Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
811 PM EST Wednesday Dec 4 2013
a weak warm front will push north of the area tonight allowing
high pressure offshore to maintain a warm moist southwesterly flow
of air across the region Thursday into Friday. A strong but slow
moving cold front pushes east into the Appalachians Thursday night
before crawling across Virginia later Friday. Strong cold high
pressure to the north will ridge down along the mountains Saturday
before interacting with precipitation returning north by Sunday.
Near term /through Thursday/...
as of 245 PM EST Wednesday...
A deep trough remains over the western Continental U.S. Providing a
bitterly cold shot of air for much of The Rockies and central
states complemented by a strong ridge of high pressure with
590 dm heights over the Gulf of Mexico allowing very warm air to
push North Well into the Middle-Atlantic States. A surface low is
located over the Great Lakes and will track north toward James Bay
tonight while the trailing cold front will make only very grudging
remains against the powerful southeastern ridge over the next 24
hours or so. Skies have cleared out over most of the County Warning Area this
afternoon with exception of NC Piedmont and portions of Southside
Virginia where low overcast has persisted. Expect low clouds to fill
back in this evening or overnight as boundary layer cools with
nearly saturated profile in place up to around 900 mb. Overnight
temperatures will be quite mild again..running in the low to middle-50s as
air mass remains unseasonably warm 850 mb temperatures +12 to +14c and
clouds acting to suppress overnight cooling. Record high mins are
still possible (see climate section). Thursday will see little
real change in the air mass properties with still very mild
southerly flow across the County Warning Area but likely to be more widespread
cloudiness which may be the only thing holding back some record
Short term /Thursday night through Saturday night/...
as of 300 PM EST Wednesday...
Slow moving cold front again the main focus as weak low pressure
passes to the north Thursday evening allowing the low level
boundary to sag southeast across the far northern sections by early
Friday. However the deeper swath of moisture/lift and associated
precipitation will still be along the 850 mb portion of the boundary and
mainly just west of the region. Therefore have slowed probability of precipitation up some
Thursday night with a swath of likelys extreme west...chance Blue
Ridge...and only isolated coverage southeast. Main aspect will be with
very mild overnight lows given clouds...mixing ahead of the front
under the strong jet aloft and rather moist dewpoints. This could
result in record high min temperatures in the 50s to lower 60s per
climate section below.
Models agree on the front basically remaining stuck across the
far west/north Friday as another wave rides up the boundary
keeping strong warm advection east and oozing cold advection
north-NW. Main axis of showers looks to still be from the western
mountains and points west during the day with perhaps a few bands
across the south/east where guidance indicates some shallow
instability. Cant totally rule out a rumble of thunder along the
leading edge of the heavier showers west as well but will leave
out all areas for now. Showers could also fold south across the NE
sections where winds may shift to the NE around lyh before
reaching bcb/roa Friday evening. Despite clouds...very warm highs
in store Friday with some low/middle 70s possible southeast and 60s back to
the Blue Ridge.
Wave will boot east/southeast Friday night allowing the boundary to finally
accelerate a bit more espcly Sat when the stronger drier advection
from the northwest will be ongoing. Latest ec suggests a swath of
weakening showers will head southeast with the front Friday night...then
perhaps enhance some over the southeast late where the wave may slow the
front down a little. Thus will have Cat/likely probability of precipitation
overnight...then mainly chance probability of precipitation Blue Ridge south and east
Saturday when many solutions show things drying even more from northwest
to southeast. Colder air may catch the precipitation just before it ends far northwest
early Saturday but little more than spotty wintry mix at this
point. Temperatures begin to crash by Sat with highs only 35-40 west and
mostly 40s Blue Ridge east as The Wedge starts to dig in.
Any drying will be short as yet another wave heads up west of
the mountains and the strong Arctic high to the north entrenches
down east of the Blue Ridge by Sunday morning. Some guidance has
things drier with moisture slower to return north Sat night while
the GFS remains the fastest with a large axis of moderate precipitation
lifting across the area late Saturday night ahead of the wave.
This along with timing of the best low level dry/cold advection
makes timing of potential icing iffy Sat night with consensus
indicating this scenario developing from NE to SW mostly after
midnight. Soundings show a typical below freezing layer around
2-3k feet which will make for the best icing along the Blue Ridge in
Virginia and across the far NE closer to the parent high. Appears
most of these areas excluding northwest NC into Southside Virginia along with
the far SW...will see at least a period of decent elevated type
icing by Sunday morning as temperatures fall into the upper 20s
north/eastern slopes to the low/middle 30s elsewhere. Otherwise will follow
more of a blend of probability of precipitation with most of the SW half seeing likely
probability of precipitation late with high chances elsewhere for now.
Long term /Sunday through Wednesday/...
as of 400 PM EST Wednesday...
Will start the day on Sunday with a weather pattern ideal for
freezing rain across much or our area north of Highway 460...and
possibly sleet north of Interstate 81. Arctic high pressure will be
wedged against the eastern face of the Appalachians...resulting
in a cold northeasterly wind across the piedmonts...shifting
southeasterly from the Blue Ridge westward across the mountains.
Meanwhile...an upper level trough will approach from the plains...
with strong southwesterly flow off the surface carrying warm Gulf
moisture northward along the appalachian chain.
Precipitation will already be moving across the area by sunrise
Sunday morning...which will result in a very slow warmup from from
early morning lows...with the warmup taking place from southwest to
northeast. As such...areas from the Greenbrier valley into the
Piedmont north of Interstate 81 are expected to remain below
freezing through much of the day...and will likely see periods of
freezing rain through the day. Models are still jumping around a bit
concerning precipitation amounts...so will give it one more day
before we start mentioning ice accumulations.
Warmer air will continue to work in from the southwest Sunday
night...with the last pockets of freezing rain across the Alleghany
Highlands and Shenandoah Valley ending by sunrise Monday. Will
then see a cold front push across the area during the afternoon.
Have added a slight chance of thunderstorms for portions of
Southside Virginia and the North Carolina Piedmont for a few hours
during the afternoon due to strong shear associated with a 60kt
low level jet passing across the area...which may result in
locally strong surface winds in the convection.
The cold front may stall along the coast Monday night and Tuesday
morning as models continue to hint at a wave developing along the
boundary. By Tuesday afternoon...the front will continue its
eastward push...ending precipitation across our area and bringing a
return of dry high pressure from the west. Upper level troughing
across the northeast will keep temperatures on the cool side during
the middle of next week.
Aviation /01z Thursday through Monday/...
as of 745 PM EST Wednesday...
High pressure is still positioned off the coast. Meanwhile a
strong low pressure system situated in the northern plains will
make its way toward the Great Lakes on Thursday...accompanied by
a trailing cold front.
Overnight...bands of stratus will continue to build in from the
southeast as flow off the Atlantic increases into the County Warning Area. These
will keep visibilities from falling much below 2sm to 5sm
range...although fog is still likely for airports along the Blue
Ridge. Downslope flow near blf...combined with the overall
southwesterly flow aloft across the entire County Warning Area will keep them VFR
to high MVFR through the night and on Thursday. The remaining taf
sites on Thursday will not see breaks in the low ceilings until
the late morning into midday. Mainly low end VFR conditions will
persist throughout the afternoon...although patchy MVFR will still
linger across the area. Winds will remain generally light at 5kt
Extended aviation concerns...a cold front will make slow progress
across the area Friday...resulting in MVFR and IFR conditions in
addition to widespread rain showers...before the front passes east
Friday night. May see a brief improvement to VFR on Saturday as
Arctic high pressure builds in...however IFR will return Saturday
night as another system approaches from the west...bringing
freezing rain to the area Sunday and Sunday night for most
airports. A cold front will move through on Monday...bringing a
slow improvement to VFR through the day as high pressure returns
from the west.
warm temperatures the next couple days will approach records at the
climate sites. Below are the current records.
December 5 current record high temperatures (year)
December 5 current record high minimum temperatures (year)
December 6 current record high minimum temperatures (year)
lwb 52 (1998)