Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 040006

Area Forecast Discussion
National Weather Service Blacksburg VA
706 PM EST Sat Dec 3 2016

High pressure will shift east into the area tonight before sliding
offshore on Sunday. An upper low across the southwest states will
slide east into the lower Mississippi Valley into early next
week. Several disturbances ahead of this feature will bring
periodic chances of mainly rain Sunday through Tuesday.


As OF 700 PM EST Saturday...

Forecast pretty much on track. Really do not foresee much of any
measurable precipitation making it into the southwest part of the
CWA until closer to daybreak. Temperature and dewpoint readings
were right on track, so no significant changes needed at this
time. Adjusted the cloud cover up a bit in eastern areas.
Basically, mid/high clouds have overspread the entire region and
from this point on it will be overcast, so nearly 100% cloud cover
next 24-36 hours at least.

As of 300 PM EST Saturday...

12Z sounding from Blacksburg this morning showed dry air below 500
MB. Bufkit forecast showed mid and high level clouds remaining in the
area this evening then the column saturating from top down and
becoming moist enough for precipitation in northwest North Carolina
between 09Z/4AM and 12Z/7AM with the precipitation spreading
northwest across northwest North Carolin and southwest Virginia
through the day Sunday,reaching Lynchburg and Lexington by mid to
late afternoon. Models have been consistent with the timing and
amounts of precipitation for the past several runs.

Clouds had limited temperature rise today. Expecting a small drop in
temperatures after sunset then plume and meteogram forecasts
forecast a nearly steady temperatures for the rest of the night.
However, may get some evaporative cooling since starting with dry
air near the surface as the precipitation develops. Most guidance
only showed a limited coverage of any freezing temperatures late
tonight and on Sunday...mainly at the highest elevations of far
southwest Virginia and northwest North Carolina and along the spine
of the southern Blue Ridge. So have brief periods of snow and sleet
mixing in where there are colder surface and near surface
temperatures.  Have followed a non-diurnal trend for temperatures
tonight and lowered maximum temperatures for Sunday.


As of 300 PM EST Saturday...

Dual corridors of rainfall will continue to cross the region Sunday
night as split northern/southern stream waves ride through weak
shortwave ridging across the region. Appears best coverage will reside
more with the southern stream impulse over the southern and southwest
sections early on, then perhaps across the southeast into the overnight
where guidance has weak diffluence along the edge of the theta-e
gradient. Farther north quite iffy in seeing little more than spotty
light rain where will still be deeper into the initial dry air. Could
see spots across the higher elevations of Bath/Greenbrier wetbulb back
close to freezing and with isothermal forecast soundings that are near
freezing at the surface, added in some patchy light snow/fzra mention
but quite iffy. Otherwise likely to chance pops overall with some split
possible central sections. Lows mainly in the 30s with some slow rises

Deeper moisture exits with the passing mid level trough Monday leaving
the area under weak high pressure that will shift in from the west
early and orient into wedge formation by the end of the day. This
influx of drier air aloft likely to slow down the next round of
rainfall that will be heading northeast toward the area from the main
upper low ejecting out of the southwest states. Some models remain
rather fast but blended trend of the GFS/EC would suggest going slower
into the wedge with little pop except chance far south/west by late
Monday which may be generous. With rather warm temps aloft and more of
a northwest trajectory under perhaps some sun before the wedge digs in,
expect a little bump in highs with 50-55 east and 45-50 west.

Surface low progged to track just west of the mountains later Monday
night into Tuesday with lots of lift/deeper moisture over the deepening
wedge. This will make for a chilly and increasingly wet Monday night
and a very cool Tuesday before things dry slot from southwest to
northeast late in the day. Guidance showing quite a warm nose aloft as
well that should keep most precip liquid except perhaps again in the
highest elevations across Bath county where may mix in some wet snow at
the onset along with spotty icing ridges if surface temps get caught
around freezing early Tuesday. Other concern could be some cutoff in
the degree of QPF per convection to the south/southeast and a
subsequent secondary surface wave off the Carolinas on Tuesday which
may act to funnel drier air into the wedge. Otherwise looking at
categorical pops for a soaking rain event including QPF of an inch or
two espcly south and along the southern Blue Ridge where upslope aided
at the onset. Southeast winds will also increase over the far west
around BLF but appears not strong enough per less mixing for headlines
at this point. Kept highs quite cool on Tuesday, mostly 40s although
may struggle to get out of the 30s north, while a few 50s possible on
the western edge of the wedge.


As of 200 PM EST Saturday...

Remnant upper low responsible for the early week rainfall will depart
Tuesday evening followed by a weak cold front of Pacific origin early
Wednesday. Lingering moisture trailing this system likely enough for a
few added showers east early on Tuesday night, then across the far west
overnight. Otherwise clouds early then decreasing in all except the
mountains overnight with lows mostly in the 30s. Will be in between
systems midweek as brief zonal flow gradually evolves into a deepening
upper trough regime with a large dome of arctic air heading south
through the Midwest by early Thursday. This may allow more sunny
periods Wednesday with temps rebounding into the 40s and 50s which
will be the warmest for a good while to come.

Very strong cold front associated with the 500 mb trough will cross the
region Thursday with some timing differences in models as slightly
faster trends continue. Lobe of deeper moisture right along the front
may bring a band of showers west, with a quick change to snow showers
far west as seen with similar arctic fronts in the past. Some
of this coverage may even spill east of the mountains in liquid
form so running with decent to high chance pops west and slight
coverage east. Pending timing, temps to fall from west to east by
afternoon with the core of the cold air arriving Friday into
Friday night under strong northwest winds, and lingering snow
showers despite very dry air by weeks end. Gradient given depth of
the cold advection perhaps enough to warrant advisory level winds
in spots over the mountains otherwise very cold. Appears highs
wont get out of the 20s to around 30 mountains Friday and 30s east
with lows teens most spots Friday night and even single digits in
the deeper valleys. This should also push wind chills to around
zero if not colder in spots by early Saturday. Thermal trough
including the coldest air aloft will lift out by Saturday but
still highs only 30s to low 40s despite sunshine.


As of 700 PM EST Saturday...

Generally VFR conditions expected to prevail until the later
portion of the TAF valid period. Extensive mid/high clouds will
overspread the region in association with a southern stream system
moving across the Gulf Coastal states. Light precipitation from
this system will spread into areas south of the NC/VA border
during the morning hours, then northward into the bulk of the TAF
sites toward the end of the TAF valid period. Ceilings should
remain VFR until the rain arrives, thus at least through 18Z,
possibly as long as 20-21Z Sunday, and may remain VFR for KLYH and
KROA until closer to 00Z Monday. However, cigs will progressively
lower from the current 180-250 into the 040-080 range by 18Z Sun.

Visibility is expected remain VFR through the TAF valid period
with no fog development given lower-level dry air and extensive
mid/high clouds cover.

Winds will be light and variable overnight as weak high pressure
drifts across the region, trending more toward the ENE-ESE through
the day Sunday with speeds mostly 5kts or less all areas.

Medium to high confidence in cigs/vsbys through the TAF valid
High confidence in vsbys through the TAF valid period.
Medium to high confidence in wind speed/direction through the TAF
valid period.

Extended aviation discussion...

Rain, mostly light, will spread northeast Sunday evening and will
continue Sunday night.

A break in the rain into Monday may allow for some VFR with sub-
VFR returning late Monday night ahead of low pressure moving into
the Tennessee Valley with widespread rain expected into Tuesday.

Expect a bit of improvement after Tuesday as front moves offshore
with another front entering from the west, with mainly VFR
expected then though some MVFR cigs BLF/LWB lingering with good
upslope flow behind an Arctic cold front.




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