115
FXUS62 KGSP 251940
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
340 PM EDT Tue Jun 25 2024

.SYNOPSIS...
High temperatures will peak on Wednesday ahead of a cold front.
Shower and thunderstorm chances increase Wednesday into Thursday
as the front moves across the area.  Behind the front, afternoon
shower and thunderstorm chances will persist through Saturday.
Hot and humid conditions are expected on Sunday with numerous
showers and thunderstorms possible as another cold front enters
the area.  Typical summer weather is expected behind the front to
start the next work week.

&&

.NEAR TERM /THROUGH WEDNESDAY/...
As of 150 PM Tuesday...Dry high pressure will remain dominant across
the area, before upper heights begin to fall ahead of an incoming
weak cold front Wed afternoon. Column moisture remains quite low and
only expect fair-wx Cu this afternoon as the mid-level cap will be
hard to break with no triggering available. So, continued the no
PoP mention in the grids into Wed. With upper dynamics increasing
and a sw/ly sfc flow developing, pre-frontal convective activity
will have an increasing probability of occurrence by Wed afternoon
and early evening. Not expecting much coverage, mainly isol activity
and mostly confined to the mtn ridges due to limited moisture
transport until the very late period. The op models have backed off
on a precip response with the ECMWF now dry thru the period and the
CAM models not enthused outside of discrete ridgetop cell
generation. Thus, PoPs were adj down, esp across the non/mtns thru
the period. Lows will linger a few degrees abv normal, while highs
bump a cat or so warmer most areas with readings in the u90s across
the non/mtn and l90s within the mtn valleys. Still not anticipating
much of a heat index issue as sfc td/s vertically mix out a little
better (arnd 800 feet higher than this afternoon) due to the warmer
sfc temps.

&&

.SHORT TERM /WEDNESDAY NIGHT THROUGH FRIDAY NIGHT/...
As of 230 PM Tuesday: The upper-level pattern will evolve Thursday
into Friday as a shortwave trough axis slowly drops south and east
of the area and heights build ahead of another shortwave trough
entering the Upper Midwest.  This will allow the heat ridge to
expand east across the Southeast and into the Atlantic for Friday.
Guidance has been a bit inconsistent from run to run regarding the
progression of a cold front through the area for Thursday with
continued disagreement on whether or not it will clear the CWA.
Given that the causative upper-level energy is shearing out over
Southeast on Thursday, blended guidance is a reasonable solution
at this time, which stalls the front somewhere over or just to
the east of our Piedmont zones.  However, as mentioned in previous
discussions, the front will become increasingly ill defined.  It
will therefore serve more as a weak focusing mechanism for scattered
to numerous showers and general thunderstorms on Thursday, but we
lack sufficient forcing and instability with poor mid-level lapse
rates, minimal SBCAPE, and anemic bulk shear values.  Therefore, QPF
and any strong to severe storm threat is insignificant.  Highs on
Thursday are generally unchanged from earlier packages, but the
moderation behind the front will be a bit more noticeable given
the increasing forecast max temps on Wednesday.  Highs will be 4-6
degrees "cooler" for Thursday, but still 2-4 degrees above normal.
The influence of the front will be a bit more discernible in the
mountain valleys, with highs 5-7 degrees cooler than Wednesday,
though these highs will still be slightly above normal.

Guidance continues to advertise the cutting off of a weak mid-level
low as upper energy departs the area later Thursday into early
Friday and heights build to the north and west.  Therefore, weak
surface low development along the remnant front over the central
Carolinas is possible during this timeframe, which may serve to
maintain chance PoPs near the proximity of the front overnight,
primarily east of the mountains.  On Friday, the upper-level ridge
axis will settle over the Southeast as surface high pressure
propagates across New England.  Resultant northeasterly flow
early on Friday should help to push whatever remains of the
front south and east of our area and usher in a more seasonable
airmass, though highs will still be about 2 degrees above normal.
The transient nature of the high will allow near-surface winds
to veer easterly then southeasterly during the day, which should
promote modest subtropical moisture advection.  PWs gradually
begin to increase during this time as evidenced by increasing
moisture content below 650mb in forecast profiles.  Therefore,
even with mixing, dewpoints are expected to remain elevated into
the afternoon and moist upslope flow should promote scattered
shower and thunderstorm development across the area with terrain
enhancement over the mountains.  Some convection should be able
to produce much-needed rainfall in isolated locations.

&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
As of 230 PM Tuesday: The extended period will feature the
strengthening of the upper-level anticyclone, potentially reach
598-599 dm over the Southern Plains in the Sunday-Monday timeframe.
The ridge will stretch over our area for Saturday, but height
falls in response to the next shortwave over the Upper Midwest will
erode the ridge overhead later Saturday into Sunday.  An attendant
cold front will be dropping south and east across the Ohio Valley
on Saturday, as we continue to advect moisture into the column.
PWs continue to climb during the day on Saturday in response,
with guidance suggesting values approach 2 inches.  As a result,
SBCAPE values remain unimpressive for significant convection,
but ample moisture and increasing mid-level and low-level forcing
from the approaching shortwave and cold front suggests scattered
to numerous showers and storms are possible Saturday afternoon.
High temperatures should be similar to Friday though they may be
influenced by higher convective coverage.  Regardless, dewpoints
are not expected to mix out much if at all during the afternoon
hours given the moisture aloft and this would result in notably
uncomfortable humidity values during peak heating, with heat index
values reaching the low 100s across much of the Piedmont.

Expect chance PoPs to persist overnight Saturday into Sunday
given the approaching cold front and ample low-level moisture.
The front is expected to directly impact the area on Sunday,
but as with previous packages, there remains about a 12-18 hour
timing discrepancy between deterministic guidance.  The faster GFS
passage would cause the best combination of heating and forcing to
be south of our area, so this is something that needs to be watched.
Nevertheless, the overall impact remains the same, with numerous
to widespread showers and thunderstorms expected during the day.
With several days of moisture advection in place, this frontal
passage looks to be the best opportunity for appreciable rainfall
we have had in quite some time, especially over the mountains and
northern Piedmont.  Unfortunately, this is relative given how dry
we`ve become; GFS and ECMWF ensemble probabilities of an inch or
greater remains well below 20 percent for most of the forecast area.

It looks like the worst combination of heat and humidity continues
to be on Sunday, with National Blend guidance nudging afternoon
dewpoints even higher.  Given the widespread dewpoints in the
lower to even mid 70s in isolated locations, heat index values
may approach Advisory criteria (>105) in the Charlotte metro and
Upper Savannah River Valley.  However, a mix out of even a couple
degrees in afternoon dewpoints will notably impact the heat index
values; therefore, confidence in Advisory-criteria indices remains
too low to add to the HWO at this time.

Following the front on Sunday, we expect modest improvements in
humidity and highs closer to normal for Monday and Tuesday, but
an active upper-level pattern supports continued above climo PoPs
each afternoon.

&&

.AVIATION /20Z TUESDAY THROUGH SUNDAY/...
At KCLT and elsewhere: No good chance of flight restrictions across
all TAF sites thru the period. The atmos will remain quite dry and
only allow for Ci and some afternoon Cu today and Tue. A cold front
will approach the wrn Carolinas late in the period, but any pre-
frontal precip or lowering clouds will not affect KAVL or KHKY by
18z. Northeasterly to se/ly winds will remain light this evening
with little to no gust potential, before going calm overnight. Winds
pick up out of the southwest Wednesday ahead of the cold front and
remain rather light, except for more moderate flow developing at
KCLT by the late period.

Outlook: A cold front will track over the terminals Wednesday night
into Thursday before stalling south of the area on Friday, keeping
SHRA/TSRA chances around the region.

&&

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

&&

$$

SYNOPSIS...JMP
NEAR TERM...SBK
SHORT TERM...JMP
LONG TERM...JMP
AVIATION...SBK