Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 211824

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
224 PM EDT Wed Sep 21 2016

Weak low pressure will drift around Georgia and the Carolinas over
the remainder of the work week. An upper ridge will build over the
Southeast over the weekend as temperatures rise above normal again.


As of 215 PM EDT Wednesday: The remnants of Julia continue to spin
near the NC/SC coast, bringing intermittent rounds of moisture into
the Piedmont with light showers continuing mainly east of I-77. This
trend should continue through the afternoon. Meanwhile a a broad and
ill-defined area of high pressure will continue to dominate the Ohio
Valley northeast toward portions of New England, with some drying in
the mid-levels and weak confluent flow aloft resulting in poor lapse
rates. A weak ridge of high pressure extending south over the higher
elevations will help to turn the winds up against the escarpment
just slightly more easterly with a bit of an upslope component, so
expect some showers to develop across the Blue Ridge this afternoon.
Instability is weak and limited mainly to the Piedmont areas where
latest SPC mesoanalysis showed some pockets creeping upwards of 500
J/kg, must surface thermal inversion underneath the moisture/cloud
cover is keeping most instability at bay, and elevated at that.

As we move into tonight, the upper low will gradually weaken and
transition into more of an open wave while the surface low begins to
slowly work its way northeast. A slug of low level moisture will
rotate around the low overnight, with clouds overspreading the area
and keeping lows a good 5-8 degrees above seasonal normals. The
moisture will break up somewhat Thursday afternoon giving way to
some sunshine and temperatures fairly close to normal. Have kept
previous trend of low chance pops across the eastern Piedmont with
slight chances spreading into the foothills during the afternoon,
but waning toward the end of the period.


As of 200 PM EDT Wednesday: The lingering 500 mb trough over the
coastal plain of the Carolinas will continue to slowly fill and lift
northeastward Thursday night into Friday. Any lingering low to mid
level moisture should just brush the piedmont early Friday before
departing. Otherwise, northerly 850 mb flow will provide drying for
the area - with just isolated afternoon diurnal convection possible
along the highest ridge tops where 850 to 500 mb lapse rates are a
bit steeper than elsewhere.

A prominent ridge axis over the central CONUS Friday night will
drift eastward toward the Appalachians through the day on Saturday.
Only the higher ridges will once again see the potential for
isolated diurnal convection given steeper mid level lapse rates over
the mountains. Steering flow will be very light under the ridge.
Anticipate temperatures to continue well above climo.


As of 210 PM Wednesday, with this period starting 00Z Sunday. Weak
low pressure is shown Saturday evening over south GA with a 500mb
ridge axis from Mobile to Louisville. The GFS gradually dissipates
this low as it moves it north toward the western Carolinas through
Sunday night. The ECMWF builds the ridge over our area into MOnday
while the GFS shifts the ridge east. Both the GFS and ECMWF have SE
upslope low level flow against our mountains early in the week with
the GFS strongest going into mid week. The backdoor cold front is
now doubtful to cross our area and may not even reach our NE zones
in the Interstate 77 NC piedmont. The upper support for the front
has moved off to the NE as the large upper trough will have great
influence on New England bot almost none on our area. Overall
confidence in the Day 6 and 7 day period of this forecast is rather
low as each day there have been significant changes in the late
period of the medium range models. If the GFS pans out, we can
expect good changes of rainfall in mid week. If the ECMWF pans out,
any significant rain will not arrive until late Thursday into
Friday. According to the GFS, late Monday and late Tuesday the
better instability will be to our south and some weak to near
moderate shear will be in place but perhaps not overlapping. There
is some potential for areas of heavy rain first in the NC and GA
Mountains early Thursday then maybe other areas later.

Temps several degrees above normal Sunday under the ridge, then
perhaps falling to around normal if that front settles in plus the
effects of the SE low level flow in the first part of the week with
the increased cloud cover and upslope rain.


At KCLT: Persistent low near the NC/SC coast continues to bring
moisture into the Piedmont, with MVFR cigs continuing at KCLT.
Expect some gradual lifting to low VFR through the afternoon with
VCSH and potential -SHRA. Removed the TEMPO TSRA as instability is
limited. As another round of moisture moves in, expect cigs to drop
again overnight to solid MVFR and flirting with IFR, though have not
introduced that just yet. Marginal improvement through the end of
the period. NNE to NE winds and gusty this afternoon but generally
remaining at or above 10kt through the period.

Elsewhere: Trends in general fairly similar to KCLT but more
optimistic. Precipitation should remain to the east with mainly VFR
cigs this afternoon and evening, dropping to MVFR overnight and
improving after sunrise on Thursday. Fog looks likely to be confined
to the Little Tennessee Valley and and other typical mountain
valleys. As with KCLT, winds should remain generally NE but speeds
dropping to between 5-10kt overnight.

Outlook: Meandering low pressure along the Carolina coast will
persist thru Friday. This will allow isolated to scattered diurnal
convection to continue especially east of I-77, and periodic chances
for restrictive cigs. Meanwhile, a new cold front will be
approaching from the northeast Sunday, but with sparse precipitation
expected at this point.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High  96%     High  88%     High  94%     High 100%
KGSP       High 100%     High 100%     Med   77%     High  88%
KAVL       High 100%     High 100%     High  95%     High  94%
KHKY       High 100%     High  93%     High  80%     High  95%
KGMU       High 100%     High 100%     Med   77%     High  90%
KAND       High 100%     High 100%     High  88%     High  85%

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:




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