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791
FXUS62 KGSP 260459
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1259 AM EDT Wed Jun 26 2024

.SYNOPSIS...
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High temperatures will peak today ahead of an approaching cold front
which will bring shower and thunderstorm chances to the area this
afternoon into Thursday. Hot and increasingly humid and unsettled
conditions are expected Friday through Sunday as another cold front
impacts the area this weekend. Typical summer weather is expected
behind the front to start the next workweek.
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&&

.NEAR TERM /THROUGH TODAY/...
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As of 100 AM EDT Wednesday...No major changes were needed this
update as the forecast remains on track. Did populate the latest OBS
into the current forecast to get temps and dewpoints back on track.
Also tweaked PoPs and thunder chances through the period based on
the latest high-res guidance. Starting to see some upper cirrus
track into the southern NC mountains but clear skies prevail
elsewhere, along with dry conditions.

Otherwise...dry high pressure will remain dominant across the
area, before upper heights begin to fall ahead of an incoming weak
cold front this afternoon. With upper dynamics increasing and a
sw/ly sfc flow developing, pre-frontal convective activity will have
an increasing probability of occurrence by this afternoon and early
evening. Not expecting much coverage, mainly isolated activity and
confined to the mountain ridges due to limited moisture transport
until the very late period. The 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 adjusted down, especially across the non-mountain zones
through the period. Lows will linger a few degrees abv normal, while
highs bump a category or so warmer most areas with readings in the
upper 90s across the non-mountain zones and lower 90s within the
mountain valleys. Still not anticipating much of a heat index issue
as sfc dewpoints vertically mix out a little better (~800 feet
higher than this afternoon) due to the warmer sfc temps.
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&&

.SHORT TERM /TONIGHT 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 reaching
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 will 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.  Given mean flow and anomalous PWs
exceeding 2 inches across much of the Piedmont, isolated areas of
heavy rain are certainly possible.  Unfortunately, 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.  The latest National Blend guidance nudges
afternoon dewpoints even higher, reaching the lower to even mid-70s
in isolated locations. This continues to be on the upper end of
the guidance, but if these values verified heat index values would
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 would notably impact the heat index values;
therefore, confidence in Advisory-criteria indices six days from
now 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 /06Z WEDNESDAY THROUGH SUNDAY/...
At KCLT and elsewhere: Nice VFR TAF period in store with no
precipitation expected outside the mountains; expect SKC/FEW skies
tonight for all the terminals amid winds becoming increasingly
SEly, SSWly by morning.  FEW/SCT cu field should break out by
late morning, but won`t amount to much.  Some widely scattered
ridgetop showers are expected across the NC mountains...but not
warranting more than a token VCSH at KAVL.  Winds will pick up by
afternoon...but models ensembles aren`t particularly impressed by
gust potential, so will leave out for now.  Despite an approaching
cold front, precip will struggle to get going, at least through
Wednesday evening.

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...AR/JMP
NEAR TERM...AR/MPR/SBK
SHORT TERM...JMP
LONG TERM...JMP
AVIATION...MPR