Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 231720

Area Forecast Discussion
National Weather Service Blacksburg VA
120 PM EDT Wed Aug 23 2017

A cold front will move across the Appalachians and central mid
Atlantic region today, bringing some showers and thunderstorms
to the region, especially east of the Blue Ridge. A large high
pressure system will move out of central Canada and build
eastward behind the front tonight, bringing significantly
cooler temperatures to the region into the weekend.


As of 935 AM EDT Wednesday...

Cold front in the mountains this morning will move southeast
this afternoon into tonight. This boundary will create scattered
showers and thunderstorms especially east of the Blue ridge. For
the morning update, adjusted temperatures with the latest
surface obs and trends, then shaped values towards lav headings
into the afternoon. Will hold with current high temperatures,
but readings will be dependent on cloud cover and convection.
Most of our area is under general thunderstorms with a small
portion in the southeast clipped by the marginal severe weather
on the SPC Day One outlook. In any case, the best chance for
severe thunderstorms will be off to our east with best
instability and richer Capes. Adjusted pops for this morning
with latest radar trends, then blended in the HRRR for this
afternoon into tonight. More changes later...

As of 300 AM EDT Wednesday...

Showers/some thunder out ahead of a cold front approaching
from the northwest are having little success advancing into
the region. Given the lack of upper support believe this will
continue to be the case until the actual frontal boundary
arrives later this morning and starts working across the area.
This timing will allow for diurnal heating to generate some
surface based instability and help reinvigorate convection along
the front east of the Blue Ridge this afternoon. However, the
best low level winds and dynamic support will be moving by to
our north which will greatly limit the potential for any
thunderstorms to become severe. There may be a window later this
afternoon when enough surface based instability can pool just
ahead of the front to give storms a boost in the far
southeastern portion of the area, but this potential is marginal
at best. Will trend convection in the grids from west to east
with improving conditions indicated behind the front as winds
become northwest.

A large high pressure system will move out of central Canada
and build eastward behind the front, bringing an end to any
lingering showers early tonight. Slackening winds west of the
Blue Ridge and a cooler airmass should help generate some late
night valley fog.

Highs today will generally be in the upper 80s east of the
Ridge, while the earlier passage of the front keeps readings in
the mid/upper 70s to the west. Lows tonight will be notably
cooler with mid 60s east, low/mid 50s west.


As of 350 AM EDT Wednesday...

A broad upper trough will remain in place across eastern Canada
and the eastern Great Lakes, with the base of the trough across
our CWA. The cold front of today will move well to our south and
east and should extend across southeast North Carolina into
central Georgia at 12Z Thu. All associated shower activity
should be well south of our area. During the afternoon and
evening, a short wave embedded within the broad upper trough
will sweep through the Ohio Valley into PA, with the dynamics of
the trough impinging on the far northern parts of the CWA,
especially the Alleghanys. This feature should bring lots of
clouds to upslope areas of the Alleghanys, mainly north of I-64,
with even a few showers possible during max diurnal heating across
northwestern Greenbrier county. By far, the greater chance of
showers will be further north toward Snowshoe and Elkins. This
feature will track northeast of the region Thursday evening
taking any threat of showers with it. For Friday, a large
sprawling high pressure area will slide eastward across the
Great Lakes toward NY and PA. This will begin to set up east to
southeast flow along the Blue Ridge and a few showers will be
possible especially along the southern Blue Ridge from Watauga
county northeast toward Grayson and Carroll. Lots of clouds can
also be expected in these areas by afternoon. The showers should
dissipate during the evening.

A notable drop in 850mb temperatures will be noted during this
period from the current situation. Looks like the end of the
+20C 850mb temperatures for a while, perhaps most of the rest of
August as a pocket of cool air first moves into the region from
the northwest, then more from the northeast into Friday as the
surface high moves east. 850mb temps will drop back into the
+10C to +14C range. The most notable surge of this cooler air
will come behind the upper short wave slated to move across
northern areas Thu. Max temperatures especially will drop to
just below normal levels Thu to several degrees below normal by
Friday. Min temperatures will be below normal, but not to the
degree of the max temperatures. Clouds and moisture may limit
overnight cooling, especially across the Piedmont and NC
Foothills. Look for temperatures to average about 10 degrees
cooler than current conditions with near 70 west higher
elevations to around 80 Piedmont. Lows will be mostly in the 50s
west to around 60 across the Piedmont.


As of 415 AM EDT Wednesday...

High pressure will linger across the northeast U.S. through the
weekend keeping an easterly flow across the region and enhancing
clouds and showers along the Blue Ridge, especially northwest NC
into far southwest VA. Diurnal showers appear possible Saturday
and Sunday, with a tendency for the activity to drift more in
eastern TN and western NC with time. Temperatures will remain a
few degrees below normal through the weekend thanks to the
northeast flow and 850mb temperatures hovering around +12C
through the weekend.

As we move into early next week, the pattern becomes increasingly
complicated thanks to the remnants of Harvey that are progged
to move into the western Gulf over the weekend, then inland
somewhere along the western Gulf Coast, TX/LA area. Weak
steering current aloft do not help predictability at this point.
While all of the models are in general agreement about the
initial movement of this system, considerable timing
differences arise after it moves inland and subsequently where
the remnants will track, perhaps northeast through our region or
more likely the TN/OH Valley, or potentially on a more eastward
track through the south and southeastern U.S. The GFS has the
effects of Harvey combining with the wedge in place over our
area and thus rain moving into the area as soon as Monday night,
while the ECMWF still has the tropical system along the TX/LA
coast line. Meanwhile, a weaker tropical system is poised to be
lingering off the southeast U.S. coast, which appears to further
enhance the wedge across our region. To say the least, weather
conditions will become quite interesting next week as these
systems interact and control the synoptic situation across the
eastern U.S. Some solutions would have our area becoming quite
wet by the mid part of next week, while others do not. Stay

With persistent high pressure to our northeast and an easterly
surface flow into the region, look for below normal temperatures
to continue 850mb temperatures will hover in the +14C to +16C
range into early next week. Maximum temperatures will be well
below normal with highs in the 60s and 70s, although lows will
just be near normal thanks to increasing cloud cover and a moist
maritime easterly flow.


As of 120 PM EDT Wednesday...

MVFR convection associated with cold front moving southeast
across the region this afternoon into tonight. With the front
advancing into the Piedmont during peak heating, expect enough
surface based instability to generate scattered showers and
thunderstorms along it east of the Blue Ridge this afternoon
into this evening.

Conditions improving to VFR behind the front as winds become
northwest. A large high pressure system will move out of
central Canada and build eastward behind the front tonight.
Winds will begin to come around to the northeast east of the
Blue Ridge overnight as the large surface high begins to
translate toward New England.

Enough northeast wind should develop overnight to prevent fog
across the Piedmont, but good radiational cooling and clearing
skies should promote good fog development at LWB and BCB, but
not as much confidence as usual with advection of drier air into
the region. After any morning fog or low clouds, VFR conditions
will prevail Thursday. Any isolated shower may be possible
across the extreme northern portion of forecast area.

Medium to high confidence in ceilings,visibilities and
winds during the taf period.

Extended Aviation Discussion...

Generally VFR conditions are expected Thursday night into the
weekend as high pressure builds over the region. There will also
be a diurnal trend for late night fog/stratus especially west
of the Blue Ridge with KLWB and KBCB the most likely TAF sites
to be affected. Isolated convection may be possible across the
mountains Saturday into Monday.


As of 1205 PM EDT Wednesday...

KFCX doppler radar has completed all repairs and is now back in




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