Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 300710

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
310 AM EDT Thu Mar 30 2017

A moist ridge of high pressure will build in from the north today
before a cold front sweeps across the forecast area early Friday.
Dry and warm high pressure moves in over the weekend...with
unsettled weather returning early next week.


As of 255 AM EDT: Water vapor imagery reveals a strong closed low
pressure center moving east from Kansas into Missouri. Downstream
ridging has developed ahead of this system over the eastern U.S.,
and surface high pressure stretches southward from Quebec through
the Carolinas. The surface ridge will gradually migrate offshore
today as low pressure approaches from the west and southerly return
flow moisture rides northward over the surface ridge to set up
hybrid cold air damming. Upglide and upslope forcing will be much
better over western NC and points north today, which leaves the
southern tier as the more questionable forecast for temperatures and
associated weather. The northern tier east of the Blue Ridge should
get locked into cold air damming fairly quickly, with light rain
breaking out this morning and expanding this afternoon. The southern
tier could well see drier conditions initially and more warming,
which will in turn pump up instability for the afternoon hours.
SBCAPE values should reach 1000-1500 J/kg in piedmont sections
southeast of I-85 this afternoon. A few 55 dBZ storm cores reaching
reaching 23 kft this afternoon could well produce marginally large
hail and brief damaging wind gusts in a few places once the cap
breaks. This is the early activity that supports the latest Slight
Risk area, at least for the southern tier.

As the upper low pressure system lifts east over the Ohio Valley
tonight, the associated trough will acquire a more negative tilt and
bring a relatively decent period of upper forcing to our region
overnight. Meanwhile, the numerical models have the surface wedge
front retreating northward across western NC overnight. Although
this will uncover a bit more overnight instability, it will also
help to minimize the potential for a focusing mechanism of low level
helicity. Of course, the models are notorious for eroding the wedge
layer too quickly at night, but the offshore high position is not
favorable for entrenched CAD, so the model solutions seem
reasonable. Some uncertainty remains, however, regarding the risk of
nocturnal severe thunderstorms overnight as the 40 kt low level jet
translates through - especially if the better overnight instability
in the NAM is correct. This later activity will thus also support
the Day 1 Marginal to Slight risk severe tstm threat.  The nighttime
rainfall maximum should occur in the southern upslope areas from the
GA mountains to parts of the Blue Ridge and the current belief is
that this area can handle any localized two inch totals with the
main precipitation band without flooding consequences. The better
convection should translate through fairly quickly 06Z to 12Z,
likely crossing the I-77 corridor around daybreak.


As of 215 AM EDT Thursday: Upper level s/w energy will quickly shift
to the east early Fri as the h5 low tracks across the OH Valley.
Thunder chances will remain decent before noon...but PoPs drop off
rapidly during the afternoon as a dry slot works in from the
southwest. Not anticipating much MCS maintenance nor enhancement
with these storms as the best llvl convg zone and sfc theta/e ridge
will have moved east of the area shortly aft 12z. Sfc dewpoints will
drop off about 8-10 degrees behind the frontal zone while cloud
cover decreases in the afternoon...with perhaps lingering -shra and
cloudiness across the nrn mtns. The increased insol and winds
remaining sw/ly will allow max temps to reach m70s non/mtns and u60s
mtn valleys.

The weekend will see weak but broad cP high pressure encompass the
area as an h5 ridge axis traverses the ern CONUS. No good chance of
precip with likely only fair-wx Cu until Sun afternoon when layered
clouds move in ahead of the next approaching storm system. The upper
ridge and cP high will mix lower td/s while column drying allows for
max temps near or arnd 80 F each day outside the mtns and L70s mtn
valleys. Basically...two rather nice days are in store.


As of 250 AM EDT Thursday: Upper level heights begin to fall quickly
aft 00z Mon as a strong h5 low advances toward the area. Increasing
difl flow will bring in added lift across the wrn zones where
upslope showers will begin and increase in coverage thru the morning
hrs. The GFS is more agressive and moist with the warm front than
the ECMWF/CMC PoPs have been slowed down a little early
on due to uncertainty. By the daytime Mon...the area will be in a
detabilizing warm sector while deep layered forcing increases from
the west. The 00z GFS is also stronger with this forcing than the
other op models...but regardless the dynamics shud increase
throughout the day. Model soundings show effective shear maxing out
arnd 40 kts within a modest MUCAPE environ before a frontal zone
moves into the FA arnd max heating. Thus...the potential for stg/svr
organized storms are probable thru the afternoon and early evening.
A sfc low crosses the nrn zones early Tue as a dry slot quickly
reduces the chance for tstms aft 03z...with lingering -shra across
the NC mtns likely thru daybreak. Tue thru Wed shud be mostly dry
and warm as a broad cP sfc high builds in from the north.


At KCLT: Upglide cloudiness over a developing cold air damming layer
has already started to form across the NC foothills and NW NC
piedmont early this morning. This cloudiness will expand and lower
through the morning hours as upglide moisture improves. Anticipate
ceilings lowering into the MVFR range after 09Z and then bottoming
out in the IFR range for the morning hours. There is some potential
for recovery to MVFR this afternoon as the wedge may become fairly
shallow given the unfavorable offshore surface high pressure center
position. But, there is also some potential that IFR conditions
could get locked in through the day if precipitation develops more
quickly than expected. Wind direction will be a tricky easterly
cross wind today as well, likely more ESE during dry periods, and
then ENE when any precip coverage increases. Anticipate thunderstorm
chances to be better south of the wedge boundary, but KCLT could be
close to convection if the CAD becomes shallow nearby. Shower
chances will ramp up through the evening hours. LLWS conditions look
better just north and west of the airfield tonight.

Elsewhere: Expect a fairly rapid onset of hybrid cold air damming
across the western Carolinas through the morning hours. This should
mean a fairly quick transition down through MVFR ceilings and into
IFR at KHKY, along with light rain and drizzle, and a slower
downward transition farther south from KGMU to KGSP. Moist
southeasterly flow into the mountains will also permit MVFR
restrictions to develop at KAVL this morning. Instability will
bubble up south of the wedge boundary this afternoon, with thunder
chances the best at KAND and then from KGMU to KGSP. Shower coverage
will improve and intensify as the upper support begins to push in
from the west late in the period, and some thunder mention could be
needed again overnight. Expect mainly SE winds throughout, except
for more NE at KHKY. Upstate winds could briefly back to ENE in any
better precipitation.

Outlook: A cold front will sweep east across the region Friday
morning, with associated showers, thunderstorms, and restrictions.
Drier conditions will return for the entire weekend, but with
unsettled weather again developing early next week.

Confidence Table...

            07-13Z        13-19Z        19-01Z        01-06Z
KCLT       High  92%     High  81%     High  87%     High  81%
KGSP       High  87%     Med   71%     Med   76%     High  80%
KAVL       High  91%     High  95%     Med   72%     High  98%
KHKY       Med   79%     Med   76%     Med   74%     High  92%
KGMU       High  96%     Med   66%     High  81%     Med   73%
KAND       High  98%     Med   66%     High  95%     High  86%

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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