170
FXUS62 KGSP 270505
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
105 AM EDT Thu Jun 27 2024

.SYNOPSIS...
A cold front will track across the area today leading to producing
scattered 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.

&&

.NEAR TERM /THROUGH TODAY/...
As of 100 AM EDT Thursday: Made some tweaks to PoPs through the
period and removed the likely PoP mention across the western zones
for this afternoon/early evening as coverage should remain mostly
scattered per the latest CAM guidance. No other major changes were
needed as the forecast remains on track. Otherwise, scattered
showers continue lifting north along the southern portion of the
NC/TN border this morning in association with a cold front. There is
very little lightning activity left within these showers so removed
the thunder mention through daybreak.

A fairly quiet overnight period is expected. Mid-and high-level
clouds will spill into the area through the early morning hours
along with a few isolated to scattered showers scraping across the
mountains in association with decaying upstream convection across
the Tennessee Valley. Recent runs of the CAMs suggest some degree of
showers possible pushing east of the mountains in the pre-dawn
hours, but confidence is this being realized is low owing to a very
dry antecedent airmass entrenched across the area. Area ACARS
soundings continue to depict very dry mid-levels and substantial dry
sub-cloud layer air. It will take time and more moistening of the
column to support measurable precipitation reaching the surface.
Thus, have kept the forecast dry east of the mountains and will let
the overnight shift further evaluate moisture trends. Otherwise,
overnight lows were bumped up a degree or two owing to increasing
clouds and lingering warmth from what was a rather hot day today.

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. A frontal boundary
is also draped across the TN/KY region, progressing toward 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%) of seeing showers or thunderstorms Thursday
afternoon and evening, but rainfall amounts will likely be small.
Expect a small dip in highs tomorrow with the FROPA.

&&

.SHORT TERM /TONIGHT 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
on 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.  Therefore, 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 percentile 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 low-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 /05Z THURSDAY THROUGH MONDAY/...
At KCLT and elsewhere: VFR conditions will prevail through most of
the period at all terminals. A few isolated showers will continue
over the next hour or two, but will dissipate quickly following the
loss of daytime heating. These showers have contained gusty winds up
to 40kts, but activity is isolated enough to preclude mention in any
TAF. Mid- and high-level clouds will increase in coverage overnight
with a band of showers and a couple storms approaching the mountains
from the west. Showers will be possible at KAVL and cannot be
completely discounted elsewhere, but confidence is lower. Greater
coverage of afternoon showers and storms may be realized tomorrow,
especially along and south of I-85. Will carry temporary
restrictions and TSRA at all terminals. Otherwise, winds will shift
to out of the north/northeast overnight with some terminals
returning back to southerly at the very end of the period.

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...AR/CP/TW
SHORT TERM...JMP
LONG TERM...JMP
AVIATION...TW