Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 210556

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
156 AM 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 145 AM EDT...Another pocket of showers continues to move
thru the Sandhills and Midlands, the likely effect of a vort
lobe rounding the low pressure to our east. Low level moisture is
expected to continue expanding slowly westward thru the morning,
bringing low clouds to the I-77 corridor by daybreak, as well as a
gradual increase in PoPs. Main revisions with this update are to
refine sky cover and winds per latest guidance; no major changes
to thinking.

The stationary low will continue to spin over the eastern Carolinas
today. It appears the higher POPS will continue in our eastern
zones, similar to guidance values. The question is how far west
will the precipitation spread? The trends the last couple of days
from the models have been to edge POPS down farther west. Indeed
that continues to be the case today. As a result will run solid
chance POPS east, and then taper back to slight chance along and
west of the I-26 corridor. Otherwise we have followed temperature
guide during this cycle, which continues with current trends.


As of 210 PM EDT Tuesday: A weak closed 500 mb low center over the
NC/SC border in the eastern Carolinas may retrograde slightly
Wednesday night into Thursday. This system will keep a persistent
northeasterly fetch of 850 mb moisture across our region, but deeper
layer moisture should remain fairly limited. With diurnally mixing
dewpoints, instability will be fairly unimpressive and any scattered
convective chances will once again be greatest east of I-77 Thursday

The stalled system over the eastern Carolinas should fill and
steadily lift northeast Thu night through Friday as a northern
stream wave crosses the Great Lakes and upper OH valley. A more
prominent 500 mb ridge will start to build east toward the
Appalachians through late Friday. Drier air will advect into the
region at lower levels and diurnal PoPs will be isolated at best
Friday afternoon. Min temperatures will remain above climo
throughout, with near climo maxes on Thursday rebounding to above
normal on Friday.


As of 205 PM Tuesday, the dissipating weak low which was partly
tropical system Julia will move off the Carolina coast Friday night
with the upper flow trough. An amplifying upper ridge will be
building Saturday and Sunday with axis over the Mississippi River
Valley. At the same time there will be a deepening upper trough over
eastern New England and Nova Scotia which will be pushing a backdoor
cold front down the East Coast. Very little moisture will be
associated with this backdoor cold front which will cross our area
from the NE Sunday night into Monday.  After above normal
temperatures through the weekend, max temps will fall to at or a
little below normal Monday and Tuesday. This of course greatly
depends on the strength of the surface high on the east coast to
take it as far as GA and TN. If the new GFS is correct, then the
east coast trough will hang on into mid week keeping the cooler air
in place over our area. If the old ECMWF is more right, then an
upper ridge would build over our region into mid week and raise
temps. The GFS is expected to be the more correct model. This is bad
in a way since the old ECMWF had strong flow out of the Gulf
affacting much of the SE States which could help our drought.


At KCLT: Stagnant weak low pressure over the coastal plain
will continue to advect moisture westward into the Piedmont this
morning. Some guidance, including LAMP, suggest low MVFR cigs will
set in just after daybreak. An inspection of prog soundings is not
quite as pessimistic, but still supports restrictions around 12z. No
matter how low they actually go, MVFR looks likely to persist
thru midday, with bases mixing up very slowly and possibly being
replaced by cumulus. A chance of an MVFR shower does still exist
during the afternoon, hence PROB30. A stronger push of low level
moisture occurs tonight as the low elongates and retrogrades a bit,
and that is expected to bring even lower cigs. Winds will be breezy
and from the N to NE, with gusts to 20 kt possible during the day.

Elsewhere: Restrictive cloud decks are not expected to advance
even as far west as KHKY this morning, but VFR-level cloud cover
will generally increase with time. Fog could cause issues in the
mountain valleys and the more rural areas of the Piedmont, but
KAVL has such a low crossover temp and modest dewpoint depressions
that it is unlikely to see reduced vsby. Chances of precip are not
enough to mention today, though isolated showers might form over
the Piedmont. A tighter pressure gradient will make winds a bit
more breezy than the past few days, with gusts to 15-20 kt likely
across much of the area. Expect more low clouds to advect in from
the east after sunset.

Outlook: The meandering low pressure along the coastal Carolinas
will hold sway into Friday. This will allow isolated to scattered
diurnal convection to continue, and periodic chances for restrictive
cigs. Meanwhile, a new cold front will be approaching Sunday with
sparse precipitation expected at this point.

Confidence Table...

            06-12Z        12-18Z        18-24Z        00-06Z
KCLT       High 100%     Med   67%     High  91%     High  91%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     High 100%     High 100%
KHKY       High 100%     High 100%     High 100%     High  98%
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:




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