Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
350 am EST Friday Dec 26 2014
high pressure covered the middle Atlantic and southeast states this
morning and will remain over the area through tonight. Low
pressure over the lower Mississippi Valley will track along a cold
front Saturday and Sunday and will be over the Carolina by Monday
morning. High pressure gradually builds across the eastern United
States Tuesday and Wednesday.
Near term /through tonight/...
as of 330 am EST Friday...
Southeast upper ridge builds today and tonight with modest rise in
heights over the forecast area. Short wave over the lower
Mississippi Valley will track around the ridge into the County
Warning Area late tonight. Models showed some upper level moisture
with this feature late tonight. Otherwise humidity below 700 mb
remains low and until then...little to no cloud cover is expected.
Surface high pressure remains over the area until late tonight.
Winds will gradually turn to the southwest as the high moves
offshore early Saturday morning.
Highs today will be 5 to 10 degrees above normal. Appears a
reasonable rise in temperatures from values this morning since very
dry air mass is in place. Lows tonight will again be cooler in the
valleys and warmer on the ridges. Stayed close to gridded lav
guidance for maximum temperatures today and met guidance for lows
Short term /Saturday through Monday night/...
as of 300 am EST Friday...
The center of an area of high pressure will continue to shift east
of the region during the day Saturday. Southwest flow will continue
across the area and allow for another day of very mild temperatures
for this time of year. To our west will be a conveyer belt of
moisture between our departing high...and high pressure centered
over the Central High plains. Weak areas of low pressure are
expected to track northeast along this boundary. Saturday night
into Sunday this feature will shift eastward across our
region...along with increasing chances of rain. By Sunday and Sunday
night rain is likely across the entire area with conditions still on
the mild side. By Monday into Monday night...the moisture fetch will
shift east of our area. Cooler air will start to enter the region in
the west...allowing for the potential for some light snow on the
backside of the exiting system.
Temperatures Saturday through Monday will be very mild...10 to 15
degrees above normal...for this time of the year...but trend cooler
to about five degrees above normal Monday night.
Long term /Tuesday through Thursday/...
as of 300 PM EST Thursday...
Early part of next week looks to be unsettled with large scale
trough over the western two thirds of the country ejecting at least
one final shortwave piece of energy toward southern Appalachians on
Monday...with potentially some very limited lingering moisture on
western slopes into Tuesday. Still some model differences in 12z
cycles...with European model (ecmwf) and Canadian remaining most aggressive with shortwave
and amount of moisture overrunning the rnk forecast area as surface low
tracks to the south. European model (ecmwf) is faster than wet Canadian and has any
precipitation exiting by midday Monday. GFS and at least some of its
ensembles are weaker and more progressive with this shortwave...keeping
most light precipitation to our south. In all cases...low level thermal
profiles too warm for any wintry precipitation. Best for now to go with
compromise of all these ideas...which ends up being close to European model (ecmwf)
but leaning a little toward GFS. Not going as wet slow as Canadian.
But with uncertainty through late Monday do not feel it prudent to
go with higher than 50 pop anywhere...but have highest probability of precipitation in east
Monday with exiting system...as well as lingering upslope chance
probability of precipitation far west...and keeping some very low chance probability of precipitation into Monday night
After that models now in better agreement that by middle week a flat
upper trough settles more into the eastern third of the country with
split flow over The Rockies...as significant energy cuts off from
the flow over Southern California. Good to see all the models agreeing on
this idea...yet have to be a little leery of model skill in
developing this cutoff...so still cannot rule out some weak waves
heading in our direction through middle week...but chances look better
for broad dry but cool surface high to dominate with westerly
flow...and perhaps still just enough moisture to sneak out some
sprinkles or flurries in the far west at times. Have lowered probability of precipitation
Tuesday to only slight chance in far west...and decided to take out
completely for Wednesday given model agreement of dry high pressure really
pushing in from west...although still some weakly cyclonic flow at
middle levels and likely some low level clouds in west in addition to
possibility of high level clouds everywhere.
Most likely dry into Thursday as well...but European model (ecmwf) advertising some
better energy once again trying to push into the surface ridge late in
the week...latest 12z run slowed this down a bit...so have left probability of precipitation
out for now until perhaps overnight...and then will have to see what
future runs do with this by next Friday.
Gradual cooling trend through this period due to broad but rather
flat upper trough and associated large surface high from central U.S.
Shifting toward Appalachians...and temperatures will end up pretty close to
seasonal norms by middle week. Bottom line is no major wintry weather
to deal with through New Years Day.
Aviation /09z Friday through Tuesday/...
as of 1125 PM EST Thursday...
High pressure building overhead should provide for widespread VFR
overnight under clear skies and light winds. Cant completely rule
out some spotty fog late...mainly in the river bottoms...and out
east given the wet ground from rainfall on Wednesday. However
latest models show a gradual lowering of dewpoints overnight...and
given current spread in temperature/dewpoint depressions only kept tempo
fog in at klwb/klyh where brief low end VFR to MVFR visibilities will be
possible toward daybreak.
High slides to the east Friday allowing a weak return SW flow to
develop by afternoon. However other than for some sheared warm
advection middle/high clouds the column remains quite dry so
continued good flying conditions for Friday into Friday night.
Another cold front will approach the region Saturday and be over
or just west of the area Sunday. This may allow multiple weak
waves of low pressure to ride up the boundary and across the
region later Saturday night/Sunday resulting in periods of sub-
VFR in lower cigs/rain. Boundary may linger over or just south of
the area early next week pending future progress of this slow
moving system by then. This could leave some degree of moisture
including MVFR ceilings over the taf sites into at least Monday or
Monday night before perhaps seeing some improvement to VFR on
near term...air mass