Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 112022

Area Forecast Discussion
National Weather Service Blacksburg VA
322 PM EST Mon Dec 11 2017

High pressure remains in control through tonight. An Alberta Clipper
system will pass over the region on Tuesday, ushering in periods of
snow and blowing snow, cold temperatures and gusty northwest winds
through Tuesday night. Chilly but dry conditions return for
Wednesday before temperatures begin to slowly moderate on


As of 319 PM EST Monday...Modest warm advection regime underway
across the Appalachians into the central mid-Atlantic region,
ahead of a potent Alberta Clipper system and lead parent mid-
level trough. Another shortwave is evident in water vapor
imagery north of the Great Lakes associated with a strong shot
of Arctic air. Each of these features will go to produce a
period of wintry weather, cold temperatures and gusty northwest
winds, especially along and west of the Blue Ridge Tuesday.

Several hazard headlines have been issued with this package. Winter
Weather Advisories have been hoisted for western Greenbrier County
in West Virginia, and in Ashe and Watauga County in western North
Carolina for accumulating snow and low wind chills (low wind
chills primarily for Tuesday night). A Wind Advisory has also
been hoisted along the southern Blue Ridge from Roanoke down to
Boone. Each of these products run until 12z Wednesday.

Quiet weather anticipated for the first part of the evening. Will
start to build clouds in progressively with time from the east as
clipper system approaches from the west. Should see increasing
chances for snow showers into the pre-dawn hours of Tuesday from the
mountains of NC northward into the central Appalachians and into
western Greenbrier County. Low temperatures tonight should be in
the mid 20s to near freezing.

Guidance generally agrees on Arctic frontal passage tied to the
clipper system from 12-16z Tuesday. By most indications, it should
be a well defined and strong cold frontal passage as indicated in
simulated reflectivity progs. Noticed on the NAM-12km that steep low-
level lapse rates, CAPEs of 50-100 J/kg and Snow Squall Parameter
values > 1 unit support an environment conducive to short-duration
snow squalls with the Arctic frontal passage, especially along the I-
81 and I-77 corridors during the morning commute. As the air mass
behind the front is cold, snow in western counties should be powdery
in nature with snow-to-liquid ratios likely greater than 16:1.
Pattern then transitions to one of typical northwest flow snow
setups, with a Great Lakes moisture connection to Lake Michigan
indicated in the higher-resolution NWP. I`ve left open the
possibility for light accumulating snow to make it into the New
River and Roanoke Valley, but the most persistent snows will be
confined to the NC Mountains and in southeast West Virginia
(particularly in western Greenbrier County). Including continuing
snows for tomorrow night, forecast accumulations run from 3-5" in
western Greenbrier County, 1-4" across the rest of southeast West
Virginia, 1-3" in Ashe and Watauga Counties in NC, and a coating to
an inch in more intermittent snow showers roughly along the I-81
corridor. Specific to winds, these will tend to peak immediately
behind the frontal passage. While breezy conditions will exist in
most locations west of the Blue Ridge, the strongest winds capable
of causing minor damage appears more likely along the southern Blue
Ridge with gusts up to 55 mph possible. With the powdery nature of
the snowfall and breezy conditions, this will lead to periods of
blowing/drifting snow. For temperatures Tuesday, opted for a non-
diurnal temperature trend with early high temperatures in the 30s to
mid 40s before sharply falling to the mid 20s to upper 30s by late
afternoon under strong cold advection.


As of 319 PM EST Monday...

Strong cold advection will persist Tuesday night as the center of
the cold pocket aloft passes to the north. This should continue to
provide a favorable fetch of upslope driven moisture to result in
bands of snow showers overnight espcly western slopes but also with
some possible spillover out to the Blue Ridge per strength of the
jet. Expect coverage to slowly fade later Tuesday night as soundings
show moisture depth decreasing but still likely another 1-3 inches
far northwest slopes to an inch elsewhere with only light
accumulations farther east. Strong northwest winds will also still
be an issue with another round of subsidence overnight when the
coldest 850 mb air should be helping lower the inversion. Therefore
will need to keep or expand going wind headlines into early
Wednesday morning at this point. This combined with lows in the
teens to around 20 likely to spell wind chill issues from the
foothills west so keeping mention in the HWO for now. Otherwise
should see clearing take shape out east Tuesday night and
elsewhere by Wednesday with this lingering into Wednesday night.

Next weak clipper will then arrive from the west late Wednesday
night and moreso Thursday as this impulse shears east and offshore
Thursday afternoon. System appears lacking moisture per lack of
digging so only including more clouds and slight pops northwest late
Wednesday night and mainly low chance pops Thursday before lift
gives way to weak upslope. Otherwise more clouds north and less
south with highs 30s north to 40s south.


As of 319 PM EST Monday...

Upper flow will again undergo a quick transition from zonal flow to
start Thursday night back to a deeper eastern trough regime on
Friday before again flattening over the weekend into early next week.
This in response to development of split flow ahead of the last in a
series of digging clipper systems that will attempt to phase with
energy dropping out of the Rockies. However latest models show quite
a spread with this system, from basically dry with a split between
the clipper passing farther north and low pressure offshore, to
somewhat better linkage with low pressure closer to the Carolinas
and thus at least some light precip per the GFS. Boundary layer
appears rather marginal for snow outside of the mountains but could
arrive early Friday so something to watch. Otherwise will include
some low rain/snow pops on Friday most sections for now.

Lingering upslope snow showers again likely Friday night before
heights build allowing for some temperature moderation on Saturday
under warmer westerly flow which should push most into the 40s. This
in advance of yet another system swinging northeast out of the
southern plains that will link with the next upstream cold front and
bring rain showers into the area late Sunday into Monday before
coldere drier air follows later on Day 7. Appears most precip will
be liquid given warming aloft ahead of the system Sunday night with
only perhaps some snow showers at the end far west Monday


As of 1256 PM EST Monday...

VFR through at least 06z, though ceilings will steadily lower
from west to east, especially into the evening/overnight hrs.
Winds southwest 4-8 kts.

We`ll then expect deterioration in flight categories as
approaching Alberta Clipper and shot of modified Arctic air
introduces periods of MVFR to LIFR snow showers, along with
gusty northwest winds. These conditions are more likely west of
Roanoke along the southern Blue Ridge in NC up into
southeastern West Virginia. May see a period of short-duration
snow squalls associated with the Arctic frontal passage between
11-14z at Bluefield, Lewisburg and perhaps Blacksburg. Ceilings
expected to drop to sub-VFR levels by early morning along and
west of the Blue Ridge. VFR conditions expected away from the
mountains, with gusty post-frontal northwest winds developing.
Sustained winds between 10-15 kts with gusts up to 30 kts,
highest at Roanoke.

Extended Discussion...

A continuation of MVFR to IFR snow showers are expected west of
the Blue Ridge through Tuesday night. Weak high pressure builds
in Wednesday followed by another weaker clipper Thursday that
passes to our north.

Into Friday, potential for a stronger clipper system to re-
energize off the Atlantic coast, inducing a period of sub-VFR
with light to moderate snow.

Conditions then trend VFR for Saturday.




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