Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 230113

Area Forecast Discussion
National Weather Service Blacksburg VA
913 PM EDT Sat Jul 22 2017

A cold front will cross overhead during tonight into Sunday,
which should spark showers and thunderstorms. This front should
pass south of our region by Monday. High pressure will arrive
to bring slightly cooler conditions by the middle of the week.


As of 910 PM EDT Saturday...

A short wave moving by to our north is helping the frontal
boundary sag a bit closer to the area early tonight. This will
allow convection along the front to brush the northern portion
of the region mainly along and north of the Interstate 66
corridor. However, the entire overnight period is rather muddy
as the remnants from waves of convection upstream move in our
direction and may be able to affect the area, while lingering
boundaries from earlier convection may be able to generate some
new activity in residual pools of instability. Believe the best
course of action is to play the POPs early from convection
dropping in from the north, then allow broader POPs in the west
later tonight.

Convective debris clouds will have an impact on overnight low
temperatures. Believe there will be enough cloudiness around to
keep us a degree or two above last night with low/mid 70s east
of the Blue Ridge and mid 60s/near 70 to the west.

Previous discussions...

745 PM EDT Saturday...

Activity associated with a wavy frontal boundary to our north
will remain very close to the boundary and gradually work its
way south during the night in a much weaker fashion.

Confidence in activity for tomorrow is still only in the chance
range, much of which will depend on how much cloud cover and
convection there is in the morning. Will not make any changes to
tomorrow`s pops for now. SPC still holding a SLIGHT RISK for

330 PM EDT Saturday...

High resolution models hint at another round of convection
arriving west of I-77 by early Sunday morning. Cloud cover from
upstream convection could limit high temperatures from climbing
as high by Sunday afternoon compared to Saturday afternoon.
Still, it will be quite hot and humid, and the cold front may
reach the VA/NC border by the afternoon. This situation should
allow enough instability and lift for more scattered showers and
thunderstorms to develop in this area toward the evening.


As of 230 PM EDT Saturday...

For Sunday evening, weather forecast models are hinting at a line of
thunderstorms entering our far western Virginia/northern North
Carolina counties during late evening as an upper level disturbance
passes overhead. Believe any storms would diminish in intensity
quickly as they cross the ridges and move into downslope flow. Have
introduced a period of higher rain chances across the west for the
possibility of this system, however admit that confidence of the
occurrence/timing/location of any organized line of storms moving in
from the west is marginal. The coming overnight forecast shift will
have the benefit of an updated set of models to have a look at and
will make adjustments to these rain chances accordingly. Outside of
these storms, convection is expected to diminish after sunset with
the loss of daytime heating.

Widely scattered showers and thunderstorms can be expected for
Monday despite a cold front approaching from the northwest. Slightly
drier air and stronger northwest wind flow will help to inhibit
convective development, though those storms which do develop may
pulse up to strong/severe intensity for brief periods of time,
capable of producing localized damaging winds. The cold front will
enter the southeast West Virginia counties during late Monday
evening, and gradually loose momentum as it continue south across
our area.


As of 1230 PM EDT Saturday...

Looking to Tuesday, long-range weather models are indicating that
the cold front may not pass completely across our forecast region,
possibly stalling near the Virginia-North Carolina border as high
pressure shifts from the Great Lakes toward New England. Will hold
close to the previous forecast of a higher chance of spotty
afternoon showers/storms south of Highway 460...closer to the front.

Passage of high pressure off the coast on Wednesday will bring
cooler easterly windflow to the mid-Atlantic, resulting in likely
the coolest afternoon temperatures of the week. Upslope wind flow
will make for the best chances of shower/thunderstorm activity
across the mountains.

Winds will shift more southerly by Thursday, resulting in a return
of warmer temperatures and higher humidity, and greater coverage of
afternoon showers/storms, more so across the mountains. A cold front
will slowly make its way across the mid Atlantic on Friday and
Saturday, again resulting in scattered showers and thunderstorms
both days, a few of which may become severe for brief periods of


As of 750 PM EDT Saturday...

No convection left in the CWA at this time. Meso-high has built
across the area and left us with mainly just a high cirrus deck
from upstream convection. HRRR suggests little will happen until
after 08Z, when some showers/thunderstorms may begin to spread
back into the area from the west/northwest. Current
thunderstorms to our north extend along a wavy quasi-stationary
frontal boundary that will move little overnight. Southward push
of convection will be largely from cold pool outflow and
density. However, the overnight trends will determine how active
Sunday becomes in terms of convection. HRRR, as well as larger
synoptic-scale models, through tomorrow afternoon is not excited
about much convection in our CWA, largely in part because of
downslope westerly winds that develop in the morning in the wake
of potential MCV. We could see a situation where clouds and
early morning showers relocate to stronger convection in the
afternoon mostly across eastern/southern areas. For now, have
left VCTS/TS out of the TAFs until there is greater consensus
among the synoptic scale and short range models.

All this being said, generally looking at a VFR forecast through
the TAF valid period. Not expecting fog/low cloud development
tonight/early Sunday because of high temperatures today, large
T/Td spreads, and extensive mid/high clouds overnight from
upstream convection and possible convection later tonight.

Winds will be SW-WSW through the period at speeds of 4-6kts
overnight and 8-10kts after 14Z Sunday with low end gusts
possible at most sites.

Medium confidence in ceilings through the TAF valid period.
Medium to high confidence in visibilities through the TAF valid
period. High confidence in wind speed/direction through the TAF
valid period.

Aviation Extended Discussion...

The cold front currently to our north should be near the NC/VA
border by Monday. This leaves a baroclinic zone in or near the
area through much of the extended period. Additional upper-level
disturbances are progged in northwest flow to impact the area,
leaving an unsettled weather pattern in place. The most likely
areas to see convection, at least during the first part of the
week would be near the NC/VA border, mainly affecting KDAN.
Late/night early morning fog can be expected in the usual
locations, otherwise outside these concerns and scattered
convection, mainly in the south, look for mostly VFR cigs/vsbys.




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