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000
FXUS61 KRNK 171931
AFDRNK

Area Forecast Discussion
National Weather Service Blacksburg VA
331 PM EDT Thu Aug 17 2017

.SYNOPSIS...
Warm, muggy conditions to continue tonight and into part of Friday
ahead of a cold front. Scattered to numerous showers with scattered
thunderstorms continue late this afternoon into early this evening.
A few storms may produce localized downpours along with gusty winds.
Another opportunity for scattered showers and thunderstorms, a
couple of which may be strong east of the Blue Ridge. Still
rather warm into the weekend, but with slightly less humidity.

&&

.NEAR TERM /THROUGH FRIDAY/...
As of 331 PM EDT Thursday...

Very warm and humid out there this afternoon with feed of dewpoints
in the upper 60s to mid 70s in place across the Appalachians and
VA/NC Piedmont. With mostly sunny skies, atmosphere has destabilized
with surface-based CAPEs anywhere from 2000 - 3000 J/kg. Regional
radar shows widely scattered showers and thunderstorms from the Blue
Ridge foothills to the Mtn Empire into southeast WV. Storms become
more numerous in association with leading edge of 500 mb height
falls and focused by a pre-frontal trough which extends from
southeast OH through central WV to just northwest of Knoxville.

Rest of this aftn/tonight: Shower and thunderstorm coverage should
be primarily focused from Wilkesboro to Rocky Mount to Lynchburg
westward, due to southeasterly orographic lift along the Blue Ridge
and the pre-frontal trough. Isolated thunderstorms can`t be ruled
out in the Southside and the upper NC Piedmont, where air mass is
more unstable, mainly from activity drifting off the southern Blue
Ridge. With precipitable water values from near 2" to 2.3", primary
concern is for localized heavy downpours and potential short-fused
hydro concerns in slow-moving cells. Dual-pol radar rainfall mosaic
estimates small footprints of near 2" rainfall (1-1.5" hourly rate)
from slow convection over eastern Watauga County. Not everywhere
will see rates this high but the potential is there. Severe threat
is minimal given poor vertical wind profile/shear and think mainly
sub-severe/SPSable cells with gusty downdrafts should predominate.
Given pool of better instability (MUCAPEs as much as 3200 J/kg) from
Patrick-Pittsylvania counties southward to the NC Triad, that could
be one area to watch for possible marginal severe. By 7-9 pm,
guidance shows a general west-east decrease in shower coverage with
waning instability and passage of pre-frontal trough. Leftover
showers and storms could linger a couple hours after that point into
the NC Piedmont/Southside VA. Should see brief clearing from the
Blue Ridge west, and given background humid air mass that could lead
to a good coverage of patchy fog. Eastern and southern counties may
struggle to fully shake free of clouds overnight with patchy fog
mainly in cloud breaks here, while enough southwest wind flow across
the Mtn Empire and southeast WV to limit fog coverage. Warm, muggy
lows from the upper 60s to mid 70s.

Friday: Should have a good amount of early-day cloud cover in place
east of the Blue Ridge, ahead of the primary cold front which 12z
NAM/GFS progs to be near our western doorstep by sunrise. This is
maybe a little sooner than recent guidance had indicated. Question
for Friday is on convective coverage and strength given the earlier
frontal timing and potential for morning cloudiness to slow
destabilization in the east. Should have more limited chances for
thunderstorms west of I-81 perhaps as soon as mid to late morning,
but with an increasing trend as the front makes its eastward
advance. The better chance for strong to localized severe storms is
to the northeast in northern VA where deeper shear and strongest
lift is located, but a corridor of stronger storms could exist from
Amherst County to Surry County eastward. Heavy downpours certainly
possible along with localized gusty winds in strongest cells. Front
should be clearing the forecast area by mid to late afternoon, with
notably drier air filtering in on northwest flow. Until then, expect
another warm and muggy day in the Piedmont (heat index values 95-
98). Earlier timing of frontal passage leads to lower highs in the
upper 70s/lower 80s from I-81 westward, with highs mid/upper 80s
east, though a few potential low 90s in the central VA/upper NC
Piedmont.

&&

.SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY NIGHT/...
As of 331 PM EDT Thursday...

Cold front will cross east of the area Friday night taking most showers
out of the region by late evening if not sooner as evidenced by progged
lowering 850 mb moisture/theta-e early on. Thus only leaving in some
low chance pops east of the Blue Ridge during the evening before
clearing overnight. Also may see a few upslope induced showers far west
behind the front but should be quite isolated with only a brief mention
needed. Lows mostly 60s except lingering muggy low 70s southeast.

Boundary will briefly stall just to the southeast Saturday
ahead of a rather strong trailing upstream shortwave trough that will
pivot across Saturday afternoon. This may cause some backing of the
flow including a bit of a northward jog in moisture before better
subsidence arrives behind this passing wave Saturday night. Given
rather dry forecast soundings and weak northwest flow, appears can
leave out pop mention for now, with mainly just scattered cumulus
around.

Upper flow will then flatten under slowly building heights on Sunday
with surface high pressure building in from the northwest. Forecast
profiles again indicate quite a bit of dry air aloft that should again
limit shower chances. However with moisture nearby to the south and the
low level flow turning more southeast in the afternoon, cant totally
rule out isolated convection across the far south or southwest where
guidance shows some return instability into Sunday evening. Otherwise
partly to mostly sunny at this point.

Will remain quite warm/hot through the weekend given little change in
temps aloft behind the front and weak downslope flow until late Sunday.
Should be a little less humid Saturday before becoming muggy again
later Sunday with highs both days in the 80s mountains to lower 90s
piedmont.


&&

.LONG TERM /MONDAY THROUGH THURSDAY/...
As of 331 PM EDT Thursday...

Flat upper ridging will remain in place to start next week as surface
high pressure shifts from the Mid Atlantic on Monday to off the
southeast coast by midweek. This along with warming aloft within a west
to southwest trajectory should keep things quite warm and increasingly
humid from later Monday into Wednesday. Convective chances early on
appear iffy with only faint forcing later Monday when higher PWATS
begin to advect back north, followed by gradual height falls on Tuesday
when deeper moisture will be somewhat back in place. Most guidance
showing only isolated showers/storms mainly south/east late Monday with
perhaps a scattering of storms Tuesday espcly where aided by
orographics. Highs likely around 90 east Monday/Tuesday and 80s
elsewhere except possibly a little lower Tuesday far west pending
clouds/showers.

500 mb pattern will once again become more amplified as another upper
low crossing north of the Great Lakes helps dig out an even deeper
eastern trough by the end of the period. This feature will also propel
a strong cold front toward the area Wednesday with the boundary likely
just south of the region during Thursday. Lead pre-frontal trough axis
likely to spark better coverage of showers/storms Wednesday
afternoon/evening, followed by cooler/drier air during Thursday.
Therefore will include higher chance to low likely pops Wednesday and
then mostly residual showers southern sections Thursday pending timing
of the frontal exodus by then. Should see some cooling due to clouds
including more showers Wednesday, then cool advection behind the front
resulting in highs only 70s mountains to mid 80s east Thursday.

&&

.AVIATION /20Z THURSDAY THROUGH TUESDAY/...
As of 158 PM EDT Thursday...

Generally VFR conditions, though noting increase in shower and
thunderstorm coverage along the southern Blue Ridge northward
to just east of Bluefield. This is in association with a pre-
frontal through. Given humid airmass, brief and localized IFR
to LIFR visibility in heavy downpours can be expected, along
with localized gusty winds and lightning in thunderstorms.
Indicated VCTS for now and will be amending TAFs with TEMPOs as
no threat presently imminent. Best chance from Blacksburg,
Lewisburg, Roanoke through late aftn and perhaps Lynchburg into
early evening. Outside of storms, winds should be southwest to
west- southwest 3-7 kts.

For tonight, lingering showers or storms in the central Piedmont
should dissipate. May be a brief period of clearing between,
and with weakening wind field, could see a large coverage of
patchy fog. Indicated IFR vis patchy fog at Blacksburg and
Roanoke where cloud clearing could be greatest, with MVFR vis
patchy fog at Danville and Lynchburg where there could be a
greater coverage of clouds. It`s less certain on fog coverage in
the WV TAFs, where southwesterly winds may remain up enough
ahead of cold front to mitigate development.

Into Friday, should have mix of flight categories from VFR to
potentially leftover IFR cigs/vis. Best chance MVFR/IFR cigs
in the eastern Piedmont/Danville. These should gradually erode
ahead of a cold front. Renewed thunderstorm activity ahead of
the beginning as early as late morning/early afternoon, though
better shot exists after 18z Friday. Indicated VCTS to indicate
potential storms around, though VCSH indicated at Lewisburg and
Bluefield given earlier frontal timing and less opportunity for
destabilization.

Extended Aviation Discussion...

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.

&&

.EQUIPMENT...
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.

&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...None.
NC...None.
WV...None.

&&

$$
SYNOPSIS...AL
NEAR TERM...AL
SHORT TERM...JH
LONG TERM...JH
AVIATION...AL/WP
EQUIPMENT...RAB/WERT