Area Forecast Discussion
Issued by NWS Blacksburg, VA
FXUS61 KRNK 271920
Area Forecast Discussion
National Weather Service Blacksburg VA
320 PM EDT FRI MAY 27 2016
High pressure remains off the mid-Atlantic coast tonight and expands
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.
.NEAR TERM /THROUGH SATURDAY/...
As of 320 pm EDT Friday...
A southwest flow of warm and moist air continues around the high
pressure center off the Mid Atlantic coast this afternoon into
Saturday. The combination of heating,instability and terrain
creating scattered showers and thunderstorms this afternoon. SPC
Mesoscale Analysis at 18z showed SBCAPE of 1000 to 2000 j/kg. SB LIs
were minus 2 to minus 4 this afternoon. The best chance for showers
and thunderstorms will occur along the Blue Ridge and Alleghanys
this afternoon into this evening. With limited shear and
instability, not expecting any severe weather this afternoon.
However, slow moving thunderstorms in weak upper flow could produce
locally heavy rains. Combined the HRRR and NAM for pops this
afternoon, then shaped towards Superblend for tonight. With the
loss of solar heating, convection weakens and ends after midnight.
With moist low level and some clearing expect areas of fog to
develop, especially in the valleys and where it rains during the
afternoon/evening. Low temperatures remain mild with readings
from the upper 50s in the mountains to the mid 60s in the
For Saturday and beyond, the main concern continues to focus around
a tropical system/low, that the models are in general agreement on
drifting northwest from the Bahamas into South Carolina by early
next week. In advance of this feature Saturday...there will be an
area of subsidence and a definite lack of precipitation. However,
with heating and some instability expected isolated to scattered
showers and thunderstorms to develop in the northwest portions of
the forecast area. High temperatures Saturday afternoon will range
from the mid 70s in the mountains to the mid 80s in the Piedmont.
.SHORT TERM /SATURDAY NIGHT THROUGH SUNDAY NIGHT/...
As of 320 pm EDT Friday...
Please refer to products and bulletins from the National Hurricane
Center regarding tropical low. The ridge along the Mid Atlantic
coast will start shifting east Saturday night as tropical
disturbance/tropical storm makes landfall on the South Carolina
coast. Rain showers should 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. Increased pops and QPF for Saturday night
especially with upslope flow along the southern Blue Ridge
mountains. Low temperatures on Saturday night will only drop into
the upper 50s in the west to the mid 60s in the east.
Sunday will be very unsettled with periods of rain across the
forecast area. The rain will be heavy at times. Played the
highest pops along the southern Blue Ridge with the upslope
enhancement. PWATS climb upwards into the 1.5 to 2.0 inch range
and an area of deep tropical moisture appears evident on most
models from the tropical low northward across the Piedmont. The
amount of thunderstorms is in question as instability is minimal
given extensive cloud cover and a saturated atmosphere. Lowered
high temperatures Sunday to around 70 degrees in the mountains to
the upper 70s in the Piedmont. Showers and a few thunderstorms
continue Sunday night with best chances along the Blue ridge
mountains. Low temperatures Sunday night will vary from the mid
50s in the northwest mountains to the mid 60s in the piedmont.
.LONG TERM /MONDAY THROUGH FRIDAY/...
As of 230 pm EDT Friday...
Model guidance varies on the position and timing, but each offer a
solution for an area of low pressure along the east coast Monday
morning, somewhere between South Carolina and North Carolina.
Moisture on the western flank of this system is progged to influence
our region to various degrees through Tuesday, model dependent. The
GFS and Canadian solutions are slow to eject the system northeast
from the the SC/NC border. The GFS offers a solution that ejects a
piece of the low northeast Monday, but with the main system
remaining anchored near SC/NC. The European and NAM solutions eject
the low a little more quickly towards the northeast. In different
ways, both of these scenarios yield similar forecast solutions for
the region. If the slower solution pans out, it will mean the bulk
of the moisture will remain south of the forecast area, and the
region will experience subsidence on the north side of the system.
If the quicker solution pans out, any precipitation the area
receives will be on the backside of the system, skirting the eastern
portion of the region. Our forecast will reflect the best chances of
precipitation across the southeastern portion of the area Monday
into Tuesday, with limited or no precipitation across the far
Tuesday night into Wednesday night, the low will continue to make
slow progression to the northeast. The biggest difference will be
with the European solution that started its progression just
slightly sooner. At the same time, an upper trough will be
approaching from the west. The uncertainty in the departure track of
the low, plus increasing dynamics from the approaching trough, and a
boundary layer rich with moisture will allow for isolated to
scattered showers and storms across the forecast area.
Chances gradually increase Thursday into Friday for one of two
reasons. If the GFS solutions comes to pass, the upper trough will
deepen, closed off, an stall across the center of the country. This
would allow for plenty of moisture to stream north from the Gulf of
Mexico into the region. Pieces of energy ejecting eastward from the
main low will act as trigger mechanisms for daily convection. If the
European solution is correct, precipitation chances will also
increase as the upper trough remains progressive, and moves eastward
along with its associated cold front into and through the region.
Temperatures during this portion of the forecast will average above
normal with upper 70s to around 80 common for highs across the
mountains and low to mid 80s across the piedmont. Lows in the upper
50s to around 60 will be common across the mountains with low to mid
60s across the piedmont.
.AVIATION /19Z FRIDAY THROUGH WEDNESDAY/...
As of 136 pm EDT Friday...
VFR ceilings will prevail this afternoon into tonight, with MVFR in
convection and low clouds. Isolated convection starting to form
in the warm unstable air across the higher terrain. Best focus for
afternoon showers and thunderstorms will be across the mountains
and west of the Blue Ridge. There are some capping aloft and the
lack of a significant trigger which will hinder convection and its
Placed the mention of VCTS to BLF/BCB/LWB in the tafs with the best
chance for MVFR thunderstorms this afternoon into this evening.
Decided to add a VCSH to ROA with cloud development this
afternoon. Overnight...any showers/thunderstorms will dissipate
quickly after 00Z leaving SCT-BKN mid/high clouds. Fog will
develop...mainly west of the Blue Ridge...and most likely in any
areas that see late day rain.
Winds...mostly SSW-SSE through the period...speeds 5-7kts or less.
Medium to High confidence in cigs and vsbys through the TAF valid
period. Medium to High confidence in wind spd/dir through the TAF
valid period. Low to Medium confidence in thunderstorm potential.
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