Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 252040

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
440 PM EDT MON JUL 25 2016

Broad surface high pressure will linger in the southeastern CONUS
before sliding offshore as a surface low develops and moves
northeast across the Midwest and Great Lakes regions. A surface
front/boundary will remain off to our north and linger within a
region between the Ohio Valley, lower Great Lakes, and Mid-Atlantic


As of 430 PM EDT Monday:  Isolated/Scattered convection continues
across the region with the deepest of which residing over the
NC high terrain where subsidence is weakest, and mid level lapse
rates are steepest.  As for the low terrain, latest radar trends
indicate isolates showers/thunderstorms across the NC/SC piedmont
as well as NE GA, however given proximity to the H5 ridge, mid
level lapse rates are rather poor.  Consequently, said convection
is struggling to reach levels even supportive of lightning.  Thus,
think these trends will continue for a few more hours before heating
subsides allowing for llv lapse rates to begin to flatten out.
Given the abundant DCAPE, any deeper convection could still present
a damaging wind threat, however that is looking less likely.
As for this update, tweaked temps/dews and pops to account for
recent ob/radar trends and left the remainder of the fcst as it was.

Previous Discussion:  Convective temps have been reached per
visible and radar trends. However, showers have struggled to
become deep enough for thunder so far. It seems the upper ridge
is in a position to keep things somewhat suppressed. The latest
SPC mesoanalysis shows SBCAPE around 2000 J/kg, with some higher
amounts along a lee trough in western NC. This instability may be
enough for at least a few TSTMS to develop. And with DCAPE between
1000-1400 J/kg, any storms that can get an icy core may produce
wet microbursts. Another concern this afternoon is heat indices,
which are running a little higher than this time yesterday. Dew
points are remaining in the lower 70s across much of the Piedmont,
as temps reach the mid 90s.  So far heat indices are generally
between 96-104 across the Piedmont.

Tonight...convection should wane with loss of heating this evening,
leaving some debris cloudiness around. Patchy fog will likely
develop again in the mountain valleys. Lows will remain a couple
categories above normal (upper 60s mountains to mid 70s piedmont).

Tuesday...little change in the overall set up and air mass from
Monday. Model guidance has slightly higher PoPs, perhaps due to
a frontal boundary approaching from the north. In fact, most of
the guidance keeps dew points more elevated across the area, with
widespread lower to mid 70s even during peak heating. This should
result in a little more CAPE. Temps are forecast to be a degree
or two cooler, but with the increased humidity, heat indices will
again range in the upper 90s to 104.


As of 230 PM EDT Monday: Subtropical ridge will remain in place
across the Southeast through the short term, with Bermuda high off
the Atlantic Coast keeping the warm and moist airmass in place
across the region (typical summertime conditions). As has been the
case the past few days, lee troughing will remain in place along the
eastern slopes of the Appalachians. A stronger upper high over the
Desert Southwest will retrograde slightly westward through the
period as a weakness moves onshore over the Lower Mississippi Valley
and connects with a low-amplitude wave traversing the northern tier
of the country, which will drag a weak front into the Ohio Valley at
the start of the period. The ridge will remain strong enough to keep
the front stalled to our northwest, but a reinforcing shortwave
coming out of the Rockies into the Plains late in the period will
make some headway in actually moving the front towards our area, to
impact us in the extended.

As for sensible weather impacts, the heat and humidity will remain,
though heat indices will continue to hover a few degrees below
advisory criteria levels. Typical summertime afternoon showers and
thunderstorms will continue, though the pattern will be slightly
enhanced on Thursday over the mountains as the front slowly sags
closer to the area and enough of a shortwave pushes through aloft to
enhance the lee troughing (the GFS actually closes off a low over
the NC Piedmont). Because of the approach of the front and
shortwave, some deep-layer shear actually pushes into northwest
zones Thursday afternoon (right around 30kt 0-6km along with sbCAPE
values on the GFS between 2000-3000 J/kg, pretty impressive given
the GFS is usually a little less robust on CAPE values), so with the
enhanced convective potential we could also see a slight uptick in
the potential strength of storms, with maybe more than just
"isolated" severe thunderstorms possible. For now, we remain in
General Thunder for the Day3 outlook but would not be surprised to
see that go to Marginal in later updates.


As of 210 PM Monday... The general consensus is that typical weather
for July will dominate the sensible weather pattern throughout this
coming weekend. Have gone with mainly to perhaps slightly above
climo POPs and temps for the period, with moderate heat and
instability remaining in place across the CWA. A surface-based
boundary/front will remain off to our north and draped across an
area between the Ohio Valley and lower Great Lakes regions. Closer
to home, a Bermuda High centered off the Atlantic coast will
continue to support the aforementioned heat and instability. A 500
mb trough axis will also be in place across the Midwest. This trough
then begins to breakdown and eject northeastward by the end of the
period. Diurnal POPs should generally favor the mountain zones as
westerly to southwesterly flow dominates with the greatest moisture
axis remaining just to the west of the CWA along and just ahead of
the previously mentioned trough. This may lead to some weak
downslope flow that may briefly suppress convective activity in the
foothills and piedmont areas during the early afternoon hours. Upper
ridging then builds across the Great Lakes region by the beginning
of next week.


At KCLT and elsewhere...Convection beginning to become a little
deeper around the mountains and along a piedmont trough, but still
limited to SHRA at time of 18z TAF issuance. Expect a few of these
cells to become TSRA, but mainly in the mountains. Only KAVL will
mention TS, but will keep VCSH at KAND as it is surrounded by small
SHRA. Confidence is too low elsewhere, but timely amendments will be
made, if any TS manage to develop near the other sites. Otherwise,
expect VFR conditions with cumulus bases around 6000 ft and a light
SW wind, becoming somewhat VRB tonight. The one exception being
possible fog and/or stratus at KAVL late tonight due to valley fog
potential. Tuesday looks similar to today, with isolated convection
in the Piedmont and scattered to numerous in the mountains.

Outlook: Isolated/scattered afternoon/evening showers and
thunderstorms will continue across the mountains/piedmont through
much of the week, with coverage increasing a bit each day. Morning
fog and low stratus will continue in the mountain valleys around
daybreak each day, and also be possible in locations that receive
heavy rainfall the preceding afternoon/evening.

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 100%     Med   75%     High 100%
KHKY       High 100%     High 100%     High  91%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     High  95%     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:



  STATION      HIGH        LOW         HIGH        LOW
  -------  ----------  ----------  ----------  ----------
   KAVL      92 2010     68 2000     70 2012     51 1895
                2005                    2011
                1987                    2010
   KCLT     101 2010     69 2000     79 2010     62 1947
   KGSP     100 1995     72 2000     77 2010     57 1911


  STATION      HIGH        LOW         HIGH        LOW
  -------  ----------  ----------  ----------  ----------
   KAVL      94 1949     72 1911     71 2012     49 1911
                1940                    2010
   KCLT     100 2005     74 1920     76 1992     60 1904
                1940                    1940
                1914                    1936
   KGSP      99 2010     76 1920     76 2005     53 1911
                1995                    1940




LONG TERM...Staarmann
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