Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 190707

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
307 AM EDT Tue Sep 19 2017

A weak trough of low pressure will cross the region from the west
through Wednesday. High pressure will then return from the north
Thursday through Friday, but with more moisture in easterly flow.
Drier and stronger high pressure will settle over the region through
the weekend as both Hurricane Jose and Hurricane Maria remain well
offshore in the western Atlantic.


As of 300 AM EDT Tuesday:  Deep layer ridging will highlight the
pattern once again today as an upper ridge axis remains draped along
the Appalachians, and surface high pressure stretches southward from
New England into the southeast.  Meanwhile, Hurricane Jose remains
at Cat 1 strength centered approx 230 miles east of Cape Hatteras
NC with a northward movement of around 8 mph, thus moving away from
the fcst area.  A look at sat fog products this morning correlates
fairly well with observations as radiation fog has and continues
to develop in the mtn valleys as residual mid level stratocu
remains thin.  Also seeing some patchy fog across the northern
NC piedmont where weak showers occurred late on Monday evening.
Still think some further expansion of fog is possible along the
I77 corridor, however not all that confident that it will reach as
far south as Charlotte given current Tdd`s are at approx 5 degrees.

Otherwise, today looks very similar to days past as far as sensible
weather goes, yet with improved chances for ridgetop convection
given slightly lower upper heights and improved instability through
the profile.  Thus the fcst will feature slight chance pops along
the Blue Ridge, with low end chances favored atop the Upper French
Broad valley and dry conditions across the low terrain.  Wouldn`t be
shocked to see a few thunderstorms amongst this convection, with
such remaining at general levels with lightning and gusty winds
being the primary threats since convection will be rather shallow.
Temperatures today will remain above normal levels, however probably
a degree or two cooler than days past given expected abundance of
fair wx cu this afternoon.  Tonight looks to be a repeat of recent
nights given slow erosion of daytime/evening cu/stratocu and the
potential for patchy radiation fog for some locales.


As of 300 AM EDT Tuesday: A mid-level trough will cross the
Appalachians from the west on Wednesday. Slightly improved moisture
is evident in model profiles along and east of the trough axis, and
this will raise shower chances. Also anticipate 850 to 500 mb lapse
rates steepening up a bit above 6.5 deg C/km by Wednesday afternoon
to warrant an isolated to scattered thunder mention. Warm maximum
temperatures should reach 5 to 8 degrees above normal Wed.

The trough axis will settle southeast of the forecast area toward
the southeast coastline on Thursday, with drier air settling into
our region from the northwest. Warm conditions will continue, but
with temperatures perhaps a degree below Wed. values.


As of 300 AM EDT Tuesday: A prominent central CONUS ridge axis will
build from the southern plains to the Great Lakes on Friday,
wrapping over in Rex block fashion atop the lingering trough axis
along the southeast coastline. At lower levels, flow will become
more easterly on Friday around the surface high to the north,
producing greater cloudiness and shower chances in our region -
especially in eastern upslope mountain areas.

The ridge will become dominant north of the region through the
weekend, while Hurricane Maria spins well offshore - east of the
coastal waters of FL/GA/SC. Subsidence over the southeast along with
surface high pressure along the southern Appalachians will keep the
region dry.

The complicated interaction of Jose and Maria over the western
Atlantic will make for high levels of uncertainty for early next
week. However, both GFS and ECMWF solutions keep any circulations
east of the Carolinas through Monday. The forecast will remain dry
with slightly above normal temperatures through the period.


At KCLT and Elsewhere: Conditions very similar to mornings past
as VFR is expected to dominate aside for chances of patchy ground
fog and possible cigs across portions of the NC piedmont, as well
as in the mtn valleys.  Once again guidance seems to be picking
up of fog development along the I77 corridor as it did yesterday
morning, however given less stratocu in place, think chances are
little better for such as does the guidance given that 4 sources
favor it.  Therefore, opted to include prevailing 6sm at KCLT around
daybreak with a 2hr tempo for MVFR fog and sct008. Further north
at KHKY, hit them a little harder with prevailing 5sm, tempo`d
to 3sm sct003. As for KAVL, went with very similar conditions to
KHKY with MVFR tempo`d to IFR, albeit with lower confidence given
min temps are fcst to fall just to the xover temp.  Otherwise,
the remaining sites should stay restriction free with occasional
passing of low/mid stratocu, with all sites favoring light nely/ely
or calm flow.  Moving into the daylight hours, high pressure will
remain in control of the pattern as it`s center shifts slightly
south, possibly setting up nearly atop the region.  Therefore all
sites remain dry with llv moisture sufficient for widespread fair
wx cu amidst light/vrb flow.  Chances for convection will be a bit
higher across the terrain today as upper heights fall slightly,
however kept any wx from KAVL at this time.

Outlook: Chances for diurnally favored showers and possibly a
thunderstorm will increase each day this week.  The best chances
look to be Thursday and Friday as flow veers easterly and marginal
moisture advection returns across the region.  That said, the best
chances for restrictions will be from early morning fog across
the northern NC piedmont/foothills, as well as in the mtn valleys.

Confidence Table...

            07-13Z        13-19Z        19-01Z        01-06Z
KCLT       High 100%     Med   71%     High 100%     High 100%
KGSP       High  96%     High  96%     High 100%     High 100%
KAVL       High  95%     High  88%     High 100%     High  90%
KHKY       High  93%     Med   61%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High  96%     High  96%     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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