Scientific Forecaster Discussion
fxus61 krnk 200831
Area forecast discussion
National Weather Service Blacksburg Virginia
431 am EDT Fri Sep 20 2019
high pressure at the surface and aloft will continue to build
over the area through the weekend. A weak cold front will drift
into the area from the northwest early next week. Weather
conditions will remain dry through the period with cool nights
and warm afternoons.
Near term /through tonight/...
as of 420 am EDT Friday...
High pressure at the surface across the northeast U.S. And a
ridge of high pressure aloft across the Ohio/Tennessee Valley will
continue to drift toward our region. These will be the main
synoptic features through the weekend and into the first of next
week. Thus, little change is expected in the weather. Perhaps
the only notable change will be a gradual increase in
temperatures through the weekend into early next week as the
high aloft builds over the area and 850mb temperatures slowly
creep back toward +20c as they were a week or so ago.
The persistent stratus or stratocumulus deck we have seen
because of the northeast to east flow/marine layer in place the
last several days has been relegated to the far
southwest/upslope areas of the Blue Ridge. With the surface high
continuing to build southwest, the synoptic pattern will
continue to become less favorable for such cloud development. In
addition, any such clouds today into tomorrow should be
confined to the far southwest, leaving most of the County Warning Area clear or
mostly clear throughout the next 48-72 hours.
Any patchy Valley/River fog this morning will dissipate quickly
after sunrise, with a slightly greater likelihood of such fog
developing late tonight/early Saturday morning.
/Confidence in forecast parameters/
temperatures - moderate to high,
precipitation probabilities - high (0% through the period)
winds - moderate to high,
thunderstorm probabilities - none.
Short term /Saturday through Monday night/...
as of 330 am EDT Friday...
While an upper level ridge is expected to remain in place across the
region for the weekend, a weak shortwave trough is progged to make
some headway into the northern extent of the ridge. Its influence is
expected to impact more of Ohio/PA/NY, but the southern tail of this
feature may clip northern parts of the forecast area. The result is
expected to be some isolated showers across mainly the Alleghany
Highlands. This weak disturbance is expected to exit as quickly as
it arrived, bringing a return to a dry forecast on Sunday for the
For Monday, while timing and impact vary from model to model, each
to some degree offer a solution of a cold front entering the area,
bringing isolated to scattered showers to at least western parts of
the region by Monday afternoon, with limited coverage continuing
into Monday evening.
The presence of the upper ridge and dry ground should help continue
to provide temperatures averaging some five to ten degrees above
normal. Have trended slightly lower on guidance for afternoon dew
points/relative humidity values given our regions dry pattern.
Confidence in the above portion of the forecast is moderate to
Long term /Tuesday through Thursday/...
as of 300 PM EDT Thursday...
High pressure should return by Tuesday. Tuesday night and
beyond poses more significant disagreement between the models.
The GFS shows another chance of rain due to a cold front
arriving on Wednesday, because it keeps an upper level trough
entering the northwest as an open wave while quickly crossing
across the country. The GFS also keeps the east under weak
troughiness aloft through Thursday. Meanwhile, the European model (ecmwf) cuts
off this trough into a closed upper level low that slowly tracks
over the southwest during the same timeframe. The European model (ecmwf) also
quickly builds a strong upper level ridge overhead with surface
high pressure throughout the east during Wednesday and Thursday.
Given the recent dry trend and the fact that other models such
as the Canadian are also drier than the GFS, the forecast was
steered toward the European model (ecmwf) solution. Thus, unseasonably warm and
dry conditions will likely continue well past the autumnal
equinox of 2019, which will occur Monday morning at 3:50 am EDT.
Aviation /09z Friday through Tuesday/...
as of 150 am EDT Friday...
High pressure building eastward from the Tennessee Valley and surface
high pressure building southwestward from the northeastern U.S.
Will remain in control of our weather through early next week.
The resultant continued dry advection, yet easterly flow, has
resulted in the low/mid clouds from upslope along the Blue Ridge
to gradually progress further southwest each day this week, with
erosion on the northeastern side. Thus, clouds tonight are
confined mainly to the northwest NC mountains and far southwest
Virginia. This May Creek back as far northeast as the I-77/I-81
Junction overnight, but will erode further to the southwest
during the day as dry air advection with northeast flow
continues. Therefore, expecting mainly VFR conditions through
the taf valid period.
Clouds that develop in the upslope areas will generally have
ceilings in the 030-050 range. In addition, with radiational
cooling enhanced tonight, expect valley and river fog
development, which is already quite evident along the upper New
River basin and Yadkin River basin, and could develop further
north late, such as in the Greenbrier River basin, although the
air overall is drier there.
A mid-level deck of cumulus or SC may develop in the western areas as
warm advection/moisture attempt to return east under the upper
ridge. Nonetheless, ceilings are expected to remain VFR
throughout the remainder of the taf valid period after 13z this
Winds generally favoring southwest west of the Blue Ridge to
southeast east of the Blue Ridge, but direction will be more
variable today than previous days as the surface ridge axis
slowly shifts southwestward over the forecast area. Speeds will
be mostly 7kts or less, except for a brief period of gusty southeast-
south-southeast winds around kblf and kjfz this morning.
/Confidence in forecast parameters/
ceilings - moderate to high,
visibility - moderate to high,
winds - moderate,
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. A weak cold front may bring
scattered MVFR showers on Tuesday, mainly to the western
mountains. However, the latest trends of the models hint at