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Area forecast discussion 
National Weather Service Blacksburg Virginia
756 PM EDT Wednesday may 6 2015

Synopsis...
a backdoor cold front will slowly make its way into and through our
region tonight and Thursday...all while high pressure continues its
grip on the southeast United States. A slow moving area of low
pressure will move north along the coast of the Carolinas this
weekend. A cold front will move across the area on Monday or Tuesday
of next week.

&&

Near term /through Thursday night/...
as of 730 PM EDT Wednesday...

Rain showers/thunderstorms and rain widely scattered from southeast WV into the Piedmont...with
best axis of higher convective available potential energy extending from Floyd/Roanoke County
northeast toward District of Columbia...where the better instability lies. The
narrow band of thunderstorms and rain moving south toward Floyd-
Franklin/Pittsylvania will begin weaken through 8-9pm with loss of
sunshine. The airmass will still be conducive for isolate to
widely scattered showers into late evening with best coverage staying
north of a blf-lyh line.

Previous valid discussion...

The last of the showers are expected to end around 200 or 300 am
on Thursday across the Alleghany Highlands....east to near
Lynchburg.

We will need to watch northern and eastern parts of the region
tonight for an increase in low level cloud cover on the backside
of the advancing front. Some guidance is hinting at this to
occur. Our cloud cover forecast will allow for this potential to
some degree with the greatest coverage of clouds towards sunrise
Thursday being north and east of Lynchburg. Patchy fog will also
be possible to a greater degree late tonight and early Thursday as
compared to this morning....given more areas will have received
some precipitation today...light winds...and increased low level
moisture behind the backdoor front. Low temperatures will range
from the middle 50s to near 60 degrees over much of the region. The
higher elevations will be cooler with lows around 50.

On Thursday...the backdoor front is expected to take up residency
over the far western portion of the area. Look for the focus for
shower and thunderstorm develop to shift west from the Blue
Ridge...to be in the vicinity of the front. Coverage will be
scattered near the front and isolated east to the crest of the
Blue Ridge. Highs will only be slightly cooler than conditions of
today. Highs will range from the middle 70s to the upper 70s across
the mountains with upper 70s to around 80 across the Piedmont.

&&

Short term /Friday through Saturday night/...
as of 345 PM EDT Wednesday...

Short and long term forecast complicated by movement of potential
tropical storm along the southeast U.S. Coast. Latest NHC outlook
has a 60 percent chance of this developing into a
tropical/subtropical system within the next 48 hours somewhere off
the SC/Georgia coast. As with previous model runs...the GFS remains
the most aggressive in bringing this system into the central and
eastern portions of our County Warning Area underneath a broad upper ridge across
the eastern U.S. And weak steering currents. However...all models
indicate that we will see at least some effects in the way of
clouds and at least limited moisture into the eastern sections
within this time frame as a result of this system. To the west...a
broad upper trough persists in the western U.S. And is expected to
remain well west of the County Warning Area through this time frame.
Meanwhile...whatever is left of the backdoor front would appear
to be across either far SW Virginia into northwest NC or back in eastern
Tennessee...generally being absorbed into the large scale circulation
around the tropical system.

Focus for showers/thunderstorms should be either in the far
west...generally west of I-77...or perhaps near outerbands of the
tropical system in areas of differential heating. Cannot justify
any more than chance probability of precipitation at this point. With respect to
thunder...instability is limited and generally west of the region
across the Ohio/Tennessee Valley. Clouds and showers associated with the
tropical system will minimize instability across the Piedmont...so
overall thunder through Saturday will be limited.

Temperatures will be quite warm through the period...but may be
held down somewhat across the Piedmont pending cloud cover
associated with the tropical system. Generally look for lows in
the 50s and 60s with highs in the 70s to lower 80s...overall about
10 degrees above normal.

&&

Long term /Sunday through Wednesday/...
as of 400 PM EDT Wednesday...

Sunday appears to largely be a repeat of Saturday...with effects
of remnant tropical system across the east...and the frontal
system still well at Bay to the west. Monday appears to be the
best day for widespread convection and any threat of
severe...albeit that remains low...as the frontal system and
associated upper trough push through the area. GFS appears too
quick with pushing the front south of the area and bringing much
cooler/drier air into the region from the north. Have leaned more
toward the European model (ecmwf) solution of slowing this down...holding onto low
chances for convection across the southern counties into Tuesday. Much
of the upper support on Monday tracks either north or south of the
region...and there still remain questions as to how much
subsidence or interaction with the front the remnant tropical
system will have at that point. Have not advertised any likely
probability of precipitation at this point...sticking with middle to high range chance probability of precipitation.
Temperatures should remain well above normal Sunday and
Monday...trending more toward north readings Tue-Wed. Another
stronger system appears destined to approach from the southwest
toward the very end of the period...but for now have just
advertised slight chance probability of precipitation northwest NC.

&&

Aviation /00z Thursday through Monday/...
as of 735 PM EDT Tuesday...

Isolated/widely scattered rain showers/thunderstorms and rain early this evening will fade toward
midnight. The Airport that will stay close to any activity will be
lwb so left vcsh in it. Not seeing any thunder threat at the
airports...though thunderstorms and rain exists between roa/mtv as of this afd.

Overnight...the backdoor cold front will make progress into the
region and bring low level winds around to the NE-E. Some guidance
is allowing for the potential for some MVFR ceilings to arrive in
mainly north and eastern parts of the area...though confidence is
low at this occurring so left scattered in. Patchy light fog will
also be possible...especially in areas that received rain
today...and also in some of the mountain and river valleys...with
locally dense fog toward morning at lwb.

On Thursday...isolated to scattered shower and storm development
will again be possible along the leading edge of the backdoor
front and during the peak heating of the day. This would place the
greatest coverage across southeast West Virginia...south into the
northern mountains of North Carolina. For now will have vcsh in
blf/lwb/bcb tafs Thursday afternoon with VFR ceilings.

Forecast confidence Thursday night through Saturday is medium.

Afternoon and early evening showers and storms will be common
through Saturday...but the trend for the axis of best coverage
will slowly work its way west of the region. Conditions will
mainly be VFR except for some late night and early morning patchy
light fog and sub-version patchy MVFR clouds.

Forecast confidence Saturday night into Monday is low.

Model guidance is still showing a decent amount of variability in
their solutions regarding the Route a slow moving upper low along
the coast of the Carolinas will take late this weekend into early
next week. Accordingly...the timing and approach of a cold front
from the middle-Mississippi Valley will be impacted by the position
of the low as well. A consensus forecast places increasing
coverage of showers and storms over the area during this period
with a mix of MVFR and VFR conditions with the best coverage on
Monday.

&&

Rnk watches/warnings/advisories...
Virginia...none.
NC...none.
WV...none.

&&

$$
Synopsis...ds
near term...ds/wp
short term...rab
long term...rab
aviation...ds/wp

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