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Area forecast discussion 
National Weather Service Blacksburg Virginia
927 PM EST sun Feb 7 2016

Synopsis...
deep low pressure off the Carolinas will continue to shift slowly
northeast and away from the region overnight into early Monday.
Another area of upper level low pressure across the Great Lakes
will swing a strong cold front across the middle Atlantic
Monday...followed by a secondary surge of Arctic air Tuesday into
Wednesday. Weak impulses combined with northwest flow and deep
moisture will then result in a prolonged snow shower event
especially over the mountains into midweek.

&&

Near term /through Monday/...
as of 925 PM EST Sunday...

The region is currently between systems with cirrus shield from
large storm bombing out off the coast covering the eastern part of
our area...while the approaching Arctic front is still a ways off
to our west. Do not expect much in the way of precipitation to
reach the western slopes before daybreak and increasing low chance
probability of precipitation for the far west late tonight have the situation covered.

Previous afd...

Deep low pressure off the Carolinas has continued to fan a deformation
middle/high cloud canopy across the region this afternoon but finally
seeing signs of thinning with dry air working in from the north. Expect
as the system continues to wrap up will see this axis of clouds finally
shift east...exiting the eastern zones later this evening per latest
GFS cloud forecast. This should allow for a brief period of subsidence
clearing between the exiting offshore surface low and the next upstream
upper trough for much of the night. Thus went below MOS temperatures given a
period of clear skies and diminishing winds late before clouds arrive
across the far west late.

Strong upper low across the Great Lakes pivots east Monday allowing an
associated cold front to work east into the region Monday afternoon.
This will bring an axis of lift into the west by midday Monday with
this pre-frontal band of moisture possibly working across the County Warning Area by
the end of the day. Concerns remain with timing and the degree of
boundary layer warming that could occur...and just how much quantitative precipitation forecast will be
able to be generated as moisture works into leftover subsidence behind
the offshore low. Latest GFS remains the wettest with over a quarter
inch liquid over the north/west while the NAM quite dry and the
sref/12z ec in the middle. Since there is some good backing ahead of
the boundary...and a rather strong but brief period of lift...think
there could be a decent band of snow over the western third if arrival
is as fast as the GFS. Elsewhere may see precipitation start as a brief period
of rain closer to the Blue Ridge...with mostly liquid heading east
excluding the higher elevations such as the southern Shenandoah Valley
counties and Skyline drive where more snow possible up top.

Therefore trending probability of precipitation with a band of likely/categorical working
east...reaching the Blue Ridge around midday...and the Piedmont
middle/late afternoon. Provided too much warming doesn't occur and the
band stays together crossing the higher elevations...expect a range
from 1-3 inches far west to up to an inch out to parts of the Blue
Ridge and little elsewhere. Given amounts basically near advisory
levels...will go ahead and include a second period advisory for the far
western slope counties for Monday for now. High temperatures tricky with
potential to rise well into the 40s or warmer east given warm
advection...while perhaps getting stuck in the low/middle 30s far west if
precipitation is faster. This supports running with a blend while nudging
values closer to the warmer MOS out east.

&&

Short term /Monday night through Wednesday night/...
as of 340 PM EST Sunday...

Models showing next surges of colder air coming in on Monday
night...then again on Wednesday night. Persistent northwest winds
from the surface through around 700 mb...therefore resulting gin snow
showers. There may be occasional lulls in the snow showers...but
anticipate more time with precipitation than without. More
questionable is when and how far east bands of snow showers may
develop during this time frame. With good cold air advection and
forecast soundings showing prime ice Crystal growth...will trend
toward at least some probability of snow showers or flurries nearly
as far east as the Blue Ridge. Appears that deeper moisture will
erode from south to north on Wednesday and Wednesday night...with
the snow showers ending first in the northwest North Carolina
mountains and eventually in southeast West Virginia. Also enhancing
the available moisture...hysplit trajectories off the GFS showed
between air 2000 and 300 feet above ground level may be picking up some moisture
from Lake Michigan.

Issued a Winter Storm Watch for several counties in the far western
County Warning Area...since total snowfall from Monday night through
Wednesday night will be enough to impact travel. Plus...enough cold
air...combined with moderate winds...to result in hazardous wind
chill values. Highest snow amounts will be on the higher west facing
slopes and coldest wind chills will be at higher elevations
Wednesday night. Stayed as cold or slightly colder than guidance
through Wednesday night.

&&

Long term /Thursday through Sunday/...
as of 250 PM EST Sunday...

Closed 500 mb low remains over southeast Canada but heights rise
over the region. Long wave trough remains over the east through
Sunday with models showing difference in timing and intensity of
short waves embedded within the trough.

Surface high pressure will be centered over the forecast area on
Friday morning. More Arctic air comes in Saturday and Sunday. Both
GFS and European model (ecmwf) are showing variations of a 1040 mb high building well
into the Ohio Valley and middle Atlantic region during that time frame.

Snow showers will be ending in the northwest County Warning Area.
Will have a low probability of snow in the forecast for Friday or
Saturday that is association with a somewhat stronger short wave
tracking through the base of the upper trough. But at that time
frame...location and timing are likely to change.

&&

Aviation /03z Monday through Friday/...
as of 700 PM EST Sunday...

Still dealing with the high level cloud canopy from the deep
coastal low off the NC coast. This system will lift northeast of
the region overnight. The cloud canopy will pull northeast away
from the region...but it looks like this will be a little slower
to occur than earlier thought. Generally looking at the back edge
to reach the kdan/klyh area by 06z. At any rates...ceilings will
remain well above 10kft.

Overnight...the County Warning Area should generally be sandwiched in between the
departing coastal system and a deep upper low sagging southward
through the Great Lakes. This system also appears to be somewhat
slower than expected yesterday. Consequently...deeper associated
moisture should not reach the western County Warning Area until around
daybreak...after daybreak east of the Blue Ridge. So largely...all
taf sites will see VFR ceilings/visibilities throughout the first 12 hours of
the taf valid period. Ceilings will begin to lower into the MVFR
category kblf-klyh and kbcb in the 12z-15z time frame with -sn
associated with a band of stronger dynamic forcing and initial
frontal boundary. Although this initial band of precipitation will
spread into the Piedmont...temperature profiles suggest that it may
largely be rain in those areas. Have carried only -ra for
kdan...with a -rasn mix klyh/kroa...with low confidence in snow
there. Kbcb should see mostly -sn...but some -ra possible at the
onset. The initial band of frontal forced precipitation should
move east of the County Warning Area by afternoon. At that point...and with colder
Arctic air beginning to filter into the region from the
west...precipitation should generally end east of the Alleghany
front except for a few flurries later in the evening...but -shsn
should commence across the alleghanys with MVFR ceilings lowering to
IFR. Ceilings should generally improve to MVFR or better east of the
Blue Ridge.

Winds will remain chaotic through the overnight hours as the
region remains sandwiched between the two larger scale synoptic
systems. Generally a north-northwest wind will prevail in most areas. Toward
daybreak...winds in the west should transition to SW 5-7kts and
this will spread east through the morning. By evening...winds will
become west-southwest-west-northwest 7-10kts behind the frontal boundary.

High confidence in ceilings through 12z...then medium confidence
through the remainder of the taf valid period.
High confidence in visibility through 12z...then medium confidence
through the remainder of the taf valid period. Medium confidence
in wind direction...medium to high confidence in wind speed
through the taf valid period.

Extended discussion...

Sub-VFR should become more confined to the western slopes Monday
night into midweek with prolonged MVFR/IFR conditions at kblf/klwb
in snow showers through at least Wednesday night as Arctic air
arrives. Potential also for periods of MVFR at kbcb/kroa as
impulses rotate across the mountains and bands develop during the
overnight hours. Accumulating snow is likely in the mountains
west of the Blue Ridge. Flurries may also occur as far east as
the Piedmont where intervals of VFR to MVFR ceilings could also
occur.

Conditions should briefly return to mainly VFR Thursday/Thursday
night as weak high pressure builds in and moisture decreases.

&&

Rnk watches/warnings/advisories...
Virginia...Winter Storm Watch from Monday evening through Thursday
morning for vaz007-009>012-015-019-020.
Winter Weather Advisory from 7 am to 6 PM EST Monday for
vaz007-009-015.
NC...Winter Storm Watch from Monday evening through Thursday
morning for ncz001-018.
Winter Weather Advisory from 7 am to 6 PM EST Monday for
ncz001-018.
WV...Winter Storm Watch from Monday evening through Thursday
morning for wvz042>044-507-508.
Winter Weather Advisory from 7 am to 6 PM EST Monday for
wvz042-043-508.

&&

$$
Synopsis...jh/PM
near term...mbs/jh
short term...air mass
long term...air mass
aviation...jh/PM/rab

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