274
FXUS62 KGSP 160602
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
202 AM EDT Mon Sep 16 2024

.SYNOPSIS...
A low developing off the South Carolina coast is expected to track
northwestward into the Carolinas later today, possibly become
a Tropical Storm, bringing heavy rain and gusty winds. The low
will slowly drift inland tonight through Tuesday, and weaken. By
Wednesday, the remnant low will stall out over the Mid-Atlantic,
while high pressure begins to build into the region from the north
and west late in the week.

&&

.NEAR TERM /THROUGH TODAY/...
As of 200 AM EDT Monday: Broad deck of lower clouds has expanded
across the eastern two-thirds of the CFWA. Some areas of precip is
showing up on radar, but likely having a hard time reaching the
surface the further north this activity travels. Otherwise, the
forecast remains on track.

The broader area of precip (associated with the potential tropical
system just off the SE Coast) remains well to our SE. I still can`t
rule out a few more brief, upslope showers developing in the ELY
flow regime over-night, but any coverage would be minimal at best.
NE winds will re-main gusty, especially over the Piedmont. Low temps
should remain at least a category above normal.

Otherwise, the coastal low which is now Potential Tropical Cyclone 8
per the Hurricane Center, is currently centered about 130 miles off
the coast of Charleston, SC, although it remains difficult to
actually locate a center of circulation as the system remains poor-
ly organized. The 18z operational GFS remains notably faster moving
this system inland on Monday and has it centered over the Columbia
area by roughly 00z Tuesday. Most of the other operational models
barely have the system over the SC Coast at this time. Regardless,
the general consensus remains more in-line with the more progressive
GFS solution with the center of the low moving onshore Monday
evening and turning more northward as it makes landfall. Expect
deformation zone moisture/forcing to wrap into our eastern zones
during by this time, with the outer precip bands moving over the I-
77 corridor by mid-afternoon with the potential for heavier rain
increasing during the evening hours. In contrast, our
western/southwestern zones only carry a slight chance PoP at this
time. A significant gradient remains wrt QPF amounts across our CWA,
with the I-77 corridor still expected to see roughly 2 to 4 inches
from Monday thru early Wednesday, while most of the Upstate and NC
mtns will see half an inch or less. These amounts could change
significantly going forward depending on how fast this system moves
over our area. The other main concern will be the gusty winds. Much
of the short-term guidance still depicts a 40+ kt 850mb jet
developing N to NW of the cyclone center on Monday. Gusts of 30 mph,
and perhaps some occasional gusts of 35+ mph are still expected
across the NC Piedmont Monday afternoon and evening.

&&

.SHORT TERM /TONIGHT THROUGH WEDNESDAY/...
As of 230 PM Sunday: An unusual amount of uncertainty remains in
the short term forecast given the much discussed low pressure
system off the Carolina coast, though consensus appears to be
building for at some notable impacts gusty winds and moderate
to isolated heavy rainfall) to the northeastern half of our CWA.
The timeframe for these impacts will likely include the beginning of
the period Monday night into early Tuesday or Tuesday afternoon.
The 12Z GFS has not appreciably changed its solution from the
06Z run, bringing a surface low across NE SC toward the Charlotte
metro by early Monday evening then quickly weakening the low and
propagating it into the eastern Ohio Valley later Tuesday, as
the parent upper low becomes trapped under an expansive longwave
ridge over the eastern ConUS.  There is general agreement in
the GFS ensemble suite and increasing support from CAM guidance,
including the 12Z HRRR and HRW, that areas along and to the north
and east of the surface low will at least see a swath of moderate
rainfall (generally 1-2 inches) with isolated heavier totals of 2-4
inches possible through Tuesday evening.  An ensemble of CAM and
NationalBlend guidance suggest 40-80% probabilities of storm-total
rainfall exceeding 2" across the northeastern half of the CWA,
with the highest amounts focused along and near the I-77 corridor.
This rainfall is expected to fall over roughly a 12-18 hour period,
and with probabilities of 6-hour rainfall expected to exceed 2"
or more of about 20-40% over the Charlotte metro, if the guidance
pans out, an isolated flash-flood threat would be anticipated.
The areas that need the rain the most, across the western half of
the forecast area, are the least likely to see appreciable rainfall
from this system.

Not to be outdone, the pressure gradient between any surface low
and ~1027mb high pressure centered off the New England coastline
suggests strong wind gusts will persist into Tuesday morning as
well.  HREF probabilities of 30-50% for wind gusts greater than 45
mph (Wind Advisory criteria) across much of I-77 and adjacent I-40
corridors suggests we will have to monitor the forecast closely.
Although high-resolution guidance tends to overforecast winds,
these probabilities suggest winds will nevertheless have an notable
impact through the Tuesday morning commute.  The strength of wind
gusts will be dependent on the presence and eventual strength of the
surface low, and again, despite being 24-36 hours out, there remains
the potential of significant shifts either west or east in the axis
of greatest impacts as well as in the severity of those impacts.

Conditions will improve rapidly later Tuesday into Wednesday, as
whatever materializes for Monday night is expected to occlude if
not dissipate well to our north, leaving the area on the southern
edge of a large closed-low with the best moisture and forcing
displaced to our north and east.  Nevertheless, attendant lapse
rates and weak areas of vorticity rotating around the closed low
support slight chance to chance PoPs for widely scattered showers
through the period.  Skies should remain mostly cloudy or overcast
on Tuesday with high temperatures 5-7 degrees below normal across
the northeastern half of the CWA.  High temperatures further
west and away from any surface low are expected to be normal
to slightly above normal under partly sunny skies. With skies
gradually becoming partly to mostly sunny by Wednesday afternoon,
highs should return to near normal.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/...
As of 230 PM Sunday: The deterministic guidance is in general
agreement that the closed upper low will gradually propagate
south and east toward the Southeast coast through the rest of
the work week, possibly getting absorbed into a longwave trough
on the eastern side of a developing Omega block over the western
two-thirds of the ConUS.  Heading into the weekend there remains
decent agreement that an upper low will become separated from
the longwave pattern and close off near Florida as upper-level
ridging builds over the Ohio Valley and Great Lakes, evolving the
Omega block into a Rex blocking pattern.  The evolution of this
upper-level pattern suggests that isolated to scattered showers are
possible, especially across the northern half of the CWA, as the
upper low drops south and east through the Carolinas on Thursday
and Friday.  Meager but non zero instability suggests a few isolated
thunderstorms cannot be ruled out each afternoon.  The pattern and
partly cloudy skies suggests seasonable high temperatures both days.

By next weekend, the forecast area is expected to be north and
west of the mean trough axis, placing us in a more stable and
drier airmass.  Surface high pressure centered over southern Quebec
on Saturday, with a weak surface trough axis located just off the
Carolina coast, suggests a decent pressure gradient and support for
a wedge-like pattern across the CWA Saturday night and persisting
through perhaps Monday.  In response, blended guidance supports
highs at least several degrees below normal at the end of the
period on Sunday under partly sunny (southwest) to mostly cloudy
(northeast) skies and within a stout northeasterly flow.

&&

.AVIATION /06Z MONDAY THROUGH FRIDAY/...
At KCLT and elsewhere: Moist easterly flow just above the surface
layer in response to Potential Tropical Cyclone Eight, located
offshore the Grand Strand will support BKN/OVC MVFR low clouds
across most of the area through daybreak Monday. Conditions may
improve at KAVL and the Upstate terminals during the afternoon,
but should continue to slowly deteriorate at KCLT and KHKY as the
tropical disturbance inches closer to the coast and eventually
making landfall late Monday. As a result, rain chances will increase
by the afternoon and remain in place through the evening, which is
reflected in the prevailing lines at these TAF sites. Low-level
wind shear may become an issue at KCLT and potentially KHKY,
but confidence is higher at KCLT for a TAF mention. Confidence
lowers at the upstate sites and KAVL as model guidance differ on
the timing and extent of the rainfall, but rain and associated
restrictions should eventually reach KGSP/KGMU/KAVL by the end of
the forecast period. Showers are possible at KAND, but confidence
is too low through the 06Z forecast period to include nothing more
than VCSH in the prevailing line due to the sharp cutoff of precip,
but still expect for MVFR cigs to filter in before the end of the
TAF period. Eventually, MVFR/IFR should overspread all terminals
by Monday night, with vsby restrictions being included, especially
with any ongoing rainfall. Winds will take on a northeasterly
component, but may back more northerly by late Monday evening,
depending on the location of the center of Potential Tropical
Cyclone Eight. Gusts in the 25-30 kt range are possible, mainly at
KCLT and KHKY, with some gusts potentially exceeding 30 kts. 20-25
kt gusts are included at the upstate terminals and KAVL as the
storm system pushes towards the region late in the period.

Outlook: Gusty winds and some instances of low-level wind shear
will impact KCLT and KHKY through Monday night and even into
parts of Tuesday. The best chance for rainfall and associated
restrictions will reside at KCLT and KHKY, based on the current
forecast of Potential Tropical Cyclone Eight. MVFR/IFR restrictions
are likely at all TAF sites by late Monday evening through at least
daybreak Tuesday. Still some uncertainty on the extent of rainfall
coverage and associated restrictions beyond daybreak Tuesday,
but winds and precip will begin to taper off at some point during
the day Tuesday. Spotty showers and occasional restrictions will
be possible through the remainder of the week.

&&

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

&&

$$

SYNOPSIS...ARK
NEAR TERM...CAC/JPT
SHORT TERM...JMP
LONG TERM...JMP
AVIATION...CAC