Scientific Forecaster Discussion
fxus61 krnk 190658
Area forecast discussion
National Weather Service Blacksburg Virginia
258 am EDT Thu Sep 19 2019
high pressure builds over the area tonight into Thursday. High
pressure center will then remain in control through the weekend
with cool nights and warm days.
Near term /until 6 PM this evening/...
as of 730 PM EDT Wednesday...
Quiet evening weatherwise. Clouds have been steadily dissipating
from NE-SW with only remaining cloud elements over western NC
into the Grayson Highlands of Virginia. Where it has cleared, expect a
quick cool down, temperatures quickly retreating to near the
dewpoint values which are mostly in the 50s. Where clouds
persist, the dewpoints are still closer to 60. As such, when it
does clear, it will not take much to resaturate the boundary
layer resulting in late night fog and/or redevelopment of
Looking upstream, surface high pressure over New England will
continue to build southwest and into the forecast area from the
northeast. The easterly flow favors redevelopment of the upslope
stratus over along the Blue Ridge especially more-so southwest
of Roanoke where the dewpoints are higher. To the northeast, the
advection of dry air from Nova into PA may be enough to offset
the stratus development. However, the rivers are warm, so late
night fog development is still possible within the river
valleys. Any fog/stratus development should easily dissipate
Thursday as increasing subsidence an much drier air, dewpoints
in the 45-55 degree overtake the entire forecast area.
Low temperatures Thursday morning are expected to range from
the mid 40s in the cooler mountains valleys to the lower 50s in
the Piedmont. Under near full afternoon sunshine, high
temperatures on Thursday will vary from the upper 60s in the
mountains to the mid 70s in the Piedmont.
Short term /6 PM this evening through Saturday night/...
as of 330 PM EDT Wednesday...
High pressure will gain control over the mid Atlantic during
Thursday night into Friday, and it should remain overhead through
Saturday night. Meanwhile, an upper level ridge will build over the
mid Atlantic, which should create a vertically stacked area of
ridging throughout the atmosphere over the County Warning Area. Thus, there is high
confidence in dry conditions during this part of the forecast.
Because of the expected dry weather, a wide diurnal range in
temperatures will be expected with lows dipping into the 40s and
50s and highs climbing into the 70s and 80s.
Long term /Sunday through Wednesday/...
as of 315 PM EDT Wednesday...
Both the GFS and European model (ecmwf) models reveal high pressure edging offshore
while a notable upper level ridge remains overhead during Sunday.
Thus, the unseasonably warm and dry weather will continue through
the remainder of this weekend. The models diverge with respect to
the timing of an approaching upper level trough and associated cold
front over the Great Lakes beyond Sunday night. While the GFS brings
the cold front over the mid Atlantic during Monday into Monday
night, the ecwmf does not bring the cold front overhead until Monday
night into Tuesday. Because of the fast bias that the GFS normally
has for weather systems in the extended portion of the forecast, the
European model (ecmwf) was favored for the timing and the overall big picture.
The cold front appears rather weak as most of the dynamical energy
and support will pass well to the north in both models. It is quite
possible that the Piedmont may not see any rainfall at all from this
frontal passage. The best chance for any rain will remain along and
west of the Blue Ridge, but confidence is low that any significant
rainfall will occur that could help to reverse the overall dry trend
from the past couple months in the County Warning Area. Any showers should dissipate
by Tuesday evening according to the ecwmf model, and high pressure
will return over the mid Atlantic by Wednesday.
Aviation /07z Thursday through Monday/...
as of 245 am EDT Thursday...
MVFR ceilings are mainly confined to the Blue Ridge mountains
southwest of Roanoke and into western NC. Elsewhere skies have
cleared out or at least have become scattered at VFR levels.
Expectation is that the clouds will work their way back some to
the east and likely involve roa at a minimum, less likely lyh
and Dan. However, these ceilings may remain above 030, thus VFR.
Fog development seems to be very questionable this morning
because of advection of drier air and 40-50 degree dewpoints
across the northern areas. Have reduced fog threat in the 06z
Regardless of the fog/stratus outcome this morning, strong
subsidence and advection of dry air into the region Thursday
under high pressure surface and aloft should result in quick
dissipation of cloud/fog elements leaving US with widespread
clear air and VFR.
Winds remaining northeast to east-southeast through the period
at speeds mostly 9kts or less.
/Confidence in forecast parameters/
ceilings - moderate to high,
visibilities - moderate,
winds - high,
thunderstorm threat - none.
Extended aviation discussion...
High pressure to our northeast will continue to build southwest
over the region promoting VFR into the upcoming weekend.
Only exceptions will be late night and early morning fog in the
mountain valleys which would impact the usual terminals (e.G.,
Klwb and kbcb).
Confidence is high that VFR will be the dominant flight
category into the upcoming weekend. Next chance scattered MVFR
showers would accompany a front on Tuesday, and mainly impact