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Area forecast discussion 
National Weather Service Blacksburg Virginia
1234 PM EST sun Dec 28 2014

a weak cold front will drift southeast into the mountains today
before sliding south of the region later Sunday night. Weak waves
of low pressure tracking along the boundary today into Monday
morning will bring periods of mostly light rain today through
early Monday. Cold high pressure will slowly build east into the
region behind the front for the middle of next week.


Near term /through tonight/...
as of 955 am EST Sunday...

A broad and moist southwesterly flow continues across southeastern
and Middle-Atlantic States this Sunday morning with regional radars
showing a fairly disorganized shield of rainfall covering much of
that area...but mainly south of our forecast area. A couple of
factors have led to a backing off on rainfall amounts from this
initial shortwave. Precipitation seems to be having a very
difficult time overcoming the dry air over much of the County Warning Area... with
12z krnk sounding showing several dry layers in the column. High-
res models nmm and arw both showing the current radar trends
fairly well with the heavier rainfall splitting north and south of
the County Warning Area and these have been followed more for todays rainfall.
Adjusted probability of precipitation downward some to reflect this pattern which is
forecast to persist much of the day but many areas may still pick
up measurable rainfall..just a lot less than was forecast even
yesterday...many areas may see only a few hundreths of an inch
today. A second short wave forecast to move northeast overnight
may offer a better chance for more substantial rainfall...possibly
0.25 to 0.50 inches and the bulk of that may come with second
shortwave forecast to move through tonight.


Short term /Monday through Tuesday night/...
as of 130 am EST Sunday...

Cold front should be well to the southeast of the area Monday with low
pressure offshore keeping an axis of deeper moisture back into the
region espcly eastern sections into Monday afternoon. This will
likely keep some steadier rain going east of the Blue Ridge during
the morning with probability of precipitation tapering from west to east during the
afternoon as the wave exits. However as the final main piece of
shortwave energy passes Monday morning...will see a leftover
residual inverted trough take shape over the mountains in response
to another faint strung out middle level wave riding up the
Appalachians. This may act to keep some -ra going mountains even
as the main precipitation shield pulls out by early afternoon. Cold
advection looks to slowly ooze in behind this feature with perhaps
a few locations over the far northwest mountains seeing a change to light
snow. Otherwise likely probability of precipitation east early and chance probability of precipitation elsewhere Monday.
With slower cooling at 850 mb...opted for the slightly warmer side of
MOS which results in highs mostly 40s and perhaps low 50s southeast.

Leftover but somewhat deeper moisture will be the main concern
espcly west Monday night into Tuesday morning as the residual
inverted trough looks slow to weaken. Models continue to print out
a tenth or two of quantitative precipitation forecast along western sections of the Blue Ridge
with the NAM by far a wet outlier in developing a better surface
wave. Colder air draining into this weak convergence zone will
eventually make for ptype issues espcly north with forecast
soundings showing enough moisture depth for snow or sleet later
Monday night northwest...and perhaps some freezing rain late as the
moisture becomes more shallow. Given uncertainty with both quantitative precipitation forecast and
depth of the cold air under such a weak system...will leave mostly
-ra and keep some snow/mix mainly north of Highway 460 and along
higher elevations into Tuesday morning before drier/cold advection
ends precipitation in all except the extreme north in the afternoon.
Light snow accumulations could occur mainly at elevation but no
more than an inch or so with some light icing possible when/if
surface temperatures get cold enough and moisture Shallows out. Otherwise lows
Monday night upper 20s/low 30s north half to around 40 south.
Highs Tuesday mostly 30s mountains to low/middle 40s east.

Large dome of cold high pressure over the central states will
finally ridge eastward and allow for some clearing Tuesday night
but cold with lows in the 20s and perhaps some teens in the


Long term /Wednesday through Saturday/...
as of 430 PM EST Saturday...

Long range least for Wednesday and Thursday are in
agreement with fair continuity as we transition into the New Year.
Broad upper level trough is prognosticated to extend from eastern
Canada...southwest through the central Continental U.S. Into a cutoff area of
low pressure over Southern California. A high amplitude ridge will
be positioned off the Pacific northwest coast. This pattern favors cold
temperatures persisting over the northern Continental United
States (conus) and southward over the plains...and milder air
confined to the extreme southeast Continental U.S....I.E. Florida. For the
middle Atlantic we will likely end up in the transition zone...or
pretty close to normal to finish out the week. Worst case...we may
end up 5 degrees or so below normal for New Years Day...but
nothing extreme.

After Thursday the models diverge...which is to be expected since
they do not handle cutoff lows very well...and especially those
that eject from the Desert Southwest. Within the evolving mean
pattern the primary feature of interest will be the upper low
which all guidance closes off over/near California/Arizona initially but then
diverge on when the low opens up and moves east during the Friday
Saturday time frame. There is a fair amount of model/ensemble
spread regarding the timing and to a somewhat lesser degree track
of the upper low. The operational run of the GFS is 12-24 hours
faster than the European model (ecmwf)...which makes a difference in whether it
precipitates here on Friday vs Saturday. This high spread/low
confidence forecast favors an ensemble mean approach for both
Friday and Saturday...the forecast reflecting a blend...with chance
precipitation both Friday and Saturday...the greatest probability of precipitation favoring
Friday night where the blended solutions overlap. Timing and track
of the system will also determine p-type. Temperatures look to be
borderline critical Friday...suggesting at least some wintry mix
if precipitation arrives Friday morning. If onset holds off until later
Friday...the p-type will be mostly rain.


Aviation /18z Sunday through Friday/...
as of 625 am EST Sunday...

Light rain expected at taf sites this afternoon with mainly MVFR
ceilings/visibilities although occasionally IFR in heavier passing showers.
Could see a break this evening before another weak wave brings
rain and lower flight categories back into the area overnight. In
between these waves...expect restrictions to remain marginal with
fog and drizzle and low clouds. Precipitation from this last wave
pulls out from west to east tomorrow morning but with only slowly
improving ceilings as the middle-level flow remains southwest while
colder air at the surface slowly pushes to the southeast.

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 afternoon for eastern sites to return to VFR while
lingering low clouds could cause MVFR ceilings to persist over the
west until late in the day. Some spotty light rain or wintry
precipitation may also occur early Tuesday as a weak trough of low
pressure redevelops over the mountains. Otherwise high pressure builds
east Wednesday into Thursday with good flying conditions returning
under VFR.


Rnk watches/warnings/advisories...


near term...pc
short term...jh/pc
long term...PM

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