Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
1204 am EST sun Mar 9 2014
high pressure across the southeast states will weaken and pass
offshore tonight as a weak cold front to the northwest drops
southeast through the middle Atlantic region by early Sunday. High
pressure should follow the front Sunday afternoon into Tuesday
ahead of another complex storm system that will bring the next
chance of precipitation by middle week.
Near term /through Sunday/...
as of 756 PM EST Saturday...
Still expecting scattered rain showers...and some snow showers
across mainly western Greenbrier and southeast West Virginia for a few
hours near dawn. Coverage of light quantitative precipitation forecast may be overdone by the
models...as is typically the case. Still going with light accums
across western Greenbrier. Any precipitation will end rather quickly.
Clouds will increase into dawn. Some spots may decouple before
thicker clouds arrive...with slightly rising temperatures into the
high pressure responsible for the mild temperatures this afternoon will
weaken across the southeast states this evening ahead of a weak cold
front that should drop southeast through much of the region by early
Sunday. Moisture remains limited ahead of the boundary at this time and
expect precipitation to stay rather disorganized as it encounters current
lower dewpoints and deep westerly flow across the region. Latest
guidance printing out up to one tenth of an inch quantitative precipitation forecast over the far
west mainly midnight-daybreak...with the deeper moisture drying up
crossing the Blue Ridge early Sunday. Thus will maintain the
overall trend of chance probability of precipitation western third overnight with only a
light shower or sprinkle possible farther east by daybreak.
Boundary layer also looks too warm for much snow until very late
when the higher elevations of northwest Greenbrier could go over to a
period of snow showers before the precipitation ends so kept some very
light accumulations there. Otherwise becoming cloudy far west...and
increasing clouds east late with lows mainly 30s to low 40s given
some mixing...and warmer at elevation per increasing gusts with
the frontal passage after midnight despite weak cold advection.
Some Post frontal showers possible far west/SW early Sunday otherwise
punch of much drier air to kick in by afternoon allowing clearing
with gusty winds diminishing as weak high pressure builds in. Should
be a bit cooler given passing 850 mb cool pocket with highs likely a
good 5-10 degrees below those seen today despite downslope and
Short term /Sunday night through Tuesday night/...
as of 230 PM EST Saturday...
Relatively uneventful weather expected over most of the region
Sunday night into Tuesday as northwesterly flow aloft gradually
transitions to more of a zonal flow by Tuesday...with short wave
energy in northern stream remaining well to the north of the
Blacksburg forecast area.
Southwesterly surface winds between mean low pressure over the
Great Lakes...with surface ridge remaining poised to our south
during the period...coupled with partly to mostly clear skies
should support strong daytime heating...but with good radiational
cooling at night as boundary layer decouples.
Southwesterly surface flow and associated compressional heating
tends to yield warmer temperatures than longer-term guidance would
imply by day. However...gradient not strong enough to maintain
sufficient mixing at night...and with relatively low dew point
temperatures...guidance likely not projecting low temperatures
low enough at night...especially in the mountain valleys and low
lying areas of the Piedmont. As a result...trended grids cooler
than most guidance output at night...and warmer than guidance
projections by day through Tuesday.
Upper flow over the area will gradually back into more of a
southwesterly component late Tuesday into Tuesday night as closed
upper low currently over the southern rockies drifts east into
the Gulf Coast states...while next northern stream long wave trough
approaches the area from the west.
Main area of forcing and associated precipitation shield with
southern stream system will likely remain to our south on Tuesday
night. However...tap of middle-level moisture feeding northward as
winds aloft back in advance of approaching northern wave will
likely result in secondary area of precipitation in vicinity of
baroclinic zone/developing low pressure over the Ohio/Tennessee
Valley...possibly overspreading at least part...if not most...of
the Blacksburg forecast area by/toward daybreak Wednesday.
Unlike recent systems...the Blacksburg forecast area will be on
the warm side of this system...at least initially...such that any
precipitation that falls later Tuesday night will be wet rather
Long term /Wednesday through Saturday/...
as of 300 PM EST Saturday...
A rainy and perhaps stormy Wednesday is expected...with the GFS
now joining the Canadian and European model (ecmwf) with regards to Stout low
pressure moving roughly just west of the mountains through West Virginia
during the day. Jet streak may pass over our region as the strong
cold front arrives late Wednesday or Wednesday night...possibly
some gusty thunderstorms through the Piedmont and Southside. 850 mb
temperatures plunge behind this front to -18c...as additional strong
upper wave crosses into Thursday 12z/7a. Although several days
away...pattern recognition says this should be a good set up for
accumulating upslope snows along the western slopes. The European model (ecmwf) MOS
values look too warm...so followed the mex MOS more
closely...especially Thursday night when high pressure is overhead
and decoupling may occur...sending some of the colder spots into
the teens. Arctic blast departs quickly with 850 mb temperatures above zero
again by next weekend.
Aviation /06z Sunday through Thursday/...
as of 1200 am EST Sunday...
An Alberta clipper type system will move quickly through the County Warning Area
overnight...reaching our western border around 06z frame...then
move to the eastern border toward 11z. A few sprinkles may
accompany the front or an hour or so behind it across the upslope
areas of eastern WV...otherwise little if any precipitation will make it
east of the Alleghany front because of downslope effects and very
limited available surface moisture. The bigger problem through this
taf valid period will be development of MVFR ceilings in upslope areas
across eastern WV. Expect 015-030 broken-overcast ceilings to develop across
these areas shortly after frontal passage in the 06z-08z time
frame...then linger well into the afternoon before scattering
out around 18z-20z...then becoming sky clear by 00z sun. East of the
Alleghany front...there may be a brief period of MVFR ceilings for bcb
around daybreak...otherwise expect ceilings to remain above 050 for
the most part and these will only linger through mid-morning. All
taf sites should be sky clear by 00z Monday. Visibilities should remain VFR
through the taf valid cycle. Have removed mention of MVFR visibilities
across eastern WV as precipitation will be too limited to produce such.
Winds...west to west-southwest 5-7kts prior to frontal passage...then west-northwest-northwest 8-12kts
with low end gusts through the afternoon sun before becoming west-southwest
once again with speeds 7kts or less.
Medium to high confidence in ceilings through the taf valid period.
High confidence in visibilities through the taf valid period.
High confidence in wind direction through the taf valid period...medium
to high confidence in wind speed through the taf valid period.
Extended aviation discussion...
VFR continues for early next week before another complex storm
system approaches from the west Tuesday night and
Wednesday...bringing the possibility of widespread sub-VFR into
Thursday including lingering mountain snow showers. High pressure
following this system should provide a return to VFR for the end
of the week.