Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 242350

Area Forecast Discussion
National Weather Service Blacksburg VA
650 PM EST Tue Jan 24 2017

An upper ridge will pass over the mid Atlantic on Wednesday,
resulting in temperatures well above normal for late January. A
cold front will move through on Thursday, bringing a return to
near seasonal January temperatures for the remainder of the


As of 640 PM EST Tuesday...

Very little change needed to previous forecast. Per shallower
moisture in the upslope areas, added some drizzle/freezing
drizzle to portions of northwest Greenbrier County this evening.
All in all not enough to warrant any statements. Should see the
8h thermal trough lift northeast this evening which will set the
flow more west and scour out the low clouds a bit. Another surge
of gusty winds will occur through late evening as an 8h 40+kt
jet moves across with 6 hr pressure rises ending at 03z on the
order of 4 to 5 mb. Will see some gusts in the higher elevations
of 40 mph. By midnight, winds should be weaker.

Previous valid discussion...

Weak high pressure will track over the central Appalachians
overnight. The temperature forecast for tonight remains
challenging. Higher ridges will start the night cooler than
lower elevations, however as the upper ridge builds in, bringing
warmer air aloft, expect higher ridges to become warmer than
the mountain valleys and the Piedmont by daybreak Wednesday.

The upper ridge will be well established across the mid
Atlantic on Wednesday, with the combination of abundant sunshine
during the morning and afternoon and warm southwesterly
windflow pushing afternoon temperatures into the 60s for much of
the area, possibly touching 70 in spots across the Southside.
Cloudcover will increase during mid afternoon from the west
ahead of another approaching cold front.


As of 330 PM EST Tuesday...

Large 500 mb ridge builds eastward with increasing 500 mb
height falls from west to east tomorrow night. A 990-ish mb low
pressure centered over eastern MI trails a cold front down the
Appalachian Divide by 00z Thursday which progresses eastward
across the forecast area overnight Wednesday and into the first
part of Thursday. Maintained high chance to low Likely range
PoPs for Tazewell County up through southeast West Virginia
overnight/early Thursday, with lower Chance range into the
southside of Virginia and the North Carolina piedmont region. In
the interior, pretty much on either side of the Blue Ridge -
there`s some indication in the 12z model consensus QPF fields of
lower QPF, perhaps reflective of a hydraulic jump of the cold
front. So in central sections I`ve kept PoPs more in the slight
chance neighborhood. Still, little in the way of QPF is expected
- generally a tenth of an inch or less. Precip stays as all
rain except at higher elevations (at/above 3000 ft) through
early Thursday as the 850 mb thermal gradient more or less lags
the surface front by a few hours.

However, the cold advection will go to produce sharp, steep
low-level lapse rates and result in increasing/gusty northwest
winds with a northwesterly low level jet of 45 kts. Will include
a mention in the HWO of gusty winds at elevations above 3000`,
as winds may approach or exceed Wind Advisory criteria overnight
into the first part of Thursday. Breezy, but sub-Advisory level
gusts anticipated below this elevational band.

Into Thursday, flow aloft becomes cyclonic as a large 500 mb
trough sets up across the eastern two-thirds of CONUS.
Northwesterly flow generally diminishes but keeps 850 mb
temperature profiles falling through the single digits below
zero Thursday night and then to values between -8 to -12C by
Friday. As the northwest flow helps to tap into Great Lakes
moisture and advects it southeast, it sets the stage for what
may ultimately be a long-duration period of at least light
accumulating snowfall starting later Thursday afternoon into a
good portion of the extended (more on that below) for the
favored northwest flow upslope areas in western Greenbrier
County, parts of southeastern West Virginia into the Mountain
Empire/Grayson Highlands of Virginia extending at times into the
high country of North Carolina. East of these areas however,
temperatures begin to trend closer to typical late January
normals with lows Thursday night into the 20s to middle 30s and
highs on Friday in the 30s to lower 40s. It sounds odd to say
given the winter season, but the late week period may be
something of a shock given how mild our temperatures have been
up through this point.


As of 330 PM EST Tuesday...

Much of the extended will feature mean 500 mb height fields
consistent with a developing positive Pacific/North American
(PNA) teleconnection pattern...that being large ridging along
the West Coast and a deep trough for the central and eastern
CONUS. Temperatures generally trend near to below normal for
much of the period.

A continuation of accumulating snowfall in the northwest flow
upslope areas (western Greenbrier County, parts of southeastern
West Virginia into the Mountain Empire/Grayson Highlands of
Virginia and at times into the high country of North Carolina)
would appear to be the greatest sensible weather impact during
this period. Identifying periods of more focused, persistent
upslope snowfall is difficult at this time range as embedded
shortwaves and vort max energy in the deep 500 mb longwave
trough would govern that to some extent. A very early look at
snow parameters in these western areas (per the 12z GFS) depict
shallow moisture in the snow growth region which may limit
accumulation, but cold temperature profiles may support higher
SLRs (12-15:1) late in the week into the weekend. Given the
potential long-duration nature of accumulating upslope snow,
feel confident enough to include mention in the HWO with more
specific details to ultimately avail themselves in the coming
days. Increased PoPs in these areas into the Chance range Friday
night through Monday for now. Stay tuned.

For the rest of the region, while there may be occasional
periods of light snow showers making it into the Blue Ridge,
that should be the exception and it looks to be a generally dry
period. Look for several days in the forecast period of highs in
the 30s-lower 40s and lows in the 20s.


As of 628 PM EST Tuesday...

Cloud cover continues to erode from east to west this evening
into tonight, with VFR conditions in place for ROA/LYH/DAN. LWB
just rose to VFR. Meanwhile...MVFR clouds holding at BCB and
IFR clouds at BLF. Overnight, high pressure will build
overhead, allowing for continued clearing and a return to VFR
for BLF/BCB.

Wind speeds will continue diminishing for much of the area,
eventually decoupling at lower elevations while ridgetops will
remain somewhat gusty 35kt to 45kt. May see wind shear set in
toward dawn Wednesday as surface winds diminish to near calm.
Widespread VFR is anticipated for Wednesday, with winds
shifting southwesterly, holding generally below 10kt.

Medium confidence in ceilings, visibilities and winds
during the taf period.

Extended Aviation Discussion...

High pressure will move east of the area during the day
Wednesday, warm southwest winds developing on the back side of
the high. A band of MVFR showers is possible along/behind a
cold front Thursday into Thursday night. Colder west to
northwest wind flow will develop behind this front with mountain
snow showers likely for the weekend associated with MVFR




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