Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
1221 PM EST sun Jan 25 2015
an Alberta clipper will approach the region from the west by early
this evening...before moving across the central Appalachians
overnight into Monday. This system will bring periods of snow and
rain to much of the region later tonight into Monday. Strong
northwest flow should follow this feature Monday night into
Tuesday before high pressure builds in for the middle of the week.
Near term /through tonight/...
as of 940 am EST Sunday...
Increasing warm advection pattern to develop through this
afternoon with winds turning more south/SW ahead of the digging
upstream clipper that will be approaching from the west late this
evening. Latest temperature guidance continues to show added warming
this afternoon but highly dependent upon the degree of clouds
since overall mixing looks to stay rather light. However even with
passing middle/high clouds and some increase in low level moisture
late in the far northwest...thickness values supporting middle/upper 40s
most of the mountains...and 50-55 from the Blue Ridge east. Thus
bumped up highs a degree or two more especially east. Otherwise
slowing up any precipitation chances up until this evening...while going
with a period of more sun from late morning into middle afternoon for
most sections as dry air aloft gradually mixes out residual
As clouds thicken by tonight...should keep temperatures from
dropping too quickly and question is will boundary layer temperatures get
cold enough for any precipitation to fall as snow that comes in ahead of
surface low...generally timed for midnight or later in the west.
While this classifies as a clipper system...it has been trending
to dig a little deeper and slow down a bit with new model
cycles...so measurable precipitation might not reach east of the Blue
Ridge until closer to dawn Monday morning. Thermal profiles of NAM
and other high resolution models suggest most areas will not see
snow tonight except for highest elevations...3000 feet or even
higher. I may even have temperatures too cold at these highest elevations
and changing to some accumulating light snow too early. An inch or
so is certainly possible by Monday morning at places like Beech
Mountain NC...Montana Rogers Virginia...to western Greenbrier County WV and
higher ridges in the Alleghany Highlands of Virginia...but much of
this depends on if precipitation rates can briefly get high enough to
start sticking...with the ground getting fairly warm Sunday. By
12z Monday the surface low is still in the far SW part of the forecast
area and colder air not expected to filter in behind with better
upslope flow until later in the day Monday. For Monday morning
lows...going with milder met guidance given warm advection
continuing and solid cloud cover.
Short term /Monday through Tuesday night/...
as of 340 am EST Sunday...
As the clipper approaches the southern Appalachians Monday
morning...a secondary low will develop off the Carolina coast.
During the day Monday...the clipper will become a coastal low. As
the coastal low becomes dominate by late morning...low level flow
will turn easterly...then northerly in the afternoon...dropping
temperatures across the northern half of rnk County Warning Area. The cold front
should stall across the southern Ohio Valley Monday morning...then
race across the region early afternoon. Mountain temperatures will
peak by middle morning then fall in the afternoon. Models continue to
run warm Monday but lowered them with increasing easterly flow in
the morning and the cold front zipping across the area in the
afternoon. The resulting p-type would be mostly rain in the morning
turning to snow across the higher elevations around noon then across
the rest of the mountains by rush hour Monday evening.
Additional accumulations Monday around an inch.
Monday night...the once clipper turned coastal low tracks north and
becomes a bonafide nor-Easter. With the track close to the
coast...wrap-around moisture may keep rain across the Piedmont
through midnight...while the mountains have scattered snow showers.
Late Monday night...a significant short will drop south out of the
Great Lakes...bring more accumulating snow to the mountains Tuesday
morning. This wave will be strong enough to over come downsloping
and push snow east of the Blue Ridge Tuesday morning. Accumulations
across the mountains could range 1-3 inches and under an inch east
of the Blue Ridge into the foothills. By Tuesday afternoon...the
snow stops with the exception of western slopes where snow showers
will continue into Tuesday night. Not looking at much upslope with
northerly winds and high pressure building in by Wednesday morning.
Once the nor-Easter bombs off the middle Atlantic coast Monday
night...a pressure gradient will remain over the region until high
pressure builds in Wednesday morning. This gradient will keep breezy
conditions over the region. Pressure rises behind the front on
Monday and again Tuesday afternoon and evening will increase winds
and gusts but should not exceed advisory criteria. Wind chills drop
into the teens Monday morning and to near 10f above Wednesday
Long term /Wednesday through Saturday/...
as of 230 PM EST Saturday...
A high amplitude upper trough along the East Coast will lift out by
Wednesday evening into Wednesday night. Meanwhile...a 1026 mb surface
high pressure will slide east across the region Wednesday into
Wednesday night. As high pressure moves east across the area...the
low level winds back around Wednesday...bringing an end to any
lingering upslope snow showers or snow flurries with temperatures
High temperatures Wednesday will range from the upper 20s in the
mountains to the lower 40s in the Piedmont. On Wednesday night...the
high center will be located across eastern North Carolina...and a
low pressure center in the Chicago area with an approaching trailing
cold front in the Ohio Valley. Low temperatures Wednesday night will
generally be from the upper teens to the upper 20s.
A cold front will move east across our area Thursday afternoon into
Friday. High temperatures Thursday will warm into the upper 30s in
the northwest mountains to the upper 40s in the piedmonts. Will keep
ptype as rain/snow mixture with highest probabilities in the
northwest...as indicated by thickness and temperature profiles.
Enough cold air arrives by Thursday night for mainly snow showers or
snow flurries. Low temperatures Thursday night will vary from the
middle 20s in the mountains to the lower 30s in the Piedmont.
The cold front pushes into the Atlantic Ocean on Friday as the upper
trough setups along the East Coast. A weak shortwave will rotate
east across our area with some scattered showers for the east. Highs
Friday will range from near the freezing mark in the northwest
mountains to the upper 40s in the Piedmont. With the northwest flow
aloft may be some snow flurries in the northwest upslope locations
Friday night...but models looked relatively dry. Low temperatures
Friday night will generally be in the middle teens to the middle 20s. The
upper flow becomes westerly Saturday...with highs in the middle 20s in
the northwest mountains to the middle 40s in the southeast. Moisture
slowly approaches from the west Saturday evening into Sunday
Aviation /17z Sunday through Thursday/...
as of 1205 PM EST Sunday...
Overall continued VFR this afternoon with mainly passing middle and
high cloud canopy until late when some lower VFR ceilings may arrive
across the far western sites. Steady southwest winds should remain
in the 7-15 knot range before diminishing this evening. Ceilings
should then quickly lower to MVFR in light rain ahead of the
clipper system...starting at western sites by 03z/10pm...and
after 06z/1am eastern sites. The lower level southerly flow
should help lower ceilings may result in widespread IFR to LIFR
conditions before daybreak...but precipitation should remain
rather light. At kblf and klwb there may be snow mixed in at
times...and even may start briefing as snow...and then closer to
dawn Monday may switch over to all snow espcly at klwb as model
continue to trend colder maker for a bit higher confidence of more
snow at kwlb where some accumulation possible by early Monday.
Monday...expect MVFR to IFR ceilings and visibilities to persist with
the clipper and the associated light rain/snow. Flow turns more
westerly by midday and colder air filter in behind surface
front...and rain or rain showers will change over to snow at kblf
while remaining mostly snow at klwb...and intermittent snow
showers at kbcb...perhaps kroa as well. Late Monday and Monday
night some added very light accumulations of snow are more likely
at kblf/klwb with northwest winds increasing across the region as well.
MVFR to VFR conditions should return at kroa...klyh and kdan
during the day Monday.
Tuesday...a reinforcing shot of energy comes in through the
northwest upper flow. High confidence for MVFR conditions in the
mountains both Monday and Tuesday with occasional IFR due to scattered
snow showers. Lower confidence the MVFR ceilings will spill over the
mountains into the Roanoke valley and Southside Virginia.
By Monday night...any ptype will be snow due to colder
temperatures. Winds will also be gusty Monday night and Tuesday
behind the reinforcing surge of cold air.
By Wednesday...high pressure will provide a day of tranquil
weather and region-wide VFR conditions before another clipper
arrives Thursday afternoon with possible added sub-VFR in light