Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
806 PM EDT Thursday Jul 10 2014
weak high pressure will spill south into the region Friday and
Saturday. Deeper moisture will return later in the weekend and
early next week ahead of a much stronger cold front that will be
passing through the region by next Tuesday.
Near term /until 6 am Friday morning/...
as of 750 PM EDT Thursday...
Theta-E ridge situated over the scntl part of the County Warning Area and then
heading into portions of southeast WV. Radar coverage has been isolated
at best with a few showers and storms moving over the NC mountains and
another weak batch over Rockingham County. Watching storms just
north of Amherst to Rockbridge with one cell into Bath.
Overall...the latest models have been too aggressive with
coverage. There is an upper shortwave pushing into the central
Appalachians. With loss of heating and weakening of low level lapse
rates...feel the coverage into this evening will stay isolated at
Adjusted sky cover also to account for less clouds in portions of
the New River valley into southeast WV.
Previous valid discussion...
upper trough axis will continue to shift eastward overnight.
Weak high pressure will build from a 1024 mb high centered over New
England overnight and should see slightly drier air filter into the
area during with dewpoints below 60 in the west to middle 60s east.
Patchy fog likely especially in the areas that received significant
rainfall today. Far southeastern County Warning Area received the best rainfall
today with over one inch across parts of Halifax and Pittsylvania
County and also in the NC mountains. Lows will bottom out upper 50s
to middle 60s which is fairly close to early-middle July normals.
Short term /6 am Friday morning through Sunday night/...
as of 400 PM EDT Thursday...
Friday the surface front will be well to our southeast with much drier
air filtering in behind it...especially aloft. Surface dew points will be
a little slower to dry out but with good mixing down of dry air
aloft they should fall more rapidly by the afternoon. Given this
more obvious push of drier air behind the front now...have lowered
probability of precipitation and raised temperatures slightly given trends in guidance. Forecast
soundings suggest it will be hard to get much surface based cape
except for over higher terrain...and mainly mountains of northwest NC and Virginia
Blue Ridge south of roa. Keeping a slight chance pop up elsewhere
includingpiedmont which is a little closer to frontal boundary
and slightly deeper moisture...but we may be able to drop this too and
keep any convection limited to mountains...and isolated. Really not
seeing any chance for severe thresholds with any of these storms
given limited instability and very weak shear.
Rest of the weekend the ridge dominates with slowly increasing
temperatures and slow return to moisture again as easterly flow
shifts slowly back to southwest behind ridge...especially by
Sunday. Still...have trended lower on probability of precipitation given relatively dry
atmosphere...keeping it dry in Piedmont but some areas of chance probability of precipitation
in mountains. Again not seeing enough instability or any forcing
for much in the way of severe threat. Storms will be fairly widely
scattered. Sunday the moisture return is a little more
pronounced...and storms will be a little more widespread but still
chance category with no forcing. NAM showing increasing SW winds
while GFS and European model (ecmwf) suggest a little more westerly component so
some question about how far east storms may develop or shift to
later in the day. Much more organized and possibly severe storms
well upstream closer to next frontal boundary should remain well
to the west through the night...especially with westerly winds at
ridge top level.
Seasonal temperatures over the weekend with highs in the middle 80s west to
lower 90s east. Could be even middle 90s Sunday in Piedmont if
downslope is a little stronger and clouds stay away.
Long term /Monday through Thursday/...
as of 1230 PM EDT Thursday...
Monday a low pressure system will be centered over James Bay with a
high pressure system to our south across the Florida Panhandle. A cold
front will stretch through the Ohio Valley to our west. With this
setup...Monday will be warm and humid with plenty of instability to
set off some afternoon and evening showers and thunderstorms.
Waves of upper level energy association with an upper level trough and
associated cut off low over the northern Great Lakes will bring
additional showers and thunderstorms ahead of the front on Tuesday.
Timing differences remain on when exactly the cold front will cross
our region...however it will happen either late Tuesday or early
Behind the cold front...high pressure builds in and a noticeable
drop in humidity will be felt as dewpoints crash into the 40s and 50s.
High temperatures Wednesday and Thursday will be several degrees below
normal. This means middle to upper 70s across the mountains and low 80s
east of the Blue Ridge. Depending on cloud cover...would not be
surprised to see a few mountain locations drop into the middle to upper
40s for night time lows especially on Thursday.
Aviation /00z Friday through Tuesday/...
as of 745 PM EDT Thursday...
Appears that upper trough will finally shift far enough east to
remove most convection from the County Warning Area Friday. For this
evening...isolated rain showers/thunderstorms and rain continue to develop along and east of
wind shift line across eastern WV from ekn-bkw. Too isolated to
include in any taf sites at this time. The possible exception
would be kdan...where isolated rain showers/thunderstorms and rain from NC Piedmont may come
near as they move NE. Main concern through the taf valid period
will be potential for overnight fog and low clouds. Air
mass...especially west of the Blue Ridge...slightly drier than
this time yesterday and less rain at most sites with the exception
of kdan today. Still feel T/dew point spread low enough and air moist
enough for potential IFR-LIFR br/fog klyh and kdan...with less
potential for kbcb further west. Not expecting any fog development
at kroa or kblf. Have backed off fog development at klwb since
dewpoints down into the middle 50s and maximum-T this afternoon in the
middle 80s suggests fog development less likely with crossover
temperature of 53-54f. Winds chaotic and light...but high pressure
along middle-Atlantic coast suggests winds east-southeast-southeast 3-5kts through much
of the taf valid period...with some tendency to transition more
southeast-south-southeast Friday afternoon.
Overall...outside morning fog/clouds...expect VFR ceilings and visibilities.
Medium to high confidence in ceilings throughout the taf valid
Medium confidence in vbsys 06z-13z...otherwise high confidence in
Medium confidence in wind direction through the taf valid period...high
confidence in wind speed through the taf valid period.
Extended aviation discussion...
Continued low probability of rain showers/thunderstorms and rain Sat...as weak high
pressure slides in from the north with a relative dry air mass.
Moisture and more widespread rain showers/thunderstorms and rain likely to return for the
end of the weekend and early next week as a broad upper trough
digs into the Great Lakes region and resultant cyclonic flow
resulting in increasingly unstable/unsettled conditions across the
region. Patchy late night/early morning fog will result in local
MVFR to IFR conditions during the late nights and early
mornings...especially in valley locations and where earlier
afternoon rainfall occurred.
as of 1000 am EDT Thursday...
US drought monitor issued this morning...July 10 2014...shows
expansion of d0...abnormally dry category...across much of New
River valley and over parts of the Virginia and NC Piedmont...centered
around Danville. Pulaski County Virginia appears to be near the
epicenter of the developing conditions over the New River valley
with much of that County receiving less than 50 percent of normal
rainfall since early may.