Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 150608

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
208 AM EDT Tue Aug 15 2017

A stationary front is expected to remain across the region through
Tuesday while a second front approaches from the west.  This next
front will move into the region Tuesday night into Wednesday. Drier
high pressure is expected for the end of the week leading to
hotter conditions.


As of 150 AM EDT: Both radar trends and the latest HRRR simulated
reflectivities indicate diminishing coverage across the forecast
area through the early morning hours. Lingering showers, with a few
embedded thunderstorms, will impact mainly piedmont locations east
of the I-77 corridor for the next few hours. The main exception to
the downward trend will be continued scattered showers over the
North Carolina mountains as additional mid-level vorticity lobes
ride northeast along the mountain chain throughout the morning

Otherwise, increasingly zonal flow aloft through the day will drive
a more potent shortwave across western NC during peak heating. In
addition to the upper forcing, a surface boundary will remain draped
through the region to provide a low level focus. Deep warm cloud
layers and continued high precipitable water values in profiles will
elevate the risk for flash flooding in any training that develops.
However, there should continue to be about 20 kt of mean cloud layer
flow to keep any flooding isolated in nature. The deep moisture in
profiles should once again serve to limit instability and severe
weather seems unlikely despite the high coverage of decently strong
convection this afternoon. Will feature a rather sharp gradient in
PoP, with higher likely values over western NC, tapering down to
solid chance in the southern tier. Warmer maxes nearer 90 in SC/GA,
along with lower 70s dewpoints, will push heat index values into the
95 to 100 range there.

A shortwave ridge will develop over the southeast tonight, with any
lingering precipitation dwindling quickly overnight. Meanwhile,
Hurricane Gert will remain well out of the picture off the east
coast. With no real airmass change, another muggy night of lower 70s
mins is expected east of the mountains, with plenty of low clouds
and fog throughout.


As of 230 PM EDT Monday: The upper level flow remains rather zonal
through this part of the forecast cycle, with embedded pockets of
vorticity riding along the mean flow. Meanwhile, the evolution of
the Bermuda High will try to nudge H5 heights slightly northward.

Complicating matters is an old surface frontal boundary which the
models continue show meandered, or waffling, north and south in
parts of our forecast area (FA).

It is nearly impossible to try and time each little wave rippling
through the flow, which would aid any diurnal convection. PWATS
will continue to remain elevated 1.5 to 2.0 inches, or more, with
SBCAPES respectable.

Therefore our plan is to let one wave of energy pass by Tuesday
evening, and bring POPS down through the night. We will then restart
the diurnal cycle Wednesday in the mountains, and let cold pool
interactions drive activity south and east during the afternoon and
evening. Note, we gave a slight nod to the NAM along our I77
corridor where convergence was noted at the surface, and
therefore we started POPS a little earlier.

Once again we will then draw back POPS Wednesday night, and then
reintroduce Thursday, mostly in the chance category.

Of course in both aforementioned scenarios, a subtle movement of the
surface boundary would locally enhance convective action

Note our temperature scheme has been adjusted for a diurnal cycle,
and guide continues to nudge temperatures up. In fact, if little
mixing occurs Wednesday and Thursday afternoon ( prior to storms )
some places in NE Georgia, upstate South Carolina and western
Piedmont of North Carolina will have heat indices near or just above
100 degrees.


As of 215 PM EDT Monday: Mid-level ridging will be in place across
the southeastern states Thursday and Friday. This will allow highs
to be in the lower 90s on Thursday and Friday across the Piedmont.
With dewpoints in the lower 70s, heat index values will climb to
around 100 both days outside of the mountains. Scattered showers
and thunderstorms are expected with the highest pops over the Blue
Ridge mountains.

A trof in the mid levels will move across the Ohio Valley on
Saturday and northeastern states on Sunday. While pops will not
rise much, the lower heights will lead to temperatures closer to
normal and a few degrees cooler than Thursday-Friday.

Another mid level trof will move into the Ohio Valley next Monday
which will keep temperatures across the area less hot than late this
week. There will continue to be scattered shower and storm activity.

Concerning conditions surrounding the eclipse on Monday, August
21: Expect partly cloudy conditions during this period. There
will likely be some scattered showers and storms around during
the afternoon. Temperatures look to be in the mid to upper 80s
outside the higher elevations with a few degrees cooldown as the
eclipse occurs.


At KCLT: The last round of showers should move quickly through the
KCLT vicinity by 08Z. Abundant boundary layer moisture should then
permit ceilings to settle down through the IFR range into LIFR
before daybreak. MVFR fog is likely as well, but stratus is favored.
Anticipate light to calm winds through mid-morning. The low ceilings
will be slow to lift and scatter, perhaps just a touch quicker than
yesterday, with VFR likely returning around Noon. A stronger
shortwave should drive convection through the piedmont once again
late this afternoon and this evening, but with a quicker end to the
showers tonight. Winds will toggle back toward WSW through the day.

Elsewhere: Showers will continue to dwindle across the foothills
through the early morning hours. Additional shower development is
likely overnight in the NC mountains nearer passing upper level
waves, but little more than VCSH will be featured at KAVL with
higher chances nearer the TN border. Anticipate cigs dropping
quickly through the IFR range, with LIFR quite likely after 08Z. Fog
may also develop from KAVL to KHKY, with KHKY the most likely
suspect to receive IFR fog as well. Light winds will come up from
the NW at KAVL and from the WSW at the foothills sites as mixing
develops today. Cigs will see a slow recovery to VFR by 16Z, with
shower and thunder chances returning from the west through the
afternoon - especially from KAVL to KHKY.

Outlook: Chances for restrictions will remain elevated through late
week as a moist pattern persists. Scattered afternoon and evening
showers and thunderstorms are favored each day, with morning low
stratus and patchy fog likely each night, especially in the mountain
valleys and in locations seeing appreciable rainfall.

Confidence Table...

            06-12Z        12-18Z        18-24Z        00-06Z
KCLT       Low   56%     Med   79%     High 100%     High 100%
KGSP       Low   57%     Med   71%     High 100%     High 100%
KAVL       Low   55%     High  81%     High 100%     Med   60%
KHKY       Med   61%     Med   78%     High 100%     High 100%
KGMU       Med   60%     Med   69%     High 100%     High 100%
KAND       Low   54%     High  80%     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:




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