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Area forecast discussion 
National Weather Service Blacksburg Virginia
1139 PM EST sun Dec 28 2014

Synopsis...
a frontal boundary will move slowly southeast to near the
Carolina coast by Monday morning...while an upper wave tracks
along it overnight. This will spread another wave of light rain
across the region overnight and into the first half of Monday.
Behind that...shallow cold air will filter in from the north
Monday night and Tuesday...while another weak wave slides across
from the west. This will bring the potential for a very light
wintry mix to parts of the area early Tuesday.
&&

Near term /through Monday/...
as of 852 PM EST Sunday...

Rainfall remains at a relative minimum this evening given middle
level dry air seen off evening soundings and lack of lift as in
between the wave from earlier and the next jet maximum to the SW. This
despite the low level cold front snaking along the Blue Ridge
which looks to finally aid enough convergence with increasing
Omega espcly after midnight to overcome the drying aloft. Most
guidance continues to suggest that the baroclinic zone/Theta-E
gradient to the northwest will gradually shift southeast and combine with the
residual front and the next wave heading up from the SW late. This
should focus an axis of rainfall along and south of Highway 460
prior to daybreak with lighter amounts to the north. Thus have
adjusted probability of precipitation down to low chance west and little out east to init
through about midnight before ramping up southern/eastern areas to
likelys/categorical overnight per latest hrrr/rnk WRF and the 00z
NAM. Temperatures basically look on track with only some slight upward
tweaks east-southeast where values slow to fall and supported by the
latest slightly warmer MOS output.

Previous valid discussion as of 315 PM EST Sunday...

Cold front located across the central/southern Appalachians early this
afternoon is making slow progress against the persistent southwest
flow aloft from the stubborn ridge off the southeast Atlantic coast.
Expecting a lull in the precipitation pattern which has so far been
very underwhelming with rainfall amounts up to now under a tenth of
an inch /0.10/ or less in most areas except for the far southwestern
County Warning Area where a little over a quarter-inch /0.25/ has fallen since 12z.
The front will continue to slide slowly toward the coast by early Monday
with another weak wave forming along it and bringing possibly another
round of rainfall in the 0.25 to 0.50 inch range by midday before
tapering off by late afternoon. Overnight temperatures will hold about
steady as weak cold advection is offset by the thick overcast...
should see lows upper 30s northwest to middle-40s southeast. The cold air makes very
slow southward progress on Monday as weak cold advection continues and
clouds linger. Guidance for highs well clustered in the 40s across the
County Warning Area under mostly cloudy skies in what has been quite a cloudy month of
December.

&&

Short term /Monday night through Wednesday night/...
as of 320 PM EST Sunday...

While main surface front will be well to the south of the area...the
combination of a secondary surge of shallow colder air spilling
down from the north and northeast...along with a very weak upper
wave passing over from the west...will create some weak isentropic
lift over the Appalachians overnight Monday night and lasting
into at least part of Tuesday. Moisture is shallow and struggles
to get into the ice Crystal growth zone...so some drizzle is
possible...but also see potential for some moisture layer aloft
and thus a seeder-feeder process...and so for now will assume
most precipitation will start as ice crystals aloft. The two next
questions are how much precipitation...and what will the low level
thermal profile look like across the area.

Looking upstream to the northwest at this very cold and dry
airmass currently over the upper Midwest...think that the NAM and
other higher resolution models are going to handle the evolution
of the shallow cold air better than GFS and even European model (ecmwf). Given
this...the NAM also probably has a better handle on the shallow
lift over this cold air...including a component aided by
orographics with more of an 850 southerly component for awhile
Monday night. Thus NAM is developing more quantitative precipitation forecast...and even though will
lean in this direction...will not go quite as wet as the one third
inch or more of liquid equiv...but not nearly as dry as the GFS
either.

The combination of NAM thermal profile...and even going slightly
colder than the NAM guidance would suggest at the surface given
model biases in these cold air damming situations...along with the
blend of the model quantitative precipitation forecast...results in some light snow/sleet mix
across the northern portions of the area...changing to mostly snow
extreme north toward Tuesday morning. Have some mix as far south as
New River valley...but quantitative precipitation forecast so light and ground initially warm that
do not see a threat now for sticking except for higher elevations.
With liquid equiv quantitative precipitation forecast between 0.10 and 0... the mix of snow
and sleet...even the highest elevations should see no more than an
inch of snow/sleet. But a light coating on some roads by early
Tuesday mainly north of I-64 as well as along Blue Ridge Parkway
north of Roanoke is certainly possible. This minor event still
needs to be monitored closely for trends...mainly in quantitative precipitation forecast
potential...but really would be surprised if we end up with
advisory thresholds anywhere.

Later Tuesday behind the aforementioned upper wave...and as surface low
along the coast deepens a bit...NAM may be overdone with this
particular trend though...flow turns more westerly and some brief
upslope snow showers will be possible on far western
slopes...especially in West Virginia...late Tuesday through
possible early Wednesday morning...before cold but dry high pressure
exerts its influence on entire middle Atlantic region by Wednesday and Wednesday
night. Expect some of the coldest minimum temperatures we have seen all
of December right as the year comes to an end...with lows Tuesday
night in the 20s and by Wednesday night in the teens. I like the colder
ecwmf MOS-based guidance vs the GFS mex with this airmass in
place.

&&

Long term /Thursday through Sunday/...
as of 300 PM EST Sunday...

Period begins with split flow and modest trough over the northeast
United States with broad westerly flow over the forecast area and
a cut off low in the southwest United States. Pattern amplifies by
Friday with digging trough in the central United States that
generates a surface low in the Lee of The Rockies.

Models similar in the evolution of the low through Friday and both
the European model (ecmwf) and GFS track the low from the Mississippi Valley into the
Great Lakes. The European model (ecmwf) is faster than the GFS with the low and cold
front. Wpc stayed close to the track of the European model (ecmwf) but at a slower
speed.

Some in-situ wedging may be in place on Friday and Friday night.
Surface and 850 warm front lift north of the forecast area by
Saturday. No clear trends for precipitation type although warming
aloft supports freezing rain/sleet...depending on how cold
temperatures area at the surface.

Best moisture and lift will be Saturday and Saturday night just
ahead of the cold front. At this time expecting the front to come
through the forecast area early Sunday. This timing is likely to
change as the system GOES through a few more forecast cycles in the
models.

&&

Aviation /04z Monday through Friday/...
as of 1125 PM EST Sunday...

Rain slowly spreading back east across the mountains at this time and
should push a band east over most of the taf sites after
midnight. This along with upslope flow will keep kblf in LIFR ceilings
overnight with periodic IFR to LIFR visibilities in fog/rain. Otherwise main
concern with how fast conditions will deteriorate again overnight
as the the next wave crosses through early Monday. Most guidance
dropping all sites into MVFR if not already just after midnight
and then into widespread IFR/LIFR ceilings and MVFR to IFR visibilities
through daybreak Monday in rain/fog/drizzle and stratus. However
given current slower trends may hold conditions up a bit longer
before basically tanking ceilings/visibilities at all locations within the
rain shield by early Monday.

Wave shifts east Monday allowing the front to sink southeast and away
from the area in the afternoon. This should act to taper steady
rain back to mainly drizzle or just fog in the afternoon but given
more easterly flow appears very low ceilings will persist with mostly
IFR or worse through much of the day. Klwb being on the northern
fringe of the precipitation could see just enough drying to provide a few
hours of MVFR but iffy.

Extended discussion...

The front drifts off to the southeast Monday night into Tuesday while
leaving plenty of residual low level moisture in its wake. High
confidence in staying sub-VFR Monday night with more fog/drizzle
around overnight. Slow drying to take shape on Tuesday as a large
area of high pressure starts to nose in from the west. However may
take until late afternoon if at all for eastern sites to return
to VFR while lingering low clouds could cause MVFR ceilings to persist
over the west until Tuesday evening. Some spotty light rain or
wintry precipitation may also occur Monday night into Tuesday as a weak
trough of low pressure redevelops over the mountains. This looks
most likely along the klwb-kbcb corridor including parts of the
Blue Ridge at this point. Otherwise high pressure builds east
Wednesday into Thursday with good flying conditions returning
under VFR. The next low pressure system to the south will bring a
return of clouds and light precipitation during Friday as moisture
overruns the cool air in place. This looks to bring sub- VFR
conditions back to most of the taf sites by Friday afternoon if
not sooner.

&&

Rnk watches/warnings/advisories...
Virginia...none.
NC...none.
WV...none.

&&

$$
Synopsis...jh/sk
near term...jh/pc
short term...sk
long term...air mass
aviation...jh/pc/rcs

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