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336
FXUS62 KGSP 262002
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
402 PM EDT Wed Jun 26 2024

.SYNOPSIS...
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A cold front will approach and enter the area tonight
through Thursday, producing scattered to numerous showers and
thunderstorms.  Hot and increasingly humid and unsettled
conditions are expected Friday through Sunday.  Another cold
front moves through the area Sunday night, ushering in a more
seasonable airmass for Monday and Tuesday before heat begins to
return to the area for Wednesday.
-- End Changed Discussion --

&&

.NEAR TERM /THROUGH THURSDAY/...
As of 200 PM Wednesday: Clear skies start to fill in with cirrus and
a non-threatening cumulus this afternoon. Synoptically, an
amplifying area of high pressure out west directs upper flow more
westerly to start. A weak trough makes a run for the south from a
disturbance over the Great Lakes. This extends a cold front from the
north toward the southern states. By this afternoon, the frontal
boundary will be draped across the TN/KY region, progressing toward
the CWA. Meanwhile, a weak warm front recedes southward as moisture
continues to mix out during the afternoon period. Guidance from the
CAMs struggle to initiate convection over the mountains. HRRR and
NAM both suggests minimal shower or thunderstorm activity. Looking
at the modeled soundings from the RAP and NAM, the boundary layer
this afternoon is extremely mixed, with very steep sfc-10km
adiabatic lapse rates. Another limiting factor is the lack of
instability. RAP guidance does support decent sbCAPE, but also
suggests 1100-1400 J/kg of dCAPE, which is likely to support strong
downdrafts of any storms that can go up. These factors, combined
with the upper support lacking ahead of a front, confidence is low
for convection today for most of the CWA. Will still maintain slight
chance PoPs (25%-35%) for the mountains to account for any possible
isolated shower or thunderstorm. For tonight, moisture ahead of the
FROPA filters back into the area as sfc flow becomes more southerly.
The boundary should reach the mountains by daybreak and slowly move
through the CWA. This will increase chances for precipitation on
Thursday, but QPF response is still low. Guidance from the
GFS/NAM/Canadian/EURO all target the higher QPF across the mountains
Thursday afternoon, with limited or none in the SC/NC Piedmont.
Overall, the mountains have a better chance (50%-70%) of seeing
showers or thunderstorms Thursday afternoon and evening, but
rainfall amounts will likely be small. Temperatures today are
expected to be the hottest so far this summer, with high 90s east of
the mountains. Given the lower dewpoints, the heat index should not
be as much of a problem, but a few spots in the eastern portion of
the CWA could see 100. Expect a small dip in highs tomorrow with the
FROPA.

&&

.SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/...
As of 230 PM Wednesday: The surface pattern is a bit complicated to
begin the period Thursday night as the remnants of the cold front
which moved across the area earlier in the day lay stationary across
central SC, while a coastal front rests along the Carolina Coast.
The lack of movement on the surface pattern is due to a mid-level
trough axis which becomes detached from the mean westerlies.
Some of the guidance is trying to cut off a weak mid-level low
over the central Gulf Coast and we continue to note weak surface
low development is possible along the remnant front Thursday night.
These features may provide enough forcing/lift to sustain afternoon
convection into the overnight hours in the form of light shower
activity, especially across the lower SC Piedmont closer to the
front and low.  The pattern becomes clearer during the day on
Friday as a transient surface high slides east across New England
and east-northeasterly winds briefly advect a modified continental
airmass into the region, temporarily pushing the remnant boundary
well south of the area.  However, winds continue to veer to the
southeast through the afternoon as the high propagates offshore,
such that southeasterly surface winds begin to advect subtropical
moisture from the Atlantic and the boundary retreats north as a
warm front through the evening hours.  Weak instability persists
across the area as PWs begin to rise during the afternoon and while
the guidance isn`t bullish on strong convective development, there
is good consensus we will see scattered to numerous showers and a
few thunderstorms through at least the late evening hours Friday.
With the expected activity, forecast highs are actually a couple
of degrees cooler than they were in yesterday`s package, but
the increase in humidity due to the pattern should be notable.
As moisture increases through the column it is unlikely we will
experience significant mixing and afternoon dewpoints east of the
mountains may remain near 70, leading to heat indices in the upper
90s to near 100 in favored locations.

The remnant trough axis aloft drifts over the Gulf Coast and
the upper-level anticyclone expands across the Southeast Friday
night into Saturday.  We remain on the northern fringe of the
upper high and on the southern fringe of the active westerly
flow.  As a result, surface winds slowly veer to the south and
southwest in response to the next shortwave and attendant cold
front approaching from the Ohio Valley.  While the route is more
circuitous, moisture flux via the central Gulf continues into
the region through Saturday. PWs surge above 2 inches across
the Piedmont in response and given the pattern numerous to even
widespread showers are likely across the area.  While lapse rates
are unimpressive due to the increasing column moisture, there will
be enough instability to promote scattered thunderstorms and given
mean storm motion vectors, isolated heavy rainfall typical of June
is possible.  Keep in mind, however, that most of the rainfall
we can squeeze out of the atmosphere will be welcomed given the
extremely dry soils and well below-normal streamflows in place.

Like Friday, the latest forecast highs for Saturday are 1-3 degrees
cooler than they were with the package yesterday afternoon,
with lower 90s prevalent across the Piedmont.  National Blend
guidance continues to show its typical high bias with afternoon
dewpoints, landing in the uppermost range of all available guidance.
Therefore, the current package continues the previous trend of
mixing in dewpoints a few degrees lower than the blend.  It will
be hot and muggy, but quite typical of June and heat indices are
currently expected to stay below Advisory criteria.

&&

.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
As of 230 PM Wednesday: Guidance has come into better agreement
regarding the long-advertised cold frontal passage late this
weekend, and the consensus at this time is that passage will occur
overnight Sunday.  This will keep our area in the moist and modestly
unstable airmass ahead of the front during peak heating. At this
time, the best SBCAPE values are likely to be across the Piedmont
with pockets of 1000-2000 J/kg possible given current guidance.
PWs will peak on Sunday as well, with widespread 2"+ values across
the Piedmont and the potential for values near the climatological
max.  Storm-motion vectors continue to suggest slow cell propagation
which would also support locally heavy rainfall However, we continue
to lack large-scale forcing and have very dry antecedent conditions
to justify advertising a threat for excessive rainfall.  Instead,
the focus should be on the potential for some much-needed rainfall.

As with Friday and Saturday high temperatures, Sunday`s highs
have been trimmed a degree or two as well in response to ample
cloud cover and convective activity.  While we continue to expect
the worst combination of heat and humidity on Sunday afternoon,
the forecast continues to blend slightly drier guidance with
National Blend to bring dewpoints out of the mid-70s.  It is
often quite difficult to achieve such humidity in similar regimes.
Nevertheless, heat indices continue to flirt with Heat Advisory
criteria in the Charlotte metro and Upper Savannah River Valley
with widespread near 100 indices elsewhere, but overall confidence
remains low.

As is common for this time of year, the front will have a difficult
time making much progress past the area on Monday and should stall
our near our southern zones.  This will maintain chance PoPs
across most of our forecast area, especially south of the I-40
corridor, closer to the front.  Meanwhile, surface high pressure
dominates the weather across the Mid-Atlantic and Northeast and
the continental airmass will start to influence our area with
highs expected to be near normal east of the mountains and 2-3
degrees above normal in the mountain valleys.  The most notable
impact of the airmass will be the significantly lower humidity,
with afternoon dewpoints at least 10 degrees lower than Sunday.

Tuesday looks to be the most pleasant day of the forecast as
we finally push the front away from the area in response to
high pressure ridging down the Appalachians.  Outside of the
SW mountains where a chance diurnal PoP persists, expect dry
conditions with near-normal temperatures and pleasant humidity.
The strengthening upper-level ridge unfortunately builds back over
the area to end the period on Wednesday, and we quickly see highs
rebound into the 90s heading into the long holiday weekend.

&&

.AVIATION /20Z WEDNESDAY THROUGH MONDAY/...
At KCLT and elsewhere: All terminals start off and should remain VFR
through the entire TAF period. High pressure continues to dominate
the region today. As a cold front approaches from the west, a few
areas in the mountains could see an isolated -RA or TS. Confidence
is low with less than a 25% chance at KAVL, which is why there is
currently no mention of convection or rain in the TAF. Overnight,
the cold front is expected to reach the mountains around 09-11z. The
front will weaken as it crosses the area through the day on
Thursday, but expect a wind shift. KCLT should see winds turn more
NE around 15z and return to SE near the 22-00z range Thursday
evening. Again, the front is not expected to be strong, keeping
overall wind speeds and any gusts low end. Though gusts are low-end,
cannot rule a g15 at any terminal. With the frontal passage, there
is an uptick in shower and TS possibilities Thursday afternoon.
Confidence still remains low, but is high enough to warrant a PROB30
for KCLT at this time. Will continue monitoring at future TAF
issuances.

Outlook: A cold front stalls south of the area on Friday, keeping
SHRA/TSRA chances around the region. Another cold front will impact
terminals this weekend along with an unsettled pattern into next
week.

&&

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

&&

$$

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