Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
1046 PM EDT Sat Aug 1 2015
high pressure will build east across the region overnight and
remain in place Sunday into Sunday night. An upper disturbance
passes across the Great Lakes Monday followed by a cold front that
will drop south into the area Tuesday...then stall over or near
the area by Wednesday.
Near term /through Sunday/...
as of 1030 PM EDT Saturday...
Forecast remains on track. Made some minor modifications to min
temperatures and hourly temperatures as some of the more urban areas appeared a
little to cool for min temperatures given the warm temperatures of this
afternoon and slow decline rate at the current hour.
Otherwise...no changes were needed at this time.
As of 200 PM EDT Saturday...
Weak surface trough crossing the Blue Ridge this afternoon will bring a
reinforcing shot of drier/cooler air overnight as high pressure builds
east and settles about overhead the western mountains by dawn Sunday.
This should allow any residual cumulus to fade with loss of heating as drier
air advects in while also helping winds diminish this evening...but
perhaps not on the ridges until after midnight. This along with even
cooler air at 850 mb than seen before likely to make for great radiational
cooling west late with a light north/NE breeze lingering out east in
spots. Latest MOS similar to previous with mainly low/middle 60s east and
cool 50s to around 60 west. However a few upper 40s possible deepest
western valleys pending any late night river fog and residual western
slope strato-cumulus that may linger.
Surface high will be overhead for much of Sunday allowing for another
warm but dry day with overall low humidity under mainly sunny skies. A
weak middle level wave riding through the base of the 500 mb trough could
produce some low clouds northwest early before mixing out. Also may see just
enough return SW flow as the high slides east late to prompt a bit more
cumulus over SW sections but without any pop mention for now given dryness.
Should be another large diurnal swing to temperatures as 850 mb values start
to rebound while highs aided by heating of dry air and weak eastern
downslope. This similar to the past couple of days so staying close to
persistence with mostly 80s mountains and upper 80s/low 90s east.
Short term /Sunday night through Tuesday night/...
as of 325 PM EDT Saturday...
Broad upper trough expected to linger from the Hudson Bay area south
into the Great Lakes region through much of the short term period as
a series of short wave troughs help to slowly amplify and deepen
Most of the energy and moisture associated with these waves will
remain sufficiently north of the Blacksburg forecast area to
maintain a mainly dry forecast for most areas...but influence of
the most substantial of the waves pivoting east through the upper
and lower Great Lakes late Monday into Monday night...along with
attendant cool front...still warrants sufficient confidence to
maintain a reflection of an increase in clouds and a threat for
widely scattered late day and nighttime showers/thundershowers
into upslope areas of the western Blue Ridge north into the
Despite some model depictions...westerly low level flow and
relative dry lower troposphere ahead of the wave should limit any
potential for spill-over into downslope areas east of the Blue
Ridge on Monday night.
Southwest to westerly pre-frontal winds expected to allow 850
millibar temperatures to climb back into the +22-23c range in most
areas...driving temperatures into the 90s across much of the
Piedmont on Monday afternoon. However...unlike past several
weeks...dew points still expected to be relatively lackluster...
mainly in 60s...as opposed to more oppressive 70s experienced
As short wave trough lifts northeast and away from the Blacksburg
forecast area on Tuesday...decaying cool front likely to stall
over or near the Virginia/NC state line as upper flow reverts to more
of a zonal configuration. Proximity of old front...and some low
level pooling of moisture in region of confluence may be
sufficient to spark isolated to widely scattered late afternoon
and evening showers or thundershowers just about anywhere in the
forecast area...most notably in the mountains where differential
heating and localized upsloping will aid in vertical development...
although most areas still likely to remain precipitation free.
Some weak Post-frontal cool advection and clouds may help to lower
daytime highs in the mountains a degree or two from those
experienced on Monday...but continued good insolation and negligible
push of cooler air into Southside Virginia and north central NC should
ensure another day of afternoon highs in the 90s in those areas.
Long term /Wednesday through Saturday/...
as of 325 PM EDT Saturday...
East-west baroclinic zone expected to become established over or
near the forecast area through much of the long-term period as
additional upstream short waves and/or mesoscale vorticity centers
from decaying old Midwest convective complexes provide sufficient
energy/support to maintain threat for daily scattered convection.
Strength and movement/positioning of each wave in mainly zonal
upper flow will influence daily position of decaying frontal
boundary...extent of cloud cover...and extent of warm or cool
advection depending on position relative to front.
Highest threat for precipitation at the present time appears to
come on Wednesday into Wednesday night...and again on Friday when two
of the more notable upstream waves arrive...and this reflected
accordingly in grids for these timeframes. Otherwise...a lower and
more generalized threat for daily precipitation will be maintained
into Saturday until regional/mesoscale features become better
Aviation /03z Sunday through Thursday/...
as of 725 PM EDT Saturday...
High pressure will help yield VFR conditions for most locations
during the 00z-00z/8pm-8pm taf time period. The exception will be
late night and early morning patchy Mountain Valley and river fog
development. Any patchy fog will dissipate soon after sunrise
Sunday. Wind will be light or calm through the period.
Extended aviation forecast...
as of 1230 PM EDT Saturday...
Mostly VFR outside of any convection through the period. A front
will set up in the middle Atlantic which could bring a few rain showers/thunderstorms and rain
by Tuesday...with perhaps a bit more coverage later Wednesday at
this point. Thus the threat of sub-VFR remains low until around
midweek...except for the typical fog potential at klwb/kbcb each
morning between 09-13z.