Special Weather Statement, NWS Greenville-Spartanburg, SC

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
240 PM EDT Sat Jun 5 2021

.SYNOPSIS...
The Bermuda high will remain entrenched over the western Atlantic
while a cutoff low tracks over the south-central United States for
the first half of the upcoming week.  This setup will drive
increased shower and non-severe thunderstorm coverage each day from
this weekend through late next week.

&&

.NEAR TERM /THROUGH SUNDAY/...
As of 220 pm: Persistent southerly flow between a Bermuda high over
the western Atlantic and low pressure across the south-central Conus
will support an anomalously moist air mass through the period...
PWATs of around 1.75", or about as high as they get around these
parts this time of year. This will yield rather active convective
weather this afternoon/evening and again on Sunday.

Scattered deep convection got an early start across the SC Piedmont,
where moderate instability developed in association with surface
dewpoints of around 70. Weak shear has resulted in classic pulse
behavior with these cells, with a strong, albeit sub-severe updraft
development near the time of initiation followed by rapid weakening.
This trend will continue into the evening, and with relatively
modest sbCAPE values, any brief severe updrafts will likely be
limited to cell mergers. Of greater concern is that cells have not
been in any hurry in light of the weak deep-layer flow, and have
therefore been dropping up to 2 inches of rain in an hour or so.
Isolated runoff problems will therefore probably be an issue through
the evening, esp in urban areas.

While convective coverage is forecast to steadily diminish this
evening, the high PWAT air will likely allow some degree of coverage
to persist through the overnight. More of the same tomorrow,
although with more in the way of morning low cloud cover expected,
convection may not get quite the early start on Sunday that it got
today. Nevertheless, another day of solid scattered to numerous
coverage is expected, with locally excessive rainfall and perhaps a
pulse severe storm or two again being a concern. Min temps tonight
will be several degrees above normal, while forecast maxes Sunday
are right around climo.

&&

.SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY/...
As of 200 PM EDT Saturday: A diurnally-driven summertime pattern
will remain in place for the duration of the short term.  At the
start of the period on Sunday night, a deep upper low will be
hanging over the southern Great Plains, while a broad ridge axis
will be situated nearly parallel to the U.S. East Coast.
Consequently, synoptic forcing will be limited through the period.
Nonetheless, with a Bermuda high entrenched off the Atlantic coast
and fueling steady southerly flow in the low- to mid-levels, we
should see well-saturated profiles with RHs pushing 80-90% there
throughout the period.  Thus, low-level cloud cover and rainfall are
likely throughout Monday and Tuesday.

Modest destabilization is likely both afternoons, with models
predicting several hundred Joules of SBCAPE each day.  With
insignificant shear and no lifting mechanism to speak of, it's
likely this will result in afternoon pulse storms, as surface
parcels reach their convective temperatures and ascend to the CCL.
Thus, severe threat is low at this time.  The main hazard will be
heavy rainfall - with several models now pushing PWATs exceeding
1.5", and saturated low-level profiles as mentioned above, we could
see heavy and/or long-lived rainfall in some spots.

Temps should remain a few degrees below normal on Monday, climbing
to around normal on Tuesday.  Despite sustained WAA driving a
warming trend all week, the cloud cover and limited insolation
should ensure that this warming trend is a gradual one.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
As of 215 PM EDT Saturday: No major changes to the forecast in the
extended.  Still looks like the broad blocking ridge over the
eastern CONUS will limit activity through the end of the week.  Mid-
and low-level moisture looks somewhat less robust in the latest runs
of the GFS and ECMWF, so heavy rainfall may be less of a concern
past Wednesday; nonetheless, PWATs remain 1"+ through Friday, and
afternoon destabilization will be possible each day.  So, showers
and diurnal thunderstorms will remain likely through to the weekend.
 Precip may be enhanced by a weak boundary to the north, but this is
contingent upon how far south said boundary sags - which based on
model output is still unclear.

The warming trend will continue as WAA weakens toward the end of the
week.  Thursday or perhaps Friday should be the hottest day, with
highs several degrees above normal.  After this, WAA begins to shut
off as flow shifts more westerly, and we could see a marked decrease
in temps over the weekend.

&&

.AVIATION /19Z SATURDAY THROUGH THURSDAY/...
At KCLT and elsewhere: The most immediate concern is the scattered
convection that has developed during the past hour across upstate
SC, which has prompted inclusion of tempos for TSRA at KAND/KGSP/
KGMU through 20Z (although the potential for convection will persist
into the evening at those sites.) Convection will eventually develop
closer to the NC sites, but confidence is too low at this time to
include a tempo...opted for VCTS/VCSH at those sites instead. A very
moist environment is expected to support some degree of convective
coverage into the overnight, but the highest chances will exist late
this afternoon and this evening.

Otherwise, increasing moisture, along with this afternoon's/
evening's potential rainfall will enhance the potential for fog
and/or stratus late tonight into Sunday morning. However, guidance
sources are quite disparate in their handling of coverage and flight
category of any restrictions during this time. Having said that, the
chances for restrictions appear high enough to advertise MVFR cigs
at most sites, while IFR visby is also carried at KAVL. Winds will
generally favor light S or SW through the period.

Outlook: Scattered to numerous convective coverage and associated
restrictions persist through at least the middle of next week.
Morning fog/stratus are also possible, especially in the mountain
valleys.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High 100%     High 100%     High  89%     High  88%
KGSP       High 100%     High 100%     High  83%     Med   75%
KAVL       High 100%     High 100%     High  81%     High  87%
KHKY       High 100%     High 100%     High  89%     High  89%
KGMU       High 100%     High 100%     High  88%     Med   78%
KAND       High 100%     High  99%     High  90%     High  87%

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:

www.weather.gov/gsp/aviation

&&

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

&&

SYNOPSIS...DEO
NEAR TERM...JDL
SHORT TERM...MPR
LONG TERM...MPR
AVIATION...JDL

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