-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KRNK 220000
AFDRNK

Area Forecast Discussion
National Weather Service Blacksburg VA
800 PM EDT Fri Jul 21 2017

.SYNOPSIS...
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.

&&

.NEAR TERM /THROUGH SATURDAY/...
As of 255 PM EDT Friday...

Broken band of convection crossing the mountains, preceded by a few
storm clusters, will continue to slide east across the region
through late afternoon before exiting. This could again result in a
few stronger cells mainly across the north per better
support/instability. Once this outflow driven band exits expect may
see added isolated storms redevelop mainly east per latest HRRR, so
will hold onto some low pops into early this evening. Will be in
between this lead feature and the next upstream MCS that will be
nearing the Ohio Valley by early Saturday. However weak shortwave in
between may result in enough lift and weak westerly convergence to
support isolated showers far west late where keeping in a 20/30 pop
western mountains late. Otherwise another very warm and humid
overnight with some patchy fog around and lows mostly mid 60s to low
or even mid 70s east.

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.

&&

.SHORT TERM /SATURDAY NIGHT THROUGH MONDAY NIGHT/...
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.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
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.

&&

.AVIATION /00Z SATURDAY THROUGH WEDNESDAY/...
-- Changed Discussion --
As of 750 PM EDT Friday...

High confidence in diurnal trends this TAF period with some
late night fog/stratus and afternoon convection. Believe fog
will be most prevalent at sites that experienced rain today,
especially KLYH and KDAN, as well as the climatologically
favored site of KLWB. There was precipitation near KBLF and a
light upslope west wind may loft some fog/stratus onto the
airfield late but confidence in this is not as high as for
other sites. Meso guidance indicates the potential for some
scattered showers to be stewing around overnight feeding of
residual instability and weak short wave energy moving through
aloft, but do not expect activity to be very widespread so will
go with a dry forecast through daybreak and amend if showers
become imminent at an airfield. Tomorrow will be similar to
today as upstream convective remnants/outflow moves into the
region and helps trigger another round of storms. Will include
VCTS all sites for the latter portion of the period. Expect a
well mixed boundary layer Saturday will allow for some low end
gusts at TAF sites but nothing overly problematic for aviation
interests.

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.
-- End Changed Discussion --

&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...Heat Advisory from 11 AM to 7 PM EDT Saturday for VAZ044>047-
     058-059.
NC...Heat Advisory from 11 AM to 7 PM EDT Saturday for NCZ005-006.
WV...None.

&&

$$
SYNOPSIS...JH/WP
NEAR TERM...JH
SHORT TERM...PW
LONG TERM...PW
AVIATION...MBS/JH