Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 202045

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
445 PM EDT Thu Jul 20 2017

An upper level ridge will build over our region from the west
through the end of the work week bringing hot temperatures and
isolated showers and thunderstorms each day. A broad upper level
trough develops over the weekend and into early next week and
is expected to bring cooler temperatures along with greater chances
for rain.


As of 430 PM EDT Thursday: Only minor tweaks to grids for the late
afternoon update, mainly to adjust pop trends through the evening,
but nothing that would result in substantial wording changes.
Temperature trends look good, though with afternoon mixing dewpoints
have dropped a little more than forecast so updated trends there as

Otherwise, convection should dissipate early this evening with loss
of heating. Expect another round of mountain valley fog, some dense,
overnight. Could be some patchy fog outside of the mountains, but
this should be limited to normally fog prone areas. Lows will be on
the warm side, up to around 5 degrees above normal.

Moderate instability expected again Friday, but with little change
in lapse rates as mid level temps remain nearly steady. A weak short
wave does drop into the area in the weak flow aloft which could
support slightly better coverage, especially across the mountains.
Therefore, have gone with scattered PoP over the mountains and
isolated elsewhere. DCAPE values remain high, so isolated damaging
downbursts remain possible. Highs will be a couple of degrees warmer
5 to 10 degrees above normal. With the continued high dew points,
heat index values will rise above 100 and approach 105 in some
locations. If dew points mix out in a normal pattern, heat index
values should remain below advisory criterion. That said, cannot
rule out a small area of 105 values if mixing is less.


As of 145 PM Thu: Early in the short-term period we will
remain under the influence of a subtropical ridge, although
it will gradually weaken as heights fall over the Midwest and
Northeast. With very warm sfc to low-level temps, 850-700mb lapse
rates remain fairly large overnight Friday night. NAM/GFS depict
a weak vort max crossing the Appalachians at that time, seemingly
the remnant of earlier convection over the middle Ohio Valley. That
feature as well as weak northwesterly flow result in some overnight
convection in the model output, mainly over the mountains. PoPs
will ramp down only gradually as a result.

Uninhibited sfc based instability develops quickly again Saturday
morning, suggesting isolated to scattered storms across the
CWFA. Dry and weakly sheared vertical profiles imply the typical
midsummer pulse storm threat will exist. Guidance is more supportive
of nocturnal stabilization and a corresponding lull in activity
Saturday night. Sunday afternoon, heights will have fallen a bit
more, and model CAPEs are a little higher under similarly hot
and humid sfc conditions. PWAT values also are expected to have
increased slightly, but sfc-to-midlevel theta-e lapses still look
large enough to expect pulse storm wind threats under continued
weak shear.

Max temps will be as much as 8 degrees above normal on Saturday,
trending down over most of the area Sunday under reduced
thicknesses. Afternoon heat indices are expected top out in the
low 100s, per the guidance T/Td sources which performed best in
last week`s brief hot spell. At this time no Heat Advisory is
warranted, as we don`t look to meet the criteria of 105 for 2
consecutive hours.


As of 155 PM EDT Thursday: The upper trough dips down from the Great
Lakes at the end of the weekend with axis from the Great Lakes to
roughly Memphis at 00Z Monday. With falling heights, increasing low
level moisture, low level convergence and upper support means better
chances of rain for our region to start the week.  The trough axis
will be closer to the Appalachians Monday evening with the surface
cold front getting closer as well. The surface front is expected to
be roughly along the Ohio River early Monday and reach Kentucky and
the Virginias early Tuesday. The front will stall out somewhere west
to east over our region for Wednesday and Thursday. Instability is
forecast to have CAPE values 1500 to 2000 Sunday evening, then rise
a bit more late Monday due to the approaching cold front. CAPE
values Wed and Thurs will depend on where the front settles. The GFS
has much lower values Wed and Thurs while the ECMWF has around 2000
CAPE especially over the western zones. Of course, temperatures will
be cooling off from near normal to start the week to just below
normal for max temperatures.  Min temps are forecast to remain a
little above normal due to the extensive cloud cover.


At KCLT and elsewhere: Only change to the KCLT TAF for the 21z AMD
is to reduce sky cover. Other trends look good. Otherwise, best
chance of restrictions overnight will be in the mountain valleys.
That said, fog and low stratus have stayed away from KAVL even
though sat pix showed it nearby. Will go with persistence at this
point and mention MVFR vsby and few LIFR stratus near daybreak. VFR
at the rest of the sites. A little better chance of convection
Friday, but still early to include. For now, show developing
cumulus. Winds are tricky with the weak pressure gradient in place.
Have gone with lee trough formation for winds outside of the
mountains and valley wind at KAVL.

Outlook: Chances for diurnal convection begin a steady increase on
Saturday, with this trend continuing into early next week ahead of
an approaching cold front on Monday. Overnight restrictions will
continue in the mountain valleys with chance increasing elsewhere.

Confidence Table...

            20-02Z        02-08Z        08-14Z        14-18Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High  93%     Low   37%     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  93%     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:




SHORT TERM...Wimberley
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