Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 301422

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1022 AM EDT Fri Jul 30 2021

Hot and dry high pressure persists across the region through Friday.
A cold front will approach the area, gradually sinking south across
the western Carolinas through the day on Friday.  This front stalls
over the Carolinas or Georgia and acts as a focus for shower and
thunderstorm chances through the weekend.  Early next week, another
front arrives and helps drier weather filter in toward the end of
the week.


As of 1000 AM: The decaying MCS hat moved into East TN and far
W NC earlier this morning has now diminished to a band of light
stratiform precip. The cirrus shield blankets most of the area,
though warming cloud tops and thinning upstream suggest it
won`t be in place all day. Of course, the impact on temps could
be significant. HRRR/RAP runs thru 12z cycle did not reflect a
significant cooling effect, but we are eagerly anticipating the
13z. Current temps fall between where the 06z and 12z NAM suggested
we`d be. All that said, forecast performance this morning suggested
we drop max temps a little bit, but retain dewpoints similar
to what we had. Not confident enough to cancel Heat Advisory in
our far south, but more confident we made a good decision in not
hoisting one any further north.

The frontal system that drove the development of the MCS
will continue to shift south across the CWA today. The best
instability develops south of the front, mainly across the southern
mountains and along and south of the I-85 corridor. A moderate
to very unstable air mass develops but shear will be better to
our east. Similar to what was noted for the temps, it will be
interesting to see what the new 13z HRRR depicts for convection;
previous runs didn`t seem to think the cirrus would have much of a
stabilizing effect. DCAPE values will be high south of the front
and drying mid levels make for relatively strong surface delta
Theta- E values as well. Some strong and isolated severe storms
will be possible, especially from Greenwood to Monroe. Updated Day
1 outlook actually expanded Marginal Risk a bit further west, but
with update kept PoPs in chance range at best, actually slightly
less than before given that the front will make more progress thru
the area before the cirrus thins enough to allow initiation.

Expect convection to drop off quickly this evening as the front
moves south of the area. Mountain valley fog and stratus expected
once again tonight, with lows a little above normal.


As of 215 AM EDT Friday: At daybreak Saturday, a 500mb trough axis
will be anchored across the Ozarks and into the Ohio Valley, as
a weakening shortwave lifts out over New England.  An amplified
ridge remains in place over the western CONUS.  At the surface,
a weak frontal boundary will be situated to our south, with our
CWA experiencing light northerly to northeasterly surface winds
as drier cP air builds in from central Canada and the Midwest.

On Saturday, a lobe of DPVA will round the trough and interact with
the frontal zone to our south to spin up a weak surface low along
the preexisting favorable environment of the boundary.  This forcing
continues through the first part of Sunday.  Concurrently, the 250mb
field demonstrates support for cyclogenesis, with the Carolinas
beneath the right entrance of a jet streak by Saturday night.
The location of best upper divergence is as-of-yet unclear, but
there`s certaintly a case to be made that upper support may enhance
cyclogenesis on Sunday.  As a swath of moisture swings into the
mid- and low-levels on Sunday, accompanied by increasingly upward
omegas, we should see PoPs ramp up, eventually peaking out Sunday
afternoon as the deepening surface low passes south of us and
pushes off the Atlantic coast.  Models agree on this progression
fairly well, with the GFS, CMC, and NAM all depicting such a system.
The ECMWF, although it agrees on PoPs, remains fairly unimpressed
by the setup and fails to develop a robust closed circulation.

Given the synoptic setup, convection isn`t favored for GSP
this weekend.  On Saturday, with the boundary to our south,
instability will be fairly limited and late-to-develop, while
soundings indicate that subsidence on the high pressure side of
the boundary will produce an inversion aloft that should result in
LFCs as high as 4-5 kft.  Prospects look somewhat better on Sunday,
as the weakening boundary allows the inversion aloft to vanish and
good forcing aloft should somewhwat destabilize the atmosphere.
So, despite less-than-ideal lapse rates, SBCAPE will be in the
1000-1200 J/kg range and shear could even approach 30kts, such that
some thunder isn`t out of the question.  The real determining factor
in all this will be the positioning of the boundary - should it sag
farther south than expected, convection will be severely limited;
should it pull a bit more north, we could be dealing with a better
chance of storms.

In any case, most models depict some kind of PoPs throughout
Saturday afternoon and Sunday, only letting up toward Sunday night.
Temps are still on track to be a tad above normal on Saturday
and Sunday.


As of 230 AM EDT Friday: The 500mb trough remains anchored in
place throughout the medium range period.  Canadian high pressure
slowly builds in and gradually forces the boundary farther south,
allowing drier air to filter in from the north.  Still, with a
series of vort lobes traversing the CWA along the broad trough,
and moisture taking its time to exit the region, PoPs will remain
in the forecast daily.  With the boundary`s position uncertain,
this remains a difficult-to-predict scenario, except to say that
we should generally see drying toward the end of the week.


At KCLT and elsewhere: Mid and high level cigs holding across
the area. Expect early VFR cumulus development today with
continued mid/high level cigs. Have TEMPOs but KHKY where TSRA
expected for afternoon convection along the southward-sagging
surface front. Generally WSW wind expected south of the front
and NW to the north. Convection should end quickly early in the
evening with winds turning N then NE. Mountain valley fog and
stratus expected again and will threaten KAVL.

Outlook: Only isolated convection expected Saturday with drier air
briefly moving in. Convective chances increase Sunday and continue
through the middle of next week week as a trough develops over the
east. Fog and low stratus will be possible each morning.

Confidence Table...

            14-20Z        20-02Z        02-08Z        08-12Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     High 100%     Med   70%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     High 100%     High 100%

The percentage reflects the number of guidance members agreeing
with the scheduled TAF issuance flight rule category. Complete hourly
experimental aviation forecast consistency tables are available at
the following link:


GA...Heat Advisory until 8 PM EDT this evening for GAZ026-028-029.
SC...Heat Advisory until 8 PM EDT this evening for SCZ011-019.


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