Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
947 am EDT Sat Aug 29 2015
high pressure will remain centered over the Delaware-Maryland-Virginia region for
much of today. A weak upper level disturbance will lift north
along the Appalachians Sunday into Monday ahead of a weak cold
front to the west. Upper level high pressure will strengthen in
the wake of this feature...resulting in more diurnal nature
showers and storms into the middle of next week.
Near term /through tonight/...
as of 945 am EDT Saturday...
Morning visible satellite imagery and surface observations indicate
valley fog has just about dissipated but middle clouds have blossomed
and high clouds are streaming up from the south. Believe this
extra cloud cover will filter insolation just enough to slightly
inhibit warming...so will adjust maximum temperatures down just a hair. The
rest of the forecast will remain unchanged with slight/low chance probability of precipitation
warranted in the far west for diurnal orographically driven
isolated showers...mainly from the mountain Empire of Virginia into the
Greenbrier valley and Alleghany Highlands.
Instability remains limited...however may see a few showers
develop from the Alleghany Highlands through the mountain Empire.
With subtropical ridging nudging its way closer to the middle
Atlantic...we can expect slightly warmer temperatures for today...
with highs ranging from the low 80s across the mountains...to the
upper 80s across the Piedmont.
High clouds will increase during this evening...making for mostly
cloudy skies for tonight. Warmer and muggier...overnight lows will
generally fall into the low to middle 60s...with upper 50s for a few
of the deeper valleys across the mountains.
Short term /Sunday through Monday night/...
as of 230 am EDT Saturday
Water vapor satellite imagery depicts a broad upper level area of
low pressure over the northern Gulf of Mexico with a plume of
moisture in advance of this system streaming into the southeast U.S.
Most of the deterministic models are in general agreement that this
low opens and lifts into our area Sunday night/early Monday before
pushing east of the region by late Monday...before upper heights and
higher precipitable water values build Tuesday. Model trends have been slightly
slower and further west with this feature.
For Sunday...a weak upper level shortwave in advance of the
aforementioned low enters far southwest Virginia on Sunday. Forecast
soundings still indicate a dry low level atmosphere with little to
no cape. Kept a small chance for showers and storms in the far west
where best uvm is forecast along with highest forecast Theta-E
values. Nudged temperatures down a degree or two with the expected
increased cloud cover.
Better chance for precipitation appears to be Sunday night with
surge of deeper moisture and best upper dynamics arrive. As a
result...increased probability of precipitation during the overnight hours Sunday
It should be noticeably more humid Monday and warmer thanks
to an increase in 1000-850mb thicknesses...and leaned toward the
warm NAM MOS values for highs on Monday. While the initial upper
level short wave will be exiting our area during the day...and
taking most of the precipitation with it...lingering instability and
weak low level convergence in the mountains should trigger diurnally
driven widely scattered showers and storms.
Precipitable water values continue to surge Tuesday as an upper ridge builds...
leading to a humid and warm day with more afternoon/evening showers
Long term /Tuesday through Friday/...
as of 1240 PM EDT Friday...
Overall persistence forecast at least early on as upper ridging
strengthens across the region in the wake of the passing wave aloft and
in increasing subsidence well north of Erika. This should result in a
more diurnally driven trend of daily showers and isolated storms each
afternoon/evening with a gradual increase in coverage/expanse from
Tuesday through Thursday given better precipitable waters /moisture and slight cooling
aloft. However think low/medium chance probability of precipitation confined mainly along/west
of the Blue Ridge per orographics...and more isolated elsewhere
especially given moisture over top a lingering surface ridge. Thus
followed closer to trends from wpc with a gradual boost in probability of precipitation each
day but little during the overnights into Thursday. Things become more
uncertain by day7 with models lowering heights across the Appalachians
and from the NE with a possible associated backdoor cool front. This
could help focus moisture given a better easterly trajectory to the
north of Erika or its remnants on Friday. Therefore going with more
widespread chance probability of precipitation per potential tropical type precipitable waters over the area
during heating Friday afternoon.
Otherwise little change in daily temperatures given such a humid/warm
airmass...supporting highs mostly in the 80s and lows in the 60s except
perhaps a bit cooler Friday pending clouds/showers.
Aviation /14z Saturday through Wednesday/...
as of 745 am EDT Saturday...
High pressure remains centered over the Delaware-Maryland-Virginia this morning...
maintaining light south southeasterly windflow across the area.
Low level moisture is gradually increasing as this windflow draws
Atlantic air upslope into the mountains...which has resulted in
more widespread reports of mainly fog/br across the area compared
to previous mornings. Surface observations across the mountains
and foothills have detected scattered ceilings in the 5kft to 8kft
Fog will burn off quickly after 29/14z with daytime heating...
resulting in VFR conditions areawide. Increasing moisture will
make for a scattered cumulus field developing late morning...possibly
becoming broken across the mountains during late afternoon...with
ceiling bases 5kft to 8kft. Instability is limited...but may be
sufficient to trigger isolated showers across southeast West
Virginia...possibly as far east as kbcb. Even then...shower threat
is not expected to result in sub VFR conditions where they occur
for more than a few minutes. Convection will wane toward sunset.
With increasing upper level clouds during late Saturday evening...
expect redevelopment of fog/br will be more on the patchy side.
Regardless...River Valley fog will be locally dense...pushing
visibilities to 1/4sm or less in spots. Scattered low clouds will
also remain across the mountains and foothills...with bases in the
4kft to 6kft range...but possibly high end MVFR in spots.
Extended aviation discussion...
short wave energy moving up from the south will support an
increased chance for convective activity in the Sunday/Monday
time frame. Generally expect the best chances for MVFR/IFR to be
late night/early morning due to fog and stratus development from
the Blue Ridge west especially at klwb/kbcb.