Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 211620
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1220 PM EDT SAT MAY 21 2016

.SYNOPSIS...
A cold front will sweep across the area today. Drier high pressure
slowly builds into the region through early next week. A moist
southerly flow develops by mid week and continues into the weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 1215 PM...cloud cover east of the mtns has resulted in slower
warming than the current forecast trend. I will update the forecast
to adjust temperatures to the latest LAV guidance. Otherwise...the
earlier adjust to PoPs continue to look good.

As of 935 AM...latest satellite images and sfc observations
indicated that most of the foothills and Piedmont was under low
stratus and patchy fog. Across the mtns...valley low clouds linger
with cloud free conditions across the ridges. Low clouds and fog
will mix through the rest of the morning...lifting by late morning.
I will update the forecast to adjust sky timing and add chc pops
across portions of the upper Savannah River Valley to cover a
passing shower.

This afternoon...CAMs are consistently pointing to one general
solution. It is expected that morning sunshine across the peaks and
NC foothills will yield weak instability by mid day. Convergence and
instability will likely support the formation of a line of showers
along the east facing slopes by 2 pm. This lines is expected to
track east through the afternoon...passing over the CLT metro area
between 3-5 pm. Cloud to ground lightning could be common along the
southern portion of the band during the late afternoon. I will
update the forecast to time the band across the foothills and
Piedmont.

As of 640 AM: Cold air damming is in its waning stage at this point;
the parent high sits offshore from NY/NJ. A deep upper trough will
swing from the Ohio Valley toward the East Coast today, leading
to veering low level winds and a loss of isentropic lift over
the wedge. This process is already underway, as cloud bases are
lifting in many areas (and some blue sky seen out our window). VAD
winds from KGSP have veered to near due west and the downsloping
does seem to have started scouring the low clouds just below the
Blue Ridge. Dense fog has developed as expected in a few spots,
particularly the Unifour area, and near Rutherfordton, as the
stratus have departed. An advisory appears unlikely at this point
as most of the area will not clear out until after the sun comes
up a bit.

The best upper Q-vector convergence over the area will be this
morning, but the presence of the trough overhead later in the
day will bring good mid to upper level lapse rates. Cyclonic flow
aloft could also promote lift to some degree. Model consensus is
that as much as 1500 J/kg of SBCAPE will develop by mid-afternoon
over the Piedmont. Several WRF runs show convection firing along
the Ridge and moving/propagating southeast into this area as
a multicell cluster. Indeed consensus is for 40-50 kt of 0-6km
bulk shear which would support long-lived updrafts and upscale
growth. Freezing levels are fairly low for the season, and the
mid-upper levels become very dry as the trough axis passes. It
appears a few storms may be capable of strong wind gusts and large
hail. Coverage however will be limited by downsloping flow. SPC
has expanded the marginal risk area into much of our Piedmont with
the new Day 1 outlook issued at 06z. Max temps will warm back up
to 3-5 degrees below normal.

Low level winds veer to northwest as the trough moves east of
the area this evening, and this ushers in cold advection and
strengthens downsloping over most of the area. Drying/clearing
conditions are expected, though profiles will still support clouds
and a few showers along the spine of the Appalachians, near the
TN/NC border. Temperatures will be near normal, with downsloping
offsetting the effects of the incoming cooler air.

&&

.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
As of 230 AM Saturday, an upper low drops into Eastern VA Sunday and
to Eastern NC Sunday night. Short wave rotate around the upper low
and across the area. Cold mid-level temps will rotate into the area
as well. Despite the colder temps...surface dew points will be
relatively dry keeping instability weak. Best chance of convection
will be across the NC Mountains where NW upslope flow will help
produce lift. Second area for showers will be the I-40 and I-77
corridors closer to the center of the upper low. Expect windy
conditions across the NC Mountains and breezy conditions elsewhere
as deep mixing taps the relatively strong low level winds. Gusts
should remain below advisory level. The upper low pulls away from
the area Monday and Monday night bringing warmer mid level temps and
rising heights. This should bring and end to any convection chances.
Winds will diminish as well. Below normal temps on Sunday will rise
back to near normal for Monday.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
As of 305 AM Saturday, weak upper ridging builds over the area
Tuesday and remains through the end of the period. There will be
some weak short waves moving through the ridge and across the area
from Wednesday on. At the surface, dry high pressure builds in on
Tuesday then slides off shore on Wednesday remaining there through
the end of the period. Weak but increasingly moist southerly flow
develops through the period as a result. This will lead to
increasing chances of diurnal convection with the best chances
across the mountains and foothills where moisture will be greatest.
Temps rise a little above normal on Tuesday then rise a few degrees
through the end of the period.

&&

.AVIATION /18Z SATURDAY THROUGH THURSDAY/...
At KCLT and elsewhere: Cold air damming dissipating over the
region. Cigs are generally lifting over much of wrn NC as well
as KGSP/KGMU, but low clouds extending from GA into midlands
SC will advect over KAND/KCLT over the next few hrs. It will
probably be midday before all the IFR is history. Widespread
cumulus are expected to develop with diurnal heating, perhaps
initially at MVFR levels but forming VFR cigs by early aftn. SCT
SHRA/TSRA subsequently will develop, with convection allowing
models in agreement that initiation will occur near the Blue
Ridge in the early aftn and propagate southeastward into the NC/SC
Piedmont. Timing of PROB30s has been aligned accordingly; 15z or
18z TAFs will refine these for TEMPOs if current model trends hold
up. Strong wind gusts and a couple of hail producers are possible
with TS this aftn. By the time winds pick up they will generally
be WSW to SW over the Piedmont and NW at KAVL. All winds veer to
NW this evening as cold front effectively passes. Enough moisture
remains banked against the Appalachians that KAVL likely will see
some low cigs early Sunday mrng.

Outlook: Conditions should gradually dry out Sunday and Monday. AFTN
showers and TSTM chances may return on Wednesday.

Confidence Table...

            16-22Z        22-04Z        04-10Z        10-12Z
KCLT       High  94%     High 100%     High 100%     High 100%
KGSP       High  94%     High 100%     High 100%     High 100%
KAVL       High  81%     High 100%     Med   72%     High  84%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High  81%     High 100%     High 100%     High 100%
KAND       High  84%     High 100%     High 100%     High 100%

The percentage reflects the number of guidance members agreeing
with the schedule 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...NED/RWH
NEAR TERM...NED/Wimberley
SHORT TERM...RWH
LONG TERM...RWH
AVIATION...Wimberley



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