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Area forecast discussion 
National Weather Service Blacksburg Virginia
459 am EST Monday Dec 29 2014

a frontal boundary extends from the Tidewater of Virginia to the
Florida Panhandle. A wave of low pressure was rippling northeast
along the front providing widespread rain to much of the middle
Atlantic region. This frontal system will slowly move east and
off the coast...colder air filtering into the region tonight and
Tuesday. An upper level disturbance is expected to pass from west
to east across the area early Tuesday. This feature may bring a
light wintry mix of precipitation to parts of the area...with the
potential for an inch or two of snow across the Virginia and West
Virginia Highlands.


Near term /through Tuesday/...
as of 430 am EST Monday...

A cold front slipped through the area late Sunday and is now
slowly drifting toward the middle Atlantic coast. A wave of low
pressure was rippling northeast along the front...providing areas
mainly south of Interstate 81 with a widespread wetting rain...quantitative precipitation forecast
of a third /0.33/ to two thirds /0.66/ of an inch. North of
I-81 the rain was not so widespread...and amounts will likely
average less than a tenth of an inch.

Once the wave passes later this morning...rain should end or
taper to drizzle. Clearing is not anticipated per the development
of northeasterly low level winds which will keep clouds banked
against the east side of the Appalachians. Temperatures today will
be steady or falling...cooler air infiltrating the area from the
north or northeast and countering any sort of diurnal temperature

While the main surface front will be well to the southeast of the
area tonight...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 and
lasting into at least part of Tuesday. Moisture is shallow and
struggles to get into the ice Crystal growth zone. Both the GFS
and NAM do indicate a 4 to 8 hour window late tonight into early
Tuesday where some ice Crystal growth takes place...both models
spitting out a tenth to two tenths of an inch of quantitative precipitation forecast.

So the forecast challenge for late tonight and early Tuesday is
weather the precipitation that develops falls as drizzle/freezing
drizzle...or as light rain/light snow. Temperatures tonight at the
surface are forecast to reach freezing along and north of Route
460. This would be the general region where the freezing drizzle
or light snow would occur. South of 460...the boundary layer would
most likely remain warm enough to support drizzle/light rain.

Since both the GFS and NAM are in general agreement that a period
of light precipitation will occur...will err on the side of caution and
forecast at least a tenth of an inch of quantitative precipitation forecast. For areas where
temperatures are cold enough...mainly north of Highway 460...this
quantitative precipitation forecast will fall as light snow or a mixture of rain and snow. The
higher elevations...Highlands of VA/WV...would have the best
chance for seeing something that would accumulate...our gridded
forecast reflecting 1-2 inches of snow for elevations above 2500
feet. Since accumulations are not expected to exceed 2
headlines planned at this time.

The isentropic lift is forecast to weaken by Tuesday
afternoon/evening...any precipitation coming to an end. Similar to
Monday...temperatures Tuesday will experience little change
remaining nearly steady throughout the day.


Short term /Tuesday night through Thursday/...
as of 400 am EST Monday...

A short wave will track across the Ohio Valley to the middle Atlantic
coast Tuesday evening. Once this wave passes east...drier colder air
will filter into the region overnight. Considering the mild weather
the past couple of weeks...this cold air will seem like an Arctic
blast to some...but we will only be dropping temperatures back to
seasonal levels. The cold surface high will reside back over the
plains Tuesday night and slowly move east in zonal flow through the
rest of the week. No precipitation is expected during the period.


Long term /Thursday night 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

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


Aviation /09z 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...


near term...PM
short term...rcs
long term...air mass

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