Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
1200 am EST Wednesday Jan 28 2015
an upper level disturbance will drop southeast across the region
tonight bringing continued gusty winds along with some mountain
snow showers or flurries. High pressure builds in on
Wednesday...followed by another clipper area of low pressure for
Thursday. Cold high pressure should again follow this system with
another round of colder air including strong northwest winds and
mountain snow showers for the end of the week.
Near term /through Wednesday/...
as of 1155 PM EST Tuesday...
Current temperatures running 1-3 degrees cooler in many areas than
advertised...so lowered and adjusted min temperatures more in line with
European model (ecmwf) MOS mins...which were the coldest among the three models and
appeared to be on track. Snow flurries have been fairly persistent
from lwb down to bcb in conjunction with northwest-southeast upper disturbance
on back side of deep NE U.S. Upper trough. Retained advertised
probability of precipitation/-shsn across eastern WV...but added snow flurries into the
Alleghany Highlands...New River valley...and down into northwest
NC...coincident with ongoing low clouds...just west of the Blue
Ridge. Sky condition looked good...so did not make any changes to
As of 710 PM EST Tuesday...
Short wave energy and moisture will rotate around the back side
of broad upper low driving the big nor Easter off the New England
coast and move through our region tonight. Latest fog product
satellite images show clouds corresponding with these features
moving in from the northwest this evening. All of this is captured
nicely by the models and probability of precipitation verify nicely with light upstream
radar returns and latest surface ob from klwb with indicates 10sm in
Expect snow showers to enhance a bit as they encounter terrain
with accumulations of less than an inch for western Greenbrier
County. Perhaps trace amounts down the far western slopes further
south into Tazewell County. A few stray flurries may also be
encountered through the New River Valley/Mountain Empire region
but probability of precipitation not high enough to warrant a mention.
A good number of observation sites showing wind gusts in the 20kt
to 30kt range. Descending inversion will squeeze the low level
flow and continue these gusty conditions with gusts near 40kt
possible for some of the ridges...especially from the Grayson
Highlands and along the Blue Ridge from Floyd County south into
the North Carolina mountains through about midnight...then winds
will slacken toward daybreak.
Temperatures continue to fall with readings already in the lower
teens for the high spots of western Greenbrier. Clouds moving in
will help keep temperatures from falling much farther but areas further
south and east of the Blue Ridge will still lose another 10f to
15f by morning.
High pressure finally slides in from the northwest Wednesday under
building 500 mb heights allowing for sunny skies and diminishing
winds. With very dry air in place and gradual 850 mb warming...expect
highs to be closer to the warmer mav MOS especially east where
some middle/upper 40s likely...with middle 30s to around 40 west.
Short term /Wednesday night through Friday night/...
as of 330 PM EST Tuesday...
Things to remain very progressive for the middle of the week with the
upper Ridge Crossing the area Wednesday night ahead of the next fast
moving clipper approaching from the upper Midwest. Expect increasing
warm advection aloft ahead of this system to spread a middle/high cloud
canopy in from the northwest along the upper jet especially mountains
overnight but dry. Surface high sliding east in combination with clouds
should act to hold temperatures up outside the deeper valleys with lows mostly
in the 20s including a few teens northwest pending cloud coverage.
Center of the low pressure system prognosticated to pass well to the north
later Thursday while swinging another fast moving cold front into the
region Thursday afternoon. Given best dynamics and lift heading by to
the north...expect moisture to be limited under only brief/weak return
low level flow so precipitation chances look iffy outside the mountains.
However does appear that a band of snow/rain will arrive over the far
west by midday and then perhaps clip the northern Blue Ridge upon
attempting to cross the ridges under a quickly veering westerly flow.
Ptype likely to start as light snow mountains before switching to
more liquid nature as both 850 mb and boundary layer warming take over.
Thus keeping a narrow swath of likely probability of precipitation northwest Thursday with chances
elsewhere west...but mainly slight probability of precipitation south as well as the eastern
third. Snowfall amounts of perhaps an inch or so western Greenbrier
with overall quantitative precipitation forecast of less than a tenth of an inch...mainly west.
Expecting high temperatures to get a boost Thursday afternoon given the
slowing front and perhaps more sun out east where could see values
around 50. Think only the far northwest and higher elevations to remain in the
30s...pending timing of clouds and any precipitation. However could see most
push into the 40s...as usually warmer just south of these clippers
passing way north with models too wet...and too cold so stayed above
MOS for highs.
Cold front from the clipper scooting by to our north will move
through the region early Thursday night...ushering in much colder
air for the end of the workweek and into the weekend. This will
set up what looks to be pretty decent northwest flow snow showers west of
the Blue Ridge. Accumulations for the far western slopes look to
be an inch or two...but western Greenbrier County should pick up a
total of around 5 inches before things start to wind down Friday
morning into Friday afternoon. If later model runs are as robust
with the quantitative precipitation forecast and snow this will require a Winter Weather Advisory
at least for western Greenbrier Thursday night/Friday morning.
The other big story will be the very strong winds developing
Thursday night and continuing into Friday. The low level flow will
veer around from west top northwest and increase to 50kts or more
per the latest GFS. NAM soundings on BUFKIT show the inversion
lowering to around 850mb by Friday morning with 50kt winds right at
the top of the shallow mixed layer below the inversion. If this
scenario plays ot as currently advertised by the models it will
produce wind gusts around 50mph...possibly higher. We will be
keeping a close eye on this in later model runs to see if a high
wind watch is necessary for the Thursday night/Friday time frame.
With the combined wind and cold will also be looking at some wind
chill values in the teens to single digits west of the Blue Ridge.
Long term /Saturday through Tuesday/...
as of 330 PM EST Tuesday...
Below normal temperatures trending to much below through the
extended...along with possibility of more strong northwest winds
Sunday night through Monday.
Upper level pattern for the extended will be split flow with a
dominant northern stream trough in the east and ridge for the Pacific northwest.
There is enough general agreement between the GFS and European model (ecmwf)
members...which are supported by naefs anomalies...to buy off
on this which will be a cold solution.
Saturday looks like fair weather under cold high pressure. Next
system develops to our west and tracks into New England with a
trailing cold front sweeping through the region Sunday night. Will
not split hairs about ptype this far out but the general track to
our west and north will keep US on the warm side of the storm.
Possible hybrid wedge in place at the onset may help cool things for
wintry mix at onset going to rain for the bulk of Sunday then back
to wintry as precipitation tapers off...though typical northwest flow upslope snow
showers west of the Blue Ridge become established in the cold air
behind the cold front. May also be looking at another strong northwest wind
event as we go through Sunday night and into Monday but there is
plenty of time to watch this and see how later model runs handle the
evolution of low level winds.
Aviation /06z Wednesday through Sunday/...
as of 1145 PM EST Tuesday...
Deep coastal low northeast U.S. And associated upper
trough/cyclonic flow still dominating the weather across the
region. A couple of embedded northwest flow short waves and
upslope flow continuing to enhance clouds across the
alleghanys...with clouds occasionally spilling over into the New
River/Roanoke valley. This should be the trend through the night
as these systems track southeast. Lyh/Dan should remain mostly
sky clear...with scattered-broken VFR clouds at roa...sct-bkn MVFR clouds for
bcb...and broken-overcast IFR ceilings for lwb/blf. The upper short wave
tracking through WV/along with upslope may support occasional -sn
mainly for lwb for a few more hours. Lwb has been reporting an
MVFR visibility for several hours...which seems hard to believe given
radar coverage...but possible since radar overshooting very low
topped echoes. Will include a tempo MVFR group for a few hours at
beginning of taf valid period...then expect as upper
support/upslope diminishes...VFR visibilities should prevail. VFR
visibilities are expected elsewhere through the taf valid period.
VFR ceilings should return to eastern WV middle-late morning Wednesday as well
as high pressure builds in from the west. Expect VFR ceilings at all
taf sites after 15z Wednesday morning.
Winds will continue to be an issue for a few more hours. Northwest winds
10-13kts with gusts 18-23kts persist west of the Blue Ridge...and
likely will through the night. Models show an enhancement area to
the winds near Roanoke/Floyd counties through the nighttime. Even
Dan has been seeing low end gusts for the past few hours. Winds
will veer to the north-northwest through the day Wednesday and decrease to 5-7kts by
evening...becoming calm/light and variable most sites by 23z.
Medium to high confidence in ceilings through the taf valid period.
High confidence in visibilities through the taf valid period...except
medium to high confidence at lwb.
High confidence in wind speed/direction through the taf valid period.
Another clipper system arrives on Thursday...bringing a chance of
sub-VFR ceilings and visibilities...with a chance for -shra...-shsn in the
mountains. MVFR ceilings likely to linger over the mountains Thursday
night into Friday in the wake of this system with strong northwest winds
also persisting into Friday evening.
Another area of high pressure shifts east into the area Saturday
with VFR expected into Saturday night. The next low pressure
system heading out of the SW states will bring a return of clouds
and some precipitation on Sunday with widespread MVFR to IFR