Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 182008

Area Forecast Discussion
National Weather Service Blacksburg VA
308 PM EST Sat Feb 18 2017

An upper level disturbance will increase cloud cover tonight and
bring a period of rain showers, mainly concentrated in western
locations. The upper low moves east of the region early on
Sunday, leading to a couple days of mild and dry weather.
Temperatures are expected to remain above normal for the next 5
to 7 days. After tonight, the next chance for rain is not until
Tuesday night.


As of 307 PM EST Saturday...Visible satellite reveals mid-
level cloud cover has begun to overspread the central
Appalachians and Blue Ridge, while skies in the Piedmont remain
generally clear. Filtered sunshine through much of today has
resulted in temperatures well into the 60s to the low 70s for
many locations this afternoon, though is also accompanied by dry
low-levels (mid 20s to low 30s dewpoints common along with
large dry layer below 700 mb sampled well by 12z GSO/RNK RAOBs).
Increasing cloud shield is associated with pronounced moisture
plume seen on water vapor imagery on the eastern end of an upper
low centered over western TN. Regional composite radar mosaic
and upstream METARs show a south-to-north band of light to
occasionally moderate rain extending from near central KY
southward through GA into FL.

For Tonight: Clouds will continue to lower and thicken with time.
Band of rain should also advance northeastward, but it will be
running into aforementioned dry layer of air that will only slowly
saturate up. High-res guidance pivots this band northeastward
through midnight across the forecast area, though given how dry the
preceding air mass is, I`m not sure many areas east of the Mountain
Empire and southeast West Virginia will see much of any measurable
precip as the initial band pivots north. As the upper low pivots
into eastern KY/TN after midnight, nearly all higher res and coarser-
res global model guidance depict an enhancement in QPF/simulated
reflectivity along a roughly east-west 700-500 mb deformation axis
as the primary band of rain pivots back northwestward across
southeast WV, the Mountain Empire in VA and perhaps into the NC
mountains. As the rainband rotates NW, a dry slot should produce at
least partly to mostly cloudy skies for the Piedmont and perhaps as
far west as the foothills of the Blue Ridge. Thus, the best chance
for rain in far western sections where the highest PoPs are tonight
where QPF amounts are around one-tenth of a inch. Temperature
advection aloft is pretty much near neutral, but transitions back to
cold advection especially toward morning west of the Blue Ridge.
Think we`ll be looking at lows only falling into the 40s tonight.

For Sunday: Upper low opens up into an open wave and exits off
the Atlantic coast Sunday morning. While many areas will likely
start off mostly clear to partly cloudy, the leftover showers
and overcast in far western areas begins to improve as shortwave
ridging aloft builds back over top of the forecast area later
in the day. Due to "cold" advection (850 mb temps only falling
to around +4 to +6C) and steepening low-level lapse rates,
northwesterly winds should turn breezy. I`ve also lowered
dewpoints somewhat a bit more than raw and statistical guidance
would indicate during the afternoon hours given some dry air
mixing potential. Highs may be only a degree or two lower for
the Piedmont, foothills into the Roanoke Valley compared to
today - upper 60s to low 70s, but closer to the upper 50s to low
60s due to some cloud cover for areas along/west of the Blue
Ridge. While well above normal, projected forecast highs are
just below record highs for our long-term climate sites. See the
climate section for more details on these high temperature


As of 307 PM EST Saturday...

Surface high pressure over the Tennessee Valley will move east
Sunday night and Monday and will be offshore on Tuesday. Some
return flow around the high off the Atlantic but low pressure
over the northern Gulf of Mexico will limit return of Gulf
Moisture. 500MB ridge over the eastern United States until
Tuesday then a weak short wave in the northern stream will bring
a weak front through at the surface. GFS has this feature
moving east faster than some of the other guidance. Not out of
the question to have the probability of precipitation arrive in
the mountains late in the day Tuesday. Maximum temperatures on
Monday will be above normal. Based on enough sunshine and a dry
air mass have gone slightly above guidance.


As of 307 PM EST Saturday...

Best probability of precipitation from the northern stream short
wave will be on Tuesday night. 500 mb pattern becomes more amplified
on Friday and Saturday with a deepening trof moving across the
central United States and more spread showing up in the modest by
that time frame. Increasing southwest winds on Friday will increase
moisture and keep temperatures above normal ahead of the front.
Timing of the frontal passage across the Appalachians looks to be on
Saturday morning with precipitation along and ahead of the boundary.

Much colder air comes in behind the front for the end of the
weekend. The combination of a modest low level jet and strong cold
air advection will result in strong gusty winds behind the front,
possibly through Sunday morning.


As of 1240 PM EST Saturday...

VFR with gradually lowering/thickening mid-level cloudiness
through the rest of the afternoon. A continued lowering in
VFR cloudiness expected from southwest to northeast tonight
with a period of likely unrestricted rain showers advancing
northward through midnight.

Rain band then pivots back to the northwest after 06z,
affecting BLF and perhaps LWB with MVFR to IFR ceilings and MVFR
vis in potential areas of mist/light showers through 12z.
Improvement anticipated by mid morning for these two western
TAFs. Elsewhere, expect more breaks in OVC VFR from southwest to

West to southwest winds trend light westerly overnight
(variable at times at BLF). As upper low shifts to the southeast
tomorrow, northwesterly winds pick up to around 6-10 kts with
gusts to 22 kts at ROA.

Medium to high confidence in cigs/vis through 00z, then medium
to low confidence in cigs/vis 00-12z, medium confidence in
cigs/vis 12-18z. Medium to high confidence in winds.

Extended Aviation Discussion...

High pressure should bring VFR Sunday night through Tuesday.
MVFR rain showers are possible Tuesday night into Wednesday with
the next weak cool front crossing the region midweek.


As of 307 PM EST Saturday...

Record High Temperatures and Year of Occurrence

Feb 19
Blacksburg, VA..65 in 1994
Bluefield, WV...66 in 1994
Danville, VA....73 in 1981
Lynchburg, VA...76 in 1939
Roanoke, VA.....77 in 1939

Feb 20
Blacksburg, VA..68 in 1984
Bluefield, WV...67 in 1986
Danville, VA....74 in 1971
Lynchburg, VA...76 in 1930
Roanoke, VA.....75 in 1939




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