Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Greenville-Spartanburg SC
1052 am EDT sun Jul 5 2015
a broad upper level trough will remain over the southeast through
Monday...providing lift and maintaining elevated rain and
thunderstorm chances each day. Slightly drier conditions return
Tuesday and Wednesday...before a front becomes nearly stationary
just north of our region late in the week. High temperatures will
remain below average into Monday...then warm to around seasonal
normals for the rest of the week.
Near term /through tonight/...
as of 1030 am EDT...satellite imagery shows the upper low continuing
to spin eastward toward the Appalachians late this morning. Regional
radar mosaics show plenty of showers rounding the circulation...but
with little activity on the east side of the system over the
southern Appalachians. Radars also show convection suppressed
southward toward the Gulf Coast. The showers around the low should
steadily fill in over the mountains through the afternoon...and the southern tier
convection may wrap up along the eastern fringes of our Piedmont
counties as well...so a general uptick in coverage is expected.
However...only scattered coverage of any convection is anticipated
across NE Georgia and the western upstate...so the Flash Flood Watch has
been cancelled from Toccoa to Elberton and across the western upstate.
Confidence is not high for flash flooding in the southern
Appalachians...but there should be several more rounds of
showers/thunderstorms there...so will keep it going until 00z.
Otherwise...morning low clouds have been slow to lift and
scatter...but laps SBCAPE is rebounding nicely with plenty of 1000
to 1300 j/kg values east of the mountains with any breaks at
all...scattered thunderstorms should manage to fire as the upper support
increases from the west. The best chance for strong convection will
be east of Interstate 77 where the instability will be greatest
through late day. Coverage should gradually wane through tonight as
our forecast area gets on the negative vorticity advection side of the upper trough.
Short term /Monday through Tuesday night/...
as of 300 am Sunday...upper low begins to fill and lift northeastward Monday
as a larger scale shortwave and surface low move across the north central
Continental U.S.. at least through early afternoon the low will still be in position to
enhance lift over the northern half of the County warning forecast area. However some significant
differences are noted in the NAM and GFS depictions of the feature.
GFS more quickly brings in subsidence in its wake...so far as to have
resolved a weak surface high moving over the area during the afternoon. The
NAM features more favorable lapse rates due to lower heights during
the day...suggesting better convective coverage than the GFS. It is
difficult to tell how the better forcing early in the day will
balance with the expectation for more instability late. Probability of precipitation reflect
somewhat of a compromise...with mentionable values throughout the
morning...and a peak earlier than the time of peak heating. Precipitable water values
will take a hit as drying occurs behind the departing low...but
otherwise some heavy rain threat will remain mainly due to slow storm
motion vectors. Maximum temperatures will warm up a category or so but remain
Coverage should diminish Monday evening...but southwest flow continues
into the area Monday night...with weak warm advection over the southeastern
states ahead of the system moving across to our north. This warrants a
Low Mountain pop overnight. Mins will be near climatology Tuesday morning.
By Tuesday the subtropical ridge will have regained footing over the
southeast and no significant upper features are expected to affect
the County warning forecast area...though heights fall slightly Tuesday night as the northern stream
low moves east. So diurnal instability will be the main driver for
Tuesday probability of precipitation...chances ending up close to climatology for early July. Profiles
will be substantially drier in the upper levels and the flow will
have weakened allowing shear parameters to return to seasonably small
values. Pulse storms producing gusty winds and hail will return as
our main concern. Temperatures will top out around normal. Wednesday night low
probability of precipitation were maintained near the tenn border with a cold front possibly
sinking into the Ohio/Tennessee valleys.
Long term /Wednesday through Saturday/...
as of 200 am Sunday...the medium range begins 12z Wednesday as a
frontal boundary steered by an upper trough over the
Central Plains/Great Lakes region continues its eastward progression
towards the forecast area. 500 mb heights on Wednesday and Thursday will be
roughly west to east with a broad 591 height closed off anticyclone
over the Gulf coastal region. On Friday into Saturday this pattern
will strengthen further into a closed 594 height. This 500mb pattern
center will be over roughly MS and la on the GFS and over North Texas
on the European model (ecmwf). Most of the expected convective systems are expected to
mainly affect the NC mountains as the flow over the top of the ridge brings
impulses across Kentucky and Virginia. The trend in the latest model runs are to
keep the surface front to the north of our region until at least the
weekend. This is evident in both the GFS and European model (ecmwf). Temperatures will be around
normal from Wednesday through Friday...then may go slightly above normal on
Saturday as the ridge builds. However...if the ridge builds farther
west as a dip in The Heights occurs over the Carolinas...that would
tend to lower our temperatures a bit.
Aviation /15z Sunday through Thursday/...
at kclt...morning restrictions are gradually lifting and should
scatter more rapidly 15z to 17z. The hrrr continues to advertise a
round of rain showers/thunderstorms and rain developing west of the airfield 18z to 19z...but
it has been overdone on recent days in this airmass. A narrow tempo
group for thunderstorms and rain could be needed at some point this afternoon since SBCAPE
values are already near 1500 j/kg near the airfield. Anticipate SW
winds to increase with low end gusts with mixing. Any scattered
convection will end this evening...with high uncertainty again
overnight for low clouds toward daybreak Monday.
Elsewhere...any residual low cloud restrictions will continue to
lift/scatter through late morning...with mainly middle level ceilings by
noon. Lower VFR ceilings will fill in with any heating...and shower
coverage will increase from the west with the approach of the upper
trough. Thunderstorms and rain chances will be best near kavl given the upper
support...but isolated to scattered thunderstorms may form throughout with SBCAPE off
to a good start despite the morning low clouds. SW winds will gust
to 15 to 20 knots across the upstate sites this afternoon with mixing.
Expect coverage to diminish this evening...with another round of low
clouds possible early Monday morning.
Outlook...the pattern will finally return to more of a diurnal
nature on Monday as the upper low ejects northeast. This diurnal
precipitation trend will persist through most of the work week.
Restrictions will remain a possibility each afternoon due to
shra/tsra...and also each morning where previous rains have occurred.
15-21z 21-03z 03-09z 09-12z
kclt high 100% high 100% high 100% high 100%
kgsp high 85% high 100% high 100% high 100%
kavl high 100% high 100% low 55% high 80%
khky high 100% high 100% high 100% high 89%
kgmu high 85% high 100% high 100% high 100%
kand medium 75% high 85% 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)
Georgia...Flash Flood Watch until 8 PM EDT this evening for gaz010-017.
NC...Flash Flood Watch until 8 PM EDT this evening for ncz048-051-