Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 201427
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1027 AM EDT Thu Jul 20 2017

.SYNOPSIS...
An upper level ridge will build over our region from the west
through the end of the work week bringing hot temperatures and
isolated showers and thunderstorms each day. A broad upper level
trough develops over the weekend and into early next week and
is expected to bring cooler temperatures.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 1025 AM: Forecast generally on track with latest observations
and guidance supporting previous thinking. 12Z observed soundings
show very high freezing level but a large separation between those
and low wet bulb zero levels due to the large amount of mid level
dry air. Very high DCAPE values are shown as well along with the
potential for moderate instability even with the warm mid levels and
poor lapse rates. The HRRR has backed off on the NC Piedmont
convection and is now closer in line to the other guidance.
Therefore the isolated coverage focusing on the southern mountains
and southern CWFA still in line. High heat with heat index values
below 105, and mostly below 100, on track as well as the high
morning dew points should mix out given the dry air aloft.

Otherwise...The flow aloft will become increasingly anticyclonic, as
ridge associated with expansive subtropical high over the central
Conus continues to nudge its way into the forecast area. Resultant
warm mid-level temps of -5 to -6C will yield very poor mid-level
lapse rates, but forecast soundings are generally not capped, and
still reveal some modest buoyancy, especially across the southeast
third or so of the forecast area. Convection-allowing models depict
a smattering of small convective cells across the southern Piedmont,
and to a lesser extent across the high terrain. The concern with
that is that the CAMs have been too reserved with diurnal convection
for the past 2-3 days, especially over the Piedmont.

Instability is such that I would expect isolated convection to fire
along the buoyancy gradient that is forecast to set up between the
Midlands and the northern Piedmont this afternoon (the HRRR features
this to some extent), so 20 pops will be advertised roughly along
the Hwy 72 corridor in GA/SC. The environment is expected to become
increasingly hostile to convective development as one travels
farther northwest across the area, but again, forecast soundings
across the mtns are uncapped and feature sbCAPE in the 1000-1500
J/kg range. Even with such meager ingredients, it`s hard not to get
at least isolated convection across the southern (i.e., srn NC/GA/
SC) mtns in late July, so a 20 pop will also be forecast from the
Balsams southwest into our GA mtn zones. Considering the forecast
degree of instability and downdraft CAPE, would not be surprised to
see a microburst or two across the Piedmont. Another two degree-ish
increase in max temps is expected this afternoon, with the vast
majority of the Piedmont and foothills expected to reach the
mid-90s. Nevertheless, dewpoints mixing out in the mid-to-upper 60s
should keep heat indices contained within the 95-100 range this
afternoon. Min temps tonight will average a couple of degrees above
normal.

&&

.SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
As of 310 AM EDT Thursday: the short-term fcst picks up at 12z on
Friday with very broad upper ridging in place over most of the CONUS
and a weak H5 low, that has separated from the mean flow, just to our
south. The low proceeds to drift SW towards the Gulf of Mexico and
by early Sat, what`s left of the low is centered over the northern
gulf coast with flat upper ridging in place over the fcst area. By
early Sun, another shortwave upper trof begins to dive down over the
Western Great Lakes as the upper ridge amplifies over the western
CONUS. At the sfc, the Bermuda High will be in place offshore with
warm SLY flow over the region. The overall synoptic pattern is not
expected to change much thru the period with weak high pressure
persisting to our north into Sat and some degree of weak lee trofing
expected over the CWFA each day. By the end of the period early Sun,
a low will deepen over the Great Lakes and its associated cold front
will begin to approach the fcst area. As for the sensible fcst, no
significant changes were needed with near climo PoPs for Fri and
slightly above climo PoPs for Sat where lapse rates should be greater.
Temps will be well above normal thru the period with heat index
values reaching triple digits over the lower elevations each day,
however values are not expected to reach heat advisory criteria.

&&

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
As of 245 AM EDT Thursday: the extended fcst picks up at 12z on
Sunday with flat upper ridging over the southeast and an upper trof
beginning to amplify and dig down over the Great Lakes. The long
range models move the trof axis to our north by late Monday or early
Tuesday depending on which model you believe. Most of the energy
associated with the system is expected to remain to our north as
height falls over the fcst area are minimal. The trof begins to
move off the New England Coast by mid-week as broad upper ridging
persists to our west. The ECMWF remains more progressive with the
pattern compared to the GFS or Canadian models. At the sfc, the
Bermuda High will be in place to our east as the models deepen a
low over the Eastern Great Lakes and move its associated cold front
towards the fcst area. For the rest of the period, the sfc pattern
is looking more ambiguous and complex than previously depicted.
The models no longer show a clean fropa on Mon or Tues, but instead
have the front stall out over the fcst area and keep deep-lyr
moisture over the CWFA well into Wed. There also appears to be
less drying behind the front than previously anticipated, with
the ECMWF keeping deep moisture over the CWFA into Thurs. As for
the sensible fcst, above climo PoPs persist thru the period as
the front impacts the CWFA with Mon and Tues still having the
best chances for widespread convection. Temps will start out a
good 4 to 6 degrees above climatology on Sunday and then steadily
cool thru the rest of the period. By early next week, they are
fcst to be back near normal.

&&

.AVIATION /15Z THURSDAY THROUGH MONDAY/...
At KCLT and elsewhere: Fog/low stratus have dissipated. Therefore,
VFR is expected to persist at the terminals through the rest of the
period. Light/vrbl or light W/NW winds are expected through the
morning, becoming light SW during the afternoon. Isolated convection
is expected to develop mainly south of the terminals this afternoon.
I can`t completely rule out something straying near KCLT this
afternoon/evening, but the probability still appears too low to
include in the TAF at this point.

Outlook: Drier air will likely continue to limit convective coverage
and the extent of low clouds and fog through Friday. Chances for
diurnal convection begin a steady increase on Saturday, with this
trend continuing into early next week ahead of an approaching cold
front on Monday.

Confidence Table...

            14-20Z        20-02Z        02-08Z        08-12Z
KCLT       High 100%     High 100%     High 100%     Low   50%
KGSP       High 100%     High 100%     High 100%     Low   50%
KAVL       High 100%     High 100%     High 100%     Low   50%
KHKY       High 100%     High 100%     High 100%     Low   50%
KGMU       High 100%     High 100%     High 100%     Low   50%
KAND       High 100%     High 100%     High 100%     Low   50%

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:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

$$
SYNOPSIS...JPT
NEAR TERM...JDL/RWH
SHORT TERM...JPT
LONG TERM...JPT
AVIATION...JDL/RWH



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