036
FXUS62 KGSP 160019
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
819 PM EDT Sun Sep 15 2024

.SYNOPSIS...
High pressure over New England and a developing low off the South
Carolina coast will maintain a gusty easterly wind flow over our
area into the early part of the week. The coastal low is expected to
drift north or northwest over the Carolinas bringing heavy rain to
the central and eastern Carolinas into mid week. The low may linger
before moving away from our region by next weekend.

&&

.NEAR TERM /THROUGH MONDAY/...
As of 5:45 pm EDT Sunday: We`re currently seeing a handful of
isolated showers make their way westward over the Upstate. The
much broader area of precip (associated with the potential tro-
pical system just off the SE Coast) remains well to our SE. Cloud
cover has become more widely sct across most of our CWA since the
mid-afternoon, while the broad cloud shield associated with the
coastal low continues to cover our easternmost zones. I still
can`t rule out a few more brief, upslope showers developing in
the ELY flow regime this evening, but any coverage would be mini-
mal at best. Clouds will increase again overnight, while NE winds
will remain quite gusty, especially over the Piedmont. Low temps
should remain at least a category above normal.

Otherwise, the 12Z guidance, including convection-allowing/hires
models, has suddenly come into better agreement regarding the de-
velopment of hybrid tropical/baroclinic cyclone development off
the Carolina Coast tonight into Monday...although there are slight
differences in the low track, and significant differences regarding
timing. However, the general consensus is that the center of the
low will move onshore near the SC/NC border and wobble N/NW just
east of the CWA, with deformation zone moisture/forcing wrapping
into our eastern zones by the end of the period. CAMs and the GFS
depict outer precip bands moving into the I-77 corridor by early/
mid aftn, with the potential for heavy rainfall quickly ramping up
into the evening hours. PoPs have been increased to 70-80% along
the I-77 corridor prior to 00Z Tuesday. Wind gusts will also become
more of a concern, with short term guidance indicating a 50-55 kt
H8 jet developing N->NW of the cyclone center. Gusts of at least
30 mph, with perhaps occasional gusts as high as 40 mph appear to
be a good bet across the NC Piedmont by the end of the period.

&&

.SHORT TERM /MONDAY NIGHT 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 /00Z MONDAY THROUGH FRIDAY/...
At KCLT and elsewhere: With the relatively moist ELY flow expected
to continue above the surface, cigs will likely drop to the low-end
MVFR range around daybreak Monday. They will likely improve somewhat
during the afternoon over the Upstate sites and KAVL, however they
will likely drop into the IFR range for KCLT and possibly KHKY as
the broader area of precip associated with the coastal low spreads
over the I-77 corridor during the aftn. As such, I have prevailing
rain at KCLT beginning around 19z and continuing for the rest of the
taf period along with visby and cig restrictions. In addition, I have
included a few hours of low-level wind shear at KCLT later tomorrow
evening as the center of the low approaches the area. At KHKY, I have
prevailing rain beginning around 22z on Monday, but I kept cigs at
low-end MVFR thru the end of the period. The other taf sites are less
likely to see prevailing showers, so I only have VCSH for them thru
24z Monday. Winds will remain gusty and from the NE thru the period.
They will likely intensify late Monday morning as the low approaches
our area with gusts in the 20 to 25 kt range. Gusts in the 25 to 30 kt
range are likely at KCLT on Monday afternoon/evening, with some gusts
possibly exceeding 30 kts.

Outlook: Gusty winds will continue to impact the area into early
Tuesday. Based on the current fcst, the heavier showers associated
with the coastal low are more likely to impact KCLT and KHKY than
the other taf sites. Winds should weaken and precip should taper
off on Tuesday, with spotty showers and occasional restrictions
possible thru the remainder of the week.

&&

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

&&

$$

SYNOPSIS...DEO
NEAR TERM...JDL/JPT
SHORT TERM...JMP
LONG TERM...JMP
AVIATION...JPT