Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 230735

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
335 AM EDT SAT JUL 23 2016

Broad deep layered high pressure will remain across the region
through early next week, with a general weakness developing over the
east coast on Monday. Moisture levels will gradually increase across
the western Carolinas and northeast Georgia by the middle of next
week as a weak cold front settles south into the area.


Little change in the overall pattern is expected during the near
term, as the forecast area will remain parked between the
subtropical high centered over the Four Corners, and a weakness in
the S.T. ridge off the Southeast Coast. While this pattern is not
obviously supportive of substantial precip chances, it`s not
entirely hostile to development of deep convection. In fact, plenty
of heat and some humidity will be realized once again this
afternoon, with NAM soundings somewhat impressive on the buoyancy
front. Max temps will be a couple of degrees warmer than on Friday
in most areas, with much of the SC/GA and southwest NC Piedmont
expected to see heat indices of 100-104.

Meanwhile, much as they were yesterday, the GFS and the RAP are
quite aggressive in mixing drier air to the surface within a lee
trough this afternoon, such that there is very little CAPE depicted
in these models` forecast soundings. This did not work out well at
all yesterday. In fact, some of the best convective coverage
yesterday occurred across the northern foothills and northwest
Piedmont, where the GFS/RAP were depicting the weakest instability.
In that same vein, none of the mesoscale/convection-allowing models
were excited about convection in those areas yesterday, and they are
painting a pretty similar picture for today.

If anything, the mid-level flow is expected to be more NE today, so
it would stand to reason that any convection developing within lee
trough and across the higher terrain would tend to drop south as
opposed to SE, sparing much of the Piedmont of the solid scattered
coverage that was observed yesterday. Have therefore opted to
forecast a slight chance across the Piedmont this afternoon, with
30-40 pops across the foothills, and 40-60 pops across the mtns.
Mid-level dry air and a well-mixed/deep boundary layer will result
in impressive downdraft CAPE once again this afternoon, thus
microbursts will be a concern with the stronger cells.

Convection should wind down this evening, with another warm
overnight expected.


As 250 AM Saturday: On Sunday, the axis of the H5 ridge will build
over the forecast area, increasing to 594 dm by the afternoon. At
the sfc, little to no pressure gradient will exist across the
western Carolinas and NE GA during the day. In fact, model guidance
indicates that wind speeds are forecast to remain below 5 kts from
the sfc up to H7. High thicknesses and good insolation should
support high temperatures from around 90 degrees within the mtn
valleys to upper 90s to around 100 across the I-85 corridor.
Forecast soundings show the mixed layer deepening slightly above H8
during the heat of the afternoon. The mixing may lower sfc dewpoints
from around 70 in the morning to the upper 60s during the afternoon.
Heat Index values may peak between 101 to 106. In addition, forecast
soundings indicate that llvl CIN will linger east of the mtns
through most, if not all, of Sunday afternoon. Thermal profiles
appear supportive of mtn ridge convection as early as mid day.
Storms may propagate by developing along outflow boundaries. This
process should support sct coverage across the mtns and isolated
coverage east. Convection should ash out shortly after sunset, with
debris gradually clearing into the late night hours. Low
temperatures Sunday night are forecast to range for the 60s across
the mtns to the low to mid 70s east.

On Monday, a sfc front is forecast to slowly slide across the Ohio
River Valley, with a lee trof across the foothills and Piedmont.
light south winds should result in a one to two degree rise in
afternoon dewpoint temperatures. As a result, llvl CIN appears
weaker than Sunday across the foothills and Piedmont. NAM forecast
soundings for KGSP indicates CAPE above 2500 J/kg and PW rising to
1.8 inches. Winds are forecast to remain less than 10 kts from the
sfc to H4. I will time chc PoPs across the CWA by mid day, rising
very quickly across the mtns around lunch, then spreading east across
all of the Piedmont. A few areas may see high Heat Index values
between 100 to 106 degrees during the mid to late afternoon.


As of 3 AM Saturday: Tuesday through Thursday, the center of the H5
ridge will shift over the Deep South. Models indicate that a series
of weak mid level S/W will ripple across the forecast area. Each
day, moderate instability and weak CIN will exist across the region
during the afternoon and early evening. I will continue to indicate
diurnal chc to ridge top likely PoPs each day, schc east. I would
expect that PW values will remain above normal as a front becomes
stationary to our north. Temperatures should range within a couple
of degrees of normal, cooler than Sunday and Monday.


At KCLT and the Upstate SC terminals, expecting some MVFR visby this
morning at KGSP, which did experience some rainfall Friday
afternoon. Otherwise, VFR conditions are expected through the
period. Generally expect light S/SW winds to persist through the
period. Isolated to scattered convection is expected to develop once
again Sat afternoon, esp near the Upstate SC terminals, and PROB30s
are carried there from late afternoon into the evening. Cannot at
all rule out convection near KCLT either, but coverage should be a
bit more sparse in the Piedmont.

At KAVL/KHKY, another tricky forecast at KAVL, with the prospect of
a light surface wind once again preventing fog/low stratus formation
later this morning (as was the case yesterday). The preponderance of
the evidence suggests that a light wind will keep conditions from
crashing too harshly, but there`s still enough of a potential to
continue a tempo for 2SM/SCT003 from 10-12Z. The forecast for KHKY
is also a bit tricky, as heavy rain did fall there Fri afternoon,
and the temp/dewpoint spread was only two degrees at TAF issuance
time. Opted to include a tempo for 3SM/FEW004, but wouldn`t be
surprised if worse (or better) conditions are experienced there.
Otherwise, PROB30s for convection will be carried during the
afternoon/evening, as cells should fire once again along lee trough
and across the high terrain.

Outlook: Scattered/isolated afternoon and evening showers and
thunderstorms will continue across the mountains/piedmont through
early next week. Morning fog and low stratus will be possible each
day in the mountain valleys and also in locations that receive heavy
rainfall the preceding afternoon/evening.

Confidence Table...

            07-13Z        13-19Z        19-01Z        01-06Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High  96%     High 100%     High 100%     High 100%
KAVL       Med   75%     High 100%     High 100%     High  96%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High  96%     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 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


  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




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