Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
339 PM EDT Friday Jul 31 2015
high pressure will build east into the middle Atlantic region tonight.
This high center will stay over most of region this weekend. A
couple of upper disturbances may bring a few spotty showers to the
mountains late Saturday or Sunday...but the majority of our area
will stay dry.
Near term /through Saturday/...
as of 152 PM EDT Friday...
High pressure centered around Missouri this afternoon will
build eastward tonight into Saturday. Scattered afternoon cumulus will
diminish or dissipate by sunset with the loss of solar heating.
Tonight...the flow starts to back to the west as a weak upper
shortwave pivots under the broad trough across the middle Atlantic
region. Airmass is too dry to generate anything more than clouds
with any shortwave rotating around the upper trough. Great night
to open your windows. The combination of light winds and some low
level moisture may produce patchy fog in the favored valleys.
Leaned towards the cooler MOS coop guidance for low temperatures
tonight. Low temperatures tonight into Saturday morning will range
from around 50 degrees in the coldest deeper valleys to the lower
60s in the Piedmont.
High pressure will bring lower humidity again Saturday. Abundant
sunshine and light downslope wind flow will warm high temperatures
Saturday into the upper 70s in the northwest mountains to the
Short term /Saturday night through Monday night/...
as of 335 PM EDT Friday...
Broad upper trough will remain anchored from the James Bay area
south into the upper and lower Great Lakes Saturday night into
Monday morning...with weak surface high pressure system lingering
over the middle appalachian region.
This setup will ensure a continuation of dry weather and
relatively low dew points that will support efficient daytime
heating...with good radiational cooling at night...resulting in
marked diurnal temperature swings of around 30f.
By Monday afternoon into Monday night...approach and eventual
passage of an upstream shortwave trough diving southeast through/out
of the upper Great Lakes will bring an increase in clouds along
with a threat for widely scattered showers/thundershowers into
upslope areas of the western Blue Ridge north into the Alleghany
Highlands. However...westerly low level flow ahead of the wave should
prevent any potential for spill-over into downslope areas east of
the Blue Ridge.
Long term /Tuesday through Friday/...
as of 335 PM EDT Friday...
Energy associated with Great Lakes trough expected to be northeast of
the Blacksburg forecast area by daybreak Tuesday. Attendant cool
front will likely slow and then stall over/near the middle
appalachian and middle Atlantic region later Tuesday into Tuesday
night as trough spoke lifts northeast and away from area and as upper
flow in its wake gradually flattens to more of a zonal
East-west baroclinic zone will then become established over the
forecast area for the rest of the long-term period...with
additional upstream short waves and/or mesoscale vorticity centers
from decaying old Midwest convective complexes providing
sufficient energy/support to maintain threat for scattered
convection through the period.
Gradually rising dew points will slowly compress the diurnal
temperature curve back to around 20f as sultry airmass slowly
reestablishes itself over the area. Expect overnight lows during
the period to range from the 60s in the mountains to around 70 in
the Piedmont...with highs ranging from mainly 80s in the mountains
to Lower-Middle 90s across the Piedmont.
Aviation /19z Friday through Wednesday/...
as of 132 PM EDT Friday...
Great flying conditions expected this afternoon into the weekend.
VFR conditions will prevail over taf sites during this period. The
only exception would be patchy morning fog. Klwb and kbcb have the
potential for some fog late Friday night into Saturday morning.
Any morning fog will burn off or mix out by 9 am.
High confidence in ceilings...visibilities and winds during the
Extended aviation forecast...
Mainly VFR this weekend with high pressure in place. A few upper
disturbances may bring a few VFR ceilings to the mountains later
Saturday into Sunday. The threat of showers will be low though a
backdoor front may allow for some again later Monday or Tuesday.
With sustained winds staying light to calm at night and mainly
clear skies...the fog threat will exist in the typical predawn
hours at klwb/kbcb by early next week.