Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 111127

Area Forecast Discussion
National Weather Service Blacksburg VA
627 AM EST Sun Dec 11 2016

High pressure across the region will pass offshore today into
tonight bringing a gradual increase in moisture by this evening. A
cold front will then move across the area Monday resulting in a
period of rain later tonight into Monday, with the possibility of
spotty freezing rain across the mountain valleys. A series of
weak disturbances will then skirt the region through midweek before
another Arctic front crosses the area later Wednesday night into
early Thursday.


As of 230 AM EST Sunday...

Arctic air across the region will begin to depart today as the surface
high passes offshore allowing increasing southerly flow to take shape
this afternoon ahead of the next weak cold front approaching from the
west tonight. This should also result in gradual moistening from south
to north through the day before much deeper moisture arrives ahead of
the trough to the west overnight. However questions in just how fast low
level moisture works in from the south later today, and whether or not
can moisten things enough to at least produce some spotty light rain or
drizzle far south late. Latest guidance a bit drier and farther east
with the very light precip band with mainly the Nam printing out some
light QPF across northwest NC by early this evening. This typically
overdone into such a dry airmass but since ensembles hold
onto some light pop will bump up mention with 20ish pops far south.
Soundings also of concern in showing some spotty icing potential
valleys but should be light and very brief if it even occurs since
precip likely too light to wetbulb/cool the column too much overall.
Otherwise expecting periodic warm advection mid deck today to make
for intervals of clouds and sun before low level moisture now across
Georgia arrives late. Highs mostly upper 30s to lower 40s except could
reach the mid/upper 40s far west.

850 mb winds really ramp up this evening with very strong warm
advection taking shape on a 50+ knot jet that will rapidly erode any
lingering below freezing temps espcly after midnight. However appears
timing of pre-frontal precip from the west may arrive in time for
valleys across the far north to see some spotty freezing rain before
all turn to rain overnight. Could also see some light icing given such
cold ground even with temps above freezing, so included some light
accumulation valleys mainly in Greenbrier/Bath counties where
will mention in the HWO. Elsewhere appears combo of clouds
arriving early on and rising dewpoints to limit the icing threat
to localized colder spots during the evening. Best rain coverage
likely to progress from west to east after midnight with
likely/cat pops western third late and chance elsewhere out into
eastern sections.

Soundings also show the strong south/southwest jet not very far above
the ridges with potential for some of the higher western peaks to get
into advisory type gusts before the more widespread rain arrives.
Therefore plan to include in the HWO as well as if winds are slower to
veer more from the south/southeast to the southwest then higher speeds
could work down the ridgelines a bit more espcly BLF vicinity. Expect
temps may cool initially this evening before rising overnight with all
above freezing and perhaps into the 40s ridges by morning. Therefore
running with a non diurnal curve tonight.


As of 430 AM EST Sunday...

A broad upper low anchored across the south central Canadian
provinces will keep a broad westerly, near zonal flow in place
across the entire U.S. through the period. The key through the
entire period will be on subtle ripples that will bring weather
systems in or near the area, as well as drive a couple of cold
front, including another significant Arctic intrusion through the

Monday morning a cold front will be moving through the region.
Significant warming at 850mb and a strong southwesterly LLJ will
quickly erode any remaining cold air that would have been in place
overnight. Current grids have any frozen precipitation ending
before 12Z Monday. So, will have all liquid precipitation during
this portion of the forecast. Given the zonal nature to the flow,
the front is expected to stall across the southeast states, where
it will serve as a notable baroclinic zone during the remainder of
the period. The front is expected to push far enough south by
Monday evening to leave the area with dry conditions. Upslope
potential is limited due to mainly a west post frontal flow versus
northwest flow and nominal cold advection. The air mass behind
this front is mixed Pacific and Canadian, which will just drop
850mb temps back into the 0C to -4C levels.

For Tue and Wed, the key to the forecast will be on how far north
southern stream moisture, associated with the baroclinic zone
across the southeast states, shifts back north. The ECMWF is the
most aggressive with this, while the GFS is less so. The most
likely period for precipitation to be pulled back north is in
advance of a northern stream short wave Wed that will support the
next surge of Arctic air in the region late Wed into Thu. For now,
have confined pops to slight chance and focused mainly on the
Piedmont, especially until the Arctic front arrives Wed evening,
at which point chance pops are more in order in some areas.
Confidence in pops and timing of pops through this period is low,
but there will definitely be periods of light precipitation
skirting the southern and Piedmont areas much of the period, and
in advance of the Arctic front all areas Wednesday.

The earlier indicated frozen precipitation potential for Wednesday
morning looks very limited at best with overnight warming
expected, similar to what we will see tonight/early Monday. Thus,
it has been removed. As the Arctic front moves into the area
Wednesday night, rain will change to snow west of the Blue Ridge,
but additional potential for upslope again appears relatively
limited because of predominate westerly flow and quick drying.


As of 500 AM EST Sunday...

Another significant surge of Arctic air is expected to spread
across the region Thursday behind the aforementioned cold front.
850mb temps are progged to fall into the -18C to -20C range, so
likely a few degrees colder than this most recent spell. With the
exception of a limited period of upslope snow showers west of the
Blue Ridge, and mainly across the Alleghanys, precipitation will
be limited behind the front. The Arctic high will build in for
Friday, which will be the coldest day of this spell. Gusty
northwest winds behind the Arctic front early Thursday may meet
wind advisory criteria west of the Blue Ridge according to a
local study, so this has been highlighted in the HWO.

The next Arctic front will approach the region next weekend.
There is a potential for a Miller B storm system to develop by
that time with a large region of overrunning and mixed winter
precipitation across the region sometime between Friday and
Saturday night. However, timing has been variable in the models
for the past few runs and a big surge of warm air advection will
greatly influence precipitation type. Current model trends suggest
a period of snow Saturday morning transitioning to mixed
precipitation as warming aloft occurs, then to all rain by
Saturday evening as the significant surge of warm air arrives from
the Gulf coastal region on the heels of a strong LLJ. So while
there may be some winter weather issues, the time frame for such
to be significant appears limited. Hopefully, future models will
paint a clearer picture of this potentially first notable winter
weather event of the season.


As of 610 AM EST Sunday...

VFR will continue for much of today with periodic mid clouds this
morning and then strato-cu from the south as low level moisture
increases this afternoon. However still appears that most cigs
will stay above 3000 feet until late in the day when KDAN could
lower into MVFR.

Winds will start to increase this afternoon and especially this evening
from the south and eventually southwest late in the day. Some
gusts over 30 kts will be possible at KBLF heading into this
evening. Strong low level jet and limited winds at sfc may also
lend to low level wind shear issues toward midnight. Given the
strength of the jet aloft will go ahead and include LLWS at all
locations excluding KBLF overnight where winds will mix to the

Cloud bases will lower from the south and west overnight as the
combination of return low level moisture and a cold front
coincides. Appears most will lower into MVFR espcly after midnight
with IFR possible around KDAN and KLWB. Timing of rainfall also to
be mostly after midnight with best chances west of the Blue Ridge
by early Monday so included a high end MVFR vsby restriction west
of KROA.

Extended aviation discussion...

A cold front will arrive later Sunday night into early Monday which
will likely produce sub- VFR conditions at times, and possibly a
wintry mix in the mountains. The cold front crosses the area
Monday with continued threat of showers and sub- VFR at times,
then somewhat of a break Monday night and maybe Tuesday, with
another system arriving midweek with more precip, and more than
likely sub-VFR at times. High pressure arrives Thursday although a
northwest flow with the center of the high over the midwest will
keep some stratocu around in the BLF area possibly MVFR. Winds
will also be gusty and strong Thursday.




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