Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
448 am EST Monday Dec 9 2013
a weak frontal boundary will slide east across the region today
before stalling south of the area this evening. Low pressure will
develop along this front tonight...and pass to the coast Tuesday
resulting in another round of precipitation later tonight into
Tuesday afternoon. Cold high pressure will finally build in for
the middle of the week.
Near term /through tonight/...
as of 245 am EST Monday...
Persistent wedge finally showing signs of weakening with decent
pressure falls ongoing within the cold pool and much warmer temperatures
now seen on many of the ridges. However with precipitation...although
patchy in spots...still sliding across on the eastern edge of the
upper jet per vapor loop...will likely maintain going headlines a
while longer until at least those seeing adding rain can get above
freezing. Appears best ice accretion early on will remain along the
eastern slopes of the Blue Ridge and across the NE counties but
should be mainly spotty after sunrise. Models show rain becoming
more patchy after middle morning but maintain a belt of deeper
moisture espcly southern and eastern sections into the afternoon
as the passage of a weak front from the west and the approach of
the upstream 500 mb trough shifts the convergence axis east. Thus will
maintain likely to Cat probability of precipitation this morning and slowly taper back to
chance all except the far south by late in the day as more of a
weak westerly flow kicks in. High temperatures very tricky as MOS looks
too warm given NAM depiction of a weak wedge remaining out east so
tweaked down a few degrees as parts of the north-NE may not get
above 40 while the SW could jump above 50 if mixing is better.
Any decrease in probability of precipitation which looks to occur early this evening will
be short as the final shortwave with the upper trough arrives late
tonight and helps bubble deeper moisture along the old boundary
back north by daybreak. Just how far to the west this next impulse
travels key to both quantitative precipitation forecast and eventual ptype as late night cold
advection starts to develop. Guidance remains spread pending where
the 850 mb boundary ends up with the GFS/ec solutions colder than the
NAM which would support more wintry mix to snow across the northwest
third right before daybreak and a cold rain elsewhere. Since tendency
in this scenario for the cold air aloft to be a bit slower went
with the colder solutions but delayed the mix a couple hours given
the forecast sharp 850 mb temperature gradient over the west late. Otherwise a
quick rebound from some brief evening drying to likely/Cat probability of precipitation
all areas shortly before daybreak. Lows critical to more freezing
ptype issues central/west...but thinking enough clouds to hold
readings up long enough for limited mix from the western New River
valley and points northwest. Likely enough snow/mix for a possible
advisory northwest third late but wont hoist another headline at this
point given the current system and uncertainty late so adding to
the severe weather potential statement at this point. Otherwise going closer to the latest mav MOS
giving low/middle 30s mountains and upper 30s to lower 40s east.
Short term /Tuesday through Thursday night/...
as of 400 am EST Monday...
Will start Tuesday off with an upper level trough moving east from
the Mississippi River region...pushing another round of Gulf
moisture northward along the Appalachians. The trough will develop
an area of low pressure along the cold front over central North
Carolina...causing it stall through the morning. As such...the Gulf
moisture aloft will overrun the shallow layer of cooler high
pressure building in at the surface...resulting in another round of
rain for our forecast region.
The temperature profile of the lowest few thousand feet of the
atmosphere will not be as complex Tuesday morning as what we saw
early Monday morning. Surface temperatures east of the Blue Ridge
will start above freezing...resulting in rain for that region.
Further west...may see a few pockets of light freezing rain and
sleet return for the New River valley and the Alleghany Highlands...
with mainly snow across the mountain Empire and southeast West
Virginia where there will little to no warm air aloft above
freezing. Wintry precipitation accumulations are expected to be
At any rate...the upper trough will pass across our area during the
afternoon...pushing the surface cold front further east and ending
precipitation by evening. High pressure will settle across the
central Appalachians by early Wednesday morning...resulting in
mostly clear skies and light winds. A clipper cold front will push
across the area Wednesday night...resulting in gusty northwest
winds...increased clouds and light upslope snow showers across
southeast West Virginia. Cold Canadian high pressure will build in
behind the front...passing across our Thursday and Thursday night.
Long term /Friday through Sunday/...
as of 300 PM EST Sunday...
During the Wednesday night through Friday portion of the
forecast...our region...along with most of the north central and
northeast portions of the country will be in a pattern dominated by
long wave trough with the associated upper low situated over Hudson
Bay. Closer to the surface...high pressure will make its way
eastward from the middle-Mississippi Valley to over the central
Appalachians. This will keep US in a below normal temperature regime
along with a dry forecast. A shortwave trough within the main
longwave trough pattern will cross the area Thursday...but any
associated precipitation will be light and remain well north of the
region. The most notable impact on our region will be an increase in
wind speed/gusts as 850 mb winds are prognosticated to be anywhere between
25 and 35 kts. There also will likely be an increase in cloud cover
Wednesday night into early Thursday.
During the upcoming weekend...there is considerable differences
between the operational models in regards to the sensible weather
across our region. What is somewhat common among the solutions is
for the Hudson Bay low to move eastward...and thus allowing for a
more zonal flow across our area. Also...both the GFS and European model (ecmwf)
solutions develop a trough across the central U.S. Along with an
enhanced southerly jet that rounds the base of the trough and
translates into the southeast U.S. And eventually middle-Atlantic
region by Sunday. Increasing cloud cover and chances of
precipitation for our region are also common among the models.
What varies significantly is the track...speed...and any
deepening development of a shortwave trough that progresses along
this southerly jet. The GFS is much slower...and allows a greater
period of time for warm air to advect into the region on southerly
winds ahead of and concurrent with the arrival of the
precipitation. For our area this solution offers the potential of
a rain versus wintry mix forecast. The European model (ecmwf) is much more
progressive in timing...so much so...that the parent upper
trough/low ejects eastward with the southern stream energy and
brings much colder air aloft over the region. All this is
happening while the shortwave trough spawns cyclogenesis off the
coast of the Carolinas and our region would have more of a wintry
mix versus snow forecast. The folks at wpc have opted to follow
close to a ensemble GFS/European model (ecmwf) blend. Our forecast will reflect
numbers close to theirs in regard to temperatures and
precipitation chances...and allow the forecast to be a rain versus
snow forecast with at or sub freezing surface temperatures dictate
the areas of snow. There will be no regard given at this time to
what the low level T/dew point profile will be.
Aviation /10z Monday through Friday/...
as of 1245 am EST Monday...
Continue to see waves of precipitation pushing in from the
southwest this morning. Temperatures will remain above freezing
at blf through the night...ending the threat of freezing
rain. Bcb...lwb...roa and lyh will all remain below freezing
through the night...and will be subject to rounds of freezing
rain/severe clear icing. Dan is currently just above freezing...
but could too experience freezing rain for a few hours if
temperatures wet bulb below freezing as rain pushes east. Low MVFR
and IFR ceilings will remain solid through the night areawide.
Models continue to underestimate the strength of the cold air
wedge hugging the surface...so kept the chance of freezing rain in
most tafs through late morning...when daytime heating will finally
get temperatures rising. A cold front will begin to push east
across the area during early afternoon...causing a westerly wind
shift that will allow IFR ceilings to lift...and rain to become
more showery in nature. By sunset...low clouds across the
piedmonts will diminish to the point where expect ceilings to flip
flop between VFR and high MVFR. For blf and lwb...northwesterly
flow will bank low level moisture against the nearby terrain...
maintaining IFR ceilings.
A secondary push of Arctic air is expected into the region behind
a new cold front Monday night. This will bring upslope -shsn to WV
sites by Tuesday...with a period of -ra/-sn expected across the
remainder of the County Warning Area Monday night/early Tuesday. Mostly MVFR conditions
expected Monday night/Tue...especially for the WV mountains. High
pressure will build in for Wednesday and Thursday with a return to
Virginia...Winter Storm Warning until 9 am EST this morning for
Freezing Rain Advisory until 9 am EST this morning for vaz043-
NC...Winter Storm Warning until 9 am EST this morning for ncz001-
Freezing Rain Advisory until 9 am EST this morning for
WV...Winter Storm Warning until 9 am EST this morning for