Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 181750

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
150 PM EDT THU AUG 18 2016

High pressure over the Western Atlantic will weaken and move
eastward while allowing a weak front to become stationary across
our area to end the work week. This front will slowly dissipate
as it acts as a focus for afternoon showers and thunderstorms into
the weekend. A stronger cold front will move in from the northwest
early next week, bringing cooler and drier air in by the middle
of the week.


As of 150 PM...convection developing across the CWFA at this time.
Have updated PoP for likely coverage over the mountains early this
afternoon and likely outside of the mountains later. Still expect
some severe downburst winds during the afternoon and early evening.
Morning observed PW values are around 1.7 inches indicating the
threat for heavy rain or even flash flooding continues.

High temps will still be above normal with the decreasing convective
debris. The showers and storms should slowly diminish with loss of
heating during the late evening hours. Temps will remain warm


As of 230 AM Thu: Cold front will lay out from west to east near
the NC/SC border Friday, remaining stalled in this area until it
starts to lift north again Saturday in advance of the digging
central CONUS trough and developing Midwest sfc low. The front
is pretty weak and one might argue it will wash out, but there at
least appears to be some broad convergence along it. Furthermore a
weak 500mb wave will move overhead late in the day. I will maintain
PoPs in high chance range. Moist profiles imply the main threats
from thunderstorms will be hydrologic. PWAT values are progged
to best Thursday`s values, flirting with 2 inches. Unidirectional
flow, as well as the front acting as a gentle focusing mechanism,
could lead to training cells and localized flooding issues. It is
difficult to pinpoint those locations at this time as it will partly
depend on the antecedent conditions created by activity today.

On Saturday, the overall pattern will have changed little. Another
upper disturbance is expected to move atop the frontal zone. With
the base of the approaching mid-upper trough nearing from the
west, low level flow is a bit more backed, which aids in moisture
advection. PWATs rise slightly higher on model progs. PoPs edge
somewhat higher as well. Saturday night, as the trough and its
cold front move still closer, I will keep a chance PoP throughout
the night over the mtns.

Max temps both days will be near normal, with mins a category or
so above normal due to overnight cloud cover and excessive moisture.


As of 100 AM Thu: On Sunday the forecast area will be in the warm
sector of the low pressure system over the Great Lakes region. GFS
profiles are modestly unstable and very moist. PWAT values are
progged to be near 2 inches. With the base of the parent trough over
the Tenn Valley, 500mb flow is 30-35 kt during the day. While the sfc
front is not expected until Sunday night, a prefrontal MCS producing
very heavy rain may impact the area during the day.

The consensus of global models has trended faster with the fropa and
accordingly brings in the continental high for Monday; 48 hours ago
that was not depicted until Tue night. Cooler and less humid
conditions will persist thru the remainder of the fcst period, with
subsidence inversion also keeping convection largely if not entirely
suppressed. Expect max temps a degree or two below climo Monday,
trending down thru Wed.


At KCLT...convection developing south of the airfield a little
earlier than expected. Have started the TEMPO at 20Z now. Winds
still a little questionable. Guidance now developing a lee trof
which turns winds south of west during the afternoon. With light
winds remaining, outflow boundaries will play a big role in the wind
directions into the evening. Therefore, have kept winds north of
west for now with gusty variable winds in the TEMPO. Winds remain
light overnight into Friday, but should turn more SW. Does not look
like a great night for fog despite the expected precip. Should be
quite a bit of convective debris through the night and forecast
soundings are not very favorable.

Elsewhere...The day should be similar to KCLT but with earlier
timing on the thunderstorm potential. In addition, some mountain
valley stratus expected but should be brief like this morning. Later
forecast will likely need a short IFR TEMPO at KAVL.

Outlook: Thunderstorms will gradually increase in coverage Friday
afternoon and into the weekend as another cold front approaches from
the northwest. The front should arrive early next week to continue
the thunderstorm activity through Monday. Morning fog/stratus will
be possible each day in the NC mountain valleys, and possibly in
locations that receive soaking rains the previous aftn/evening.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     Med   64%     Med   70%
KHKY       High 100%     High 100%     High  97%     High  97%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     High  97%     High  93%

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:




SHORT TERM...Wimberley
LONG TERM...Wimberley
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