756
FXUS62 KGSP 272311
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
711 PM EDT Thu Jun 27 2024

.SYNOPSIS...
Seasonable temperatures are expected Friday and Saturday as
shower and thunderstorm chances increase ahead of an approaching
cold front.  Hot and humid conditions on Sunday precede the cold
front with numerous showers and thunderstorms possible.  The front
moves through the area Sunday night, ushering in a drier and more
seasonable airmass for Monday and Tuesday before heat returns to
the area later next week.

&&

.NEAR TERM /THROUGH FRIDAY/...
As of 708 PM Thursday: Isolated afternoon showers and storms have
all but dissipated early this evening with only a stray shower or
two hanging on across the mountains. This trend is expected to
continue with the loss of heating and a quiet evening is in store.
Patchy fog will be possible across the mountains and I-40 corridor
early Friday morning where light rain occurred today. Otherwise, the
ongoing forecast is largely on track and only a few minor tweaks to
PoPs and temperatures were needed.

Synoptically, a slowly deamplifying area of high pressure in the SW
CONUS begins to spread eastward while a weakening trough over the
eastern U.S. gets cutoff. Once the approaching cold front clears the
mountains, guidance suggests the boundary somewhat stalling and
dissolving just to the south of the CWA. Dry and moist air is
expected to mix tonight and with a moisture uptick into Friday and
beyond. CAMs also hint at diurnally driven convection for Friday
afternoon with a few isolated thunderstorms possible (30%-55%).
Winds will remain light and variable, and become more SE by Friday
afternoon. Temps increase a few degrees Friday, teetering near the
upper 80s and low 90s east of the mountains.

&&

.SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY/...
As of 230 PM Thursday: The upper-level pattern is a bit of a
mess over the Southeast to begin the period, with the ridge axis
overhead, a remnant trough axis over the Gulf Coast, and active
westerlies just to our north.  The models continue to advertise
broad cyclonic flow at the surface with a weak warm front draped
across the CWA.  The near-surface flow is generally southerly
to southeasterly so we continue to advect a subtropical airmass
into the region.  The environment certainly supports chance PoPs
into the overnight hours for light shower activity.  During the
day on Saturday, the upper-level anticyclone regains control of
the southern ConUS while shortwave energy approaches the southern
and central Appalachians.  Surface and low-level flow continues
to veer to the southwest ahead of the next cold front approaching
the Ohio Valley strengthening moisture transport into the region.
PWs gradually increase through Saturday, approaching 2" across
much of the Piedmont with RHs generally above 70% from 800-600mb.
A pocket of dry air will remain in place in the vicinity of 500mb
associated with the anticyclone and sounding profiles suggest a weak
subsidence inversion may further limit overall instability/updraft
strength.  Therefore, the pattern certainly supports numerous
showers across the area, but with SBCAPE values below 1000 J/kg
and 60-75% sky cover, it seems likely that thunderstorm chances
will be suppressed.  Therefore, this is currently shaping up to be
a high-PW/low-forcing situation which will minimize the likelihood
of heavy rainfall over most locations.

The abundance of moisture and cloud cover will also suppress
max temperatures and unlike forecasts from earlier this week,
confidence in near-normal highs is increasing.  Unfortunately,
the profile also means it will be difficult to mix out dewpoints
much during the afternoon.  Nevertheless, we continue the trend of
blending in slightly drier guidance to bring the official forecast
closer to the mean of available solutions.  This will still allow
heat indices to reach 100 in favored locations.

A stronger shortwave trough will be propagating through the Great
Lakes later Saturday into Sunday and will drive a cold front
from the Ohio Valley into the Appalachians by Sunday evening.
The forecast continues to advertise dissipating activity overnight
Saturday given near-surface decoupling and increasing temperatures
in the lower and mid-levels to stabilize the atmosphere.  However,
a few light showers cannot be ruled out in the moist airmass.

Guidance continues to forecast the development of downsloping
winds developing just before peak heating well ahead of the cold
front and main shortwave trough in response to a vort max/weak
shortwave propagating over and east of the area.  In theory, we
should have decent SBCAPE (1000-1500+ J/kg) on Sunday with height
falls supporting better lapse-rates.  However, a downslope wind
will work to suppress lift, which is why guidance is more bullish
about convective development and coverage from our easternmost
zones towards the coast.  However, ample moisture remains in place
for Sunday with PWs expected to be near climatological records.
Given the uncertainty of the pattern, the forecast maintains likely
PoPs given the frontal-induced low-level convergence, with isolated
heavy rainfall possible given the support for stronger updrafts in
a very moist low-level column.  At this time, it appears the best
chance for robust convection would be closer to the I-77 corridor.
Bulk shear is minimal so while a few strong downbursts are possible,
the severe threat is very low.

Finally, we continue to battle the guidance spread on dewpoints and
continue to blend in drier guidance to back a few degrees away from
the very aggressive National Blend, which continues to forecast
74-77 dewpoints.  Downsloping flow will support higher max temps
(currently expected to reach the mid-90s across the Piedmont)
but promote mixing.  Widespread 100-104 heat indices and isolated
105-106 heat indices remain possible.  Overall, there remains
tremendous uncertainty in reaching Heat Advisory criteria, but
given the impact of the heat on vulnerable populations, we may
entertain an HWO mention later this evening or overnight.

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
As of 230 PM Thursday: The actual cold front will move through
the area at the beginning of the period Sunday night as high
pressure builds across the Great Lakes and Northeast during the
day on Monday.  The front will be slow to push into the coastal
plain and offshore later Monday and Monday night, but it will
remain progressive thanks to shortwave ridging over the Northeast
and the expansion of the upper-level anticyclone across the
Deep South.  Therefore, northeasterly flow will advect a mostly
stable and pleasant airmass into the region Monday into Tuesday.
The proximity of the front and the timing of the seasonable and
dry air advection across the forecast area from NE to SW on Monday
may keep high temperatures slightly above normal and dewpoints
a bit uncomfortable across the western zones with near normal
highs and very comfortable humidities further north and east, as
dewpoints drop into the upper 50s and lower 60s.  As a result,
chance PoPs are maintained across the southern half of the CWA
given decent instability/lapse-rates.  Any storms that manage to
develop may be capable of producing strong downbursts given the
much drier column, especially across the lower Piedmont.

Surface winds begin to veer to the east during the day on Tuesday
into Tuesday evening as the surface high pushes into New England
and offshore.  The upper-level ridge axis will likely be directly
overhead but the surface ridge axis extending down the Appalachian
foothills from VA to the Carolinas will sustain a continental
airmass.  At this time the flow does not yet support subtropical
moisture flux so Tuesday looks to be a beautiful day with highs
struggling to get to normal and unseasonably low dewpoints in the
upper 50s to near 60 outside of the Savannah River Valley.

The summer heat will return for Wednesday and Independence Day as
we begin to feel the full effects of the upper-level heat dome and
attendant subsidence.  Humidity will increase given the developing
southerly flow but not oppressively so.  Expect highs to rebound to
a few degrees above normal on Wednesday and 5-7 degrees above normal
on Thursday, which will result in upper 90s across the Piedmont.
It looks to be a hot one for outdoor celebrations.  The GFS is
suggesting that the next cold front will be on our doorstep later
Thursday, so National Blend PoPs Thursday afternoon are a bit
above climo in response.  The current forecast backs off on this
for now given the lack of consensus.

&&

.AVIATION /23Z THURSDAY THROUGH TUESDAY/...
At KCLT and elsewhere: VFR conditions are expected to prevail at all
terminals through much of the period. Showers and a few storms from
this morning and afternoon have mostly dissipated with only a few
stray isolated showers, mainly over more remote portions of the
mountains. Heading into tonight, the main focus will be for patchy
fog across the mountains and I-40 corridor where light rain occurred
today. Thus, will maintain a mention of temporary visibility
restrictions from fog at KAVL and KHKY. A few isolated showers and
thunderstorms will be possible Friday afternoon, but confidence in
coverage is too low to warrant mention outside of KCLT. Otherwise,
winds will be light and variable overnight with several wind shifts
possible between north of east and south of east. Southeast winds
will return and prevail by Friday morning.

Outlook: Another cold front approaches the area Sunday night,
keeping unsettled weather around through early next week.

&&

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

&&

$$

SYNOPSIS...JMP
NEAR TERM...CP/TW
SHORT TERM...JMP
LONG TERM...JMP/Wimberley
AVIATION...TW