Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 181907

Area Forecast Discussion
National Weather Service Blacksburg VA
307 PM EDT Fri Aug 18 2017

A weak cold front will cross the mountains from the west this
evening before sliding southeast of the region overnight. The
front will remain over the Carolinas Saturday into Sunday before
dissipating. High pressure works in from the Ohio Valley during
the weekend, then overhead Monday resulting in drier weather
across the region.

As of 245 PM EDT Friday...

Surface cold front now just west of the mountains will slide
slowly east reaching the Blue Ridge late this evening before
exiting to the southeast overnight. Best instability remains
across eastern sections where the flow has backed a bit and
dewpoints remain higher. Elsewhere rather well mixed westerly
flow has tended to limit convergence/development with more
widely scattered coverage expected through late afternoon over
the mountains. Best coverage including isolated severe
potential appears across the piedmont espcly northeast sections
where slightly stronger winds aloft under a passing vort tail may
enhance bands a bit. Thus keeping low likely to chance pops
mainly east into early evening with most showers out of the
southeast around midnight as seen off the latest HRRR. Weak
cool advection along with slightly lower dewpoint air will
follow the front in from the northwest by morning allowing lows
to dip well into the 60s mountains, while lingering closer to 70
southeast. Fog likely limited to the deeper valleys and eastern
sections that possibly see more showers through early evening.

Front sags into the Carolinas before stalling Saturday in
advance of the next shortwave trough that will approach the
region during the afternoon. This should basically put the area
in between deeper moisture to the southeast, and lift beneath
the cold pool with the shortwave crossing the Ohio Valley.
Latest guidance supports an overall dry scenario with dry air
aloft off forecast soundings, and low level northwest flow
beneath rather meager instability Saturday afternoon. Cant
totally rule out a sprinkle or shower reaching the far northwest
counties late in the day but not enough to include a pop mention
for now. Otherwise partly to mostly sunny and a bit less humid
with highs still 80s to near 90 east, as weak downslope offsets
slight cooling aloft.

As of 240 PM EDT Friday...

An upper level trough will pivot over the region Saturday night. Not
much cool air is behind this trough, therefore temperatures Sunday
will warm back above normal with mid 80s west to near 90F east. The
only notable change will be slightly drier air (dew points in the
low 60s west to upper 60s east) with a westerly breeze.

Dry high pressure with increasing heights aloft will continue to
keep most of the region dry through Monday night. Some low level
moisture may creep into the area from the south Monday. This
moisture and upslope flow may generate a few storms across the North
Carolina High Country late in the afternoon, fading in the evening.
Under mostly sunny skies, temperatures will warm a little above
normal Monday with 80s west to lower 90s east.

As of 255 PM EDT Friday...

A western Atlantic upper level ridge will track westward over the
Gulf states through early next week. This ridge will push
temperatures 5F to 10F warmer than normal. With an increase in heat
and humidity, scattered showers and thunderstorms are expected
Tuesday afternoon across the mountains and then moving and fading
across the foothills in the evening.

On Wednesday, models are wanting to bring a cold front south into
the Gulf states, pushing the upper level ridge into the Gulf.
Considering climatology for the time of year, models look to be over
zealous with moving this front into a hot and humid air mass. I
would not be surprised to see a slowing trend with the frontal
passage being more towards Thursday. For now, kept timing close to
guidance while keeping PoPs low. Until the front clears the area,
temperatures will stay 5F to 10F warmer normal. Following the front,
temperatures will be 5F or cooler than normal.

As of 1240 PM EDT Friday...

Most showers with isolated storms continue to linger across the
far western areas, mainly just west of KBLF-KLWB early this
afternoon where seeing pockets of MVFR cigs.

Latest models continue to develop showers/storms across and
espcly east of the mountains this afternoon into early evening
ahead of the upstream cold front. Appears most of this will
initially be scattered in nature over the west before perhaps
organizing into more bands/clusters along/east of the Blue Ridge
by mid afternoon. Shra/tsra should then push into eastern
sections by early this evening prior to exiting shortly after
00z/8 PM if not sooner. Thus running with mostly VCTS/VCSH at
all locations with tempo groups for lower MVFR conditions in
showers across the west, and deeper convection east where IFR
vsbys may occur.

Appears fog will be stuck in the valleys and eastern locations
that see heavier rainfall this afternoon although uncertain
given gradual advection of dry air late. For now included sub-
VFR in fog at KLWB/KBCB/KLYH late tonight while leaving out
elsewhere. Some stratus also possible with the fog and perhaps
even at KBLF with weak upslope flow.

Outside of convection, winds will be mainly west to southwest
at 5-15 kts ahead of the front this afternoon before turning
light/variable overnight.

Weak high pressure will build into the area Saturday bringing
VFR conditions under northwest winds at 5-10 kts.

Extended Aviation Discussion...

Mainly VFR conditions for the second half of weekend under weak
high pressure. Scattered thunderstorms and periods of MVFR
conditions return next week, with late night/early morning fog
possible almost any day. Better potential for sub-VFR will come
Wednesday when more widespread showers and storms arrive with
the next cold front.


As of 400 AM EDT Friday Aug 18th...

KFCX doppler radar expected to be down for the rest of the
month due to a failing bull gear. Technicians are working on the
radar, through next week and the radar will most likely stay
down completely as the repairs are being made.




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