Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
445 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/...
430 PM update...the forecast is on track. Made slight tweaks to
pop/sky per satellite and radar observation...and latest available hrrr which has
backed off its expectations for convection near the Blue Ridge.
Convection should continue to bubble near the tenn border through
As of 200 PM...no big change is anticipated for the overnight as
high pressure continues to ridge southwest from the middle-atalntic
region into the southern Appalachians. However...the subtropical low
off the East Coast of Florida will drift northward with the biggest
affect for our area patchy high cloudiness. Low temperatures tonight will
remain mild with lows from 55 to 60 outside the mountains and 50-55
mountain valleys. Some fog is likely to develop again in the
mountain valleys...especially in the little Tennessee Valley where fog is
likely to develop shortly after midnight and persist through sunrise.
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 /21z Wednesday through Monday/...
at kclt...it will be somewhat difficult to nail down winds this
afternoon/evening under very weak forcing. Overall pattern suggests east-southeast
winds will back to NE tonight as a subtropical low moves northward
off the East Coast of Florida. NE winds will continue tomorrow.
Quiet conditions will continue through the taf period with generally
just patchy cirrus...joined by a few rather high based cumulus for the
remainder of the afternoon.
Elsehwere...generally VFR with light winds. The exception will be
for a few hours early Thursday where fog may reduce visibility/S at kavl
and khky to MVFR.
Outlook...generally VFR outside the mountains except for patchy
morning fog. Over the mountains as moist upslope flow
persists...isolated to scattered diurnal convection is expected
through the weekend which will produce local restrictions. Low
clouds and or fog restrictions will be possible each morning.
20-02z 02-08z 08-14z 14-18z
kclt high 100% high 100% high 100% high 100%
kgsp high 100% high 100% high 100% high 100%
kavl high 100% high 100% high 91% high 100%
khky high 100% high 100% high 83% 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)