Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
755 PM EDT Wednesday may 6 2015
weak high pressure will remain across the region through Thursday...
as low pressure organizes east of Florida. The low may develop
into a tropical or subtropical feature by this weekend...as it
drifts north toward the South Carolina coast. There is still plenty
of uncertainty as to whether or not the system remains offshore or
moves inland over the coastal plain of the Carolinas this weekend.
The low will eventually drift east of the area...as a cold front
pushes in from the west on Tuesday.
Near term /through Thursday/...
as of 730 PM...high pressure continues to ridge southwest from the
middle-Atlantic region into the southern Appalachians. However...the
subtropical low off the East Coast of Florida will drift northward
tonight with the biggest affect for our area being patchy high
cloudiness. Low temperatures tonight will remain mild with lows from 55 to
60 outside the mountains and 50-55 mountain valleys. Low level
profiles remain pretty dry...and dewpoints mixed out enough this
afternoon that fog looks unlikely following ups technique. Removed
fog mention from forecast.
On Thursday...the subtropical low is forecast to continue to drift
northward to off the South Carolina coast by late in the day. Models
indicate that the non-mountain areas should remain dry as they will
be in an area of general subsidence along with drier air advecting
in from the NE. However...the mountains should see some scattered
afternoon convection thanks to low level upslope flow and a spoke of
vorticity rotating around the upper low into the area along with
convective available potential energy increasing to 500-1000j/kg. Hence...will continue to carry scattered
probability of precipitation in the mountains. Temperatures will be a couple degrees lower than
today due to increased cloud cover...but still above climatology.
Short term /Thursday night through Saturday/...
as of 130 PM Wednesday...the main forecast challenge for the short term
will be how much impact the tropical (or subtropical low) will have
on our weather across the western Carolinas as it approaches the SC coast.
The models are in a little better agreement on their depiction of
the feature...gradually organizing it into a closed surface low by
Thursday evening...with outer bands of convection expected to start
affecting the NC/SC coasts. It looks like the most likely scenario
is for the low to meander near the SC coast Friday and
Saturday...keeping our area in the subsidence band on the northwest side of
the circulation. Guidance does allow for some instability and mountain-top
convection both days...with the I-77 corridor possibly being
affected by some rain bands. The Blue Ridge...upstate and NE Georgia look
to be in the minimum of pop both days. Temperatures will be 2-3 categories
above normal Thursday night through Saturday.
Long term /Saturday night through Wednesday/...
as of 215 PM Wednesday...the medium range starts at 00z
Sunday...with the tropical or subtropical low somewhere in vicinity of the
SC/NC coast. The models still diverge on where the low is steered.
The latest wpc preference is close to the 00z European model (ecmwf)...which has the
low drift north into eastern NC on Sunday...then turn slightly NE off
the Virginia convective available potential energy. The GFS is a slow and western outlier...having the low
drift SW on Sunday toward Savannah...then NE along the coastal plain
on Monday. If the GFS verifies...we would probably see more enhanced
afternoon/evening convection. However...given the consensus/wpc track...the
western Carolinas will likely stay on the subsidence/back side of
the system...with mainly just some mountain convection and isolated Piedmont
activity Sunday and Monday. I will undercut the superblend probability of precipitation both
days about 10-20 percent...keeping chance pop in NC...and slight chance pop
south. Temperatures will continue to be above normal...about 7-10 degree.
Tuesday and Wednesday...the persistent western Continental U.S. Trough will eject
into the plains...and lift north into the Great Lakes. The flow will
flatten the southeastern Continental U.S. Ridge...but not completely.
Still...both the GFS and European model (ecmwf) show a cold front associated with the
upper wave pushing through the area on Tuesday. The GFS has it through by
early morning...while the European model (ecmwf) pushes it across Tuesday evening.
Again...the wpc preference is more in line with the ec...so will
continue the chance probability of precipitation for showers and thunderstorms...with above normal temperatures
Tuesday. Then dry Wednesday with slightly cooler temperatures.
Aviation /00z Thursday through Monday/...
at kclt...VFR. Dry low level profiles and surface dewpoints will be
unsupportive of restrictions Thursday morning. Winds are most likely to hold
east-southeast through the evening before backing to northwesterly...ahead of
subtropical low pressure system moving toward the NC coast. The northeasterly
winds will continue through the day Thursday. As the system nears and the
gradient amplifies in the afternoon...concurrent with fairly deep
mixing...winds are expected to become gusty. Increasing/thickening
cirrus will be seen as the day progresses.
Elsewhere...VFR expected this period with conditions remaining dry
enough to preclude morning restrictions. Winds will be mainly calm
overnight but should come up from the north or NE Thursday morning. Kavl will
flip to southeasterly later in the day. A few low end gusts are likely...but
appear infrequent enough to omit from the tafs. Few-scattered rain showers/thunderstorms and rain
are expected over the mountains...but chance at kavl is too low to mention.
Outlook...moist upslope flow persists into the area...so isolated to
scattered diurnal convection is expected through the
weekend...producing will produce local restrictions. Low clouds and
or fog restrictions will be possible each morning.
00-06z 06-12z 12-18z 18-00z
kclt high 100% high 100% high 100% high 100%
kgsp high 100% high 100% high 100% high 100%
kavl high 100% high 100% high 100% high 100%
khky high 100% high 100% high 100% high 100%
kgmu high 100% high 100% high 100% high 100%
kand high 100% high 100% high 100% high 100%
The percentage reflects the number of guidance members agreeing
with the scheduled taf issuance flight rule category. Complete hourly
experimental aviation forecast consistency tables and ensemble forecasts
are available at the following link: (must be lower case)