Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
944 am EST Friday Dec 20 2013
high pressure centered along the southeast Atlantic coast will
drift drift very slowly offshore today and Saturday. A southwesterly
flow of very mild air will provide above normal temperatures to
the southeastern United States through this upcoming weekend.
A slow moving frontal boundary will approach the area from the
west Saturday...crossing the area Sunday. This system will result
in considerable cloudiness this weekend and a threat for showers.
The best chance for measurable rainfall will occur Sunday.
Near term /through tonight/...
as of 940 am EST Friday...no major changes to the forecast
currently. Moisture continues to spread northeastwardly in advance
of an approaching cold front. For our region today...this will
yield plenty of middle to high level cloud cover across the region.
The least abundant amount will be across the far southeast portion
of the region. Hourly temperatures are trending a bit cooler than
earlier expected...but we are still counting on strong warm air
advection to help US realize high temperatures in the upper 50s to
lower 60s despite the cloud cover. If any aspect of the forecast
has the greatest potential for errors today...it would be the
temperature forecast. We will continue to monitor and make
adjustments accordingly as need be.
As of 200 am EST Friday...
1026 mb surface high was centered along the southeast Atlantic
coast. A cold front stretched from the lower Great Lakes into the
Central Plains. Between these two weather features...unseasonably
warm air was flowing north from the Gulf of Mexico...flooding the
southeastern United States with higher dewpoints and warming
temperatures. This mild airmass will advect into our region
promoting above seasonal warmth the next several days.
Temperatures this afternoon should equal or exceed yesterdays
numbers...readings headed for the 60s. Per increasing
dewpoints...lows tonight should be 5 to 10 degrees warmer.
Upper level moisture is on the increase per connection to the
strengthening upper level trough over the southwest Continental U.S.. thin
veil of cirrus will be over the area today...varying in thickness
attms producing a dirty sky sort of appearance. Call it what you
wish...partly sunny/cloudy...the message is the same.
The surface cold front will gradually make inroads into the Ohio
Valley tonight. Clouds will continue to thicken as it gets
closer...but little or no threat of shower activity is expected
for our area just yet. Most of the precipitation today and tonight will
be confined west of our region from the Red River valley northeast
through the middle Mississippi Valley and into the Ohio Valley and
Great Lakes. There is a slight chance some light rain showers or
sprinkles could reach as far east as our WV counties tonight...but
the probability for measurable rainfall is pretty low (20 pcnt).
Short term /Saturday through Sunday night/...
as of 500 am EST Friday...
Main focus through this period will be on an amplifying upper
trough through the central U.S. As the SW U.S. Upper low merges
with a northern stream short wave. A deep surface low will track from
the arklatex region Saturday northeastward through the western
Tennessee/Ohio Valley and toward the Great Lakes by Sunday. Meanwhile a
subtropical ridge will amplify across the eastern U.S. Into the
mid-Atlantic...resulting in an increasingly SW flow aloft across
the region through the period. Through early sun...feel that most
precipitation will be confined to upslope induced -ra/-dz across the higher
elevations of the alleghanys...especially for elevations above
3500 feet as these elevations and above will likely be near the
saturation/cloud layer. With the main dynamics still well to the
west of the area...cannot see much if any meaningful dynamics for
precipitation through early sun...with dry conditions continuing east of
the alleghanys and most of the County Warning Area for that matter. By Sun
afternoon...the cold front will accelerate eastward through the
County Warning Area as the main surface low and associated upper positive vorticity advection track toward New
England...with a secondary upstream northern stream trough digging
into the north central U.S. A secondary surface low will develop
along the front in the mid-south/southeast states. The northern dynamics
will pass north of the area...with better dynamics refocusing to
the S-southeast part of the County Warning Area by afternoon/evening. This will likely
result in a relocation of the main precipitation area from WV early in the
day to the NC/Virginia Piedmont by afternoon...with a precipitation
shadow/minimum area through the New River/Roanoke valley/I-81
corridor region. Quantitative precipitation forecast still appears quite manageable...averaging
around 1.0 inch for most of the County Warning Area. Instability remains fairly
impressive for middle- December across the Piedmont Sun
afternoon...so will continue the mention for isolated/chance thunderstorms and rain in
these areas...lingering into the evening across
Halifax/Pittsylvania. Probability of precipitation are likely to categorical throughout
the County Warning Area Sun afternoon. Temperatures will continue to be
unseasonably warm through the period with 850mb temperatures near +15c
across the Piedmont. The only factor limiting potential for
widespread record highs will be limited insolation from extensive
middle/high cloud cover...then widespread rain on Sunday. No freezing
temperatures and thus no frozen precipitation of any kind are
expected through this period.
Long term /Monday through Thursday/...
as of 500 am EST Friday...
With troughing still upstream early Monday and a surface wave of low
pressure riding along the front across the southeast part of the
County Warning Area...expect cooling to be less impressive early on than earlier
expected. Thus...precipitation should remain in the liquid form
through 12z Monday. Thus...little if any upslope -shsn is expected
through 12z Monday...where even in the WV mountains it should remain
-shra except for elevations above 4000 feet which may begin to see
some -shsn toward 12z Monday. However...precipitation will also
linger longer across the southeast part of the County Warning Area than earlier expected
as well...and could take until afternoon to completely clear the
Piedmont. Thus...probability of precipitation have been increased across the southeast half of
the County Warning Area compared to previous forecasts. Cold air advection will certainly be in
place from west-east through the day Monday...but the best cold air advection will wait
until the aforementioned northern stream short wave moves into the
eastern Great Lakes and a large Arctic/Canadian surface high builds
southeastward from the northern plains toward the Ohio Valley.
Thus...by Christmas evening morning...expect a return to cold
temperatures with lows in the teens and 20s...except low 30s
eastern Piedmont. Scattered -shsn will likely continue Monday
afternoon/night across eastern WV and possibly into SW Virginia/northwest
NC...but conditions are not ideal for such to occur.
Thus...accumulations should only be an inch or less in most of
these areas...with little to none elsewhere. Christmas will be
sunny and cold...with highs remaining below freezing in the higher
elevations of eastern WV and western Virginia to the 40s across the
After Christmas...the forecast becomes somewhat problematic with
the GFS and now the 00z European model (ecmwf) both advertising the potential for a
northern stream Alberta clipper and a southern stream short wave
to merge across the southeast/southern middle-Atlantic in the Friday-Sat time
frame. Temperatures will be cold enough by this point for concern
with winter precipitation...perhaps yet once again another wedge
event resulting in at least a brief period of -fzra/sleet...then
transitioning to upslope -shsn as a weak coastal low develops off
the NC/Virginia coast Friday. At this point...moisture appears limited and
it does not appear to be a major winter weather system...but will
certainly bear watching. Another surge of Canadian air is slated
to move into the area by the weekend...which will likely result in
near to slightly below normal temperatures as we close out the
last few days of the year.
Aviation /15z Friday through Tuesday/...
as of 645 am EST Friday...
VFR continuing through the taf valid period. Extensive high clouds
will continue to stream across the region in advance of a
developing deep area of low pressure in the southwest U.S. This
will result in mostly ovc250 at all taf sites. Middle clouds will
also start to spread into the region during the last 6-12 hours of
the taf period. Visibilities will remain VFR with a dry surface air mass and
limited radiational cooling given extensive middle/high clouds and
strong SW flow. Winds...SW 7-12kts today with gusts to 20kts
roa/blf/Dan during the afternoon.
High confidence in ceilings...vsybs...and winds through the taf valid
Extended aviation discussion...
lowering cloud bases are expected Saturday ahead of the
approaching cold front. MVFR ceilings are anticipated by Saturday
night and continue into Sunday...rain showers becoming more
numerous as the front crosses the area Sunday. Some periods of IFR
ceilings possible especially blf/lwb Sunday. Isolated thunderstorms and rain are also
expected across the Piedmont Sunday afternoon.
The frontal boundary will push off the Atlantic coast on Monday.
In the wake of the frontal passage VFR conditions will return east
of the Blue Ridge. Clouds and scattered snow showers will be
found in the mountains with potential for MVFR ceilings/visibilities
lingering until Tuesday morning.
High pressure builds in for Tuesday. Dry/VFR weather expected
Tuesday night and Christmas day.
the high and high minimum records for...