Scientific Forecaster Discussion
fxus61 krnk 180609
Area forecast discussion
National Weather Service Blacksburg Virginia
209 am EDT Wed Sep 18 2019
a backdoor cold front will continue to push south of the region
overnight. Cooler high pressure will follow the front and wedge
slowly south into the area for the rest of the week. Other than
some widely scattered showers or patchy light rain overnight,
another extended period of dry weather with no precipitation is
expected through at least early next week.
Near term /through today/...
as of 845 PM EDT Tuesday...
Backdoor front has slipped just south of the area this evening
with low clouds filling in from northeast to southwest. However
bases still remain more of the strato-cu variety given mixed out
surface conditions from earlier today which has limited the
degree of saturation and subsequent stratus coverage. In
addition, evening soundings show a strong mid level cap aided by
very dry air above within deep northwest trajectory. This has
been able to limit any convection to just spotty
sprinkles/showers despite residual lingering instability over
the southwest sections. Most models have been very overdone in
showing bands/clusters of showers espcly mountains that have not
materialized this evening. Thus have delayed the onset of low
clouds along the I-77 corridor per current satellite trends,
while reducing pops to mainly isolated South-West and low
chance NC mountains that may actually occur late tonight when
more upslope develops. Also already seeing some dry air across
the far northeast perimeter which could bring some clearing late
to eastern sections. Otrw bumped up lows a little since still
rather warm under clouds and lack of a deeper cool wedge until
Previous discussion as of 300 PM EDT Tuesday...
Cool front will continue to drift southwest through the
Blacksburg forecast area for the rest of the afternoon into
early tonight - likely passing off to the south of the area
Although some of the higher resolution models are still
implying that low level forcing and thermodynamic environment
will be sufficient to break weak capping inversion that exists
around 8k feet agl, lower troposphere is quite dry, and frontal
boundary is quite shallow. As a result, any precipitation
threat for the rest of the afternoon into tonight will remain
limited, with only scant rainfall such that little to no
reprieve is expected from the incipient drought conditions that
have begin to develop over the area.
As cool air becomes better established overnight, expect to see
bank of lower clouds work west and south to encompass much of
the forecast area - likely to linger well into Wednesday
morning before slowly mixing out as somewhat drier air arrives
from the northeast as surface ridge begins to wedge more
aggressively into the area. Still can not entirely rule out a
stray/lingering shower across southwestern areas, most
particularly across the high country of northwest NC, where arrival of
drier air and easterly upsloping will be maximized.
With slow erosion of clouds, northeast to easterly onshore
flow, and continued weak cool advection, daytime highs on
Wednesday should be about 10f cooler than that of today, and
close to if not a couple of degrees below seasonal normals - in
marked contrast to temperatures averaging 10f above normal the
past several days.
Short term /tonight through Friday night/...
as of 300 PM EDT Tuesday...
Rather tranquil weather in store for the second half of the week as
a broad and highly amplified upper ridge of high pressure remains
anchored across the eastern Continental U.S. Flanked by troughs over the pacnw
and over the western Atlantic where Humberto will continue its
sojourn into the open ocean. The surface pattern will be dominated
by high pressure which will steadily build from New England and into
the mid-Atlantic region, providing a multiday period of dry and
stable air under mostly clear skies. Temperatures will favor mid-
September normals, the dry air and clear skies supporting a wide
diurnal range with lows in the 40s to around 50 at night and
afternoon highs in the 70s to lower 80s. Chances for measurable
rainfall during this period are essentially nil.
Long term /Saturday through Tuesday/...
as of 300 PM EDT Tuesday...
Overall model guidance is in agreement for the upper level ridge of
high pressure to remain in place over the southeast Continental U.S. Through
the upcoming weekend. The actual surface high will slowly move/drift
east to off the mid-Atlantic coast by Sunday - maintaining dry
conditions with slowly moderating temperatures as winds become
southwesterly. A cold front is forecast to move across the Great
Lakes and Ohio Valley early next week, crossing our forecast area
Monday. At the present time it appears little if any moisture
associated with the front will make it east of the mountains, shower
chances mainly confined to areas west of the Blue Ridge.
Temperatures this weekend, and ahead of the front Monday, are
expected to climb back above normal, with afternoon highs well into
the 80s Saturday through Monday, and lows creeping back into the 50s
to lower 60s. Depending on how much compressional warming we get
Sunday and Monday, due to the increasing southwest-west wind aloft,
it possible some locations Lee of the Blue Ridge could test 90
Aviation /06z Wednesday through Sunday/...
as of 200 am EDT Wednesday...
Another basically dry wedge front moving southwest through the
area tonight. The front during the evening has surged
southwestward as far as the Georgia/SC border and up against the
eastern Continental Divide. Light easterly maritime flow well
established across much of the County Warning Area at this hour. In addition,
low clouds, mostly in the MVFR range of 015-030 noted along and
behind the front for a couple hundred miles, or roughly back
toward lyh and Dan, where the ceilings become more broken. Some
very light sprinkles noted across the northwest NC mountains and
foothills, but otherwise no precipitation detected.
Little change expected through the taf valid period. The MVFR
ceilings are expected to hold steady generally along/west of the
Blue Ridge through early afternoon, then begin to break up from
the east as well as erode on the far western edge by afternoon.
Model soundings are rather reluctant to break up the low cloud
ceilings until mid or late afternoon. In addition, the models
want to bring the low clouds back in after evening. Not as
confident about this later point, except in the west/southwest
part of the cwa, say roughly ukf-mwk-bcb and westward. So, there
should be at least a few hours, especially across the Piedmont
of VFR conditions during the evening and early overnight hours.
Fog, other than cloud induced fog at locations with higher
elevations (e.G., Bcb and blf) is not expected, but some MVFR
visibilities in fog at these higher elevations can be expected.
Winds will be northeast to east through the period at speeds of
/Confidence in forecast parameters/
ceilings - moderate,
visibilities - moderate to high,
winds - moderate to high,
thunderstorm threat - none.
Extended aviation discussion...
Continued cool upsloping of easterly winds may be sufficient to
allow redevelopment of MVFR to possibly IFR restrictions to
ceilings on Wednesday night - especially across the eastern
flanks of the Blue Ridge. Otherwise, expecting a relatively
lengthy period of VFR weather with no flight restrictions,
outside of late night/early morning valley fog, across the
entire area for the rest of the extended period as a large ridge
of surface high pressure builds southwest into/over the area -
and then drifts east to off the mid- Atlantic coast - allowing
low level winds to veer back into the southwest and scour out
any lingering vestige of the former cool wedge.
Confidence is moderate for ceilings and visibility Wednesday
night through Sunday.