Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 151413

Area Forecast Discussion
National Weather Service Blacksburg VA
1013 AM EDT Mon Oct 15 2018

A warm front will lift north through the area today ahead of
a cold front which will be approaching from the northwest.
This will result in brief warm-up today, followed by another
cool down behind the front Tuesday. Both frontal features will
be the focus for showers, in addition to the possibility of
a thunderstorm this afternoon and evening.


As of 1000 AM EDT Monday...

There are two main items of interest impacting parts of the
forecast area this morning. The first is the advertised light
precipitation that has been impacting mainly parts of southeast
West Virginia and a couple of neighboring counties of southwest
Virginia. This is on track pretty well with the forecast.
However, the forecast is also advertising precipitation at the
current time farther east than the ongoing conditions. Both the
06Z deterministic GFS/NAM solutions and the nearer term HRRR and
RAP13 are offering solutions that keep the bulk, if not the
entirety, of the precipitation today across the far western and
far northern parts of the forecast area. While not ready to leap
to that great of a binary of a forecast, the latest update will
reflect a greater probability in the west and a lesser
probability in the east and southeast than our prior forecast.

The next item of interest is a band of fog/stratus and patchy
dense fog that has developed across portions of Southside
Virginia, south into the Piedmont of North Carolina. The 12Z
sounding out of KGSO showed a shallow, yet steep, inversion that
is expected to mix out as both strong winds aloft mix down to
the surface, and heating along the perimeter also help to
facility mixing and erode the fog/stratus from the edges inward.

For this afternoon, have introduced the potential for isolated
storms across a slightly wider area of real estate than earlier
offered. This places our area more in line with the SPC offering
of general thunder across all but the far southeast portions of
the area today.

Have also made minor tweaks in the hourly temperature, dew
point, wind, and sky cover grids to better reflect the current
conditions and expected trends into the early afternoon.

As of 500 AM EDT Monday...

Hodge podge of frontal boundaries this morning with warm front
(remnant cool wedge) lifting north per increasing southwest
winds ahead of a more bonafide cold frontal boundary which is
making its way across the Ohio Valley. This cold front is
associated with a positively tilted long wave trough which
extends from the upper Mid-West into the southern Rockies.
Orientation of the trough is allowing low level jet to transport
moisture northward from the Gulf of Mexico into TX-AR-TN. This
moisture is forecast to advect northeast ahead of the
approaching cold front today.

There are some model differences with respect to where this
deeper moisture will end up by the end of the day, the NAM
focusing most of its rainfall along a PIT-MEM line while the
GFS is farther south along a BKW-BNA line. Both models produce
between 0.50 to 1.00 rainfall within their respective forecast
corridors. WPC forecast reflects a blend with heaviest rains
favoring MGW-CRW-LEX.

All things considered, highest confidence for wetting rain is
across our northwestern CWA today and tonight, with primary
focus across our WV counties and into the highlands of VA...rain
amounts of a quarter (0.25) to half (0.50) of an inch. Gradient
drops off quickly to the southeast, although amounts of up to a
tenth of an inch are possible as far east as the Blue Ridge.
East of the Blue Ridge expectation is for amounts to average
under a tenth (0.10). Worst case would be the GFS solution, and
in spite of its potential one inch QPF scenario for the
Greenbrier Valley, it still only generates a few hundredths of
an inch for places like Danville.

Wedge of cool air over the area has become shallow and should
erode from southwest to northeast during the a warm
frontal fashion. This will result in brief warm-up today, with
temperatures rebounding into the 70s, and possibly 80 degrees
in the Carolina Piedmont. A lot will depend on the degree of
cloudiness and ability to mix during the afternoon, but all
indications suggest a big warm-up today. That said, the farther
north you go (esp north of I-64) the less chance you will have
to partake in the deeper mixing and more subject to rain cooled

The approach of the cold front from the Ohio Valley will result
in increasing wind speeds. A 30 kt low level jet is progged
just ahead of the front, so the higher elevations will
experience some healthier gusts today in the 20-30 mph range.
Can`t totally rule out a thunderstorm this afternoon. With warm
sector temperatures in the 70s and dewpoints climbing back into
the 60s SBCAPE of 500-1500 j/kg would support development of
embedded deeper convection within whatever activity moves into
the area from the west. We are not expecting anything severe at
this time, however with a 30 kt LLJ it`s possible to have some
strong surface gusts for the more robust convective cells.

The cold front is forecast to make its way through the area
tonight...winds shifting to the northwest behind the front along
with cooling temperatures and lowering dewpoints. Guidance still
differs on just how far south the front travels. That fact,
along with various timing of a shortwave disturbance moving
eastward along the front Tuesday into Tuesday night will play a
role in the sensible weather across our area through the short


As of 400 AM EDT Monday...

A frontal boundary will stall across South Carolina Tuesday into
Tuesday night as a wave of low pressure rides east along the front.
Isolated to scattered showers are possible with the best chances
across the south closer to the frontal boundary. On the northside of
the front, High temperatures on Tuesday will be cooler than Monday
with readings from the lower 50s in the northwest mountains to the
mid 60s in the piedmont.

The better kicker to propel the front farther to the southeast will
be an approaching upper level low/trough that is expected to cross
the Great Lakes region late Tuesday night into Wednesday. Followed
the nam for pops tapering off showers Tuesday night from north to
south. Low temperatures Tuesday night will vary from the upper 30s
in the mountains to the lower 50s in the piedmont.

Gusty Northwest winds will usher in colder and drier air into our
region Wednesday. High temperatures will range from the upper 40s
in the mountains to the mid and upper 60s in the southeast. By
Wednesday night the pressure gradient will have slackened enough for
winds to diminish and the center of surface high pressure will be
nearing the region. This will be the coldest temperatures so far of
the season under weak winds and limited clouds. Frost will be
possible across portions of southeast West Virginia and some of the
neighboring sections of southwest Virginia. Low temperatures will be
cold with values from around 30 in the northwest mountains to the
lower 40s in the piedmont.


As of 300 PM EDT Sunday...

Surface high pressure will keep us in a cool and dry pattern through
at least Thursday night, and probably the beginning of the day
Friday. Our pattern will change though heading into the weekend. We
expect the high to shift to the east in response to the approach of
a longwave trough pattern that is expected to settle over the north
central to northeastern sections of CONUS.  This pattern will open
up a corridor of moisture from the Gulf of Mexico to stream into the
area coincident to a baroclinic zone that will establish itself over
the area.

By the end of the weekend, the push of the northern stream trough is
sufficient enough to usher in another shot of colder and drier air
for Sunday on gusty northwest winds.

Confidence in the above portion of the forecast is low to moderate
as the movement and timing of the mentioned systems do not have good
run-to-run consistency.


As of 800 AM EDT Monday...

Flight conditions generally favor IFR vcnty of the Blue Ridge
where light southeast upslope flow is resulting in Low Cigs,
some of which is obscuring the ridges. This low cloud layer
will persist until at least mid morning.

Strengthening low level isentropic lift ahead of an approaching
cold front will result in occasional showers and patchy light
rain from WV into the OH/TN valleys. This coverage is expected
to increase today as the front begins to push into the region.
Low level mixing in combination with the daytime heating should
allow for a period of VFR during the afternoon with cloud bases
030-070. After sunset, trend will go back the other way with
lowering cloud bases favoring MVFR category. IFR/LIFR may
develop along the western slopes where winds become
northwesterly behind the front generating upslope stratus which
would impact the higher elevation terminals such as BLF.

Winds aloft will be increasing ahead of the cold front. As such,
LLWS may occur just above stable boundary layer. Once deeper
mixing occurs Monday afternoon, then break down of the stable
layer will result in a better alignment of wind direction,
although surface gustiness will increase...especially at KBLF
where wind flow will be better aligned with the ridge
crest...gusts favoring 18-24kt range.

Extended Discussion...

Confidence in the extended portion of the forecast is low as
several models show a baroclinic zone persisting across the
region into at least Tuesday or early Wednesday. This may at
least keep sub-VFR ceilings around into Tuesday night as low
level moisture persists. Does appear that some improvement back
to VFR could occur Wednesday into Thursday as high pressure
builds in from the north. This before deeper moisture starts to
rebound from the southwest on Friday ahead of another cold front
that will approach by the end of the week.




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