Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 240814

Area Forecast Discussion
National Weather Service Blacksburg VA
314 AM EST Fri Nov 24 2017

A ridge of dry high pressure will shift east over the mid-
Atlantic coast through tonight, with a return of light southwest
flow and increasing breezes along the ridges tonight. A cold
front will sweep across the Appalachians later on Saturday,
bringing only a few light showers mainly to western slopes, as
well as gusty winds along ridges behind it and cooler
temperatures for Sunday. High pressure will build back in again
for the early part of next week.


As of 300 AM EST Friday...

High pressure continues to dominate today but will shift east
to along the mid-Atlantic coast by evening with a return of
light south to southwest flow and thus milder temperatures in
store today compared to yesterday. Went close to warmest
guidance given sunny skies and return to light southwest flow.
This results in near to just above average highs for the date.

Tonight, southwest flow at 850mb shifts to westerly and
increases resulting in gusty winds along ridges and certainly
milder, and while valleys will likely decouple initially and
will see temps dropping quickly in the evening, some mountain
valleys will remain steady or might even increase overnight as
the winds pick up. Toward dawn Saturday a few mid to high clouds
may also move in to the far west ahead of the approaching
trough and surface front, and this may also help temps to steady
out or even rise slightly in the far west after midnight. Went
generally a little cooler than most guidance but this is tricky
to nail down along the ridges with the mixing increasing
throughout the night. More confident in going below guidance in
the Piedmont where no mixing or clouds are expected.


As of 314 AM EST Friday...

A cold front will move quickly from the upper Mississippi
Valley early Saturday morning to the Appalachian Mountains by
late afternoon. This front will clear the piedmont during the
early evening hours. Prefrontal showers will make a run at the
mountains through the day, but may not make much progress past
the western ridges of SE WV/SW VA. These showers will be moving
into a relatively dry air mass and winds will quickly become
westerly. Low level winds do back to the southwest as the front
crosses the area, but given lack of moisture return, measurable
rainfall should be confided to western slopes. With some
support, a few sprinkles are possible for mountain counties
bordering the Blue Ridge. Downsloping winds will severely limit
precipitation chances east of the Blue Ridge. Given the lack of
showers, temperatures will run 4F-8F warmer than normal Saturday
with highs ranging from the 50s west to lower 60s east.

The upper level trough will pivot over the Ohio Valley to the
Mid Atlantic coast during the evening. The strongest vorticity
maximum will stay north along the Mason-Dixon line. With some
lingering low level moisture, a few upslope rain/snow showers
are possible across western Greenbrier in the evening, then cold
drier air surges in Sunday morning. Behind the front are some
moderate pressure rises and a 40 kt low level jet. This coupled
with a tight pressure gradient over the mountains will bring
breezy and gusty conditions to the area Saturday night into
Sunday morning. Winds 15-25 mph with gusts up to 35 mph are
possible across the mountains, especially higher ridges along
the Blue Ridge. Temperatures will not be as cold as they could
be Saturday night, as mixing should keep temperatures in the 30s
for majority of the area, mid to upper 20s across higher

High pressure centered over the Tennessee Valley will edge into
the region Sunday afternoon, allowing the winds to subside. The
ridge center will eventually move overhead Sunday night. As the
winds subside in the afternoon, so will the cold air advection.
With plenty of sunshine, temperatures will approach normal
values (mid 40s to mid 50s) Sunday afternoon. Under high
pressure Sunday night, temperatures will drop into the 20s.


As of 110 PM EDT Thursday...

Split flow pattern remains through Day 7, Thursday. Upper long
wave trough will dig into the western United States Monday and
Tuesday with a strong short wave separating from the southern
end of the trough over the southeast United States on Wednesday.
As typical, GFS is faster with this feature. Prefer the timing
of the ECMWF which brings the short wave across the Mid Atlantic
region on Thursday. Expecting dry weather and seasonal
temperatures MOnday through Wednesday, then a chance of
precipitation and cooler temperatures Thursday. At the surface a
northern stream cold front will cross the area on Wednesday,
then low pressure associated with the southern stream short wave
tracks into the Tennessee Valley on Thursday. Will be slowing
down better probability of precipitation until Thursday.


As of 1215 AM EST Friday...

A progressive semi-zonal flow will remain in place across the
U.S. with a series of dry northwest flow short waves. The
northern stream will continue to dominate the Mid-Atlantic
region with a dry west-northwest to northwest flow aloft.

A series of mainly dry upper troughs and associated cold front
will continue to pass through the area every few days with
little or no moisture return in advance of these systems. Thus,
a dry/VFR forecast will remain in place through this TAF valid
period and beyond. Still feel that the air mass overnight will
be too dry for any radiational fog development, although it
cannot be 100% ruled out in favorable locations. Weak high
pressure will provide mostly calm winds overnight, trending
toward the SSE-SSW Friday afternoon as the high slides to the
east of the area. Speeds will remain mostly 5kts or less through
the TAF valid period.

High confidence in ceilings and visibilities through the TAF
valid period. Medium confidence in wind direction, high
confidence in wind speed through the TAF valid period.

Extended Discussion... On Saturday a cold front will cross the
area. It will be moisture starved for the most part, with at
best some isolated showers across the area and some patchy of
MVFR ceilings, with the greatest potential across southeast West
Virginia, south into the Northern Mountains of North Carolina.

High pressure regains control of our weather pattern Sunday
into Monday with a return to VFR for the entire region.

Gusty winds are expected Saturday into Saturday night in the
wake of the cold front passage. Surface gusts of 15 to 25 mph
will be possible at the higher elevations.




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