Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 060253

Area Forecast Discussion
National Weather Service Blacksburg VA
953 PM EST Mon Dec 5 2016

An area of low pressure in the Gulf of Mexico lifts northeast
tonight and reaches the Appalachians Tuesday resulting in
widespread rainfall overnight into late Tuesday. Moisture pulls
off the coast Tuesday evening allowing weak high pressure to build
in from the Midwest on Wednesday. A strong cold front will cross
the region on Thursday.


As of 950 PM EST Monday...

Lows for tonight were bumped upward a few degrees due to the
latest observation trends. Rain has only reached the VA/NC border,
but the latest HRRR seems to fizzle it in the next hour or so
before a stronger wave of moisture pushes northeastward. Low
pressure over the western Gulf of Mexico will strengthen and
move northeast along a stationary front. With dewpoints rising
overnight, the bulk of the precipitation should primarily be
liquid. A small area of frozen precipitation could be possible in
the higher elevations of western Greenbrier County, WV. However,
this update has lowered the amounts and coverage of any sleet or
snow toward daybreak on Tuesday.

A strong cold wedge east of the Appalachians will remain in place
Tuesday morning as low pressure moves northeast with strong
isentropic lift including upper dynamic support. A coastal low
pressure center will develop off the North Carolina coast Tuesday
afternoon. Widespread precipitation will continue through the
first part of Tuesday before starting to taper off during the
afternoon. There is enough warm air advecting in Tuesday to
transition any light wintry mixed precipitation in Greenbrier
County, WV to all rain. A light glaze of ice or coating of snow
may possible at the highest elevations. High temperatures Tuesday
will range from the mid 30s in the northern mountains to the mid
and upper 40s in the south.


As of 215 PM EST Monday...

Surface low along the coast should exit Tuesday evening ahead of a
secondary trailing cold front that will help boot out residual deep
moisture overnight. However enough lingering wrap around rainfall to
keep chance to low likely pops going early Tuesday night espcly western
third as well as the far northeast sections with rain tapering
elsewhere. Will see a band of more shallow nature moisture along the
cold front that looks to cross the far west after midnight and the Blue
Ridge by early Wednesday. Appears enough to warrant a chance of added
showers western slopes overnight with a gradual transition to higher
elevation snow showers late as 850 mb temps fall below 0C.
Otherwise clouds lingering mountains with downslope clearing
east and lows mainly 30s.

Wednesday into Wednesday night will be the in between period separating
a more zonal trajectory from the impending digging upstream 500 mb
trough associated with the upcoming Arctic cold shot. Models even
suggest a period of warm advection in the afternoon under sunshine as
the flow backs from a weak northwest trajectory to southwest later in
the day. This should also push highs into the 50s from the Blue Ridge
east and close to 50 west. Flow deepens from the south/southwest
Wednesday night with guidance suggesting enough moisture return for a
few late night showers south/west, that should transition to snow
showers far west early Thursday when the Arctic front nears.

Front then crosses from northwest to southeast Thursday with cold
advection/upslope driven snow showers western mountains and
possibly a few sprinkles east. However moisture appears much less
today off models per overall westerly flow, so cutting back on
pops, but leaving in some mention given light QPF showing up off
ensembles. High temps Thursday tricky pending timing of the 850 mb
front, with a possible range from 30s west to around 50 Piedmont,
before falling with a slightly slower trend likely the best bet
with the frontal passage.


As of 130 PM EST Monday...

Strong cold advection will be ongoing to start the period as Arctic air
pours in ahead of high pressure that should shift east from the central
U.S. early Friday to about overhead the region by Saturday morning.
However latest models tending a bit weaker with the associated upper
trough per less digging that likely will keep the core of the Arctic
airmass to the north. This also along with very dry air advecting in
should help limit the degree of upslope snowfall behind the front as
the strongest northwest winds will only coincide with the coldest air
aloft Thursday night into Friday at best before moisture fades.
Thus going with only low pops northwest mountains Thursday night
with coverage gradually decreasing Friday. Elsewhere should remain
mostly clear but still cold and blustery into Friday evening with
overnight wind chills near zero across the higher ridges. Still
appears that Saturday morning will see the coldest temps as the
high settles overhead, although increasing high clouds may
curtail good radiational cooling per latest warmer guidance.
Otherwise lows mostly teens to near 20 mountains both nights, and
20s east with highs 20s to low 30s mountains Friday, to mid 30s to
lower 40s east.

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 700 PM EST Monday...

Poor aviation conditions expected through the TAF valid period.
Negatively tilted short wave will move northeast from the
southeast U.S. Tuesday bringing widespread rain, fog, and low
clouds to the area through much of the TAF valid period. Local
radars indicate that light rain has already spread into northwest
NC and far southwest VA. A very dry layer of air just above the
surface to about 500mb will take a few hours to saturate, but this
should occur in the 04Z-08Z time frame from south to north. At
that times, ceilings will lower from just the mid/high cloud
shield presently in place quickly to MVFR/IFR and to widespread
LIFR by daybreak. Visibilities will quickly fall into the MVFR
range, the settle into the IFR range in the 08Z-10Z time frame and
hold there through much of the day Tuesday.

Strong short wave will lift to the northeast of the area in the
20Z-22Z time frame, at which time the steady rain will end, but
expect widespread low clouds, fog, and drizzle to remain in place
into Tuesday evening, smilier to what we saw yesterday, until
better drying can develop after daybreak Wednesday.

Winds will come around to the northeast east of the Blue ridge and
southeast to the west of the Blue Ridge, with the possible
exception of KLWB, where a northeast wind is also possible. An
850mb southeast jet will traverse the region early Tuesday, which
will likely equate to a period of gusty winds in the usual western
locations, namely KBLF. Some low end gusty winds are also possible
across the Piedmont later tonight into Tuesday morning. Otherwise,
generally look for speeds of 7-10ts through the TAF valid period.

Medium confidence in ceilings and visibilities through the TAF
valid period.
Medium confidence in wind direction/speed through the TAF valid

Extended aviation discussion...

Drizzle, low clouds, and fog will likely remain in place Tuesday
night before drier air and westerly flow evolve across the area
Wednesday. Wednesday 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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