Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
803 PM EDT Thursday Sep 18 2014
high pressure over the area will weaken and slide east offshore
tonight. A weak cool front will drop southeast across the region
this evening into tonight. Strong high pressure will build to the
north late tonight into Saturday before passing offshore on Sunday.
A cold front will cross the area from the northwest on Monday
bringing a better chance of showers to start the week.
Near term /through Friday/...
as of 250 PM EDT Thursday...
Broad upper-level trough will remain over the region tonight into
Friday...while a weak upper-level disturbance passes through in the
flow aloft. A weak cold front will slide south across the region
this evening into tonight.
Showers should drop southward and fade this evening into tonight as
the onset of a deepening east to southeast flow develops in response
to strong high pressure building in from the north. Moisture banked
up along the mountains with depth to support mention of spotty -ra
and certainly drizzle/fog by daybreak. Cool tonight with low
temperatures ranging from around 50 degrees in the mountains to near
60 degrees in the southeast.
A broad area of high pressure will move over New England and wedge
south down the East Coast on Friday. An easterly flow will create
areas of fog and drizzle along the eastern slopes of the Blue Ridge
into the early afternoon. Although limited...afternoon heating
combined with mixing may erase drizzle/fog through the
afternoon...while some light showers developing over the region into
the evening. Increased cloud cover for Friday and raised probability of precipitation
especially along the southern Blue Ridge. Friday afternoon
temperatures will be colder than normal with areas keeping
precipitation through the day. Lowered high temperatures on Friday
with readings from the middle 60s in the mountains to the lower 70s
in the Piedmont.
Short term /Friday night through Sunday night/...
as of 250 PM EDT Thursday...
Model trends have sped up the next front arriving Sunday now instead
of Monday. Until then...expect the high pressure over the New
England coast to wedge southwest into our area Friday night into
Saturday. Another cloudy sky can be anticipated across the Blue
Ridge and either side late Friday night...but skies clear out
A cold front shifts southeast over the region Sunday...with main
lift and shortwave energy staying across the Ohio Valley. This
shortwave then pivots southeast across WV Sunday night..and could
bring a few showers here. May see enough instability with the front
Sunday to allow for thunderstorms. Overall chance of precipitation is
20/30...higher into WV.
Temperatures this period will modify to seasonal levels...with lows in the
50s and highs in the 70s...except lower to middle 80s southeast Sunday.
Long term /Monday through Thursday/...
as of 200 PM EDT Thursday...
This period is shaping up to be dry and seasonal. Another shortwave
drops southeast across the Tennessee Valley Monday and could bring isolated
showers to the entire areas as flow backs aloft some. Then high
pressure across the Midwest pushes east over the middle Atlantic
Tuesday- Wednesday. Temperatures behind this system early next week will cool down
some...but not looking as chilly as earlier thought...with 800 mb
Highs will run from the 60s west to 70s east...with lows mainly in
the 40s...with some 50s across the Piedmont.
Aviation /00z Friday through Tuesday/...
as of 745 PM EDT Thursday...
Fairly classic wedge setup expected to develop overnight/Friday
with attendant lowering ceilings and patches to areas of -dz
br...especially along upslope areas of east facing slopes of the
Blue Ridge. Clouds already evident across the coastal plain of
VA/NC...and are already spreading west into central Virginia.
Meanwhile...middle clouds linger across western areas from passing
disturbance in northwest flow aloft. With east flow developing and
increasing overnight/early Friday in response to 1030mb surface high
setting up along the northern mid-Atlantic/New England
coast...anticipation is for ceilings to drop into the MVFR
range...with locally IFR ceilings through the Blue Ridge and west
toward the Alleghany front. Patches of -dz could develop in better
upslope areas...so have included such for blf/lwb/bcb/roa...but
not for the Piedmont. With abundant clouds...little if any
radiational fog expected to develop...but will need to watch lwb
and bcb in the event that middle clouds are slower to spread into the
area than currently expected. Given that such widespread clouds
are already in place...have opted against advertising any LIFR fog
conditions at this point. Although ceilings will lift some during the
day Friday...overall little improvement is expected with ceilings
remaining MVFR most sites all day...but visibilities should be mostly VFR
after 15z. Winds becoming east-northeast-east-southeast overnight at speeds of
5-7kts...increasing to 7-10kts after 14z Friday. Some potential for low
end gusts in usually favored southeast wind areas...such as
blf...after 15z Friday.
Medium to high confidence in ceilings through the taf valid period.
Medium confidence in visibilities through the taf valid period.
Medium to high confidence in wind speed/direction through the taf valid
Extended aviation discussion...
Wedge should erode fairly quickly Saturday in response to an
upstream trough. Some low clouds may linger through the morning
along with patchy fog...but overall VFR conditions expected Sat. A
frontal system will approach sun with -shra and perhaps an isolated
thunderstorms and rain west of the Alleghany front. Instability is quite limited and
so is deep moisture. With a possible tropical system developing
off the coast at the same time...we may be in a strong subsidence
area and see little more than -shra or sprinkles east of the
Alleghany front. Expect mostly VFR-periods of MVFR ceilings sun-early Monday
as the trough moves through. High pressure builds in for the first
half of next week with dry northwest flow. Thus...expect mostly VFR
conditions through the period with local early morning MVFR-IFR