Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
743 PM EDT Friday Sep 4 2015
a backdoor cold front will move into the area tonight and stall over
the southern portion of the region on Saturday. Anticipate showers
and storms through at least the early part of tonight in association
with the front and an upper level disturbance moving into the
region. Cooler and somewhat drier weather is expected towards the
end of the weekend into early next week. A resurgence of Atlantic
moisture returns to the area by the middle part of next week.
Near term /through Saturday/...
as of 600 PM EDT Friday...
Updated forecast to reflect downward trend in probability of precipitation over the
western County Warning Area...concentrating more across the eastern half of the
County Warning Area. MLCAPES at 21z in the 1500-2000 j/kg range over Southside Virginia
into the foothills. Atmosphere being worked over with outflow
outrunning the storms and cutting off inflow in the eastern
County Warning Area...with best concentration situated south of psk-bcb.
Local WRF takes this activity south of our County Warning Area by 01z...with the
showers and storms over the Piedmont northeast of lyh and north of
Lexington weakening by 0z.
Other adjustments to the forecast were taking into account rain
cooled temperatures...and cloud cover.
Previous afternoon discussion...
Showers and thunderstorms continue to spread south across the region
in association with both the approach of a backdoor cold
front...surface heating...and the passage of an upper level
disturbance from a small closed low over the area of Michigan and
Wisconsin. Coverage has been the least over parts of the New River
valley and the mountain Empire region of southwest Virginia for much
of the afternoon. This will change has we head into the late
afternoon and early evening as the activity to the north continues
to head south.
Showers and thunderstorms will continue past sunset thanks to both
the influence of the front and the upper level disturbance...however
the activity is expected to become more focused over the southern
and southeast parts of the area as time progresses. By
midnight...only isolated to scattered showers are expected to be
over parts of the area. Low level flow will become progressively
more northeast to east through a deeper layer of the atmosphere. This
will help regenerate cloud cover overnight after an initial
decrease. Low temperatures tonight will not be much different than
those realized this morning. Anticipate lower 60s across the
mountains and middle to upper 60s across the Piedmont.
For Saturday...high pressure will slowly start to nose its way south
along the Lee of the Appalachians. Notably drier air will still be a
couple days away...but the instability will be less Saturday as
compared to today. However...it will not be too weak as to prevent
the develop of isolated to scattered showers...the the greatest
concentration over the southwest parts of the region. An isolated
thunderstorm cannot be ruled out...but any that form will not be as
potent as those of today. High temperatures will be about ten
degrees cooler than those of today. Expect middle to upper 70s across
the mountains and upper 70s to around 80 across the Piedmont.
Short term /Saturday night through Monday/...
as of 300 PM EDT Friday...
Will start Saturday evening off with weak high pressure wedging
southward against the eastern face of the Appalachians...resulting
in light northeasterly windflow. The upslope winds along the ridges
may support a few showers into early Sunday...but expect any
thunderstorms left from earlier in the day to weaken by sunset with
the loss of daytime heating.
For Sunday...weak high pressure will remain in place across the middle
Atlantic...with winds shifting more easterly. The flow off the
Atlantic will make for continued cooler temperatures...with
afternoon highs ranging from the middle 70s mountains to low 80s east
of the Blue Ridge. The high pressure wedge will be shallow enough
across the Piedmont of North Carolina that a few showers and
isolated storms will develop with afternoon heating. May also see a
few showers develop along the mountain ridges due to the upslope
winds...but expect coverage will be spotty at best. Rainfall will
again fizzle around sunset with the loss of daytime heating.
For Labor Day...high pressure will gradually slide east...decreasing
its influence across the middle Atlantic. This will allow winds to
shift increasingly southerly to draw more unstable air from the
Carolinas into our area. As such...expect slightly better coverage
of showers developing with afternoon heating. Will also see
increased instability as subtle upper level disturbances pass across
the area...and therefore expect slightly better coverage of
thunderstorms. Upper level winds will be light...resulting in slow
cell movement...which would make for locally heavy rain as storms
remain parked over locations for extended periods of time.
Temperatures will take on a warming trend...with afternoon highs
ranging from the low 80s west to the upper 80s east.
Overnight temperatures through the period will remain seasonable...
generally ranging from the low to middle 60s most locations...with a
few middle/upper 50s in the deeper mountain valleys. Patchy Mountain
Valley fog will also be a common occurrence each night.
Long term /Monday night through Friday/...
as of 330 PM EDT Friday...
A general summertime weather pattern will return for the middle of
next week...with spotty showers and thunderstorms developing early
each afternoon with daytime heating...and diminishing toward sunset
as heating ends. Expect only spotty coverage of rainfall on Tuesday
due to weak supporting dynamics...however coverage will increase
each day for Wednesday and Thursday as a cold front approaches from
By next Thursday...long range weather forecast models differ as to
where the cold front will be located...with the European model (ecmwf) model bringing
the front across our area...while the GFS model keeps the front well
to our north. Given what appears to be weak upper level support to
give the front a southward push...have trended toward the GFS model
and trimmed back rain chances for that day.
Expect warm and muggy conditions for the period...with highs ranging
from the upper 70s/low 80s across the mountains...to the upper 80s
for the Piedmont. Lows will hold generally in the low to middle
60s...perhaps with a few upper 50s across the mountains.
Aviation /00z Saturday through Wednesday/...
as of 735 PM EDT Friday...
Expect a few rain showers/thunderstorms and rain around through about midnight/04z...with
roa having a light rain shower until 02z. Lyh may see a shower in
the vicinity as small area moves southwest from Buckingham County.
Strongest convection will stay in a corridor from tri-hlx at 00z
then dissipate...moving south into tnb-ukf by 02-3z.
Issue overnight will be backdoor front and lower ceilings plus
prospects of fog.
Will lean toward previous forecast combined with our local WRF
which favors some IFR/MVFR ceilings/visibilities late tonight at all
sites...with the blf/lwb/bcb area having worse conditions.
Confidence is high that sub VFR will occur...but low on how far
down the ceilings/visibilities go.
Saturday morning into the early afternoon...expect a gradual
trend of ceilings and visibilities improving to visual flight
rules levels. Isolated showers will be on the increase around
noontime...but confidence in their location is not great enough to
include any in the prevailing weather of the forecasts.
Extended aviation discussion...
High pressure will gradually work into the region by the end of
the weekend...and remain over the area into the early part of
next week. Lingering moisture and an easterly fetch will help
maintain some isolated to scattered showers across mainly western
and southern parts of the area...especially on Saturday and Sunday
Moisture starts to increase again Monday into Tuesday while the
influence of the ridge weakens. Coverage of showers...and possibly
a few storms...will likewise start to increase. Past
Saturday...ceilings and visibilities will primarily be visual
flight rules. The exception will be lingering patchy MVFR rules
across parts of the region Saturday afternoon. Also...overnight
and early mountain and River Valley instrument flight
rules/marginal visual flight rules fog/mist are expected.
as of 745 PM EDT Thursday...
The weekly United States drought monitor issued on September 3rd
shows the effect of a drier than normal August across much of the
area. Abnormally dry conditions...d0 on the drought scale have
been expanded into a large portion of Southside Virginia covering
much of the Dan and lower Roanoke River basins. Parts of the upper
Yadkin River basin in North Carolina are already in d1 or moderate
drought. August rainfall was as low as 50 percent of normal or
less in parts of the aforementioned areas. Several National Weather Service cooperative
climate stations in southern Virginia recorded well under 2 inches
of rain the entire month of August including Huddleston in Bedford
County at 1.17 inches and Rocky Mount in Franklin County at 1.36