridge will attempt to hold on through the weekend for
continued well above normal temperatures. With low level moisture
combined with these warm temperatures, this will result in just
enough instability for the possibility of isolated to scattered
mainly mountain diurnal convection. Models hinting at potential for
some warm frontal showers just to our west on Saturday so will need
to keep a an eye on that as we get closer. Otherwise, digging trough
to the west approaches and models in general agreement that
associated cold front approaches late on Monday and should be quite
a big stronger than the one late this week. If the front moves
through during max heating Monday afternoon will need to watch
potential for severe weather as well, and this will be a fairly fast
moving system.


As of 200 AM EDT Tuesday...

Poor Flying conditions will persist again today as slow moving
upper level cutoff low slowly meanders across the eastern
Carolinas. This Low is forecast to drift northeast and away from
the area Wednesday. Until then, expect predominantly low
IFR ceilings and areas of rain and drizzle. The tight pressure
gradient around the area of low pressure will also result in a
strong northeasterly low level jet, which will favor an extended
period of low level wind shear, these winds persisting until the
low moves off the Mid-Atlantic Coast later today and tonight.

Extended Aviation Discussion...

Better flying wx is expected for the second half of the week, though
another front may bring scattered MVFR showers/storms toward
late Thursday in the mountains. Drier and much warmer weather
will return for Friday and Saturday.


As of 415 PM EDT Monday....

Storm Totals Accumulations for the 72 hours from 8AM Friday through
8Am Monday were in the 2 to 6 inch range roughly south of a line
from Bluefield West Virginia to Lynchburg VA. Runoff from this
rainfall had resulted in small stream flooding with numerous roads
and low water crossings closed due to flooding. Many trees fell due
to the saturated ground and several minor rock and mud slides were

All of this runoff has now flowed into sections of the New, Roanoke,
Yadkin, and Dan River Basins. Multiple River Flood Warnings were in
effect. Additional rainfall tonight will be heaviest in the eastern
upslope areas of the Virginia Foothills and piedmont. The river
forecasts will be updated as additional information comes in from
the River Forecast centers.

A Flood Watch remains in effect west of the Blue Ridge until 8PM.
The Flood Watch along and east of the Blue Ridge was now in effect
through 8AM Tuesday.


VA...Flood Watch until 8 AM EDT this morning for VAZ022>024-032>035-
NC...Flood Watch until 8 AM EDT this morning for NCZ003>006-019-020.


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