Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 220530

Area Forecast Discussion
National Weather Service Blacksburg VA
130 AM EDT Sat Jul 22 2017

High pressure will remain over the region tonight into weekend
keeping a hot and humid airmass across the area. A frontal
boundary across the mid- Atlantic will remain just north of the
region as well into Saturday night. A second stronger cold
front will then arrive from the northwest and affect the area
later Sunday into Sunday night.


As of 100 AM EDT Saturday...

Showers fired up along weak outflow, low level moisture
convergence boundary in the piedmont early. The high-res showing
more showers potentially firing up over the New River Valley by
2-3am. Some support for this but overall feel threat is isolated
so made some minor modifications to the forecast. Watching
convection west of Charleston, WV. High-res models fade this
quickly in the next few hours as it encounters more stable layer
further east, but would not be surprised to see some of it hang
on into Southeast WV by 6am.

Previous discussion from Friday evening...

Earlier convection has dissipated, leaving lingering boundaries
to trigger isolated showers/storms in residual instability
overnight. There is some weak short wave energy moving through
as well so expect there will be some isolated activity bubbling
around, but all in all dry for the overnight with some patchy
fog developing late, especially for locations that received
rain today. Overnight lows will once again be quite warm with
low/mid 70s east of the Blue Ridge and generally mid/upper 60s

Previous AFD...

Another very warm and humid overnight with some patchy fog
around and lows mostly mid 60s to low or even mid 70s east. Next
upstream MCS will be nearing the Ohio Valley by early Saturday.

Upper heights will continue to slowly fall on Saturday as a surface
wave tracks east along the residual front to the north by late in
the day. This looks to keep the track of the next upstream MCS
heading more east than south per latest model Corfidi vectors with
less lift to the south. However will be quite unstable to the south
of this complex with only a weak cap above a rather well mixed
westerly trajectory. This may allow storms to unzip over the
mountains per weak progged convergence and eventually out east late
along the lee trough once mixing relaxes. Thus running with overall
chance pops spreading/developing east mainly during the afternoon.

Otherwise heat will be the main issue with Saturday perhaps the
hottest day so far out east given even warmer 850 mb temps and weak
downslope to start. However this likely wont be enough to mix out
dewpoints enough to preclude possible heat advisory levels in parts
of the piedmont as seen by latest Mos guidance. Therefore bumped up
highs some with even 90 degrees possible in spots across the
mountains and perhaps near 100 far east with indices around 105
degrees for a couple of hours. This along with it being the weekend
supports going ahead with a heat advisory for a few eastern counties
Saturday afternoon.


As of 330 PM EDT Friday...

Several mesoscale convective systems will track across to the north
during this period. The focus for these systems will be a stalled
frontal boundary over West Virginia and Maryland. It should be
trapped between an upper level trough passing eastward over the
Great Lakes during Saturday night and Sunday and a broad upper level
ridge over the Southeast. This ridge will keep the heat and humidity
over the Mid Atlantic on Sunday. Expect heat indices to reach over
100 degrees in the Piedmont. This heat and humidity should also fuel
the aforementioned systems. Models are showing the potential for
severe weather within these clusters of thunderstorms as indicated
by the slight risk from SPC for Sunday. It is possible that some of
these systems could dive southward toward our CWA, and model
soundings show CAPE approaching 2,000 J/kg by the afternoon.

Depending on how well the convection can push the frontal boundary
southward over our CWA will shape up the forecast for Monday. Model
spread increases considerably at this point while trying to refine
where the frontal boundary will reside. So, it requires keeping the
chance of showers and thunderstorms into Monday afternoon. The
highest chance appears to be along the southern Blue Ridge and
through the northwest North Carolina mountains. Temperatures will
likely be a couple degrees cooler for Monday as compared to Sunday.
More relief from the heat appears by Monday night as high pressure
approaches from the north.


As of 330 PM EDT Friday...

High pressure should build over the Northeast by Tuesday. The
frontal boundary will be shoved southward southward toward the North
Carolina coast by this time. Lingering moisture may still spark a
few diurnally-driven showers and thunderstorms during each afternoon
through this period. The best chances appear confined to the VA/NC
border on Tuesday, but more potential exists for Wednesday with
easterly to southeasterly flow from high pressure offshore. There
could be some wedging possible east of the Blue Ridge, so high
temperatures were pushed downward for Wednesday to allow for the
potential of upslope flow along the Blue Ridge and more cloud cover.
Temperatures should bounce back upward by Thursday as high pressure
heads further out into the Atlantic Ocean. Long range models hint at
another frontal boundary approaching our CWA by Friday.


As of 120 AM EDT Saturday...

Have a few showers in vicinity of Danville this morning, but
confidence low on other sites. Fog may impact Danville and
Lewisburg at times, but upstream blowoff of convection should
send dense cirrus shield into LWB with potential for some lower
cigs after 10z to limit density of fog there.

Models today mostly favor organized convection further north of
our forecast area. Exception being the GFS which blows up an
area near Danville after 20z. Confidence is low on coverage of
storms today, but enough support to have VCTS during the
afternoon at all sites. Still VFR, except if a storm moves over
the airport. Winds will be light from the southwest, after 14z.

Aviation Extended Discussion...

Confidence is growing in a potential more active extended
aviation forecast period for Sunday into Sunday night with
multiple opportunities for thunderstorms. May see coverage shift
into southern sections by Monday afternoon as residual outflow
from Sunday jumps into North Carolina so appears more in the way
of overall VFR possible Monday based on latest guidance.
Otherwise most of the period will be VFR but with better
chances for storms in more locations, mainly afternoon and
evening late in the weekend, and with better coverage of
rainfall the chances for IFR fog in the early morning hours
increases again especially for KLWB and KBCB. A weak cold front
is likely to push some of this moisture off to the southeast by
Tues or Wed for at least a couple of relatively dry days.
Overall, confidence of all weather parameters is moderate during
the extended portion of the forecast.


VA...Heat Advisory from 11 AM this morning to 7 PM EDT this evening
     for VAZ044>047-058-059.
NC...Heat Advisory from 11 AM this morning to 7 PM EDT this evening
     for NCZ005-006.


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