Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
927 PM EDT Tuesday Apr 21 2015
high pressure will stay south of US tonight while a front moves in
from the northwest. This front will cross the region Wednesday and
shift offshore by Thursday morning resulting in breezy conditions
Wednesday into Thursday ahead and behind the front. By
Friday...high pressure will be situated from the western Great
Lakes...south into the southern Appalachians.
Near term /through Wednesday/...
as of 912 PM EDT Tuesday...
Evening soundings and surface observation show very dry air lingering over
the region this evening ahead of a weak trough just west of the
mountains and the next cold front in the upper Midwest. Initial
band of broken clouds now crossing the Ohio River may bring a few
sprinkles to the western slopes before fading after midnight given
deep westerly flow and a lingering well mixed dry layer just off
the surface. Appears next round of potential showers wont arrive
until Wednesday morning ahead of the front so trimming back western probability of precipitation
a bit more overnight similar to the latest hrrr trends. Otherwise
mostly clear to init then becoming partly to mostly cloudy at
times far west and pc elsewhere late. Temperatures to zoom down in the western
valleys early on as winds fade with some spots falling into the
30s before bumping up with clouds later on. Latest lav MOS slow to
drop values but likely too warm except on the ridges given such
low dewpoints this evening so only making slight tweaks in spots.
Previous discussion as of 300 PM EDT Tuesday...
Very dry at the moment with few to scattered cumulus. Upstream...surface front
working into the Midwest will be our next weather maker...and wind
will be the main culprit.
This evening into the overnight the skies will be stay clear to
partly cloudy then should see more clouds arrive after midnight into
the mountains slowed down onset of probability of precipitation given dry low levels...though will
see a few showers make it to Greenbrier to the mountain Empire around
Temperatures will be dropping quicker this evening once the winds settle
down some. Aloft however...winds should stay up keeping higher
terrain mixed. Keeping the western slopes in the 40s for lows...while
valleys in the mountains and foothills fall into the upper 30s. Not
expecting any frost/freeze concerns.
For Wednesday....the low level jet sets up from Kentucky to Maryland
during the morning/early afternoon. Surface front will make it into the
WV mountains by midday then slowly drop southeast into the Piedmont by evening.
Stability seems marginal but wind energy with showers are a concern
for gusty showers and possible thunderstorms. Storm Prediction Center has marginal risk
east of Roanoke toward lyh/ric...for damaging winds. At this time...will
have gusty wind wording in the thunderstorms for Wednesday afternoon.
The warm air advection pattern and winds aloft favor some of the higher ridges from
hsp to Boone to get close to advisory levels of 46 miles per hour for gusts.
Overall..the winds will stay below this as we usually need a 50-55
knots 800 mb jet and the models keep the best winds situated north of US.
Wednesday will not be washout and west-southwest flow at times should limit
storm/shower development east of the mountains...but enough backing
should allow for overall low level convergence not to keep it
Going toward dusk...the showers will fade in coverage. Wednesday
will be mild/seasonal with highs from the upper 50s southeast WV to lower
to middle 70 southeast.
Short term /Wednesday night through Friday/...
as of 230 PM EDT Tuesday...
A weak cold front will slide east of the area Wednesday evening. Any
lingering showers will dissipate by 10p with lose of heating and
increase drier air advection. The 12z GFS is in agreement with this
scenario...so the forecast is more heavily weighted to that
solution. High pressure continues to build east into the area
Thursday...however a modest pressure gradient will keep breezy and
gusty northwest winds over the area through the day. The center of
the high drifts over the area Thursday night with calming winds and
cooler than normal temperatures into the day Friday. Temperatures
Thursday and Friday will range from the upper 50s to lower 60s
across the mountains to middle to upper 60s east.
Long term /Friday night through Tuesday/...
as of 240 PM EDT Tuesday...
A warm front will enter the region from the southwest Friday night
bringing an increase in clouds and the chance for rain Saturday
morning. Rain chances increase during Saturday afternoon as
isentropic lift increases with warm moist air overrunning cooler
surface air. A surface wave is expected to track over the Tennessee
Valley Saturday...then across the Carolinas Sunday. This wave will
continue the chance for rain in the area through Sunday
morning...then a back door cold front brings cooler drier air into
the region for Sunday afternoon into Monday. A stronger surface wave
tracks across the Tennessee Valley to the Carolina Monday night into
Tuesday. This wave will have more moisture and dynamics to bring
some heavy rain and strong storms to the region. Temperatures
through the period will remain cooler than normal with upper 50s to
lower 60s across the mountains to middle to upper 60s east.
Aviation /01z Wednesday through Sunday/...
as of 635 PM EDT Tuesday...
A VFR mix of high based cumulus and ac will prevail into this evening
with gusty westerly winds slowly diminishing after sunset. However
given the pressure gradient ahead of the next upstream cold
front...only expecting the valley locations such as klwb/kbcb/klyh
and perhaps kdan to see winds become light late while locations
such as kblf/kroa likely maintain a SW wind at 8-15 kts through
morning. Low level jet also ramps up ahead of the front espcly
across the mountains which would support some spots of low level
wind shear late tonight. Thus kept mention at klwb and added at
kbcb toward daybreak Wednesday while leaving out elsewhere for
Increase in clouds late tonight in the 5-10kft range with the
pre-frontal band of moisture starting to enter the far western
mountains. As we head into Wednesday morning the front will be making
its track over the ridges with showers increasing over southeast West Virginia.
At this time...will have VFR to high end MVFR...but the threat of
moderate to heavy showers and even a thunderstorm arrives toward
the end of the taf period mainly mountains. Since coverage looks
rather limited outside of the kblf-klwb corridor given deep/strong
mixing...only brought showers east into kbcb in the afternoon
with a period of MVFR visibilities possible at kblf/klwb.
Expect strong westerly winds ahead and behind this front...with
potential gusts over 30-35 kts...espcly mountains...with even
some gusts to 30 kts or stronger in the east around kdan Wednesday
Front will exit by Wednesday night with high pressure building into
the region from the west Thu-Fri. Some gradient remains between
departing front and high to the west Thursday and possibly
Friday...so gusty winds will remain mainly during the daytime.
Overrunning clouds work into the area Friday ahead of a warm
front. By Saturday...enough moisture to allow for showers and MVFR
with best forcing across the Carolinas. This will be possible
into Sunday morning before the system exits and some improvement
in ceilings starts to occur later Sunday.
as of 330 PM EDT Tuesday...
Wet fuels will dry out as we head into Wednesday. A front will
approach Wednesday with good mixing out ahead of it. Gusty winds
in the 25 to 35 miles per hour range will occur at times over the Piedmont
where min relative humidity sinks to 30 percent during the afternoon. At this
time a few showers and storms could be occurring so fire danger
for now is low to moderate as 1 and 10 hour fuels have either
greened up or are greening up. Issues would be with any loose Leaf
litter that can dry out enough...though soils themselves stay