Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 201314

Area Forecast Discussion
National Weather Service Blacksburg VA
914 AM EDT Wed Sep 20 2017

A weak short wave will move southeast across our region today,
then high pressure follows for tonight through Saturday. A cold
front will approach the Mid Atlantic region the middle of next
week. Tropical Storm Jose will drift north off the east coast


As of 914 AM EDT Wednesday...

Adjusted temperatures with latest surface obs, trends and shaped
towards GLAMP for late morning into this afternoon. Morning fog
and low clouds mixing out or lifting similar to recent morning.
WSR-88d showed isolated showers in Greenbrier associated with
weak shortwave. The best chance of convection this afternoon
will be in the west with westerly winds hindering storm
development in the east. Increased cloud cover for this morning
with latest satellite images and blended late morning towards
conshort. More changes later this morning...

As of 325 AM EDT Wednesday...

GOES 16 water vapor loops show a short wave over Indiana early this
morning with a secondary vorticity maximum over western North
Carolina. The Indiana wave will track southeast to the
Tennessee/North Carolina border by this evening, and a nearly closed
off 500MB low remains over western Virginia and North Carolina

Very subtle if any surface reflection of these features, perhaps a
lee-side trof by this evening.  Overall low level winds remain
northwest today then 850MB winds become east late tonight. Forecast
location and amount of deepest moisture today varies depending on
the model. Northwest winds suggest that best probability of
precipitation will be over the mountains.

Expecting enough heating today, with only scattered to broken mid
level clouds in the morning that air mass will become unstable
enough to support isolated thunderstorms during the afternoon.
Maximum temperatures today and minimum temperatures tonight will be
similar to past few days.


As of 330 AM EDT Wednesday...

Very slow-evolving, amplified mid-level pattern remains offered by
deterministic and ensemble solutions for the late week/early weekend
period. Lower than normal mid-level geopotential heights should
predominate the western third of CONUS, with downstream amplified
ridge with accompanying milder than normal temperatures in place for
many areas east of the Rockies. Only limited chances for showers and
thunderstorms are forecast for this period, with temperatures
running some 5-10 degrees milder than late-September climatology.
Given the slow-evolving persistent pattern, bias corrected
temperature schemes should perform quite well in this period. Will
keep an eye on developments in the tropics pertaining to Jose and
later Maria, though the most likely outcome at this point is to keep
these two systems over the western Atlantic, well away from the
region. Interests are advised to continue to monitor the latest
forecast advisories on Jose and Maria from the National Hurricane

The best chance for showers in this period is on Thursday as energy
associated with weakening mid-level shortwave trough now across the
Ohio Valley gets shunted southward and westward into the Carolinas.
Differential heating and enough surface convergence along the
central Appalachians into the southern Blue Ridge may produce some
isolated to widely scattered showers or garden-variety storms. Shown
pops in the 15-30% range along and southwest of a Lewisburg-Roanoke-
Danville line Thursday. Not expecting any significant storms as
convective depth stands to be rather shallow given dry mid-level air
in place and surface-based CAPES at/below 1000 J/kg. Any showers or
storms should follow a diurnal trend and dissipate with sunset.

Beyond Thursday, we look to be entrenched in a mostly sunny and dry
weather pattern for the foreseeable future - at least through the
end of the weekend. With little change in air mass, highs through
much of this period are in the 70s to mid 80s with lows mid-50s to
low 60s, generally mildest in the urban centers such as Roanoke,
Danville and Lynchburg.


As of 130 PM EDT Tuesday...

Surface and upper ridge will be sandwiched between tropical systems
off the east coast and strong upper trough over the central/western
U.S. this period. Will see how Hurricane Maria evolves and tracks
northward off the east coast, depending on how Jose
moves/weakens and if the upper ridge pushes further off the mid-
Atlantic coast.

At present, models in decent agreement keeping us on the warm and
dry side. Highs and lows will continue to range about 3 to 8 degrees
above normal.


As of 740 AM EDT Wednesday...

MVFR to LIFR fog in the valleys this morning and MVFR fog in
the foothills and piedmont of Virginia and North Carolina.
High confidence all of the fog to dissipate by 14Z.

A weak short wave will be reaching the central Appalachians in
the morning which would result in more cloud cover and scattered
showers over the mountains. Radar already showing a small
cluster of showers just west of KLWB. Low confidence on the
probability of any thunderstorms. Any chance would be after
16Z/noon. Brief dips to MVFR ceilings in the localized
downpours will not be out of the question. Low confidence in
narrowing down the most likely location of showers and
thunderstorms this afternoon.

Extended Aviation Discussion...

Overall persistence forecast to prevail for the rest of the
week, but with a lower probability of precipitation than today.
Strengthening high pressure remaining in place between exiting
Jose offshore and a weak cold front approaching from the west.
This should maintain good flying weather outside of isolated
diurnal showers possible Thursday afternoon as upper level
energy slides across. Dry weather is expected Friday through




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