590 
FXUS62 KGSP 300539
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
139 AM EDT Fri Jun 30 2023

.SYNOPSIS...
Increasing heat and humidity is expected Friday into the
weekend as high pressure dominates the Deep South.  Shower and
thunderstorm chances return to the area Friday and persist into
Sunday as several waves of energy move from the Ohio Valley into
the southern Appalachians.  Chances for showers and thunderstorms
will continue into early next week as a cold front approaches the
region, bringing some relief from above-normal temperatures.

&&

.NEAR TERM /THROUGH TODAY/...
As of 10:25 PM EDT Thursday: We remain mostly dry across the CWA 
late tonight, save for a few sct light showers over the western
Upstate. A number of locations continue to report reduced visby 
with haze (HZ) from the Canadian wildfires burning well to our 
north. Although the haze is generally expected to diminish thru
the overnight, it may linger for quite a bit longer.  
   
Otherwise, a robust upper ridge will remain centered to our west 
and nearly stationary thru the near-term period. Well to our north,
an upper low will persist over southern Ontario and help maintain
a conveyer belt-like series of upper-level shortwave impulses that 
move over the Southeast and mid-Atlantic regions. This set-up will
help produce multiple rounds of convective complexes that fire up 
with help from the persistent upper jet that lingers just north of 
the upper ridge. Unfortunately, near-term guidance from recent CAM 
runs remains inconsistent with the timing and strength of convective 
activity thru most of the period. We're still expecting a lull in 
activity thru the overnight, with a round of sct showers and maybe
a few thunderstorms moving thru the CWA from the NW thru the morning
hours. The best chance for any thunderstorms appears to be over our
western-most zones, if the lingering instability can be realized.
For the rest of tomorrow, the latest guidance has been less bullish
on any widespread convection with only isolated to sct convection 
looking more likely for the afternoon/evening. A stronger round of
convection/MCS appears to approach the fcst area from the NW later 
tomorrow night and then move thru overnight. Again, the stronger 
storms appear more likely over our western and southern zones. As 
such, SPC still has a Slight Risk for severe thunderstorms across 
the NC mtns and a Marginal Risk for the remainder of our CWA for 
Friday afternoon thru early Saturday. The main threat from any 
stronger storms that do develop still appears to be damaging 
straight-line winds, with larger hail being less likely. The 
tornado threat remains less than 2% across our area. Otherwise,
temperatures on Friday will likely reach the low 90s for most 
of our lower terrain with heat indices peaking just above 100 
degrees for a few hours over our southwestern-most zones.

&&

.SHORT TERM /TONIGHT THROUGH SUNDAY/...
As of 230 PM EDT Thursday: At the beginning of the period Friday
night, the region will remain underneath northwest flow aloft
as the upper-level heat ridge remains anchored near the central
Gulf Coast.  The overall pattern and mean flow, combined with
ample instability and forcing over the Midwest and Ohio Valley,
continues to support the development and propagation of upstream
convection into or near the southern Appalachians.  Depending on
the various and ever-changing CAM solutions, once such system
may be entering or impacting our area around the beginning of
the period.  As such, PoPs reflect the possibility of showers
and storms from Friday evening into the overnight hours.  It is
important to stress, however, that at this time we cannot predict
with any degree of certainty the existence, timing, or intensity
of organized convection and it will be important to stay weather
aware as our understanding of the threat evolves.  Nevertheless,
the environment supports robust convection upstream and if this
convection can sustain itself in a somewhat less unstable airmass
over the mountains and especially the Piedmont, the potential for
strong to severe storms is notable.  As such, SPC has a Slight
Risk for severe thunderstorms in place through Saturday morning
for the western NC mountains as any organized thunderstorm clusters
could produce damaging wind gusts.

Saturday's weather will be dependent on the presence and evolution
of nocturnal convection.  The presence of ongoing convection, a
residual cold pool, or convective cloud cover debris may impact
diurnal heating and the recovery of the airmass for afternoon
shower and thunderstorm development.  However, absent of these
complications, the overall airmass supports slightly warmer high
temperatures and higher dewpoints for the daytime hours, and
we continue to advertise heat index values into the lower 100s
across much of the SC Piedmont and the Charlotte metro.  As has
been discussed previously, the mixing out of dewpoints in the
afternoon tends to be underdone in guidance blends, which means we
may see lower dewpoints to suppress heat indices somewhat.  In the
end it won't matter much because it will still be oppressively
uncomfortable, but we may be able avoid widespread Heat Advisory
criteria (>104).  The heat and humidity continues to support
significant SBCAPE values and above-climo PoPs over the mountains,
with PoPs decreasing to slight-chance over the lower Piedmont,
where mid-level warm air is still expected to suppress activity.

Saturday evening into Sunday, the upper-level anticyclone begins
to weaken slightly in response to a potent shortwave trough over
the central Great Plains.  The guidance is not in agreement about
the timing of this shortwave energy which may determine whether
or not we will experience at least one more night of northwest
flow and MCS potential Saturday night into Sunday morning.
The same threats and uncertainties mentioned previously apply here
as well.  Sunday still appears to be the hottest day of the 7-day
forecast period, with highs in the mid 90s east of the mountains.
There will be not much change in heat index expectations on Sunday
and confidence in reaching Heat Advisory criteria remains low.
Similar atmospheric profiles compared to Saturday maintain
above-climo PoPs over the mountains with decreasing PoPs east.
Bulk-shear values are somewhat lower for Sunday afternoon, which
may reduce the chances for organized convection.

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
As of 300 PM EDT Thursday: A potent shortwave trough will be
swinging through the lower Great Lakes region at the beginning of
the period which will suppress the infamous upper-level anticyclone
to the Gulf Coast.  With the attendant ridge axis to the east of the
region, the upper-level mean flow will therefore be southwesterly,
as a cold front enters the Tennessee Valley.  Therefore, while
this flow will end the threat of upstream MCS propagation for
the overnight period into Monday morning, the pattern will remain
unsettled as we begin the work week.  The height falls associated
with the approaching trough will improve mid-level lapse rates
across the area such that instability will improve over the
Piedmont beginning Monday afternoon, with likely PoPs over the
mountains.  The deep-layer southwesterly flow will sustain the
heat and low-level flow around the anticyclone to our south will
support continued moisture advection into the region.  Therefore,
despite the potential for increased cloud cover from shower and
thunderstorm activity, high temperatures and heat indices only
decrease a few degrees or less.

Thereafter, the average upper-level pattern becomes fairly stagnant
over the Southeast with a trough axis roughly extending from the
central Gulf Coast to the Northeast as a heat ridge over the Desert
Southwest and subtropical ridging over Florida persist through
the end of the period.  There will be shortwaves moving through
this mean flow throughout the period, while the aforementioned
cold front slowly approaches the area.  The cold front will enter
the area later Tuesday into Wednesday, at which point it may
slowly dissipate near or over the western Carolinas.  Therefore,
each day we are advertising above-climo, diurnal PoPs area wide,
but highest over the climatologically favorable mountain zones.
Bulk shear remains below 20-25kts generally each day; therefore,
while coverage may be robust, storm severity is not expected to
be at this time.  Given the proximity of a weak front and somewhat
lower thicknesses due to the upper-level pattern, high temperatures
gradually revert to near-normal by the end of the period.

&&

.AVIATION /06Z FRIDAY THROUGH TUESDAY/...
At KCLT and elsewhere: Mostly VFR conds to begin the period except 
across the NC terminals where 5-6 miles in haze will persist 
possibly into the early afternoon. An upstream MCS is progged to 
reach the nrn NC mtns arnd daybreak and have included early on VCSH 
at KAVL and KHKY. Expect good destabilization this afternoon and 
with the mlvl forcing passing thru, isol/sct showers and 
thunderstorms with MVFR CIGS/VSBY will be possible across all TAF 
sites. Picked an afternoon period for a PROB30 thunder over most 
sites except KAND where forcing may not affect the terminal until 
another round of MCS energy moves in late tonight thru the overnight 
period. Winds will remain aligned generally nw/ly at KAVL and begin 
the period elsewhere sw/ly then veer w/ly to nw/ly by the later 
afternoon. No great gust potential, yet isol low-end gusts will be 
possible in a well mixed atmos.  

Outlook: Chances for showers and thunderstorms in addition 
to early morning fog and low stratus will increase on Friday and 
continue thru the weekend.

&&

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

&&

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