Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 170346

Area Forecast Discussion
National Weather Service Blacksburg VA
1146 PM EDT Wed Aug 16 2017

Though the majority of the forecast area is dry tonight, a weak
upper-level disturbance should spark scattered showers and embedded
thunderstorms mainly south and west of Interstate 77. An even more
humid air mass settles in for Thursday, with a better chance for
showers and thunderstorms during the afternoon. A cold front
moving into the region on Friday sparks another round of showers
and storms followed by clearing along with drier conditions
for the weekend.


As of 1140 PM EDT Wednesday...

Just a minor update to bring T/Td readings in line with current
trends and adjust pops according to latest trends. Since HRRR
has a fairly good handle on the trend of showers moving into
southwest VA at this hour, will follow its trend through the
night. Activity in Smyth and Grayson counties should work its
way through the CWA near the I-81 corridor over the next few
hours, then gradually dissipate across the Piedmont by 10Z. New
activity will likely begin developing/moving into western areas
toward daybreak.

Thursday looks like an active day in terms of convection as a
short wave interacts with a very warm and humid air mass. A good
theta-e ridge evident through the heart of the CWA Thursday
afternoon. Would expect fairly widespread convection Thu.
Instability and shear values are not overly impressive, but an
isolated strong storm certainly could not be ruled out. Heavy
rainfall will likely be the more significant problem.

As of 825 PM EDT Wednesday...

Isolated showers still feeding off lingering surfaced based
instability early this evening across the Mountain Empire of VA
through the piedmont of NC. Expect generally dry conditions
overnight but some isolated activity may be wafting around
mainly east of the Blue Ridge.

Previous discussion...

HRRR and 3-km NAM solutions display good consistency in showing
ongoing shower activity trudging slowly to the east. This
general area roughly bounded along and southwest of a line from
Bluefield to Hillsville VA to Danbury NC. Otherwise, the rest
of the forecast area stays dry through the evening. Some
question if showers can linger after midnight in weak
convergence field in the NC Piedmont and Southside as modeled by
the GFS and the 3-km NAM. I am dubious as mid- level feature
should have advanced far enough east by that point to allow for
weak subsidence. After midnight, it appears that we see a
considerable increase in moisture levels and clouds amid light
southwesterly low to mid- level flow. GFS LAMP guidance
dewpoints at MOS forecast points all trend upward overnight, so
if you thought today was on the humid side, more is on the way.
I`ve indicated some patchy fog in for the overnight hours in the
Greenbrier and New River Valley, but less cooling due to clouds
cast doubt much will develop. It`s currently doubtful we see
as much fog as experienced this morning. A mild and rather humid
evening with lows from the mid upper 60s to the lower to mid
70s, mildest further southeast.

For Thursday/Thursday Night: Looking at quite a muggy air mass
areawide. Outside of the higher elevations along the Blue Ridge,
upper 60s to mid 70s dewpoints should be rather common Thursday.
This moisture extends at depth as well, reflected in PWAT values
progged from 1.9-2.2". Though we should likely have a good amount of
cloudiness, BUFKIT soundings show an air mass that is weakly capped.
So it won`t take much heating to generate enough instability to
spark at least scattered showers and thunderstorms. While 500 mb
height tendencies are neutral early in the day, they start to fall
by the afternoon and should be enough lift, augmented by a pre-
frontal or lee trough. Indicated high chance to lower Likely PoPs
for showers and thunderstorms. I wouldn`t necessarily discount a
couple of SPSable type storms tomorrow, but the deeper layer shear
and tall, skinny CAPE profiles summing to values 1000-1500 J/kg
should keep severe threat mitigated. With Corfidi vector speeds
ranging from 5 to 15 kts indicating slow cell motion and potential
for nearby cell growth on old outflow, along with the aforementioned
high PWAT air could have some potentially heavy downpours. Showed
rainfall amounts from a quarter inch up to localized 0.50-0.70"
amounts along the lee of the Blue Ridge south of Roanoke. Confidence
wasn`t high enough to address in the HWO or to add heavy rain
wording, though WPC`s overnight Day-2 outlook does have our Blue
Ridge counties in a Marginal Risk for excessive rain. A general
decrease in coverage and intensity of showers and storms should be
expected with sunset and generally ending near midnight, though may
have a good deal of cloudiness ahead of the primary cold front. Kept
highs in the mid to upper 80s with at least partly/mostly cloudy
skies, with lows still on the muggy side in the upper 60s to mid


As of 230 PM EDT Wednesday...

Weak shortwave trough should lift by to the north on Friday allowing
a surface cold front to slowly pivot east toward the region by late
in the day. Expect some degree of residual showers preceding the
boundary across the west early in the day that could jump into the
eastern lee trough during the afternoon where deeper convection
still appears possible. Most guidance still not overly impressive
with lift ahead of the front, given most upper support shearing out
to the north with the passing weak 500 mb trough. This while seeing
the boundary layer flow turn more westerly espcly mountains behind
the pre-frontal axis early on. However progged instability still
quite strong east of the mountains where models suggest a narrow
band of shortwave energy crossing into the piedmont late. Late day
timing could allow for better convergence from the Blue Ridge east
espcly if the low level flow backs southwest, and progged very high
850 mb theta ridging materializes. Therefore keeping chance pops
with highest out east. Highs again mostly 80-85 mountains to lower
90s east given downslope but also convection dependent.

Surface cold front should cross the region Friday night into early
Saturday, booted along by a stronger upstream shortwave trough that
will pass just north Saturday night. Model consensus shows some
lingering convection along the front mainly southeast early Friday
night before deeper moisture gets shunted to the east early
Saturday. However just how far southeast dry advection can get
Saturday remains iffy espcly ahead of the strong upstream shortwave.
Latest trends suggest perhaps just enough low level moisture with
heating to still spark an isolated shower/storm mainly southeast
sections later Saturday so left in a mention. Otherwise should be a
bit drier Saturday afternoon with dewpoints dropping off espcly west,
and only slight evening pops far south Saturday night for now. Still
quite warm Saturday with highs low/mid 80s to near 90 southeast with
somewhat cooler lows in the 60s overnight behind the front.


As of 130 PM EDT Wednesday...

Exiting upper trough by late in the weekend will evolve into strong
ridging across the region for early next week as Bermuda high
pressure links with building heights over the southeast states.
Appears this in combination with weak surface high pressure ridging
in from the north and lack of upper support, will act to keep a lid
on most organized convection for Sunday into Monday. However given
close proximity of at least low level moisture near the residual
front over far southern sections, will need to leave in an isolated
diurnal convective mention espcly southern Blue Ridge for now.
Otherwise will be leaving things mostly dry with more clouds
far south and less northern half into Monday.

Ridging will begin to flatten Tuesday in advance of the next digging
500 mb trough that looks to push another cold front toward the
region by midweek. Return moisture ahead of the boundary and less
cap should support a scattering of showers/storms Tuesday afternoon
especially mountains. Expect a bit more organized convection ahead
of the front and under cooling aloft Wednesday when will have medium
to high chance pops. Better ridging and warming 850 mb temps to
around +22C suggests a rather warm/hot period with highs mostly 80s,
except around 90 or warmer piedmont, before cooling slightly on Day7
per more showers around.


As of 810 PM EDT Wednesday...

Upper short wave is pulling off to the east but surface frontal
boundary continues to linger over the region with some isolated
showers/rumble of thunder. Believe there will be some isolated
showers stewing around through the overnight and expect low/mid
clouds to be on the decrease, but there is a good amount of
uncertainty in how much cloud cover will remain overnight. While
this will limit radiational cooling, ample moisture in the
boundary layer will allow for some fog formation late. Expect
LIFR cig/vsby to set in at KLWB where cloud coverage is already
scant, and a tempo to MVFR conditions elsewhere with KROA
expected to remain VFR. However, confidence in fog/stratus
formation is medium at best as it is tied to extent of higher
cloud cover. More upper clouds will result in less fog.

Better forcing and increasing moisture along the frontal
boundary tomorrow will yield greater coverage of
showers/thunderstorms by Thursday afternoon.

Winds will generally be light through the period.

Extended Aviation Discussion...

Expect a continued increase in shower/thunderstorm coverage
and strength, lasting until 03z Friday. Outside of
thunderstorms look for VFR conditions. Overnight fog is a
possibility in the river valleys and in areas that do see rain,
though cloud cover/convective debris may limit its spatial

Another round of scattered afternoon showers and thunderstorms
Friday associated with the primary cold front, though VFR
prevails outside of showers or storms. Dry advection and light
to moderate west to northwest winds behind the front Friday
night could keep overnight fog coverage limited, but a potential
sub-VFR ceiling is possible at Lewisburg and Bluefield.

Mainly VFR conditions for the weekend under weak high pressure.


As of 755 PM EDT Saturday Aug 12th...

KFCX doppler radar will continue to be down for the rest of the
month due to a failing bull gear. It will only be operational
during this time frame for brief periods if an imminent
significant widespread severe weather and/or hydrologic event
occurs. Operating the system for any amount of time until the
bull gear is replaced risks a catastrophic failure that would
further extend system downtime. From the 17th through the 27th
the radar will most likely be down completely as the repairs
are expected to be made during this period.




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