Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 270829

Area Forecast Discussion
National Weather Service Blacksburg VA
429 AM EDT FRI MAY 27 2016

High pressure stays off the mid-Atlantic coast today into tonight
expanding west toward the southern Appalachians by Saturday morning.
A tropical low shifts northeast toward the South Carolina coast  by
Sunday morning, lingering across the Carolinas through the first
part of next week.


As of 350 am EDT Friday...

Very little change in the forecast for today. Still expect most
convection should fire along the Blue Ridge and Alleghanys at least
by midday/early afternoon and drift north-northeast at 10 to 20 mph.
As with yesterday will have to rely on low lvl outflow and
convergence along the ridges to power the convection. There is some
hint of weak upper forcing so have to account for that, but all in
all still sitting at isolated to scattered coverage, with less east
of the foothills. Expect sunshine to heat things up to the upper 70s
to lower 80s across the mountains, while the east reaches the mid

Tonight, seems convection weakens and ends after midnight, though
cannot rule out isolated showers after midnight, but hard to
pinpoint any one location. With moist low lvls and some clearing
should see fog once again spring up in the valleys and where it
rains during the afternoon/evening. Lows remain elevated in the
lower to mid 60s areawide.


As of 400 am EDT Friday...

Main concern continues to focus around a tropical system...not yet
sure if it will be named...but nonetheless a low pressure area of
tropical origin...that the models are in general agreement on
drifting northwest from the Bahamas into South Carolina or
eastern North Carolina by Monday. From that point...the GFS
remains the outlier in keeping the system lingering across the
Carolinas for several days through much of next week...not exiting
the region until an upper trough arrives from the Midwest.

A spiral band of preceding moisture is slated to reach the
southeast part of the CWA by Saturday night/Sunday morning. In
advance of this feature during the day Saturday...there is a very
clear area of subsidence and a definite lack of precipitation. It
appears that much...if any...convection that develops Saturday
prior to the evening hours...would be across the
Alleghanys...perhaps far southwest VA/northwest
association with differential heating. Rain showers should then
begin to track into or develop across the southeast part of the
Piedmont Saturday evening...but would expect rain overnight into
early Sunday to focus more toward the core of the tropical system
back across eastern SC/NC.

Sunday is the day that most models agree will be the most active
and wettest day across the CWA with the tropical low likely
located across north central or northeast SC or perhaps southeast
NC. PWATS move upwards into the 1.5 to 2.0 inch range depending on
model...and an area of deep tropical moisture appears evident on
most models from the tropical low northward across the Piedmont.
There could be some enhancement of rainfall along and east of the
Blue Ridge as well. With all of this in mind...have advertised
likely to categorical pops east/along the Blue Ridge...with mostly
chance pops to the west. Southeast West Virginia into far
southwest VA will see the least rainfall from this pattern. The
amount of thunderstorms is in question as instability is minimal
given extensive cloud cover and a saturated atmosphere. So...have
opted to just include slight chance thunder for now with moderate
rain showers likely.

By Monday...the ECMWF and Canadian begin to shift the deeper
moisture and tropical low off the coast...while the GFS circulates
the remnant low across SC. Abundant tropical moisture will remain
in place across the region...but confidence in widespread heavy
rainfall less than on Sunday...unless believing GFS solution. Have
generally reduced pops to mostly high chance low end likely for
this period.

Low temperatures will be well above normal through the
period...mainly in the 60s. Maximum temperatures will be mostly in
the upper 70s to lower 80s...but it will be very humid through the
period for sure.


As of 420 am EDT Friday...

As noted above...considerable uncertainty remains in the medium
range forecast...hinged largely on the
track...movement...duration of the southeast U.S. tropical low
pressure area. Will it stall and linger per continued GFS
solutions...or move out to sea by Tuesday per ECMWF and other
models? A blocking upper ridge to the north will be the key.

If the tropical system does linger for several days with non-
existent steering currents and blocked upper flow, rainfall may
become a concern as a result. The other key player in the extended
period is a cold front and upper trough to the west, which
travels slowly eastward during the period. Per the GFS...this may
be the system that finally takes the tropical system remnants out
of the region. Again...the strength of the upper ridge will
determine how far the front makes it. In any case, we can expect
scattered to perhaps numerous diurnal convection with warm/humid
conditions through the period. In general, high temperatures will
remain mostly in the 70s mountains to the 80s elsewhere. Low
temperatures will be mainly in the 60s.


As of 127 AM EDT Thursday...

Mainly looking at VFR conditions at 06z, but looking at fog
formation at LYH/LWB/BCB through 12z.

Today should be similar to Thursday with models still favoring
scattered shower or tsra coverage for the Blue Ridge and west to
the WV. Will have VCTS at BLF/BCB per latest high-res radar
depiction, but again overall coverage is lacking. Otherwise VFR
through 06z/28th.

Extended discussion...

Saturday the region will be in between systems with the upper low
remaining west of the region and a tropical system developing off
the SC coast. Little forcing and lift plus increased subsidence
should confine any convection to the mountains...isolated to
scattered diurnal activity at best. For the remainder of the
period...the forecast will hinge heavily on the track of the
tropical low pressure area...which at a minimum will bring
increased moisture and shower chances to the area...which could
linger well into next week. This will result in periods of mvfr-
ifr cigs and greater potential for late night/early morning fog as
well. Confidence in the evolution of the forecast past Sunday is




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