Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
754 PM EST Friday Dec 6 2013
a cold front will move slowly southeast tonight. Arctic
high pressure well behind the front will wedge south along the
mountains Saturday...before interacting with precipitation returning
north by Sunday. Southwesterly flow returns Monday before a stronger
cold front arrives Tuesday morning. Cold and dry high pressure
returns by Tuesday night.
Near term /through Saturday/...
as of 330 PM EST Friday...
One wave of low pressure across County Warning Area this afternoon will lift north
along the frontal boundary tonight. A second wave of low pressure
around Georgia will lift north up the cold front as it slowly sags
southeast tonight. Initially decreased probability of precipitation in the southeast this
afternoon into tonight as focus of rain will be across the
northwest. Then...a downsloping effect may result less precipitation
through the Roanoke and New River valleys this evening into tonight
as rain redevelops through the Piedmont and Southside by late
For this evening into tonight...an isolated thunderstorm may be
possible. Otherwise the main precipitation type is rain for late
this afternoon into tonight. However...low level nature of the Arctic
airmass makes p type quite tricky across the highest parts of southeast
West Virginia late tonight. Shaped the probability of precipitation and weather towards the GFS
for tonight. Most of the precipitation will taper off during the
overnight with better chances in the southeast. Expect moisture/quantitative precipitation forecast
to dwindle off quickly behind the front. However...there could be a
light mix of first freezing rain...then sleet and possibly snow just
after sunrise Saturday above 3000 feet across southeast West Virginia. Only
minor amounts of ice/sleet/snow are possible. If quantitative precipitation forecast amounts are
higher may need an Winter Weather Advisory for late tonight into
Surface front will be south of the region Saturday with cold high
pressure building into the area from the northwest. Cold air will
channel down the east side of the Appalachians Saturday setting up a
dam of cold air that will be hard to displace Saturday night. During
daylight hours on Saturday...there is the chance for rain across
southern portions of forecast area. Used a non-diurnal curve for
temperatures tonight and Saturday. Lows tonight will range from the
upper 20s in the northwest to the middle 50s in the Piedmont.
Temperatures will fall from Saturday morning highs in the lower 30s
in the mountains to the middle 50s in the Piedmont...to readings in the
upper 20s to the around 40 degrees Saturday afternoon.
Short term /Saturday night through Sunday night/...
as of 315 PM EST Friday...
Model guidance among the preferred GFS/European model (ecmwf) solutions is still
offering a solution where a wave moves northeast through the region
Saturday night into Sunday. This wave in turn helps lift the
front...that Saturday night will be stalled just south of the
region...back across our area. Precipitation is expected to
overspread the area as as both the wave crosses the region and the
front returns. Initially...the low level T/dew point profile will be such
that most of the northern portion of the region will receive a mix
of sleet and snow through the night. Farther south...enough of a
warm nose will already be in place for a rain versus freezing rain
forecast with the surface temperature being the determining factor.
The amount of freezing rain in this part of the area will increase
in coverage as surface temperatures fall in response to the wet
bulbing effect of the precipitation.
During the day Sunday...the strength of the warm nose increases to
as much as +6 to +7 c across the area...while most of the region at
the surface remains at or below freezing thanks to a strong cold air
damming wedge that will only be reinforced by the precipitation.
This will result in a transition for the entire region receiving
only freezing rain or rain by the afternoon...with the far northern
tier counties being the last to complete the transition from snow to
sleet to freezing rain. The far southwest portion of the area may
receive enough warm air advection so that outside the influence of
The Wedge...a cold rain will become more probable as the day
progresses. Also...the higher ridge tops of the southern alleghanys
be high enough in altitude so as to be mild enough to receive rain
throughout the day.
Sunday night...the warm air advection aloft continues...thus
causing additional strengthening of the magnitude of the warm
nose aloft. The frontal boundary...and thus the best region of focus
for the greatest precipitation...shifts from being across the
central portion of the forecast area...to just intersecting the
northwest portions. At the surface...a strong wedge holds in place
with temperatures at or below freezing over the
central...northern...and northeast parts of the forecast area.
Additional freezing rain is expected in this region.
At this time...no changes are being made to the current Winter Storm
Watch configuration and timing. Would not be surprised to see any
subsequent warnings continue for the northern parts of the area into
Monday morning. While forecast quantitative precipitation forecast numbers will continue to
fluctuate...the latest thinking places 0.25 to 0.50 inches of ice
across the central...northern...and northeastern parts of the
region...and southwest along the crest of the Blue Ridge into the
northern mountains of North Carolina. Outside of this region...most
other locations would receive less than 0.10 inches of ice. Trace
amounts or only a few hundredths are expected over the far western
portion of the region. Snow/sleet is not expected to be great in
total. Most of this would occur across the northern half of the
region Saturday night into Sunday morning and be in the 0.50 to 1.5
Long term /Monday through Friday/...
as of 255 PM EST Friday...
Wedge should be continuing to weaken by early Monday with ongoing
strong 850 mb warm advection slowly causing the cold pool to fade with
time during the day as the cold front to the west approaches. Based
on latest model soundings and idea that The Wedge will be slow to
exit until a few hours after sunrise...will keep in some light
freezing rain mention mainly Greenbrier into The Highlands early on.
Otherwise returning deeper band of moisture should be crossing the west
through midday Monday and then likely weakening along the Blue Ridge by
early afternoon as weak energy shears by to the NE. However as the
boundary slows again heading east may see yet another impulse ride
up through the Carolinas resulting in another round of rain southeast late.
Will have basically all liquid during the afternoon as surface temperatures
warm enough to push most into the 40s if not 50s southeast.
Front pushes out to the Blue Ridge Monday night before seeing
another wave along the boundary which may result in another swath of
precipitation riding up the baroclinic zone back to the mountains by early
Tuesday. However westward extent remains tricky along with temperatures as
most guidance showing values cold enough aloft for snow far west by
early Tuesday with mostly rain elsewhere until the 850 mb 0c line swings
east during the day when the boundary layer may be too warm espcly
east. Thus running with mainly chance probability of precipitation Monday night but higher
southeast and then a gradual northwest to southeast decrease in coverage Tuesday with -sn
northwest later Monday night/early Tuesday and a mix Blue Ridge to just
rain east for now. Should finally return to dry but very chilly
weather for the rest of the week as strong high pressure builds east
reaching the coast on Friday. This will keep temperatures well below normal
with most highs in the 30s for much of the rest of the week.
Aviation /01z Saturday through Wednesday/...
as of 745 PM EST Friday...
Poor aviation conditions expected through the first 12-15 hours of
the taf...then some improvements through the remainder of the taf
valid period. An Arctic front was hung up along the western slopes
of the Alleghany mountains in eastern WV...however as the surface high
spreads across PA/NY/New England...cold air was spreading down the
east side of the Appalachians. At 00z...the boundary between very
mild 60/70 degree air to the south and the colder 30/40 degree air
to the north extended from near lyh to ekn to blf...with an apex
near Pocahontas County WV. Overnight...the cold air will continue
to filter in from the west and north and should filter through
most of the County Warning Area in one direction or another by morning...but
mostly it appears from the NE. Ceilings on the west side of the front
will be mainly IFR-MVFR with blf potentially LIFR at times. Expect
roa/bcb to remain mostly MVFR at worst overnight...with Dan mostly
VFR until later tonight. A band of rain showers will move through
from the west this evening with brief periods of IFR ceilings
associated with this. Lyh should see IFR ceilings as upslope NE flow
results in fog/low clouds banking up against the Blue Ridge.
Overall...highly variable...but MVFR-IFR summarizes ceilings in most
areas. Visibilities will average in the MVFR category overnight...with
blf seeing IFR-LIFR visibilities if ceilings aob004. Winds variable...but
mostly northwest lwb/blf/bcb...and north-NE roa/lyh/Dan...except SW until
late tonight Dan in advance of the front. After 18z Sat...expect
improvement of ceilings at bcb/roa/Dan at worst...with lyh/lwb/blf
potentially remaining IFR to low end MVFR through much of the day.
Medium confidence in ceilings and visibilities through taf valid period.
Medium to high confidence in wind speed/direction through taf valid
After the brief break in the poor weather on Saturday...we get
right back into poor conditions as high pressure moves in to a
wedge position and overrunning leads to low ceilings and mixed precipitation
from S to north Saturday night...then continuing on Sunday. Temperatures
should moderate enough to end the wintry precipitation by Monday...but we
will stay with precipitation and low clouds/visibilities right into Tuesday. High
pressure finally builds in later on Tuesday with a return to VFR
for the middle of the week.
below are the current records.
December 6 current record high minimum temperatures (year)
lwb 52 (1998)
Virginia...Winter Storm Watch from late Saturday night through late
Sunday night for vaz010>020-022>024-032>035-043>047-058-
NC...Winter Storm Watch from late Saturday night through late
Sunday night for ncz001>006-018>020.
WV...Winter Storm Watch from late Saturday night through late
Sunday night for wvz042>045.