Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 300856

Area Forecast Discussion
National Weather Service Blacksburg VA
356 AM EST Wed Nov 30 2016

A cold front slides east to the mountains this afternoon, and east
of the piedmont this evening. High pressure will track from the
lower Mississippi Valley Thursday to the Southern Appalachians
Friday morning. Expect drier and cooler weather for the end of
the week.


As of 340 am EST Wednesday...

Radar has been pretty active this morning, with elevated instability
leading to thunderstorms moving across the New River Valley/Mountain
Empire and Alleghany Highlands, as well as portions of the piedmont.
The latest RAP showed this elevated instability lingering into mid
morning. There will be a break in the showers across the southeast
CWA with main axis of lift and moisture transport staying with the
front and upper dynamics across the TN Valley into the Central
Appalachians. Therefore will have higher pops in the mountains this
morning then models shift the axis east into the piedmont by late

There is a decent amount of lift and wind energy  to give most of
not all the forecast area a good rain. Unstable air is limited by
clouds and showers but with increase in the jet should be enough
dynamical forcing to string some thunder in the forecast, especially
in the foothills/piedmont. The Storm Prediction Center shows our
eastern 2/3rds of the forecast area in a marginal risk, mainly
concerned about winds mixing down with line of showers and isolated
storms. Will mention this in the HWO, but more along/east of the
Blue Ridge as think the west will be stable enough to keep stronger
winds from mixing down past 4000 ft. Some of the higher terrain this
morning could see some gusts to 45 mph, although not enough to have
a wind advisory.

Temperatures despite the clouds and showers will warm into the lower
to mid 70s east to mid to upper 60s west.

Tonight, the front moves east of the forecast area by midnight with
showers ending. There will be clearing skies as we head into late
tonight in the east while upslope keeps clouds in the mountains,
with slight chance of showers over western Greenbrier County. Colder
air will star to track in overnight west of the Blue Ridge with lows
in the upper 30s to lower 40s, while the foothills/piedmont slide
into the mid 40s to around 50.

There will be a gradient behind the front tonight but 8h winds are
around 30 kts or less, so winds should be under 30 mph in the gust
dept, except possible gusts to 35 mpg across the Alleghany
Highlands, higher ridges.


As of 320 PM EST Tuesday...

Upslope precipitation above 3000 feet will primarily be light
snow while lower elevations transition from rain to flurries
Thu morning. Pressure rises behind the front will also bring
breezy and gusty conditions to the area subsiding Thursday

The northern and southern stream becomes separated starting Friday
with zonal flow over the region. Looking aloft, the forecast area
will be under the influence of the northern stream with 85h
temperatures running -4C to -6C going into the weekend. Friday`s
high temperatures will range in the 40s across the area with 30`s
along ridgetops. Besides a few mountain fair weather cumulus clouds,
Friday will remain dry.


As of 300 PM EST Tuesday...

A closed low over north Mexico will generate a disturbance over
Texas Saturday. Moisture from this disturbance will get caught in
zonal flow and bring an increase in clouds over the region for the
weekend. An area of dry high pressure will become anchored over the
area keeping bulk of the moderate to heavy rain south across the
Gulf states this weekend.

On Monday, the closed low exits Mexico and enters the midwest. This
system will bring warm Gulf moisture over a lingering dry surface
high Monday. Precipitation falling into this dry high will create an
insitu wedge. Depending on the timing and availability of cold air,
precipitation could start out as a light snow across the mountains,
changing to rain over time. Both the GFS and ECM agree on this
solution, but differ on timing, precipitation amount and p-type. The
GFS has precipitation entering Monday whereas the ECM is Tuesday.
For now, we will carry low PoPs for the area and a rain/snow mix for
p-type mainly across the mountains.


As of 1145 PM EST Tuesday...

Expecting MVFR to IFR clouds to fill in over southwest Virginia,
northwest North Carolina and southeast West Virginia tonight. LIFR
fog developed at locations that had rain Tuesday afternoon and
stay cloud free longer. Winds will pick up overnight, especially
at higher elevations as the low level increases from the
southwest. Low confidence on the timing for when visibilities will
improve as fog erodes.

Area of upper diffluence that was providing lift for the showers
and thunderstorms over Mississippi and Tennessee will move into
the region late tonight. Guidance was in good agreement keeping
much of the area dry until after 06Z/1AM. Some warm air advection
also in the region tonight and Wednesday morning which will aide
in lift.

Next band of heavier showers and will cross the region from west
to east Wednesday afternoon allowing for widespread sub-VFR cigs
and vsbys until the front crosses Wednesday evening. Shallow
convection may enhance the winds. High confidence on the passage
of the front and associated moderate showers but lower confidence
on the exact timing.

Extended aviation discussion...

Improving conditions expected after the upper low and final short
wave move east of the area later Wednesday night. Conditions
should become VFR east of the Blue Ridge into the weekend.
Upslope clouds and potentially a weak clipper system could bring a
low chance of rain or snow showers into the western sites Thursday
and Friday along with lingering MVFR cigs before conditions
improve there as well over the weekend.




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