Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 161327
AFDRNK

Area Forecast Discussion
National Weather Service Blacksburg VA
927 AM EDT Wed Aug 16 2017

.SYNOPSIS...
Weak high pressure over our area will be sandwiched between a
front over the mid-Atlantic and a trough off the southeast
coast. This pattern stays the same into Thursday, before a cold
front shifts from the western Ohio Valley into our area Friday.
This cold front will slide into the Carolinas during the
weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 926 AM EDT Wednesday...Visible imagery and webcams
continue to show an extensive area of fog across most of the
western third of the forecast area, from the western
Applachians, Blue Ridge and the NC High Country. Visibilities at
times have as low as under a quarter mile and even into mid-
morning we are still seeing continued instances of quarter-mile
or less visibility. Since the fog should only last another
couple of hours with the sun helping to mix the fog out, and
with the morning commute over, didn`t see a need to hoist a
dense fog advisory with the mid-morning update. To raise
awareness I did opt to extend the Special Weather Statement to
15z/11 AM. Those traveling especially in fog prone areas along
interstates 81, 64 and 77 should make sure to take it slow and
not use high beams. It`s expected that by 11 am the fog should
begin to burn off with improving conditions. Also did try to
somewhat crudely account for cooler hourly temperatures in areas
blanketed by fog during this period, but otherwise no changes
to temps.

For the afternoon, am keeping an eye on the mesolow/"swirl"
noted in satellite imagery over eastern KY associated with a
relative weakness in the 500 mb height field. This feature in
conjunction with daytime heating should encourage scattered
thunderstorm development across far southwest VA (southwest of
I-77) into Ashe, Alleghany and Watauga Counties in NC.
Otherwise, most areas stay dry today per recent guidance.

Previous near-term discussion issued at 236 AM follows...

Overall airmass does not change much this period. Will see deeper
moisture/higher pwats to our south while a frontal boundary stays
draped across the mid-Atlantic into the midwest, with Hurricane Gert
veering out to sea.

This morning will be concentrating on fog, as some areas have dropped
to under 1sm, but overall still not seeing enough coverage for a
dense fog advisory as rainfall yesterday was less numerous than
expected. Will monitor conditions and issue a Special Weather
Statement at least to cover the patchy dense fog, and possibly
upgrade to a Dense Fog Advisory if it becomes more widespread.

Once fog lifts there should be a period of mainly sunshine though
some higher clouds will stream by in the westerly flow aloft. Will
see the fog turn into a cu field then some towering cu to CBs after
midday in the southwest mountains of VA into NC, as weak upper
disturbance and moisture convergence along the spine of the
Appalachians acts to form showers and storms. Coverage today will be
low, with most not seeing any rain.

With more sunshine will see warmer temperatures with highs in the
80s, around 80 NC mountains, higher ridges of WV/VA to upper 80s
east of the Blue Ridge and including Roanoke, Lexington, VA.

Tonight, the upper heights rise with weak ridging. Still models
showing weak convergence into the overnight across the southeast,
then another area of lift reaches the central Appalachians by dawn.
Very low confidence on much shower coverage overnight, after evening
showers/isolated storms fade over the NC foothills and piedmont.
Added a slight chance of showers around dawn in the WV mountains as
well. Fog will form in the river valleys as well late tonight but
increased cloud cover appears to limit coverage. Lows staying above
normal with mid to upper 60s mountains, to lower 70s out east.

&&

.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
As of 300 AM EDT Wednesday...

Weak ridging aloft will continue Thursday as a weak warm front aloft
slides back through the area from the southwest. A low level
southwest flow will advect deeper moisture north Thursday afternoon.
The combination of a boundary, low level convergence and stronger
instability will result in scattered showers and thunderstorms. The
Day Two Convective outlook keeps our area in general thunderstorms
with marginal risk to our west across the Ohio Valley. High
temperatures Thursday will range from the mid 70s in the mountains
to around 90 degrees in the piedmont.

A few showers and thunderstorms may linger into Thursday evening
before loss of heating and exodus of the weak boundary takes place
Thursday night. Low temperatures Thursday night will vary from the
mid 60s in the mountains to the lower 70s in the piedmont.

A cold front should slowly pivot toward the region Friday afternoon
preceded by a lead band of showers that could jump into the eastern
lee trough during the afternoon where deeper convection possible.
Weak lift is expected ahead of the boundary, given that most upper
support shearing out to the northeast with the passing weak
shortwave. Behind the pre-frontal axis, a west winds may hinder
convection in the mountains. However progged instability still quite
strong east of the mountains and if timing is slower then could see
better thunderstorm coverage in the east. There is enough shear,
better instabilities and healthy lift to support isolated strong
storms. High temperatures Friday will generally be from around 80
degrees in the west to near 90 degrees in the east. Temperatures
will depend on cloud cover and timing of convection.

Any convection Friday evening will taper off with the loss of solar
heating Friday night. Low temperatures Friday night will feature
readings from the upper 50s in the mountains to the lower 70s in the
piedmont.

&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
As of 300 AM EDT Wednesday...

Transition to more of an upper troughing regime will again occur
over the weekend allowing another surface cold front to push into
and perhaps south of the region by early next week. Latest models
again have differences on the southward push of drier air behind the
front espcly given another upper disturbance that looks to pass just
north by later Sunday. However appears passage of this feature may
be enough to briefly shunt deeper moisture to the south/east before
the trough flattens into broad ridging early next week. Therefore
will leave in some low pops mainly southeast third later Saturday
where the front may slow up during heating, and slight to low end
chance pops espcly southern/southwest sections Sunday as the
wave/trough passes. Monday look drier but rather hot with surface
high pressure building in from the north behind the front resulting
in better subsidence under a weak wedge. Thus limiting any pops to
start the week to mainly the southern Blue Ridge, with overall more
sunshine elsewhere for now. Added some light pops to Tuesday and
Wednesday as weak shortwaves rotate through. Confidence is low on
shortwave energy in the extended that may rotate further north
around the subtropical ridge as hinted at by ECMWF.

Despite the frontal passage, will actually see warmer high temps
given heating of somewhat drier air and warming aloft to start next
week. This supports overall above normal highs with low/mid 80s
mountains to near 90 east through much of the period.

&&

.AVIATION /13Z WEDNESDAY THROUGH SUNDAY/...
As of 717 AM EDT Wednesday...

VLIFR to IFR fog burns off by 14z, leaving VFR conditions
through tonight. Exception will be a bit of fog again Thursday
morning at LWB/BCB though increasing clouds ahead of next front
expected to limit fog.

Winds will be light through the period.

Extended Aviation Discussion...

Thursday through Sunday, we will be in an active weather
pattern with a series of disturbances or fronts crossing the
area. The most active periods will be Friday afternoon and
evening, and then again on Sunday. While during the bulk of this
time period VFR conditions are anticipated, sub-VFR conditions
are expected under any of the heavier showers or storms.

&&

.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...JH/WP
NEAR TERM...AL/WP
SHORT TERM...KK
LONG TERM...JH/KK
AVIATION...DS/MBS/WP
EQUIPMENT...RAB/WERT



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