Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 201037

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
637 AM EDT Wed Sep 20 2017

A weak trough crossing the region today will settle along the
southeast coastline Thursday through Friday. A gradual increase in
easterly flow moisture is likely through the latter part of the
week. Drier and stronger high pressure will settle over the region
during the weekend, and likely persist through early next week, as
both Jose and Maria remain off the east coast.


As of 625 AM EDT Wednesday:  Updated discussion for 12z taf
issuance.  The current fcst remains on track this morning therefore
no sig changes were needed/made with this update.  Did however
tweak t/td trends to align with recent obs.  Otherwise, patchy fog
continues to highlight the morning in the mtn valleys where visbs
are generally at/less than 1sm, and across the NC piedmont where
visbs are holding at no lower than 2sm.  Fortunately am expecting
these conditions to rapidly improve after sunrise, thus no fog
products are planned.

As of 245 AM EDT Wednesday:  Areas of mid level stratocu continue
to advect atop the region this morning, while at the surface a
few areas of patchy ground fog have developed.  Currently the
most dense fog is located in the usual mtn valley locales where
visbs are at/less than 1sm, while a few sites across the NC
foothills/piedmont are reporting visbs around 5sm.  Wouldn`t be
suprised if this continued through morning, possible filling in
a bit more with lower visbs toward daybreak as the aforementioned
stratocu advects east.

Otherwise, the synoptic pattern overall is rather similar to days
past as broad ridging dominates across the east, and troffing
prevails out west.  There is however one slight difference,
and that is a weak depression in the height field that contains
some shortwave energy working across the TN valley and into
the western Carolinas and northeast Georgia this afternoon.
With surface high pressure in place there will be a degree of
llv subsidence that will need to be overcome for convection,
which certainly seems reasonable over the mtns by early afternoon.
Said inhibition will be strongest across the low terrain, yet am
expecting convection that fires over the mtns to propagate eastward
into the foothills/piedmont regions by late afternoon aided by the
above mentioned shortwave energy.  Profiles today do exhibit the
inhibition stated above in the llvs, however deep dry air supports
modest lapse rates and abundant instability above the cap, upwards
of 1500 j/kg albeit with very weak shear.  All said the fcst will
feature chance pops atop the high terrain, with slight/chance
pops eastward with thunderstorms mentioned regionwide.  Although
general thunderstorms are expected, wouldn`t be suprised to see a
few reach strong/severe limits given the aforementioned profiles,
therefore such will be mentioned in the HWO.  Convection will die
down into the late evening hours as heating is lost, thus pops are
allowed to taper to below mentionable levels by around midnight.
As for temperatures today, quite warm for mid/lat September as
highs top out nearly 6-8 degrees above normal.


As of 300 AM EDT Wednesday: An upper level trough axis will settle
along the southeast coastline Thursday through Friday, as a 590 dm
ridge axis sets up from the Great Lakes to the MS Valley. Low level
easterly flow developing Thursday afternoon through Friday should
help to focus the highest chance PoPs along the eastern slopes of
the mountains. Thus anticipate mainly diurnal, scattered ridge
top/eastern mountain convection both Thursday and Friday afternoons
with weak forcing and limited moisture. SBCAPE values should
steadily recover to 1000 to 1500 J/kg to support a scattered thunder
mention. Minimum temperatures will remain warm - some 8 to 10
degrees above climo. Max Temps will be one to two categories above
climo on Thursday and a touch cooler on Friday.


As of 300 AM EDT Wednesday: The upper ridge will strengthen from the
north through the weekend as Tropical System Jose meanders off the
mid-Atlantic coast and Hurricane Maria moves northward well east of
coastal FL/GA. Much drier air will wrap southward from the ridge,
with very limited precipitation chances - mainly just isolated
showers Saturday afternoon in far southwest mountain sections.

The consensus solutions keep Maria far enough east off the Carolina
coast Monday through Tuesday to have no impact on sensible weather
across our forecast area. There are vast differences in the handling
of the Jose remnants that get absorbed into the Maria circulation,
but even the members with inland impacts from Jose keep any
associated moisture over the mid-Atlantic coastal region and north
of our area through Tuesday. Thus expect mostly northerly flow
around the offshore systems and generally dry conditions with
continued temperatures above climo.


At KCLT and Elsewhere:  VFR through the period at all sites aside
for the possibility of tsra induced MVFR, as well as Thursday
morning fog/visb restrictions at KAVL.  Otherwise, light/vrb flow
will prevail amidst mid/upper level clouds through late morning
with flow gradually becoming more sly.  Llv cu is expected to
dominate through the mid/late part of the day as convective
trends increase westward across the high terrain.  With that,
tafs now feature VCTS at KAVL/KHKY, with prob30s favored at the
remaining sites given slightly later toa per CAMS.  Beyond that,
any remaining low stratus/cu will gradually sct into the overnight
hours leading to another night of fog potential.

Outlook: Chances for diurnally favored showers and thunderstorms
will continue to increase as the week progresses, with the best
chances for overall coverage being on Thursday and Friday as flow
veers easterly and marginal moisture advection returns across the
region.  Chances for restrictions from early morning fog across
the northern NC Piedmont and foothills, as well as in the mtn
valleys will also be a concern. Drier weather appears to return
for the weekend.

Confidence Table...

            10-16Z        16-22Z        22-04Z        04-06Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High  83%     High 100%     High 100%     High 100%
KAVL       Med   60%     High 100%     High 100%     High 100%
KHKY       High 100%     High 100%     High 100%     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:




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