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FXUS62 KGSP 191743
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
143 PM EDT Tue Jun 19 2018

.SYNOPSIS...
High pressure will dominate our weather through Wednesday, then
begins to weaken and shift south, as a frontal boundary pushes in
from the north. The front stalls on Friday, then lifts back north as
a warm front over the weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 145 pm: Enhanced cu forming on ridgetops undergoing a
transition to deeper convection with several radar echoes over the
higher terrain. As expected...we are seeing a bit more easterly
movement to the showers and storms this afternoon. Otherwise,
the fcst was on track.

The upper ridge will remain in control across the Southeast
through the near term, although it will begin to break down/weaken
in response to northern Conus height falls (upper low wobbling
across the northern Rockies + a short wave trough digging across
New England.) As such, thickness values today will be very similar
to yesterday, which should yield max temps similar to yesterday's
readings, although weak downslope flow should add a degree or two
to maxes across much of Upstate SC. Meanwhile, see no reason to
believe that dewpoints will not once again mix out to the upper 60s
(and perhaps lower) in most areas once again this afternoon. The
"no advisory" forecast appeared to work out well yesterday, and
we will go the same route today, although max heat indices of
100-102 should be a bit more widespread, and the extended period
of hot weather will take its toll (advisory or not), particularly
on vulnerable populations.

Forecast CAPE values later today appear similar to what was
analyzed Monday afternoon, although forecast soundings suggest
slightly lower LFCs. Meanwhile, there is a bit more steering flow,
albeit only 5-10 kts. Nevertheless, convection developing along
ridge tops and/or attendant outflow boundaries will have more of
an opportunity to waft into the foothills and Piedmont late this
afternoon into this evening, but still see no need to advertise
pops higher than 20% southeast of the Blue Ridge, while mtn pops
will range from 30-50%. Min temps tonight will again be at least
5 degrees above climo in most locations.

&&

.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
As of 300 AM Tuesday: An upper low will deepen and drop SE into the
Central Plains Wednesday, then drift east into the Mid-MS Valley on
Thursday. This will effectively flatten the large upper ridge that
has been persisting across the Southeast states the last few days. A
westerly mid to upper-level flow will set up atop the CWA, while a
wavy sfc front sags south across the OH Valley to the Mid-Atlantic.
Overall, this should break the heat and increase the chance for more
widespread shower and TSTM coverage. However, the deterministic
models have some disagreements on the details. The NAM, in
particular, seems overdone with a pocket of very weak lapse rates
and a lack of convective response Wednesday. This area seems sbCAPE
struggle to get to 1000 J/kg with peak heating. On Thursday, the GFS
seems to initiating convection too quickly, and results in temps a
couple categories lower than the consensus. So with this in mind, I
stuck close to the SuperBlend, which averages out these
discrepancies. The upshot is that Thursday now looks to be a little
cooler and, hopefully, the heat wave will be a day shorter than
previously thought. PoPs will be close to climo coverage on
Wednesday (CHC mountains/SLGT CHC piedmont). Then a little higher on
Thursday. Severe threat should remain low Wednesday, but with a few
pulse severe storms possible. Shear is a little better on Thursday,
due to the approaching upper low, possibly 20-30 kts. Which may
elevate the threat for a bit more long-lasting strong storms.

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
As of 330 AM Tuesday: An upper low will bottom out around St. Louis
Friday, then start to lift slowly NE and open up into the northern
stream flow. This will place the CWA under SWLY diffluent upper
flow, with some QG forcing possible. Added to this, a
zonally-oriented frontal boundary will sag south into NC, with the
models agreeing on it stalling over the NC Piedmont. An increase in
moisture and shear will also set up along and south of this
boundary. So all these ingredients should result in scattered to
numerous storms Friday aftn. Both the severe threat and localized
heavy rain/flooding treat will be elevated, but it's still a bit too
far out to start any mention in the HWO. For now, will go with
high-end CHC to likely PoPs. Temps will be back to near normal with
the increased cloud cover and precip.

The upper low lifts into the Great Lakes as an open wave Saturday,
but additional energy will dive into a mean longwave trough over the
mid-MS Valley. This will keep the CWA within a weak, though
deep-layer SWLY flow over the weekend. The sfc front should lift
back north as a warm front, keeping the area in a warm, humid air
mass. PoPs will remain elevated above climo. Temps will return to
above normal readings heading into early next week.

&&

.AVIATION /18Z TUESDAY THROUGH SUNDAY/...
At KCLT and elsewhere: VFR conditions are expected to prevail
through the period. Convective cloud bases this afternoon
should be in the 050-060 range and should remain scattered as
a prevailing condition. Will not rule out a temporary ceiling
if some of the deeper convection moves nearby...most likely at
KHKY/KGSP/KGMU. Coverage of storms will be too isolated to mention
at TAF sites at this time. Will amend accordingly to account for
storms later in the day. Wind should be light W or NW. Convective
clouds should thin out with sunset. Cannot rule out fog during
the pre-dawn hours at any site that receives rain this afternoon,
but it will be most likely in the mtn valleys, perhaps back down
to LIFR or VLIFR in the Little TN River valley. The fog should
burn off by mid morning on Wednesday. Convection is expected to
have greater coverage in the late afternoon, so KCLT gets a PROB30
for the end of the period.

Outlook: Chances for diurnal convection will gradually ramp up
through the remainder of the week, with the highest coverage
(scattered to numerous) expected Friday afternoon/evening, before
an anticipated return to a more typical convective cycle over the
weekend. Increasing chances for convection will also mean increasing
chances for late night/early morning fog and low stratus, although
the highest chances will remain in the mtn valleys.

Confidence Table...

17-23Z 23-05Z 05-11Z 11-12Z
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% High 100%
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 and ensemble forecasts
are available at the following link:

www.weather.gov/gsp/aviation

&&

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

&&

$$
SYNOPSIS...ARK
NEAR TERM...JDL/PM
SHORT TERM...ARK
LONG TERM...ARK
AVIATION...PM
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