NWS Greenville-Spartanburg, SC wrote its last area forecast discussion before the first event at Sep 13, 2:53 AM EST, 2 hours 7 minutes earlier.
Area forecast discussion, Sep 13, 2:53 AM EST, 2 hours 7 minutes before
Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
353 AM EDT Sun Sep 13 2020
.SYNOPSIS...
An unsettled weather pattern will continue today and into Monday
ahead of an approaching cold front which will bring cooler and
drier air to the region on Tuesday. Unsettled weather returns
mid to late week as a cold front and the remnants of Sally move
near the area.
&&
.NEAR TERM /THROUGH TONIGHT/...
As of 310 am: Moist SE flow persists across the area early this
morning, and while rainfall rates have trended down significantly
over the past few hours, scattered to numerous moderate to heavy
rain showers continue to produce localized rainfall amounts of up to
.5 inch per hour in the counties along the Blue Ridge escarpment.
These rates and overall coverage of showers will continue to
steadily diminish as guidance depicts weakening and veering low
level flow through the morning. Quite a few locations along/near the
continental divide have seen heavy rainfall over the past 24 hours,
especially across southeast Transylvania County, where more than 4
inches have fallen since Saturday morning, so while the threat for
flash flooding appears to be waning, some duration issues could
still develop along tributaries of the upper French Broad River this
morning.
Otherwise, upslope showers should be all but dissipated by late
morning. NE flow is forecast to advect slightly drier/more stable
air into much of the forecast area today, which combined with a
subsident regime will limit instability and convective chances
across much of the Piedmont. However, sufficient instability
combined with terrain effects should allow for some development over
the mountains this afternoon/evening, with chances being greatest
(likely pops) across the high terrain bordering Tennessee, with pops
tapering to only 20-30% across the foothills zones. While locally
heavy rainfall again appears likely this afternoon/evening, the risk
will be greatest in areas that have been spared from heavy rainfall
over the past few days. Max temps are forecast to be right around
normal.
Convective chances persist into the evening hours across the
mountains, albeit with an expectation of steadily dissipating
coverage. Having said that, token chances are warranted near the TN
border through the night, as remnant convection associated with a
frontal zone approaching from the TN Valley could wander into
western NC prior to sunrise. Min temps will remain above climo.
&&
.SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/...
As of 320 AM EDT Sunday: A weak cool front steered by upper
troughing will be in the process of pushing across the area Monday
morning as what is now TS Sally is anticipated to be closing in
on the Gulf coast and TS Paulette well offshore will be moving
away from the eastern seaboard. With the boundary, available
moisture and instability, expect scattered showers and
thunderstorms on Monday with max temps above normal. Higher QPF
amounts will be confined to the mountains.
Per latest guidance, with the front's slow progression through the
area, expect it to become stalled on the southern periphery on the
FA into Tuesday as sfc high pressure builds in from the north and
yet another wedge forms. PoPs will decrease significantly and the
coverage of showers will become isolated in nature, tapering off
from north to south early on in the day. While the influence of the
wedge will be in place for the remainder of the fcst period, Sally
is progged to make landfall somewhere along the Gulf coast on
Tuesday. For now, guidance suggests that despite it's track inland,
mainly dry conditions will prevail across the area with the wedge in
place, though could not entirely rule out isolated showers across
the southern periphery and mountains. As with any tropical system,
any temporal/spatial changes in the fcst track could alter precip
chances for Tuesday/Tuesday night. With the cold front, conditions
will be less humid with drier air/decreasing dewpoints and max temps
will fall short of normal.
&&
.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
As of 345 AM EDT Sunday: With wedge of high pressure still in place
across the FA Wednesday morning, all eyes remain on Sally as
guidance brings it inland somewhere over LA/MS/AL at the beginning
of the forecast period. Per latest guidance, GFS continues to depict
a faster track with it's placement of Sally Wednesday and Wednesday
night as it eventually becomes absorbed with a cold front, bringing
abundant moisture to the FA for late Wednesday night into Thursday.
As for the ECMWF, a slower track and arrival time/track is progged
for Sally, but the trend of it's remnants becoming absorbed with the
front and abundant moisture/unsettled weather is in agreement. Thus,
for now have kept with the trend of gradually increasing PoPs
Wednesday morning as moisture infiltrates in from the SW, giving way
to an unsettled weather pattern from Wednesday night and through the
end of the week as the cold front moves through at some point on
Saturday, bringing drier and cooler air behind it. Anticipate the
window of higher QPF to be from Wednesday night through Friday
morning with current trends. Again, given the uncertainty with the
exact track of Sally attm, confidence remains low on actual rainfall
amounts/timing with this fcst package. Max temps throughout the fcst
period will remain at or below normal with min temps around normal,
dropping to below normal early Sunday morning.
&&
.AVIATION /08Z SUNDAY THROUGH THURSDAY/...
At KCLT and elsewhere: Showers are forecast to remain largely
confined to areas around KAVL through the next few hours...where
categorical SHRA and/or tempos are carried through 09Z. Can't rule
out a spotty shower moving across the KGSP/KGMU over the next couple
of hours, so VCSH is carried at those sites. Otherwise, expect IFR
or low MVFR cigs to develop and/or persist at the terminals through
much of the morning, with improvement to MVFR by late morning, and
VFR becoming likely by early afternoon. Slightly drier/more stable
air moving in from the NE is expected to result in minimal, if not
non-existent coverage of convection later today across the Piedmont
and foothills, where probabilities are at best 20% and therefore
insufficient to include a TAF mention. Chances increase across the
mountains, but even there the best chances are expected in areas
closer to the TN border/west of KAVL. Nevertheless, a Prob30 for
TSRA is warranted at KAVL during late afternoon/early evening.
Otherwise, gradually diminishing NE winds are expected.
Outlook: Scattered to numerous showers and storms are expected
Monday afternoon/evening as a cold front crosses the area. Drier
conditions develop in the wake of the front Tuesday into Wednesday.
Moisture from the remnants of tropical cyclone Sally may impact the
area late in the week, possibly bringing widespread restrictions and
heavy rainfall.
Confidence Table...
08-14Z 14-20Z 20-02Z 02-06Z
KCLT High 91% High 97% High 100% High 100%
KGSP High 88% Med 79% High 100% High 93%
KAVL High 100% High 90% High 100% High 86%
KHKY Med 66% High 84% High 100% High 100%
KGMU Med 68% High 80% High 100% High 93%
KAND Med 68% Med 79% 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 are available at
the following link:
www.weather.gov/gsp/aviation
&&
.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.
&&
SYNOPSIS...SGL
NEAR TERM...JDL
SHORT TERM...SGL
LONG TERM...SGL
AVIATION...JDL