Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 202337

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
737 PM EDT WED JUL 20 2016

A typical Bermuda high pressure pattern will set up across the
region through the end of the week, resulting in typical midsummer
conditions. Hot high pressure will strengthen this weekend, then a
weak cold front will approach from the northwest early next week.


As of 735 PM Wednesday...convection ongoing across the western CWFA
will linger into the evening. Still expect activity to diminish with
loss of heating.

However...there is growing uncertainty about how the overnight hours
will play out. The operational models keep developing precip over
the Savannah R basin well into the early morning hours on Thursday,
evidently owing in part to a remnant MCV left over from convection
over the Midwest, perhaps aided by the development of a light SE
flow this evening. The idea of a remnant coming down from the NNW is
supported by the overall pattern with the western Carolinas on the
periphery of the large anticyclone over the Plains. Ordinarily the
operational models might be discounted, but the CAMS all keep some
amount of convection in this area into the overnight hours, so they
all must be catching on to something. The previous forecast was dry,
so a trend toward maintaining at least widely scattered deep
convection over northeast GA, the wrn Upstate, and near the Blue
Ridge in NC past midnight. Will still allow convection to dissipate
by daybreak, but that idea is not entirely supported by the
guidance. The guidance goes on to initiate convection early on
Thursday, what with the ongoing low level forcing and increase in
low level moisture. Forecast soundings show enough mid-level dry air
and a deep enough boundary layer to suggest a potential for wet
microbursts in the afternoon, thus the recent upgrade on the SPC
Day2 outlook to Marginal appears warranted. The fcst will also trend
in that direction. More cloud cover and earlier convection should
mean temps would not get quite as warm, and the consensus of the
guidance is to lower high temps a degree or so.


As of 200 PM Wed: Monstrous upper ridge will extend virtually from
coast to coast Thursday evening, but retrogrades and is suppressed
a bit over the subsequent 48 hours as a trough crosses the NW
CONUS. We`ll see some influence from the ridge, but also from
Western Atlantic high pressure that generally will maintain moist
southerly flow into the area early in the period. Anticipated precip
chances and t-storm threats will be typical of late July. Model QPF
response hints that this flow may be able to keep some convection
going well into Thursday night, but prog profiles suggest low
level lapse rates will be rather poor. A mostly slight chance
PoP is maintained along/south of the Blue Ridge into early Fri
morning. PoPs redevelop diurnally Friday. Max temps will be 4-6
degrees above normal, with heat indices maxing out around 100.

Friday night, a weak front will settle into the area from the
north, associated with low pressure moving into the Canadian
Maritimes. It looks like the front will be too weak to have an
impact overnight, but it will slightly enhance convective chances
Saturday before washing out. Winds do veer to downslope aloft in its
wake; this combined with still higher partial thicknesses implies
even hotter conditions. Maxes in the upper 90s are fcst acrs the
Piedmont. Dewpoints are expected to mix out slightly, into the
upper 60s, and resulting heat indices top out generally just shy
of Heat Advisory criteria. For now we will maintain a HWO mention,
but there is a good chance that an advisory will be posted Friday if
not tomorrow to highlight a heat wave lasting into early next week.


As of 215 PM Wednesday...A broad upper ridge centered near Arkansas
at 00Z Sunday will strengthen eastward into Sunday with height rises
and a 594 decameters center over our region.  This ridge center will
slowly shift off the Carolina coast or just move a bit south early in
the week as some trofiness in the 500mb pattern develops.  The ECMWF
takes the 594 ridge center off the Carolina coast while the GFS
takes it from TN and NC south to SRN GA through mid week.  Hot and
very humid conditions will be in place Sunday with dewpoints in the
lower 70s in the foothills and piedmont and heat indices up to 104.
As a weak cold front gets closer from the northwest early in the
week coverage of SHRA and TSRA will increase mainly for the mtns and
max temps will be a bit lower. Heat indices of 102 in the CLT area
Monday PM and only in the 90s foothills and piedmont Tuesday.
Good CAPE values of at least 2000 for SW mtns and along and south of
I-85 each PM. Light low level SW to W wind through the period.

Highs in the 80s to lower 90s mtns to upper 90s piedmont on Sunday.
About 2 degrees cooler on Monday and still about 2 more degrees
cooler on Tuesday. Mins 60s to around 70 mtns to mid 70s piedmont.


At KCLT...VFR expected thru the period. High based Cu will dissipate
through the evening. NE wind becomes light SE this evening. Models
retain quite a bit of low/mid level moisture overnight and Thursday
morning, so some cloud cover is expected mainly at mid levels. Cu
develops again during the afternoon. However, storm chance is too
low to mention in the TAF for now.

Elsewhere...have included VCTS early this evening to cover the
ongoing convection and will amend if cells approach airfields. Rest
of the forecast remains tricky with guidance keeping a low chance of
convection lingering through much of the night. The biggest threat
would be to KAND. This could at a minimum develop and keep a low
cloud deck over the area through Thursday morning. Will start a
trend in that direction with a VCSH at KAND overnight and advise the
next shift to be on the lookout for greater potential for low
cloud/vis restrictions. Expect another round of convection but with
an earlier initiation. Have PROB30 for all sites. Light and variable
winds become S for Thursday.

Outlook: Expect to revert to fairly typical midsummer weather with
afternoon showers and thunderstorms during the latter part of the
week. Morning fog and low stratus will be possible each day in the
mountain valleys and also in locations that received heavy rainfall
in the preceding afternoon/evening.

Confidence Table...

            23-05Z        05-11Z        11-17Z        17-18Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     Med   65%     High 100%
KHKY       High 100%     High 100%     Med   75%     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:



  STATION      HIGH        LOW         HIGH        LOW
  -------  ----------  ----------  ----------  ----------
   KAVL      96 1926     67 1970     75 1983     52 1966
   KCLT     103 1926     71 1929     80 1881     58 2001
   KGSP     101 1983     70 1970     79 1940     58 1890


  STATION      HIGH        LOW         HIGH        LOW
  -------  ----------  ----------  ----------  ----------
   KAVL      95 1983     69 1974     71 2015     50 1966
                1952        1938        1999
                1934                    1983
   KCLT     101 1987     73 1880     78 1991     56 2007
   KGSP     102 1952     74 1974     79 1934     59 1966


  STATION      HIGH        LOW         HIGH        LOW
  -------  ----------  ----------  ----------  ----------
   KAVL      96 1983     68 1985     71 1934     52 1966
   KCLT     100 1983     73 1938     78 2010     59 1947
                1952                    1883
   KGSP     101 1952     70 1985     79 2010     59 1904




SHORT TERM...Wimberley
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