Area Forecast Discussion

FXUS61 KRNK 180049
Area Forecast Discussion
National Weather Service Blacksburg VA
849 PM EDT Tue Sep 17 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. 
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 
early Wednesday. 
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 
by midnight. 
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 NW 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.
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 CONUS 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.
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 CONUS 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 
degrees again.
As of 645 PM EDT Tuesday...
A cold front will push southwest through the Blacksburg terminal
forecast for the rest of this evening into the overnight - 
possibly accompanied by some widely scattered showers. As the 
cool wedge deepens over the region, MVFR to IFR ceilings are 
expected to gradually spread southwest from around KLYH, 
reaching KROA, KDAN, KBCB, and possibly KBLF this evening.
Some IFR to even LIFR restrictions are possible later tonight, 
especially over/near higher terrain, and maximized along the 
eastern flanks of the Blue Ridge as low level winds slowly veer 
into more of an easterly upslope component. 
Push of cooler air should be sufficient by late Wednesday 
morning to allow for gradual lifting and clearing of skies as 
somewhat drier air begins to advect into the area with approach 
of surface high pressure; however, persistent upsloping of winds
into the NW NC mountains may maintain threat for at least MVFR 
restrictions to persist well into the afternoon hours with a
return of VFR elsewhere.
Confidence is moderate for ceilings and visibility through 
00Z/8 PM EDT on Wednesday.
.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.