Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 301705
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
105 PM EDT Thu Mar 30 2017

.SYNOPSIS...
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.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 1100 AM EDT: Convection has developed across the Upstate, with
light rain/drizzle expanding along the Blue Ridge as damming
continues to entrench across the area. CAPE values kind of meager
this morning but expanding up the Savannah River Valley, with some
increasing muCAPE atop the cold dome over the rest of the Upstate.
Have increased pops this morning in areas already experiencing
convection. Adjusted hourly temperatures with a resulting minor
tweak to afternoon highs, but not any significant change.

Otherwise, a strong closed low pressure center continues to move
east 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.

&&

.SHORT TERM /FRIDAY THROUGH SUNDAY/...
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.

&&

.LONG TERM /SUNDAY NIGHT THROUGH WEDNESDAY/...
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 solns...so 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.

&&

.AVIATION /15Z THURSDAY THROUGH MONDAY/...
At KCLT: Update for 15z AMD is starting cigs a little lower, veering
winds a little more SE as opposed to current ESE, increasing wind
speeds with some low-end gusts overnight, and adding PROB30 to the
very end of the period. Otherwise, upglide cloudiness developing
over the cold air damming layer will expand and lower through the
morning hours as upglide moisture improves. Anticipate ceilings
bottoming out in the IFR range for the afternoon hours. There is
some potential for recovery to MVFR late day 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 Carolina foothills through the morning hours.
This should mean a fairly quick transition down through MVFR
ceilings and into IFR, along with developing light rain. 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...

            15-21Z        21-03Z        03-09Z        09-12Z
KCLT       Med   74%     High  85%     Med   72%     Med   66%
KGSP       Med   68%     Low   57%     Med   74%     Low   59%
KAVL       Med   77%     Low   57%     Med   70%     Med   75%
KHKY       Med   63%     Med   79%     High  85%     High  84%
KGMU       Med   66%     Med   64%     Med   77%     Low   53%
KAND       High  94%     High 100%     Med   79%     Med   64%

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:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

$$
SYNOPSIS...HG/SBK
NEAR TERM...HG/TDP
SHORT TERM...SBK
LONG TERM...SBK
AVIATION...HG/TDP


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