Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 031418

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1018 AM EDT Mon Aug 3 2020

Rain chances increase across the region through today as tropical
moisture from Tropical Storm Isaias interacts with a stalling front.
The storm will pass near the Georgia and Carolina coasts today and
then through eastern North Carolina tonight, enhancing our risk of
heavy rainfall and isolated flooding. A more typical summertime
pattern of afternoon and evening thunderstorms will then return for
the remainder of the week.


As of 1015 AM: Just a few tweaks based on radar trends, mainly
to keep the very high precip probs over the mtns/foothills and
to lower the chances over the NC Piedmont. That being said, a
band of showers/storms was drifting in from the southeast toward
metro Charlotte...and new showers could develop very quickly as
we destabilize through midday. Will continue to monitor the heavy
rain potential mainly over the wrn half.

Otherwise, a stalling cold front over East TN will serve
to focus convective activity this morning, driven by diurnal
destabilization and enhanced by an amplifying jet streak oriented
along the Appalachians. PoPs ramp up to categorical first over
the mountains near the TN border as a result. While the Piedmont
is currently under a lull in forcing, the approach of Isaias is
promoting moist upslope flow that should expand coverage near the
Escarpment. It also will bring likely to categorical chances back
for much of the area this afternoon. CAPE is expected to stay a
little below typical August values. However, robust mid to upper
level flow surrounding the trough will produce 0-6km shear of 30
to 40 kt over the NW half of the CWA. Therefore SPC has included a
D1 Marginal Risk across that part of our area. Loosely organized
multicells producing damaging wind would appear to be the most
likely route that severe wx would occur.

The continued weakening of the front and slightly more eastward
trend in the Isaias track no longer appears as likely to support a
sharp axis of heavy rainfall. There will be at least an isolated
risk of flash flooding across much of the area today. The
Flash Flood Watch for the northern mountains/foothills is
being maintained, however, as it is not yet totally clear the
convective trend is downward. The trough to our west will begin
to make eastward progress tonight, creeping the front eastward,
and some drying is expected both due to that development and as
winds turn more downslope around Isaias as it crosses eastern
NC. The circulation of the storm will be near enough to support a
mixed E to NE flow late tonight in the low levels along and east
of I-77. Breezy conditions likely will develop, with gusts perhaps
up to 25 mph at times. Significant impacts are unlikely aside from
a few trees falling in saturated soil; given flash-flood guidance
being above 3 inches in 6 hours, and total QPF remaining well
under 2 inches by and large in those areas, confidence is fairly
low that the soils will be wet enough for much uprooting.


As of 305 am EDT Monday: The center of circulation of Isaias should
move rapidly away to the northeast during the day on Tuesday. After
lingering morning showers and thunderstorms, mainly along and east
of I-77, a more typical summertime pattern of afternoon and evening
convection will return as the lingering, stout upper jet axis moves
slowly east across the forecast area. Low-level flow will quickly
become a westerly downslope with the passage of Isaias to the east,
but the downsloping will likely not counteract the upper forcing,
moist boundary layer, and lingering frontal zone stalled in the
area. Solid scattered coverage should easily return through the
afternoon hours in spite of the westerly flow.

General troughiness will persist west of the Appalachians Wednesday
through Wed night as additional shortwaves move east of the
Mississippi River Valley, and the stalled surface boundary slowly
dissipates in place. Better insolation will permit SBCAPE values to
recover above 2500 J/kg during peak heating to permit scattered to
numerous afternoon and evening storms to return on Wednesday.
Temperatures will return to nearly normal early August values.


As of 310 am EDT Monday: The upper trough axis west of the
Appalachians on Thursday will cross the region through Friday, with
another round of above climo PoPs for diurnal showers and
thunderstorms expected both days. Temperatures will be near or just
below climo, with a bit more clouds under the passing trough.

A broad and flat central CONUS ridge will try to build east over the
weekend, but with persistent northwesterly flow and associated
shortwaves crossing the southeast. This will keep decent, diurnal,
scattered or better coverage of showers and thunderstorms going
through the weekend. Temperatures will return to near climo under
building heights aloft.


At KCLT and Elsewhere: SHRA/TSRA are beginning to line up along the
southern mountains and Blue Ridge Escarpment as moist upslope flow
occurs ahead of TC Isaias. MVFR to IFR vsby is possible in the heavy
showers. Activity should increase in coverage in this area thru
the morning. Some mountain valley IFR fog/stratus exists aside from
the precip, and KAVL might see this fill back in between downpours
during the first hr or two of the period. Diurnal destabilization
will eventually lead to scattered SHRA/TSRA for the Piedmont as
well, mainly midday to mid-afternoon. Bases should be mostly VFR
except during heavier precip. Isaias will make its nearest pass
to our area tonight, with precip coverage waning following the
loss of heating, with the remaining chances being near KCLT/KHKY
for outer bands. It will also bring low-end gusts to KCLT in the
late evening to very early morning hours. As thicker cloud cover
clears out radiational stratus could form, mainly at KAVL and the
upstate sites.

Outlook: Daily rounds of afternoon/evening SHRA/TSRA will likely
continue most of the week. Morning restrictions can`t be ruled out.

Confidence Table...

            14-20Z        20-02Z        02-08Z        08-12Z
KCLT       High  98%     High 100%     Med   76%     High 100%
KGSP       High  93%     High  88%     High 100%     Med   63%
KAVL       Med   70%     High  80%     High  82%     Med   78%
KHKY       High  81%     High  87%     Med   70%     Low   43%
KGMU       High  92%     High 100%     High 100%     Low   42%
KAND       High  98%     High 100%     High 100%     High  90%

The percentage reflects the number of guidance members agreeing
with the scheduled TAF issuance flight rule category. Complete hourly
experimental aviation forecast consistency tables are available at
the following link:


NC...Flash Flood Watch through Tuesday morning for NCZ033-049-050-


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