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Area forecast discussion
National Weather Service Blacksburg Virginia
749 am EDT Thu Apr 27 2017

high pressure will build across the southeastern United States
resulting in a considerable boost to the temperature with above
seasonal warmth expected by the weekend. An increase in southwesterly
wind flow will bring enough moisture back into the area
beginning today to promote widely scattered afternoon and
evening showers and thundershowers over the mountains.


Near term /through tonight/...
as of 500 am EDT Thursday...

High pressure is beginning to build over the southeast Continental U.S..
this will stave off most of the energy from a vigorous short
wave trough that is lifting northeast through Illinois and into
the western Great Lakes. A surface front trails south from the
upper trough and will be moving east today, the southern portion
of the front weakening with time in response to the building
ridge of high pressure over the southeast. In spite of its
weakening trend, it will create enough lift for showers and
thunderstorms this afternoon and evening, particularly across
our western County Warning Area where SBCAPE is forecast between 1000-1500
j/kg. Some of this activity may make it east of the mountains
and into the Piedmont late in the day.

The Storm Prediction Center has the entire County Warning Area outlooked for a
marginal risk of severe storms today. The main threat would be
from isolated damaging wind gusts. Model cape is forecast into
the 1000-1500 j/kg range this afternoon, but diurnal gains in
buoyancy may be muted by in increasing cloud coverage ahead of
the front. If sufficient solar insulation does occur, then
low/midlevel flow will encourage momentum transport capable of
supporting locally damaging wind gusts.

Temperatures today will be very similar to yesterday with highs
in the 70s to lower 80s.

Loss of daytime heating will result in a demise of the
shower/storm activity before midnight. The actual surface front
is then forecast to dissipated over our area as the building
upper ridge of high pressure acts as a Road block keeping the
front from introducing any sort of change in airmass to the
forecast area.


Short term /Friday through Saturday night/...
as of 400 am EDT Thursday...

A broad and strong h5 ridge will be centered over the southeastern
U.S. In this period with 588 dm heights over Florida building to
591+ dm just off the Carolina coast by Saturday. This will provide a
very warm and fairly humid air mass across the southern appalachian
region, especially for late April. High temperatures will run 10 to
15f above normal both Friday and Saturday with highs in the upper
70s to low/mid 80s Friday to mid/high 80s Saturday. Not really
threatening records but still unseasonably warm. Precipitation-wise
do not expect to see much, if any. A strong upper trough will be
digging southward into the southern High Plains Friday/Saturday
setting the stage for a heavy rainfall event well to our west,
across parts of the southern and central Mississippi Valley. Forcing
for precipitation in our area will be weak with only a chance for
diurnally driven convection which will be widely scattered and not
hydrologically significant. This will allow the last of our flooded
rivers to continue receding. Models indicate decent ml cape of 1-2k
Saturday afternoon but only modest shear but could allow a few
strong to near-severe pulse thunderstorms in the afternoon.


Long term /Sunday through Wednesday/...
as of 400 am EDT Thursday...

Very warm and humid conditions continue Sunday with the chance for
showers and thunderstorms, especially along the Blue Ridge. By
Monday, the vertically stacked upper closed low over the Midwest
will move into the Great Lakes region. This will push a vigorous
cold front into the area from the west with a good chance of showers
and thunderstorms through Monday afternoon into Monday night. High
pressure will then build on for Tuesday with cooler weather and
windy conditions. Moisture increases on Wednesday, with better
chances of rain Wednesday night into Thursday.


Aviation /12z Thursday through Monday/...
as of 745 am EDT Thursday...

VFR conditions at all sites through 18z/2pm today before
frontal showers and a few storms arrive along the western slopes
of the Appalachians. Increasing clouds/showers are the result
of an approaching cold front. Ongoing line of deep convection
over the Tennessee Valley is forecast to weaken this morning before
redeveloping along the outflow boundary as it moves into the
western Appalachians this afternoon. Storm coverage the
afternoon is expected to be scattered in nature...thunderstorm
clusters as opposed to a solid line. Southerly winds will become
a bit gusty from mid morning on per 35 to 40 knot low level jet ahead of
the approaching front.

Showers/storms are forecast to linger until loss of daytime
heating. The actual surface front is expected to wash out and
dissipate as it crosses the mountains, so in spite of a wind
shift as it crosses the mountains this evening, very little if
any wind shift is expected over the Piedmont east of the
mountains where the winds will remain predominantly out of the
south or southwest.

Extended aviation discussion...

Mainly VFR flying weather is expected for Friday and into the
weekend as high pressure builds over the southeast states
promoting temperatures which will be much above normal. Warming
surface temperatures also means increasing atmospheric
instability. As such, potential will exist for cloud build-ups
and the risk for isolated to scattered thunderstorms along The
Spine of the Appalachians during the peak heating hours each


as of 430 am EDT Thursday...

River flood warnings remain in effect for the Dan River and lower
portions of the Roanoke River.

The Dan continues to fall rapidly at Danville, thus the warning
has been cancelled for Danville.

Downstream at paces, the river crested yesterday afternoon and is
now falling slowly. At South Boston the river is just past the
crest, which was at a stage of 30.25 feet at 12:30 am this morning.
Preliminary records indicate that this is the 9th highest flood on
record at South Boston and the highest since March 23, 2003 when the
river hit 30.34 feet.

The Roanoke River at Randolph has crested and will begin falling
this evening.


Rnk watches/warnings/advisories...


near term...PM

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