Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 260233

Area Forecast Discussion
National Weather Service Blacksburg VA
1033 PM EDT Sat Mar 25 2017

High pressure is off the coast in the western Atlantic, stretching
back across the southeastern states. A deep area of low pressure
will drift east over the Midwest tonight with a trailing cold south
into Louisiana. A warm front will reside from the Great Lakes region
to Lake Erie into Sunday.


As of 955 PM EDT Saturday...

Satellite fog product showed some breaks in the high clouds over
the mountains this evening. Seeing some low clouds forming on
the eastern slopes of the Blue Ridge as far north as Watauga
and Wilkes county. Models showed these lower clouds expanding
northeast after 06Z/2AM. HRRR and other Hi-Res guidance have
been consistent in bringing higher probability of precipitation
into the mountains around 12Z/8AM. Have trimmed back chance of
showers before then. Also adjusted low temperatures in a few
locations mainly based on the expected cloud cover.

An upper level ridge off the southeast coast will continue to play
blocker and divert a closed low over the Midwest toward the Ohio
Valley through Sunday. Meanwhile, a dry surface high pressure system has
moved off into the western Atlantic, however it remains stretched
back over the southeastern US. The combination of these two stable
high pressure systems will keep the area dry into tonight. As the closed
low enters the Ohio Valley tomorrow morning, prefrontal showers will
slowly move eastward through the day. Rain showers will cross over
the mountains in the morning, then over the foothills in the
afternoon. By late afternoon and into the evening, these showers
will track across the piedmont. Lingering stable surface high will
still cover the area east of the Blue Ridge in the morning and into
parts of the afternoon. This will limit storms from becoming to
strong in the afternoon and reduce rainfall amounts. Some wind gusts
up to 35 mph are possible in isolated stronger storms but chances
are very low.

Southerly flow around these systems and a slower entry of showers
into the area will help Sunday temperatures warm into the lower to
mid 60s across the mountains. With the later arrive of rain east of
the Blue Ridge, temperatures will be able to warm into the low to
mid 70s.


As of 330 PM EDT Saturday...

Upcoming week will be active nationwide as series of Pacific
Low pressure systems propagate east across the CONUS. Leading
system will be weakening as it moves northeast around the
periphery of an upper level ridge of High pressure over the
western Atlantic early Monday. A second shortwave will be close
on the heals of the first, passing across the area Monday night
into Tuesday. The overall result will be a threat for showers
and isolated thunderstorms across our region from Sunday night
through Tuesday night...qpf averaging about a quarter to a half
of an inch.

In spite of the weather being somewhat unsettled, it will be
mild with temperatures running about 15 degrees above normal.

Can`t rule out a strong thunderstorm or two Monday, but overall
dynamic support appears to be lacking for anything organized.
Tuesday`s trough passage may come through early enough to
preclude an organized severe threat for our CWA, except for
maybe the Piedmont. Instability and moisture are significantly
more abundant Tuesday compared to Monday, but favoring the
Piedmont and Tidewater where there will be higher CAPE and
and better dynamic support aloft.


As of 400 PM EDT Saturday...

Behind the Tuesday system, we will see a brief respite from the
parade of storm systems as an area of high pressure from Canada
filters southward into the area stabilizing and cooling the air
mass. This should lead to a relatively precipitation free and
slightly cooler period. Cannot completely rule out some light
showers due to weak ripples in the flow aloft. Temperatures and
dewpoints will drop about 10 degrees from the Mon-Tue period.

By Friday, the next, and fairly potent, upper low tracks toward
the area. However, there is considerable discrepancy between
the GFS and the ECMWF in the configuration and timing of this
system into the eastern U.S. The GFS depicts more of an open
wave reaching the CWA as early as Thursday night moving east of
the area by Friday afternoon. Meanwhile, the ECMWF is much
slower, deeper, and further south with a cutoff upper low
lifting it slowly north-northeast into the Ohio Valley by late
Friday. The difference between the two models can be accounted
for in noting the strength and blocking of an upstream ridge
across the Atlantic. This upstream ridge is much stronger per
ECMWF, which also depicts a notable wedge down the east side of
the Appalachians as the closed low lifts north into the Ohio
Valley. The ECMWF would result in a much wetter and stormier
pattern for the CWA, although it would not arrive until Friday
as opposed to Thu night per GFS. For now, have generally broad
brushed the area with high chance to likely pops beginning Thu
night and continuing into Friday. In addition to the differences
noted above, there are also massive differences between the two
models regarding instability and the potential for severe, with
the ECMWF much more noteworthy in this category. For now, have
included a chance for thunder without any further details. It
will likely take several runs for the models to resolve their
differences on this system. Another system is slated to follow
by the end of the weekend or the beginning of the next week, so
the active pattern is expected to continue into the next week.

Temperatures will be closer to normal Wed-Thu, then begin to
trend back to above normal levels Fri-Sat.


As of 709 PM EDT Saturday...

Generally VFR through the TAF period. For tonight, expect a
continued stream of cirrus clouds for the terminals. If there is
an area of lower MVFR stratus it would be along the southern
Blue Ridge into northwestern North Carolina after 05z given
increasing moisture levels and winds backing to light
southeasterly. That would lead to obscured mountain ridges, but
confidence is low that such cloudiness would affect either
Roanoke or Danville. South winds 6-8 kts become southeast 3-6
kts overnight.

Expect increasing BKN/OVC VFR cloudiness early Sunday morning,
building eastward from west to east. Appears per recent high-
resolution guidance that the earliest that showers would affect
our western airports would be during the afternoon (after 17z).
Potential for isolated thunder in the showers as well but only
limited coverage. Left out of the TAFs given uncertainty at this
late range. Ceilings/visibility should remain VFR, though
potential brief MVFR visibility in steadier showers. Southeast
winds Sunday morning increase and become southerly 6-10 kts.

Extended Aviation Discussion...

An active weather period with a series of vigorous upper-level
low pressure areas tracking from west to east across the U.S.
during the week. Showers and a few thunderstorms will overspread
the area Sunday with the first system with widespread sub-VFR
cigs/vsbys in clouds and precipitation. Monday the area will be
between systems, but a moist/unstable air mass will remain in
place and widely scattered showers and isolated thunderstorms
are possible. Areas of sub-VFR cigs/vsbys are also possible,
especially during the morning hours, in low clouds and fog. The
next upper-level low will arrive Tuesday with another chance for
showers and thunderstorms along with periods of sub-VFR
conditions. Wednesday-Thursday may bring a period of improved
aviation conditions as a weak cold front temporarily pushes the
deeper moisture south of the area. However, by Friday the next
in the series of upper-level systems arrives with another chance
for showers, thunderstorms, and sub-VFR conditions.




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