Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
1158 PM EST Friday Nov 28 2014
high pressure will remain over the region tonight into Saturday...
before slipping off to our southeast. This will bring fair weather
with a warming trend to the Appalachians and central middle Atlantic
region through Sunday. A slow moving cold front will then move
through the region late Monday into Monday night...bringing
precipitation and a brief cool down for Tuesday...followed by
seasonable weather by the middle of next week.
Near term /until 7 am Saturday morning/...
as of 1150 PM EST Friday...
Current temperatures were significantly lower than forecast in
several cases for overnight mins...E.G...lwb already down to 17
and the forecast low was 22. So...had to do some significant
lowering to min temperatures tonight. Appears that readings are more in
line with the European model (ecmwf) MOS than with a model blend or the mav/met
guidance. European model (ecmwf) MOS very close to projected trends.
As of 650 PM EST Friday...
No significant changes needed to the grids at this time...just
some minor modifications to T/dew point to bring into line with current
readings. Otherwise...forecast is on track and low clouds have
cleared in western areas.
As of 320 PM EST Friday...
Tranquil conditions expected in the near term as high pressure along
the eastern Seaboard predominates. The upper flow is much more zonal
the next 24 hours which will allow much milder air to spill eastward
from the Central Plains toward the central/southern Appalachians.
850 mb temperatures rebound from -10c readings of this morning to +5c by
midday tomorrow. Temperatures tonight will be chilly again but fighting
warm advection as well as high clouds from a short-wave passing well
to the north. Went with model blend yielding lows fairly uniform
across the County Warning Area from low to mid-20s. Could be a little cooler in the
south where clouds least likely. After the recent chilly spell
Saturday will seem comparatively mild with highs ranging from middle-
40s to low 50s which is still below climatology but about 10f warmer than
today. Clouds from the northern short-wave will be on the decrease
through the afternoon which will make for mostly sunny skies by
later in the day and good weather for outdoor activities.
Short term /7 am Saturday morning through Monday night/...
as of 320 PM EST Friday...
Quiet weather with a warming trend through the weekend as high
pressure slides off to our southeast and winds become southwesterly. Low
level jet will be setting up just to our west and expect this will
make for some blustery conditions at the higher elevations through
the weekend. With quasi zonal upper pattern in place...cold front
approaching from the northwest has no big push to it and it will
take its time moving through the region on Monday. This will lead to
slowly increasing probability of precipitation form the northwest Sunday night into Monday
with best chance of precipitation holding off until a wave along the front
ripples through Monday night into Tuesday. GFS is most aggressive
with the precipitation chances with the wave with other guidance not quite
so enthusiastic. High chance probability of precipitation should cover the situation until later
runs can converge on a solution. Very strong high pressure will build
into New England and then forcibly wedge down the eastern Seaboard
Monday night and Tuesday with much colder air moving in. This will
likely create some ptype issues as the cold air arrives. Exact depth
and magnitude of the surface based cold layer are not clear at this
point...but expect greatest likelihood will be fore some pockets of
freezing rain...so will indicate this in the grids. Guidance has
trended colder but have trimmed a bit more in anticipation of a
strong wedge situation.
Long term /Tuesday through Friday/...
as of 320 PM EST Thursday...
An upper level trough/short wave will Glide over the forecast area
Tuesday afternoon...exiting the Piedmont in the evening. Rain from
this trough will enhance The Wedge coming across the middle Atlantic
region. By Wednesday morning...The Wedge could be as far south as
the Florida Panhandle. Bulk of the moisture will exit with the
trough...however warm moist air will begin to overrun the cooler low
levels Tuesday night into Wednesday. Thick low clouds and fog will
likely engulf the forecast area Tuesday night...lingering into
Thursday morning. Warm moist air advecting in aloft and increasing
isentropic lift Tuesday night into Wednesday morning...may be enough
to produce light precipitation along and east of the Blue Ridge.
Considering how dry the parent high is over New
England...measurable precipitation may be limited to the eastern
slopes of the Blue Ridge. Confidence is not high...therefore will
keep probability of precipitation under 30 percent.
Precipitation type will be the challenge as dry cooler low level air
will advance south with warm moist air aloft advecting north. Low
level thicknesses are around 1300m over the Alleghany Highlands and
southern Shenandoah Valley Tuesday night...then gradually increase
going into Wednesday morning. 850 mb temperatures around 0c along the
southwest Virginia Blue Ridge...+2c across the North Carolina
Highlands. These 850 mb temperatures also warm going into Wednesday
morning. Viewing model soundings...the cold sub-freezing air is
around 4000 feet thick. Sounding profiles also suggesting any
measurable p-type would be sleet or freezing rain. Looking at
sounding profiles...I would lean towards a freezing fog/drizzle
scenario...a glazing of ice along the eastern slopes of the Blue
Ridge between 2500-4000 feet.. we also need to understand that this is
a day 5-6 forecast and models may change over the next several days.
For now...will keep majority of precipitation as rain. With
confidence being very low at this time for a possible trace
event...will not add to the hazardous weather outlook (hwo) at this
Following p-type concerns Tuesday night into Wednesday...the next
question to answer is...how long will The Wedge remain over the
area. Surprising...the usually progressive GFS lingers a piece of
The Wedge over the region into Thursday morning. Despite the
lingering wedge...12z GFS still has the winds turning to the south
by Wednesday afternoon. We will continue to keep The Wedge and
clouds in the area into Thursday morning with the Piedmont being the
last to clear. The Wedge will erode each period...from the southwest
to northeast and from top-down. By Thursday afternoon...a warm front
and pressure falls over the Ohio Valley will break The Wedge.
Temperatures could possibly warm 10f or so in just a few hours. Once
The Wedge breaks...we could see a day or so of warm
temperatures...then a weak cold front will bring light rain showers
to the region Thursday night and Friday. This front could stall over
the region Friday.
Aviation /06z Saturday through Wednesday/...
as of 1150 PM EST Friday...
VFR conditions expected through the taf valid period. A northern
stream short wave along with warm air advection will result in a period of
middle/high clouds skirting the northern parts of the County Warning Area overnight
and especially first half of the day Sat. As the disturbance moves
NE of the region by afternoon...mid/high clouds will decrease
leaving either sky clear or sct250. Otherwise...no ceilings below 10kft through
the taf valid period. No visibility issues through the taf valid
period. Winds becoming SW-S overnight/early Sat...increasing in
speed 8-10kts after 14z Sat with low end gusts at blf/bcb/roa/Dan as
gradient tightens between departing Arctic high and low pressure
High confidence in VFR ceilings/visibilities through the taf valid period.
Medium to high confidence in wind speed/direction through the taf valid
VFR conditions are expected to linger Saturday night and much of
Sunday ahead of a cold front that may bring MVFR ceilings to areas
from kbcb westward. This cold front will reach the region Monday-Tuesday
with a chance of -shra br and associated MVFR to locally IFR
ceilings/visibilities along the front. A wedge pattern expected to set up by
Monday night/Tuesday with widespread MVFR-IFR ceilings and -ra. Some -fzra
appears possible Tuesday morning through the New River valley into the
as of 1030 am EST Wednesday...
Visibility sensor at klyh...Lynchburg Airport...was malfunctioning
again. Parts have been order but the repair may not be until
Monday. The visibility at klyh will be augmented by staff when the
control tower is open...otherwise the visibility will be missing.