Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 250836

Area Forecast Discussion
National Weather Service Blacksburg VA
436 AM 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 300 PM EDT Saturday...

High pressure surface and aloft will remain off the coast as a
closed low slowly approaches from the west. This will bring
increasing clouds as southerly winds bring increasing moisture
into the region. Forcing is largely absent so any significant
chance for precipitation will hold off until later Saturday
night. Temperatures will remain well above normal with highs
today in the mid/upper 70s east of the Blue Ridge, lower 70s to
the west.


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 215 AM EDT Friday...

Expect VFR conditions for all TAF sites this period. High
pressure surface and aloft will remain off the coast as a
closed low slowly approaches from the west. This will bring
increasing clouds as southerly winds bring increasing moisture
into the region, but expect any cigs look to remain at or above
4kft. Forcing is largely absent so any significant chance for
precipitation will hold off until later Saturday night as the
closed low pushes in from the west, though cigs will start
trending below VFR by 06Z Sunday. Expect generally light winds
though some gusty conditions will affect KLYH and KDAN this

Extended Aviation Discussion...

Moisture continues to increase over the weekend ahead of an
upper-level low pressure area moving northeast from the southern
plains. No significant precipitation is expected until Sunday
when sub- VFR conditions become more of a possibility. Otherwise
looking at overall VFR to start the weekend with most lower
cigs remaining west of the area until later Sunday at this
point. Periods of unsettled weather will continue into the first
half of next week as a series of upper-level lows track from
west to east across the U.S.




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