Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 061742

Area Forecast Discussion
National Weather Service Blacksburg VA
1242 PM EST Tue Dec 6 2016

An area of low pressure over the lower Tennessee Valley will move
northeast today, providing a widespread wetting rain across the
central and southern Appalachians. Mixed wintry precipitation will
occur farther north from northern Virginia and West Virginia into
Pennsylvania. The low is forecast to exit the region tonight, with
a period of higher pressure Wednesday. A strong cold front will
cross the region on Thursday.


As of 957 AM EST Tuesday...

Adjusted temperatures for this morning with latest sfc obs
and adjusted down a few degrees towards glamp in the higher
elevations for late morning into this afternoon. With a few of the
coldest highest elevations, patchy light wintery mix of snow,
sleet and freezing rain may be possible in the northern mountains.
Posted an SPS for some patchy light icing along the ridges this
morning. Temperatures will slowly rise or remain nearly steady in
the wedge. More changes later today...

As of 515 AM EST Tuesday...

A wet day on tap for the mid atlantic region as low pressure moves
overhead from the Tennessee valley. Widespread rain is expected
with amounts of a half inch to an inch. Some very light wintry
precip, freezing rain may occur at the highest elevations in the
VA highlands and from the wilderness area in northern Amherst
northward toward Montebello, but coverage limited to the ridge
tops with little or no impact to the remaining 99.9 percent of
our CWA. Temperatures today will struggle to rise east of the
Appalachian divide per wedge of cooler air on the east side of the
mountains, temperatures remaining in the 30s to lower 40s for most
of the day. Warmer air will be confined to the west side of the
divide into the TN valley were readings will climb into the lower
50s Bristol and vicinity. Winds on the west side of the divide
will also be gusty, with wind gusts of 30-40 mph across Tazewell
and Smyth counties.

Primary surge in dynamic lift will occur between now and 18Z/1PM
ahead of a negatively tilted short wave trough lifting northeast
through the southern Appalachians. Once this surge of PVA passes
then precip will wane with rain tapering off from southwest to
northeast between 18z/1pm and 21z/4pm. Gradual clearing will
follow for most of the CWA tonight as winds become northwesterly
behind the departing storm system. Lingering clouds and light
precip will be found across the mountains, with potential for
drizzle and or freezing drizzle for the WV Highlands.


As of 500 AM EST Tuesday...

A fairly active period on the whole, particularly for the Thursday
and Friday time period owing to a modified Arctic cold frontal
passage Thursday, ushering in some of the coldest air yet
experienced to this point in this winter season.

Daily details below...

Wednesday: Lingering precipitation across the western hillsides in
Greenbrier County tapers down during the morning hours. Pretty stout
dry slot noted in BUFKIT soundings supports idea that any
rain/snow showers may end as a brief period of freezing drizzle.
Not likely that this will cause much of any impact in this area,
though, other than some riming on tree tops. Skies then begin to
clear out through the rest of the day, but will see increasing
high clouds across the northwestern mountains of North Carolina
and southwest Virginia late in the day. Highs should range from
the mid 40s in Greenbrier, Bath and Alleghany counties to the
mid/upper 50s in the southside of Virginia.

Wednesday Night: Clouds should continue to increase from SW to NE
through the evening ahead of the approaching Arctic front. Forecast
is generally dry until at least midnight as there should be some
antecedent dry air that should need to be overcome first. PoPs are
limited to the slight to low Chance range for areas west of the Blue
Ridge for rain and/or higher elevation snow showers with no
accumulations anticipated. I think areas east of the Blue Ridge
remain dry; though one thing to watch in later model runs is the
potential for some light drizzle in the piedmont areas in North
Carolina and into the southside of Virginia. Lows range from the low
30s for far western VA into southeast WV, mid 30s for the NRV and
northwest NC mountains and upper 30s/near 40 east of the Blue Ridge
into the VA/NC piedmont.

Thursday: Arctic front marches east across our forecast area during
the morning to early afternoon hours Thursday. As indicated earlier
and continuing with the 00z guidance suite, the trend continues to
be less enthused about precipitation chances with the frontal
passage. I really couldn`t justify going higher than Chance
associated with the frontal passage itself. Given the timing of the
front itself, timing of highs will likely vary to some extent across
the forecast area (i.e. earliest in western areas and not until
early afternoon across the central Virginia piedmont/southside and
the North Carolina piedmont). Once the front clears, expect a marked
increase in northwest wind speeds across the entire area, strong
cold advection and PoPs taking on a more NW-flow orographic
character with any rain transitioning to light snow showers. Temps
range from the low 30s west to the upper 40s/low 50s in far
southeastern sections. Temperatures should be falling pretty
steadily by Thursday afternoon, however.

Thursday Night: In a word, cold. Strong cold advection commences
during this period with 850 mb temps falling to values around -10 to
-15C across the area according to the GFS. For areas in southeast WV
and far western VA, northwest flow should be enough to wring out
some upslope snow showers, but an unsaturated -12 to -18C dendritic
snow growth layer should mitigate any significant accumulations. It
will be quite breezy Thursday night which will make it feel even
colder. Looking at apparent temps, I have wind chill values ranging
from the single digits pretty much west of the Blue Ridge and the NC
mountains to the teens in eastern and southern areas. Given lows in
the teens to low 20s and wind chill values this cold early in the
season, definitely want to dress warmly for any outdoor plans
Thursday night.

Friday: Continued cold and breezy with any leftover clouds and snow
showers across western sections eroding through the day. Despite
abundant sun, highs should only top out in the 20s to the mid 30s.


As of 430 AM EST Tuesday...

Trough lifts out later Saturday with fast zonal flow aloft
developing Sunday into Monday as the surface high shifts east with
warming/moistening return flow developing. This will allow pieces of
energy ejecting out of the Rockies to perhaps bring some chance of
precip by late Sunday into Monday at this point. However guidance
shows considerable spread with consensus having a stronger northern
stream wave push a weak surface front with faint southern connection
toward the region by Monday. This could bring some chance of precip
to mainly the western counties by Sunday evening per faster GFS
despite residual low level dry air. Appears this would be mostly
rain Sunday night into Monday given column warming with possibly
some mix late Sunday pending timing. However quite iffy espcly given
much slower timing off the 12z ECMWF. Should see temps rebound into
the upper 30s/40s Sunday and 40s to mid 50s Monday as warm advection
aloft reaches the surface.


As of 1242 PM EST Tuesday...

Poor aviation conditions expected through tonight with
improvements on Wednesday.

Negatively tilted shortwave will move northeast across
the Appalachians. Widespread rain, fog, and low clouds will
continue this afternoon into tonight. All terminals will
experience a period of ifr conditions with potential for lifr
vcnty of the mountains, ridges obscured. Drying will return late
tonight with return to VFR for Wednesday.

Expect northeast winds east of the Blue ridge and southeast winds
west of the Appalachian divide. An 850mb southeast low level jet
will traverse the region this afternoon, which may equate to a
period of gusty winds vcnty of KBLF-KTRI. Winds will become light
and variable this evening after the upper trough passes, but then
shift to the northwest toward 06Z/1am Wednesday. Northwest winds
are expected on Wednesday.

Low to medium confidence in ceilings, visibilities and winds
during the taf period.

Extended aviation discussion...

Wednesdays will be the transition day ahead of an Arctic cold
front that is expected to move through the region Thursday.
Limited moisture may result in a few showers in advance of the
front Thursday, but upslope conditions should support snow showers
across the mountains Thursday into Friday along with a significant
surge of cold air. This will lead to VFR conditions east of the
Blue Ridge through much of the period beyond Thursday, but MVFR
conditions may linger into Friday across the Alleghanys. West to
northwest winds behind the front will be strong and gusty into




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