Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 171359

Area Forecast Discussion
National Weather Service Blacksburg VA
959 AM EDT Thu Aug 17 2017

Highly moist airmass in place and residual surface trough will
be the trigger for showers and storms today into tonight. 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 959 AM EDT Thursday...Mostly sunny skies presently exist
across the forecast area in what is a rather soupy, humid air
mass (dewpoints in the upper 60s to middle 70s). Made some
downward reductions to Sky cover forecast to reflect these
conditions, which should last through much of the rest of the
morning. Looking at area RAOBs reveals precipitable water values
in the 1.5 to 1.8 inch range, a little bit lower than models
were progging, but it`s expected that these should rise through
the afternoon by almost another half-inch. Air mass is weakly
capped (surface-based CIN is < 100 J/kg), and continued
insolation should foster deepening cumulus. High-resolution
guidance consistency has been rather poor, particularly on
timing, though where they do agree area-wise is on a band of
showers and thunderstorms in association with a subtle
convergence band over eastern KY/eastern TN moving northeastward
toward the western Appalachians/western side of the Blue Ridge.
This also coincides with leading edge of better mid-level
height falls from distant upper-level disturbance. Timing on
this - as mentioned - is rather broad, between 16-22z/noon to
6pm. Couldn`t rule out an isolated thunderstorm further east
into the Piedmont. Though a few stronger/SPSable type
thunderstorms are envisioned with better prospects for stronger
convective instability, localized heavy rainers and smaller
footprints of heavy rain should be the primary hazard with
thunderstorms. Tried to better shape PoPs this way (i.e. high
chance/low likely further west, lower chance/scattered east) for
the afternoon but many locations stand to be dry for at least
the morning hours.

Previous near-term discussion issued at 301 AM follows...

Showers continue to linger and drift slowly east from the Mountain
Empire into the NC piedmont, but activity is isolated, and radar
trends showing weakening. This is associated with higher theta-e
values and moisture convergence. High-res models showing showers
arriving/developing through mid morning along a LYH/DAN corridor,
while should stay dry elsewhere, though a few shower could start
popping over the mountains by noon. Fog will be patchier than
yesterday as more clouds around, some dense fog possible though
given higher moisture content in the air.

Today, 5h ridge axis stays overhead into early afternoon, then
shifts east toward evening. Underneath, the flow will stay out of
the southwest, with PWATS around 1.5 to 2.4 inches, from northwest to
southeast. An unstable airmass and some low level theta-e ridging
and convergence should act to fire up showers and storms this
afternoon, though  upper level support is limited but some difluence
by late afternoon may increase lift and coverage. Severe threat is
limited but cannot rule out an isolated strong storm, as SBCAPES
approach 2000 J/KG, though GFS showing computed CAPES of 3-5k, seems

Models are not agreeing on which area has the best shot of storms,
varying from GFS in the mountains, NAM piedmont, and the ECMWF area
wide. High-res models this afternoon favor development in the west,
then more coverage along/east of the Blue Ridge by 21z. At the moment
based on upper support and front to the north, a higher chance
exists from the NC and far SW Va mountains, northeast toward
Lynchburg/Buckingham, with smaller more widely scattered threat over
the NC piedmont. Should be less sunshine than Wednesday but still
sunshine will be out at times to heat the airmass into the 80s. A
few locations east and south of Danville and Appomattox could reach
around 90F. With the dewpoints in the lower 70s, anticipate heat
indices upper 90s.

Tonight, upper trough axis remains to our west, and expect lower
threat of showers/storms later in the evening but upstream vorts in
southwest flow will bring another shot of showers/storms toward
WV/far SW VA by dawn Friday. Will not stay completely dry overnight
as weak sfc trough/moisture convergence also lingers in the
piedmont, but coverage should be widely scattered.

Temperatures staying muggy with lows in the mid to upper 60s
mountains, to lower 70s east.


As of 300 AM EDT Thursday...

Upper low over Great Lakes Friday morning will rotate northeast
Friday afternoon through Saturday night. Weak shortwave trough
travel to our north Friday allowing a surface cold front to slowly
travel east toward the region. Residual showers preceding the
boundary across the west early Friday should jump into the eastern
lee trough during the afternoon where deeper convection appears

Progged instability are still quite strong east of the mountains
where models suggest a narrow band of shortwave energy crossing into
the piedmont late Friday. Late day timing could allow for better
convergence, heating and instability from the Blue Ridge east
especially if the low level flow backs southwest, and progged very
high 850 mb theta ridging materializes. SPC in the day two
convective outlook has placed our area in general thunderstorms,
while a marginal risk is to our northeast where surface-3km lapse
rates could exceed 7 C/km. We can not completely rule out a strong
to severe thunderstorm with healthy cape values. High temperatures
Friday will range from the upper 70s in the mountains to the lower
90s in the Piedmont. Temperatures will be highly dependent of
downslope flow, clouds cover and convection.

Surface cold front will push east into the Atlantic ocean Friday
night as the upper trough digs into the Ohio Valley. Low
temperatures Friday night will vary from around 60 degrees in the
mountains to the lower 70s in the piedmont. Shortwave trough in the
Ohio Valley will move east Saturday and will pass just north
Saturday night. How far east the push of drier air goes Saturday
remains in question. There may be enough low level moisture with
heating to trigger an isolated shower or thunderstorm in southeast
portion of the forecast area. High temperatures Saturday will
generally range from the mid 70s in the mountains to around 90
degrees in the piedmont.

High pressure will build south across our area Saturday night.
Low temperatures Saturday night will generally be in the 60s.


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 715 AM EDT Thursday...

Any fog will be gone by 13z with VFR conditions through most of
the day. May see some MVFR at BLF this morning though.

High-res models continue to show showers/storms starting in WV
late this morning then shifting east to the piedmont in the
afternoon. Have VCTS at all sites for now. Best coverage should
be in the LWB-LYH corridor but models still disagreeing on this
so kept any predominant wx out of the tafs.

Showers/storms may linger into late evening in the piedmont,
but will take VCTS after 00z.

Added IFR fog at LWB/BCB late tonight.

Winds will generally be light through the period mainly out of
the south to southwest.

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.


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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