746
FXUS62 KGSP 141056
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
656 AM EDT Tue May 14 2024

.SYNOPSIS...
Low pressure brings rain and scattered thunderstorms to our region
today through Wednesday. Brief high pressure brings a mainly dry day
on Thursday. Showers and scattered thunderstorms return Friday and
into the weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 538 AM EDT Tuesday: Shower coverage continues to undercut
basically all of the guidance, with an axis of heavier showers
making tracks across the SC Upstate and just now entering the NC
Piedmont...but generally dry conditions elsewhere.  In response to
this, it appears that very weak sfc cooling has occurred in the wake
of this activity, but not to the extent intitially anticipated.
This may have downstream impacts on destabilization today - if
whatever wedge develops is even weaker than depicted in guidance,
it`ll be easier to destabilize and more of the CWA may wind up
open to afternoon CAPE.  Temperatures are still on track to fall
to 60-62 at most locations this morning.

Otherwise...some guidance sources, most notably the NAM and
Canadian RDPS, depict some appreciable MUCAPE atop this wedge
leading up to daybreak for the Upstate, as potent 850mb WAA
develops and steepens wedge-top lapse rates. That WAA will taper
off quickly after daybreak as the axis of best DPVA translates
eastward, but the breakdown of the wedge will be slow, at best,
sans any discernible erosion mechanism other than daytime heating,
which will be severely constrained by thick, overcast clouds.
The latest 00z HREF cycle supports >75% chance of the wedge
retreating north of I-85 by mid-afternoon, but a <30% chance of
the wedge retreating much north of the NC-SC state line.

This is important, because locations south of the wedge boundary
will essentially be in the warm sector of advancing surface low
pressure, and thus exposed to diurnal instability.  The latest
CAM runs feature 500-1000 J/kg sbCAPE across the SC Upstate, and
in an environment of some 45kts of 0-3km shear.  That`s more than
enough for severe risk for non-wedged-in areas if storms develop.
Yet, convective initiation remains a question mark, as by the
time any of this takes place, the region will be devoid of much
synoptic forcing or any real triggering mechanism.  And indeed,
all the latest CAM runs depict a pronounced lull in activity during
the afternoon hours, so much so that only chance to low-end likely
PoPs are featured for much of that time, and may still be overdone.
Nonetheless, the environment will be ripe for severe risk with
any storm that does develop, especially going into the evening
hours as the surface low enters the Ohio Valley and low-level
SRH becomes enhanced in response.  SPC`s Marginal Risk for severe
weather today therefore looks reasonable.

Anyway, highs today will only make it into the lower 70s given
widespread cloud cover.  This evening, some previous CAM runs
have depicted the arrival of semi-organized convection from
either the Tennessee Valley or from central GA; the most recent
runs of the HRRR, ARW, and FV3 all depict this activity fizzling
before it reaches us, perhaps because of waning instability and
a not-fully-removed wedge.  This`ll have to be monitored closely,
because there`s still lower-than-usual confidence for what`s going
to happen at this time range.  Lows tonight will fall into the
low 60s.

&&

.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
As of 250 AM EDT Tuesday: An upper trough crosses the area
Wednesday as short waves rotate through the trough and across the
area as well. A surface low pressure center crosses the area as
well. Guidance showing moderate instability developing during the
day. The forcing and instability will lead to numerous to widespread
showers and scattered thunderstorms. Bulk shear may reach moderate
levels which could lead to some multicellular organization and a few
severe storms. Large hail and damaging winds are the main threats.
Isolated heavy rainfall is also possible. Convection should taper
off quickly during the evening as forcing moves east and instability
wanes. Highs will be near normal and lows around 5 degrees above
normal.

A short wave ridge moves in Thursday along with a drier air mass.
This should help limit convective chances to the northern mountains
where a weak trough and lingering low level moisture could lead to
isolated or scattered diurnal showers. Highs and lows will be around
5 degrees above normal.

&&

.LONG TERM /FRIDAY THROUGH MONDAY/...
As of 315 AM EDT Tuesday: Convective chances ramp back up on Friday
as an upper low/trough and the associated surface low pressure
approach from the west. The trough and surface low continue to
slowly move into the area Saturday. Guidance has slowed and now
shows the system only slowly crossing the area on Sunday. Therefore,
have likely convective coverage Friday, solid chance Saturday, and
now, low end scattered coverage on Sunday. A short wave ridge builds
in on Monday, but lingering moisture and an approaching cold front
could keep scattered diurnal convection in the area. The magnitude
of instability and shear is uncertain through the period making
severe probability uncertain as well. Isolated heavy rainfall is
possible Friday given the amount of moisture, forcing, and
convective coverage. It`s more uncertain the rest of the period.
Temps look to be around normal Friday then rise above normal through
the rest of the period.

&&

.AVIATION /12Z TUESDAY THROUGH SATURDAY/...
At KCLT and elsewhere: Widespread IFR and even LIFR ceilings this
morning beneath a potent isentropic ascent regime.  Rain hasn`t
quite performed the way the models said it would, but still it`s
been enough for a lot of fog this morning east of the mountains -
just generally a murky morning for all.  Improvement will be almost
imperceptibly slow after daybreak, but guidance is hinting at some
breakup of low cloud cover by mid afternoon, and confidence is
somewhat reinforced by the trend toward lower coverage of showers
Tuesday in the latest model runs.  Convection seems somewhat more
likely tonight, toward the end of the TAF period, as the wedge
will be weaker/nonexistent by then, and convection over TN and GA
is progged to push into the area.  It`s unclear how far east any
activity will make it - but went ahead and kept a PROB30 for TSRA
in the KCLT TAF to address the possibility.

Outlook: Restrictions should begin to ease on Wednesday morning,
but another round of TSRA, with potentially better coverage than
on Tuesday, will return and could cause localized restrictions.
Brief drying is expected Thursday, before convection and associated
flight restrictions return ahead of a cold front over the weekend.

&&

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

&&

$$

SYNOPSIS...RWH
NEAR TERM...MPR
SHORT TERM...RWH
LONG TERM...RWH
AVIATION...MPR