-- Discussion containing changed information from previous version are highlighted. --
258
FXUS61 KRNK 220746
AFDRNK

Area Forecast Discussion
National Weather Service Blacksburg VA
346 AM EDT Tue Aug 22 2017

.SYNOPSIS...
A Bermuda high off the coast will keep a warm and moist airmass
in place across the Appalachians and central mid Atlantic
region today. This will support some showers and thunderstorms
mainly along and west of the Blue Ridge this afternoon. A cold
front will approach the region tonight before slowly moving
through the area on Wednesday, bringing a better chance for
showers and storms to the region.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 230 aM EDT Tuesday...

The latest satellite imagery shows just some high clouds moving
over the region as high pressure off the coast keeps a warm and
moist airmass in place. The high clouds are not hampering
radiational cooling much and expect a good amount of patchy
dense valley fog from the Blue Ridge west through daybreak. Some
patchy fog can be expected to the east across the piedmont as
well, especially for locations that received rain on Monday.

Fog/stratus will burn off after sunrise, but diurnal heating
will cook up convection by the afternoon. Expect pattern of
convection will be similar to what was experienced on Monday
though with a bit better coverage as model soundings are not
indicating quite the degree of capping. Orographic forcing looks
to be the primary trigger so initial activity will get started
from the Blue Ridge westward, with differential heating
boundaries and outflow interactions allowing cells to spill
eastward into the foothills.

A cold front will then approach from the northwest by early
tonight with some convection out ahead of it. However, best
dynamics will be north of the area and arrival time well after
peak heating/instability, showers and storms look to be breaking
up as they start to enter the region. Expect any severe threat
from activity ahead of the front will be marginal at best as
bulk shear parameters and hodographs are not impressive, and
arrival time is late.

The end result will be solid chance to likely POPs along the
southern Blue Ridge with chance POPs extending back westward
through the New River Valley and into the Mountain Empire region,
followed by chance to likely POPs increasing further west of
the Ridge overnight. Juicy airmass will continue to support
storms with heavy downpours which may create localized hydro
issues.

Highs today will generally be in the low/mid 90s east of the
Ridge to middle/upper 80s west. Lows tonight look quite muggy
in the east with low/mid 70s, and mid/upper 60s west.

&&

.SHORT TERM /WEDNESDAY THROUGH FRIDAY NIGHT/...
-- Changed Discussion --
As of 315 AM EDT Tuesday...

Expect strong frontal system to be located from near PIT to TYS
around 12Z Wed, with a rapidly diminishing line of showers and
thunderstorms near the VA/WV border and into SW VA at daybreak
Wed. NAMNest model has been consistent for the past couple of
days on the rapid diminishment of showers/thunderstorms as they
progress east tonight into Wed morning. However, models have
also been consistent on slower motion of frontal system through
the region Wed as the upper trough moves through the northeast
leaving a flat west-northwest flow aloft in its wake into our
region. As a result, expect redevelopment of thunderstorms
fairly quickly through the late morning and afternoon Wed
generally along and east of the Blue Ridge, although some
showers may redevelop even back toward the WV/VA border during
the late morning hours before moving east. Models in good
agreement on scenario, but have generally followed NAMNest for
timing and movement of showers/thunderstorms and associated pops
Wed.

With regard to the severe threat, SPC continues to carry a
marginal threat generally across Southside and the NC Piedmont
for Wed afternoon. This certainly appears reasonable. Although
shear is weak and the best dynamics remain well north of the
area, strong heating, and convergence along morning outflow
boundaries, cooling temperatures aloft, and the front should
provide the necessary parameters for at least scattered strong
to severe thunderstorms in the general area of Wilkesboro, to
Mt. Airy, to Danville. This activity will diminish and shift
east or southeast during the evening Wed.

Thursday, while most of the area will be dry, a secondary upper
trough will sweep through the Great Lakes. Several models depict
shower development, including the NAMNest through the Alleghanys
Thursday afternoon. This seems reasonable given the strength of
the upper trough and a pool of cold air aloft. Have introduced
a 20% pop for northwest Greenbrier, although the best chance for
showers will likely remain just northwest of our CWA toward
Snowshoe and Elkins.

Friday should be mostly sunny and dry across the entire region
behind this secondary upper trough and bring the coolest air of
the week and a taste of fall-like temperatures and conditions.

Temperatures will still be quite warm across the Piedmont
Wednesday afternoon, while a good 10 degrees cooler for most
areas west of the Blue Ridge Wednesday. However, given
increasing clouds and shower/thunderstorm chances by early
afternoon at the latest, feel that most areas east of the Blue
Ridge will stop short of the 90F mark. While Thursday will be
cooler in all areas with lows dipping well into the 50s west of
the Blue Ridge, the coolest and driest air will not arrive until
the secondary upper trough passes east of the area late
Thursday. By Friday, look for lows in the 50s west to lower 60s
east with highs in the 70s west to lower 80s east. Very pleasant
for late August. Some 40s in locations like Burkes Garden not
out of the question at all, with higher elevations in the
western mountains seeing highs in the 60s Friday with a
sprawling Canadian high moving southeast from the Midwest.
-- End Changed Discussion --

&&

.LONG TERM /SATURDAY THROUGH MONDAY/...
-- Changed Discussion --
As of 330 AM EDT Tuesday...

The extended periods present many challenges and have been
changing considerably from one model run to the next and from
one model to the next. The concerns largely focus around the
potential for multiple tropical systems to be affecting perhaps
both the western Gulf and southeast Atlantic coastal areas by
the weekend or early next week. Most models are coming on board
with this, but intensity, track, and timing of such systems very
widely from one model to the next and are complicated by the
development of a very chaotic upper-level weak pressure pattern
across the eastern two-thirds of the U.S.. Of these potential
tropical systems, the one slated to move into eastern TX appears
at some point to be the one most likely to impact our weather
down the line as its remnants could eventually track into the TN
Valley or southern/central Appalachians, mainly early next
week. Until that point, at least through the weekend, generally
looking at a large Canadian high pressure area drifting toward
New England setting up a cool northeast flow into the region.
Return flow could result in clouds and showers developing along
the southern Blue Ridge by Sunday.

At any rate, there is a great deal of uncertainty moving from
Sunday into Monday based on potential tropical systems on both
sides of our region. At this point, most of the impact, if any,
from these systems would be beyond this period. Thus, have not
made any significant changes to the pops or temperatures during
this time frame.

Given the pattern of northeast flow and 850mb temperatures
dropping toward +8C in the northern parts of the CWA,
temperatures will be unseasonably cool for late August with
widespread lows in the 50s and highs mainly in the 70s, except
around 80 Piedmont. Indeed very pleasant for late August.
-- End Changed Discussion --

&&

.AVIATION /07Z TUESDAY THROUGH SATURDAY/...
As of 215 AM EDT Tuesday...

The latest satellite imagery shows just some high clouds moving
over the region as high pressure off the coast keeps a warm and
moist airmass in place. The high clouds are not hampering
radiational cooling much and valley fog is evident in satellite
imagery and surface obs. Expect LIFR conditions to develop at
KLWB and also at KBCB where earlier showers added to the moist
environment. KLYH and KDAN also had precipitation earlier today
so expect at least a tempo to IFR through around daybreak.

Fog/stratus will burn off after sunrise and allow for a window
of VFR conditions before heating cooks up convection by the
afternoon. Expect pattern of convection will be similar to what
was experienced on Monday though with a bit better coverage as
model soundings are not indicating quite the degree of capping.
Orographic forcing looks to be the primary trigger so initial
activity will get started from the Blue Ridge westward, with
differential heating boundaries and outflow interactions
allowing cells to spill eastward into the foothills.

A cold front will then approach from the northwest late in the
TAF period with some convection out ahead of it. However, best
dynamics will be north of the area and arrival time well after
peak heating/instability, showers and storms look to be breaking
up as they start to enter the region. With the lack of discrete
features present today and the degree of uncertainty in
convection associated with the approaching front late, believe
it will be best to use a broad brush and go with VCTS all sites
with amendments today as convection declares itself and later
updates as guidance gets more precise with the approaching
front.

Winds will generally be light through the period.

Extended Aviation Discussion...

The cold front will continue moving through the region on
Wednesday. This will lead to more better potential for sub-VFR
conditions Wednesday when more widespread showers and storms
fire along the sluggish 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.

&&

.EQUIPMENT...
As of 930 AM EDT Monday...

KFCX doppler radar is expected to be back in operations as early
as Wednesday evening (Aug 23rd). Technicians have replaced the
bull gear and are reassembling the radar today and tomorrow
with calibration worked on Wednesday.

&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...None.
NC...None.
WV...None.

&&

$$
SYNOPSIS...MBS
NEAR TERM...MBS
SHORT TERM...RAB
LONG TERM...AMS/RAB
AVIATION...AL/MBS
EQUIPMENT...RCS