Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 270914

Area Forecast Discussion
National Weather Service Blacksburg VA
514 AM EDT Sat May 27 2017

Several low pressure systems will track across the central and
eastern United States today through Monday with a series of
fronts over the Mid Atlantic region. Tuesday a stronger low will
develop over the Northeast pushing a final front through the
area. High pressure builds in for the end of next week.


As of 400AM EDT Saturday...

Watching two potential areas of precipitation today. The first is
the cluster of showers and thunderstorms in Ohio and northern West
Virginia. Showers from this decaying mesoscale convective system may
cross southeast West Virginia and the Alleghany Highlands, mainly
north of a Lewisburg to Lexington line this morning. The outflow
boundary from these showers will be on of the locations that
thunderstorms develop this afternoon. Models showed differing
solutions on the location of storm development.

Next, the short wave in northern Arkansas and southern Missouri will
move through the westerly flow aloft, reaching the Mid Atlantic
region late this afternoon into this evening. This feature will also
trigger scattered thunderstorms. Expecting enough heating today,
even with the residual cloud cover from this morning, to have an
unstable airmass this afternoon. Enough instability and bulk shear
that isolated severe storms are possible.

At the surface, low pressure will track from the Ohio Valley
southeast to the Virginia coast by Sunday morning. No real change in
airmass today or tonight but moisture is increasing,with
precipitable water getting above one inch and surface dew points
rising into the upper 50s to mid 60s. Only minor changes to highs
today and lows tonight.


As of 400 AM EDT Saturday...

The second consecutive day of severe convective weather
potential should evolve on Sunday as a strong short wave,
embedded within the base of the broad upper trough centered in
south central Canada, moves through the area during peak
heating. Associated with this short-wave feature is a 100+kt jet
at 250mb. Strong to extreme instability is indicated by several
models, with the GFS likely overdone with 5000-6000 J/kg of CAPE
and lifted indices of -8C. However, all of the models suggest
good dynamics and instability will be in place as the short wave
moves into the area during peak heating. For now, the region is
largely in a marginal risk for severe, but would not be at all
surprised to see this upgraded one or even two categories before
all is said and done. The NAM NEST model shows the development
of discrete supercells developing during the mid to late
afternoon Sunday then quickly evolving into an intense squall
line that races across the Piedmont. It appears large hail and
damaging winds would be the main threat, but tornadoes cannot be
ruled out given the strong helicity/shear in place with the jet
streak over the area at the time. Will need to watch closely
with later model runs. Individual cells should be moving fairly
quickly, reducing the threat of flooding. However, we could see
high precipitation supercells at the outset resulting in
localized flooding, which won`t take much given the very low FFG
values and saturated soil from all of the recent rainfall. Of
course the caveat to all of this occurring is the potential for
an early morning much weaker squall line or thunderstorm complex
to leave behind a more stable/bubble high air mass that reduces
the potential for strong afternoon convection. Unfortunately,
this does not appear to be the favored mode, so we will
certainly need to keep a close eye on the severe potential
Sunday afternoon.

Given the speed of movement of the Sunday evening convection,
would expect any lingering activity to quickly race off toward
the coastline during the mid to late evening, leaving a weak
outflow boundary or front draped from northwest NC into
southeast VA. A broad upper low will remain across the Great
Lakes region with upstream energy/short waves still in place.
However, at this point, the most moist and unstable air mass
will be east and south of the area. Afternoon convection will
likely develop once again near the boundary, mainly along and
east-south of the Blue Ridge. SPC has indicated a marginal risk
for severe with this activity as well, but it certainly does not
seem to be as concerning as the Sunday situation.

Tuesday, the area remains just north of the baroclinic zone with
upstream energy from the broad upper trough still in place.
These features will likely continue to interact with each other
bringing a daily risk of showers and thunderstorms to the area.
The best instability and moisture will remain just south of the
area, so the overall severe threat seems low, as well as the
flash flood threat.

Temperatures during the period will be mostly just above
seasonal normals. Sunday will be the hottest day as 850mb temps
tease the +20C level. This will yield max temps in the 70s
mountains and 80s elsewhere with lows in the 50s and 60s. For
the remainder of the period, look for temperatures to run just
above normal with lows in the 55-60 range and highs in the 75-80


As of 430 AM EDT Saturday...

Broad low pressure aloft will remain anchored across the south
central Canadian provinces, shifting slowly east through the
later half of the week. This will keep a less moist and more
stable air mass across the region. As the trough axis moves
through the area Wed, look for a good chance of showers and a
few thunderstorms, but the severe threat with this activity
appears low. Thu-Fri should be dry before the next embedded
short wave moves through the region from the northwest.

Temperatures will average near normal through the period with
lows mainly in the 50s and highs in the 70s west to lower 80s


As of 145 AM EDT Saturday...

Precipitation in advance of an approaching cold front will
start to enter the western section of the area late this
morning. The result will be period of VFR conditions through
roughly 15Z/11AM. After that time, MVFR ceilings will start to
increase across the area from northwest to southeast, reaching
a KMKJ-KLWB line by 18Z/2PM. Scattered showers with mainly VFR
visibilities will accompany this cloud cover.

MVFR shower and thunderstorm chances increase across more of
the area after 21Z/5PM. Any band with heavier rainfall may
produce IFR conditions into the evening. Confidence is low at
this time as to where greatest coverage of showers and
thunderstorms will develop. Kept VCSH in the TAfs for now.

Extended Aviation Discussion...

Additional convection and sub-VFR conditions probable Sunday
into Sunday night as another disturbance crosses the area.

Convection becomes less organized Monday into Tuesday with hit
and miss sub-VFR conditions. Perhaps late night and early
morning sub-VFR river and mountain valley fog.

Better organization of convection Wednesday into Thursday with
the passage of a cold front.

Confidence concerning the general weather pattern during this
part of the aviation forecast period is moderate to high, but
confidence on specific timing of any sub-VFR condition is low.


As of 450 AM EDT Saturday...

Minor river flooding continued on the Dan River at South Boston
and on the Roanoke River at Randolph this morning.

For today localized flash flooding is possible since any of the
stronger thunderstorms this afternoon and evening will produce
high rainfall rates and Flash Flood Guidance was generally 1.5
to 2.5 inches in 3 hours. Storm motion is east around 35 mph so
locations that have repeated storms will have a higher threat
for flooding.

Depending on where any heavy rain falls today, a similar
scenario is expected on Sunday. Locations will high rainfall
rates or training thunderstorms will have an increased threat of
flash flooding and rises on smaller creeks and streams.




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