Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 261432

Area Forecast Discussion
National Weather Service Blacksburg VA
1032 AM EDT Wed Apr 26 2017

Low pressure along the Mid-Atlantic Coast will move northeast
and away from the area today. 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 by late Thursday to promote
widely scattered afternoon and evening showers and
thundershowers over the mountains.


As of 1030 AM EDT Wednesday...

Expecting a fair weather day with lots of sunshine. No major
adjustments needed.

Previous discussion...

Let the warm-up begin.

Our 5 days of drought busting rain has come to an end. The only
lingering effect will be some morning cloudiness, otherwise
looking for a return of the sun and much warmer temperatures.
Afternoon highs today will be about 15 degrees warmer than
yesterday... should hear a lot of lawn mowers going today.

Still have two main Stem Rivers in flood. See the hydro section
for details.

Synopsis... Low pressure was centered just off the Mid-Atlantic
coast. This feature is forecast to move northeast and away from
the area today. An upper ridge of High pressure is forecast to
amplify over the southeastern CONUS, while a vigorous upper
level short wave trough ejects northeast through the Mississippi
river valley.

With increasing heights and warming thicknesses, 85h temperatures
will climb to near +15 deg C by this afternoon. This surge of
warmth will translate into surface temperatures in the 70s.
With all the moisture in the ground, dewpoints will remain
elevated (50s to near 60), so tonight`s low temperatures will be
mild with lows only dropping into the 50s.


As of 400 AM EDT Wednesday...

The overarching highlight in the short-term period will be for a
unseasonably warm, summerlike temperatures. Potential exists for
showers and thunderstorms mainly along/west of the Blue Ridge
heading into the latter part of the week, though confidence is low
on timing.

For Thursday: First part of the day starts out warm and starting to
feel a bit more humid as strong sunshine works to evaporate the
moisture left behind from the multiday rain event. We should also
have a reasonably strong enough south to southwest breeze as low-
level lapse rates get steeper with aforementioned insolation warming
temperatures well into the 70s to a few lower 80s. Increasing 500 mb
height falls toward the latter half of the day will help to erode
what is modeled to be a strong capping inversion between 850-700 mb,
ahead of a surface cold front. Model guidance supports some chance-
level PoPs in western counties (from a line from Hot Springs to
Roanoke to Reidsville west) generally during the late afternoon
hours. The GFS is the earliest and feel this probably is a bit
aggressive given the stout cap that needs to be overcome. With the
cold front undergoing frontolysis as it advances eastward, I`m also
not really that sold on frontal lift, and the mid-level trough
deamplifies and lifts northeast generally west of the Appalachians.
Feel that we`ll end up seeing a weakening line of showers and
thunderstorms toward late afternoon and then weakening further into
the mid-evening hours as it progresses into the VA/NC Piedmont. Kept
lows a bit warmer than MOS in the mid 50s to mid 60s warmest in the
Piedmont and coolest west of the Blue Ridge.

Friday and Saturday feature the re-establishment of an amplified mid-
level ridge across a large part of the Southeastern CONUS, well
ahead of a powerful mid-level trough over the Four Corners Region. A
developing, though very slow moving surface low will be located
somewhere in the vicinity of the Texas Panhandle Friday and along
the Red River in Oklahoma/Texas on Saturday. A warm front will
extend northeastward from it along the Ohio Valley to perhaps as far
east as the Eastern Continental Divide. It`s really during this
period of time where global model guidance consistency begins to
suffer from timing discrepancies on when any lift generated from the
warm front may spark showers and thunderstorms. That is important
because while the air mass in low-levels is relatively capped,
progged instability aloft is substantial (-2 to -4 Showalter indices
and MUCAPES in excess of 1000 J/kg), even into the evening hours. I
could see potential for thunderstorms pretty much either day mainly
from the Blue Ridge west, though it is conditional on lift
associated with the warm front making its way into our area. As
described, confidence is quite low on timing but wouldn`t rule out
an isolated stronger storm or two if storms can develop. Kept a
rather generic 25% PoP or less along and west of the Blue Ridge for
each day as well as Friday evening until there`s better consistency.
Should see highs upper 70s to mid 80s on Friday, rather mild lows in
the upper 50s to mid 60s Friday night, and widespread 80s on


As of 300 AM EDT Wednesday...

We will be in a very summerlike pattern this weekend with very warm
and humid conditions and an increasing chance for showers and
thunderstorms each day, 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. High pressure will then
build on for Tuesday with cooler weather and windy conditions.


As of 400 AM EDT Wednesday...

MVFR ceilings with pockets of IFR/LIFR vsbys will persist until
mixing begins mid morning. The upper low which lingered over the
region for the last 4-5 days is finally moving northeast and
away from the forecast area. This will permit decreasing
cloudiness today with widespread VFR for the afternoon... winds
light and variable.

Extended Aviation Discussion...

Better flying weather is expected for the second half of the
week, though another front may bring scattered MVFR
showers/storms Thursday afternoon and evening across the
mountains. Much warmer weather can be expected through the
upcoming weekend with temperatures much above normal. Warming
surface temperatures also mean 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 530 AM EDT Wednesday...

No additional rainfall expected the next 24 hours.

Two mainstem rivers are experiencing flooding within the RNK
Hydrologic Service Area. The Lower Roanoke at Randolph, and the
Dan River. The Dan River is by-far in worse shape of the two
rivers with water levels into the major flood category
downstream of Danville.

The Dan River at Danville has crested...and the river will
gradually recede upstream of Danville toward Wentworth.
At Paces and South Boston, the situation is a bit more serious
as the river at both locations is considered major flooding.

At Paces the river is expected to crest this morning at between
28.5 and 29 feet. This is almost 9 feet above flood stage. At 30
feet water would reach the floor of the Highway 658 bridge.
Viewing the crest history, this is only the 9th time of record
that the river has crested above the major flood level of 27 ft
at Paces.

At South Boston the river will continue to rise today with a
crest near 29 feet. This is 10 feet above the flood stage of 19
feet. At 29 feet, a number of businesses in the Riverdale area
are affected.

It is interesting to note that most of the previous major
floods for the Dan River were associated with a tropical
system, such as Hurricane Agnes in 1972 or Hurricane Fran in

For additional details on the river flooding, please refer to
the AHPS page and specific FLS/FLWs issued from this office.




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