Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
309 PM EST Thursday Dec 25 2014
high pressure will bring fair weather to the region as we head into
the weekend. Then as the high pushes to our east...a front
approaching from the west will bring a chance of precipitation back
to our forecast for Saturday night and Sunday. Our weather will then
remain a bit unsettled into early next week as a high pressure
tries to build into the region from the west.
Near term /through Friday night/...
as of 300 PM EST Thursday...
We will have quiet weather through the end of the week with ridging
aloft and high pressure building at the surface through tomorrow
afternoon. Expect clear skies east of the Blue Ridge...but northwest
winds continue to generate upslope clouds west of the ridge...and
visible satellite pics show the clouds eroding from the south a bit
slower than anticipated. Model guidance is indicating that moisture
trapped under the subsidence inversion will keep the clouds
lingering into tonight. The lower clouds should be gone as we head
toward daybreak...but middle/high clouds will then overspread the
region into Friday night as the upper ridge axis pushes to our east
and a weak short wave moves through. Blustery northwest winds will
be calming down as we head toward evening...then expect winds to
remain light through Friday night. Night time temperatures will be
running a below normal with readings generally in the upper 20s to
lower 30s tonight...but Friday should warm to seasonable levels
with most locations in the 50s to near 60 degrees.
Short term /Saturday through Sunday night/...
as of 300 PM EST Thursday...
Forecast area will be under the influence of a high pressure ridge
off the southeastern U.S. Coast early Saturday. This will make for a
very mild day with highs running well above normal for late
December...generally in the 50s to low 60s far southeast. The day starts
out mostly sunny but clouds will increase through the day as a slow-
moving frontal boundary associated with a trough and short-wave over
the central U.S. Impinges on the ridge by evening. Clouds and probability of precipitation
will increase overnight Saturday which will help to hold overnight
temperatures up at unseasonably mild levels...up to 15f or more above
normal in the mountains. Hence do not expect any p-type issues at
the onset of this event as high chance and then likely probability of precipitation develop
from early to midday Sunday. Front will be slow to move east as it
encounters the deep-layered southwest flow around the South Atlantic
ridge and expect clouds and rain chances to continue much of Sunday
and on into early Monday. With no significant source of cold air
thermal profiles continue to indicate all rain for this event
although 12z GFS was colder than previous runs with far northwest
Greenbrier slipping below 0c at 850 mb by 12z Monday..albeit with
little or no quantitative precipitation forecast at that time. Gem and European model (ecmwf) are warmer and wetter
with quantitative precipitation forecast by 12z Monday showing up to 0.75 inches in the southwest
County Warning Area but used a blend of models and HPC for substantially less in the
grids by that time...generally around 0.25 to 0.50 in the southwest
and lesser amounts north. Highs Sunday will be mild again but a
little less so than Saturday due to full overcast...upper 40s northwest to
middle-50s should do it.
Long term /Monday 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 /20z Thursday through Tuesday/...
as of 1220 PM EST Wednesday...
Northwesterly winds will slacken this afternoon as high pressure builds in
from the lower Mississippi Valley. This will yield VFR conditions
east of the Blue Ridge with few clouds. However with antecedent
wet conditions and good radiational cooling as the high builds in
tonight...believe there is the possibility for some ground fog
toward daybreak. Will not mention any fog at kroa since they look
to keep a light wind overnight...but will include a tempo MVFR
visibility restriction for klyh/kdan since they should decouple
and have the best chance for fog development.
Further west...moisture is slower to pull out than earlier thought
and klwb may not break lose their ceiling. Furthermore...there are
indications that low level moisture trapped under the subsidence
inversion will yield a resurgence of low MVFR clouds for a while
overnight. Will not show a ceiling at kbcb but will reintroduce an
MVFR ceiling at kblf for several hours. With only scattered clouds for kbcb
there may be enough rad cooling for some fog here as well...and
klwb looks to finally clear out toward daybreak and this may
provide enough time for a brief period of fog to form.
Tomorrow looks like a fine VFR day with light winds and only some
middle or high clouds.
High pressure should build over the southeast...which will
provide VFR weather with lighter winds through Friday night.
Another cold front with associated low pressure will approach the
region Saturday and be over or just west of the area Sunday. Boundary
may linger over or just south of the region early next week with
possible return to VFR Monday afternoon.