Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 251725

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

High pressure off the coast will bring one more day of fair
weather and mild temperatures to the region today. However, a
series of low pressure systems will move out of the Mississippi
valley for the second half of the weekend and into the first part
of next week. This will bring a good chance for showers and
thunderstorms to the area through Tuesday, though temperatures
will remain well above normal.


As of 1020 AM EDT Saturday...

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 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. Southerly flow around these systems will help warm
temperatures into the 70s which is 15F to 20F warmer than normal
today. Overnight temperatures will also remain warmer than
normal (15F-20F) with readings in the upper 40s to lower 50s.
This moist and deep southerly flow will also bring high clouds
in the region, but not thick enough to deter the sun from
shining bright.


As of 400 AM EDT Saturday...

Active weather pattern expected through the period with a
kinematic zonal flow embedded with several vigorous upper
short waves or closed lows. In fact there are five such systems
slated to track across the U.S. from west to east over the next
seven days. Thus, there will be numerous and frequent chances
for rain. As instability increases, the threat for
thunderstorms, some even strong to severe, also increases
through the period. Given that all of the systems are of Pacific
origin, temperatures will remain mild and well above normal
through the period.

The first upper system to bring precipitation to the area will
arrive Sunday morning. While this will be a potent system as it
tracks through the Southern Plains and into the western TN
Valley today, it will be lifting north into the Ohio Valley and
weakening, as well as slowing in forward speed, as it arrives
in our region, thanks to a downstream upper ridge in the western
Atlantic. Although shear is fairly decent and dynamics are
notable, instability is marginal at best. Would expect rain
showers to arrive in the western parts of the forecast area
around daybreak Sunday, then translate east through the morning
into the afternoon. Likely to categorical pops are warranted. Enough
instability is indicated to warrant mentioning isolated thunder
west of the Blue Ridge during the afternoon where the air mass
will be more unstable. This first system will lift north of the
region Sunday evening/Monday morning, but leave an increasingly
moist and unstable air mass in its wake.

Monday, the CWA will be located between upper lows/troughs.
However, the air mass will be moist and unstable. With forcing
increasing during the afternoon/evening in advance of the
upstream short wave, would expect some scattered showers and
isolated thunderstorms to pop up across the forecast area.
Nothing organized is anticipated.

By Tuesday, the next in the parade of troughs/upper lows moves
into the region from the west during the morning hours then into
the Piedmont by afternoon. Instability and moisture are
significantly more abundant as this system arrives. In addition,
the more kinematic nature of this second/open wave system
combined with notable instability warrants the mention of
thunder areawide during the afternoon/early evening Tuesday.
Fortunately, upper-level wind fields/hence shear, appear to be
relatively weak and weakening further with time as this system
shears out toward the northeast U.S. So while a few stronger
storms cannot be ruled out, the severe threat appears low with
this system, but does appear to be somewhat greater than with
the Sunday system.

QPF for the two events should average from just under an inch in
the west to about 1/2 inch across the Piedmont. Given relatively
dry weather in the recent past, this should not cause any
hydrologic issues.

As noted above, temperatures will be unseasonably warm through
the period with lows mainly in the 50s and highs in the 60s west
to the 70s east. These temperatures are around 20 degrees above
normal for lows and 10-15 degrees above normal for highs.


As of 420 AM 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 130 PM EDT Saturday...

VFR conditions should continue for all TAF sites through 10Z
Sunday. However, extensive mid and high clouds will continue
overspreading the area, so it will be BKN-OVC all day in the
100-250 range.

Moisture will continue to increase overnight/early Sunday as a
vigorous upper-level low slowly approaches from the Midwest on
its track into the OH Valley. The continued moist south to
southwest flow ahead of this system will bring lower MVFR
clouds/ceilings to the mountains early Sunday. Showers should
not arrive until mid to late morning Sunday west and afternoon
east. Current thinking is that we will only see a low stratus
deck early Sunday and not any fog, but some MVFR in fog cannot
be completely ruled out early Sunday.

Winds will generally be S-SW through the afternoon becoming
S-SE after 00Z. Look for speeds of 5-10kts through 00Z, mostly
5kts or less after 00Z. Some low end gusts are possible once
again today across the Piedmont, but are not likely west of the
Blue Ridge.

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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