Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 201859

Area Forecast Discussion
National Weather Service Blacksburg VA
259 PM EDT Sun Aug 20 2017

High pressure across the mid-Atlantic will gradually shift east
tonight and Monday, allowing winds to turn southerly, resulting
in a return of deeper moisture by Monday afternoon. Another
cold front will approach the area by the middle of the week,
bringing with it a better chance for showers and storms.

As of 240 PM EDT Sunday...

Analysis showing only weak instability across the region this
afternoon as higher dewpoints remain slow to return north with
high pressure lingering across the area. Latest short term
guidance still shows some weak convergence and subsequent
isolated convection developing mainly Blue Ridge and points
north into early this evening but looks iffy attm. May leave in
a mention pending release trends, otherwise should see clouds
and any showers fade shortly after sunset with skies becoming
clear overnight after some evening cloudiness. Fog again likely
to develop with dewpoints rising some under clear skies, so
beefed up coverage espcly valleys and low spots out east. Lows
not quite as comfortable as last night under increasing moisture
with most only falling into the 60s, with perhaps only around
70 southeast.

Flat upper ridging will linger Monday while the parent surface
high pressure shifts east and weakens. This should allow for
weaker subsidence by Monday afternoon when more of a low level
southwest trajectory starts to return higher PWATS, including
some 850 mb theta-e ridging along the mountains. However lift
outside of orographics looks weak given little support aloft,
with early/mid afternoon instability likely dampened some by the
eclipse. Latest Nam again looks overdone with a band of deeper
convection along the Blue Ridge, although most solutions develop
at least isolated to widely scattered mountain showers/storms
as the mid level cap fades. Therefore leaving in some low chance
pops mountains per latest ensembles, while delaying the onset
and trimming back coverage during mid afternoon. Otherwise
clouds the main concern as appears could be some low clouds
early on, followed by mainly sunny skies before cumulus
development takes place around eclipse time in the afternoon.
Right now appears overall 30-50% cloud cover except locally more
where any convection does develop sooner. Similar temps aloft
to the past few days again supports highs low/mid 80s mountains
and around 90 east. Did trim back a few degrees for the eclipse
before readings spike back up late in the day where clouds are

As of 240 PM EDT Sunday...

High pressure and loss of heating in a weakly capped environment
will have any Monday afternoon storms fading in the evening. On
Tuesday, high pressure drifts further east into the Atlantic. Return
southwesterly flow on the back side of this high will bring more
heat and humidity into the region. Upslope flow along the Blue Ridge
and a lee trough over the piedmont may produce storms across the
mountains Tuesday afternoon. Increase mixing ahead of an approaching
cold front may be a deterrent to limit convection activity to
scattered coverage. Any afternoon storms will fade during the
evening. Prefrontal showers enter southeastern West Virginia after
midnight as a cold front begins to move into the Ohio Valley. Muggy
conditions are expected through the period with highs Tuesday in the
80s west of the Blue Ridge and low 90s east.

As of 255 PM EDT Sunday...

Models remain consistent with having a cold front tracking across
the region on Wednesday. One inconsistency is timing. Models have
the front stalling west of the central Appalachian mountains
Wednesday morning. It appears the front loses jet support and the
short wave axis runs parallel to the front. The surface front is
expected to slowly move across the mountains, while precipitation
hangs back on western slopes during the morning. The front will jump
to a lee trough in the piedmont during peaking heating in the
afternoon. Models have the best jet dynamics over the northeastern
US, but enough instability and lift will generate some strong to
severe storms across NW NC and Southside VA during the late
afternoon and evening. Again with the lack of jet support, the front
may stall across the coastal piedmont of southern Virgina and North
Carolina Wednesday night. This will delay the cooler and drier air
from enter the region until later Thursday when another low pressure
system pivots over New England.

Temperatures Wednesday will vary from the mid to upper 70s across
the mountains to the upper 80s towards the piedmont. Following the
front passage, temperatures will run below normal going into next

As of 1230 PM EDT Sunday...

Overall VFR will continue under light east to southeast winds
this afternoon. However should see cumulus fields start to
increase by mid to late afternoon with bases between 4-6k feet.

Models hinting at isolated to widely scattered, short-lived
showers/isolated storms developing along the Blue Ridge from
mid afternoon through early this evening, but still not enough
coverage to mention in the TAFS. Appears best chances along the
highest ridges between KROA and KTNB at this point. Expect any
convection to fade after sunset, although some clouds may
linger along the mountains and northern sections until around
midnight or so.

With winds shifting southerly this evening, expecting a
continued increase in moisture overnight. This combined with
clearing skies across the region late, expect another round of
fog espcly across the western valleys. This most likely around
KLWB where another episode of IFR/LIFR cigs/vsbys likely, while
a period of IFR possible at KBCB, although models suggest a bit
more uncertainty. Also may see low clouds advance northward
toward the eastern terminals late as tropical moisture returns.
Thus including a brief MVFR mention at both KLYH/KDAN late per
latest HREF progged cigs/vsbys. Otherwise VFR outside of any
patchy fog or stratus elsewhere late tonight.

Flow turns more south/southwest on Monday as high pressure
slides east and weakens. This should bring about a more
widespread cumulus field by afternoon once any low clouds/fog
burns off. However still appears it should remain mainly VFR
Monday afternoon with only localized MVFR under isolated to
widely scattered mountain convection.

Extended Aviation Discussion...

Moisture will continue to increase on Tuesday as high pressure
moves farther offshore ahead of another cold front that will
arrive from the northwest around midweek. This will again lead
to more afternoon clouds and isolated convection mountains Tuesday
afternoon but overall VFR at this point. Better potential for
sub- VFR will come Wednesday when more widespread showers and
storms arrive with the next cold front. Flying conditions should
slowly improve behind the passing cold front on Thursday
although sub-VFR cigs may linger across southern/western
sections a while longer.


As of 400 AM EDT Sunday Aug 20th...

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 this week and the radar will most likely stay
down completely as the repairs are being made.




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