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NWS Area Weather Forecast Discussion
County Warning Area [CWA]: RNK
Regional NWS Weather Office: Blacksburg, VA

000
FXUS61 KRNK 120527
AFDRNK

Area Forecast Discussion
National Weather Service Blacksburg VA
1227 AM EST Tue Dec 12 2017

.SYNOPSIS...
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
Thursday. Another surge of cold Arctic air is expected by
Friday.

&&

.NEAR TERM /THROUGH TODAY/...
As if 1045 PM EST Monday...

T/Td readings are so far pretty much on track. Some minor
changes were needed in areas such as LWB, where decoupling has
allowed strong radiational cooling.

Delayed the arrival of snow showers a bit per latest
NamNest/HRRR, which show very little arriving much before 12Z.
Currently, only some very light echoes showing up on radar in
central WV, with slightly more solid activity in the northern
part of the state. However, latest observations are all CLR BLO
120, so it does not appear that any of this is yet reaching the
ground.

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 west
as the clipper system approaches. 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.

&&

.SHORT TERM /TONIGHT THROUGH THURSDAY/...
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.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
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
colder 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 morning.

&&

.AVIATION /05Z TUESDAY THROUGH SATURDAY/...
As of 1215 AM EST Tuesday...

VFR conditions will continue for a few more hours through the
evening and likely until around 08Z or 10Z. Ceilings will
lower from west to east during the overnight as a strong
clipper system approaches from the northwest. For much of the
night east of the Blue Ridge it will remain VFR. Ahead of this
clipper, winds are expected to be mostly SSE- SSW 4-8kts.

As the Alberta Clipper moves into the region early Tuesday,
expect periods of mostly MVFR-IFR ceilings and MVFR visibilities
in -SHSN are expected west of the Blue Ridge, with VFR
conditions continuing to the east of the Blue ridge. A snow
begins to accumulate in the west, some locations such as KBLF,
may see BLSN even after the snow ends, thus reducing visibility
into the MVFR category. The most likely area for SHSN will be
across the mountains from SE WV and SW VA into northwest NC. May
see a period of short duration snow squalls associated with the
Arctic frontal passage between 11-14z at KBLF, KLWB and perhaps
even KBCB. Gusty post frontal winds are expected as the strong
clipper moves east of the area. Look for WSW-WNW sustained winds
of 10-15 kts with gusts up to 30 kts at KROA, with even
stronger winds possible at higher mountain peaks west of the
Blue Ridge. Winds will not decrease much until late on Tuesday
night into Wednesday morning.

Medium to high confidence in ceilings throughout the TAF valid
period.
Medium to high confidence in visibilities throughout the TAF
valid period.
Medium to high confidence in wind speed/direction throughout the
TAF valid period.

Extended Discussion...

Weak high pressure builds in Wednesday followed by another
weaker clipper Thursday that passes by mostly to our north.
Another more significant clipper system will arrive by Friday
into Saturday, with gusty winds and mountain showers, thus the
potential for periods of sub-VFR conditions.

&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...Wind Advisory from 4 AM early this morning to 7 AM EST
     Wednesday for VAZ015>017-022.
NC...Wind Advisory from 4 AM early this morning to 7 AM EST
     Wednesday for NCZ001-002-018.
     Winter Weather Advisory from 4 AM early this morning to 7 AM
     EST Wednesday for NCZ001-018.
WV...Winter Weather Advisory from 7 AM this morning to 7 AM EST
     Wednesday for WVZ508.

&&

$$
SYNOPSIS...AL
NEAR TERM...AL/RAB
SHORT TERM...JH
LONG TERM...JH
AVIATION...RAB

Forecast Discussion from: NOAA-NWS Script developed by: El Dorado Weather