Area Forecast Discussion
Issued by NWS Greer, SC

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259
FXUS62 KGSP 252344
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
744 PM EDT Tue Apr 25 2017

.SYNOPSIS...
An area of low pressure will gradually weaken and drift northeast
along the Mid-Atlantic coast through tonight. Warm and dry
high pressure will filter in behind the low through Wednesday. A
dissipating cold front crosses our region Thursday, then warm and
humid high pressure builds back in for Friday and Saturday.

&&

.NEAR TERM /THROUGH WEDNESDAY/...
As of 7 PM EDT: Areas of stratocu linger in the waning cyclonic flow
this evening, but given the very moist blyr, some fog development
remains likely overnight. It is possible there will be enough
boundary layer moisture that stratus forms, and a weak 850 mb
downslope component could offset fog and stratus development
altogether in some areas. So, dense fog is by no means certain. Will
keep a patchy mention for the overnight hours for now, along with
minimum temperatures in the 50s throughout. The southeast ridge axis
will be solidly atop the area on Wednesday. Any boundary layer
drying will be brief during the day before southerly return flow
starts up under the ridge in the afternoon. Mid-level temperatures
should be too warm to permit any convective development despite the
good heating - with temperatures rebounding into the 80s in most
areas.

&&

.SHORT TERM /WEDNESDAY NIGHT THROUGH FRIDAY/...
As of 215 PM Tue: A deep shortwave trough will cross the Mississippi
River Wednesday night. Heights fall accordingly over the CWFA thru
Thursday morning. Though the shortwave and associated sfc low are
displaced by the western Atlantic ridge, tracking toward the Great
Lakes, the system`s cold front should reach the west side of the
Appalachians Thu morning. This appears slightly sooner than depicted
on runs from yesterday. Initially we are impacted by the warm
conveyor belt, providing some upglide in conjunction with DPVA early
in the day that warrants small PoPs over the mtns, NE GA, and western
Upstate; low-level cold advection is essentially nil. Overall, the
strong subsidence inversion seen Wednesday will remain over the area,
and convection looks unlikely until perhaps late afternoon when
cooler midlevel air pushes in behind the front and erodes the capping
inversion. Instability of a few hundred joules will likely develop as
that happens, and hodographs indicate storms may organize, perhaps
becoming capable of damaging winds. Current SPC D3 outlook includes
the far western CWFA in an expansive marginal risk along the front,
but I would not be surprised if tonight`s new D2 expands the risk
area further east, seeing the current model trends. Max temps will
run above normal overall, perhaps a couple degrees in the cloudier
west and as much as 6 degrees above in the east.

By early Friday a weak area of high pressure will have built back
into most or all of the area behind the front. Over the course of the
day, heights rise above it as the western Atlantic ridge begins to
strengthen again. This most likely will form a cap over the area and
limit convective coverage to the higher terrain Friday aftn. However,
the NAM depicts a developing low near the Ozarks as lifting the old
cold front back through our FA as a warm front, with a bit of QPF
response to match. Thus a low PoP will be advertised in the morning,
over mainly NE GA and the Upstate, to handle this possibility. With
max temps nearly 10 degrees above normal, and favorable lapse rates
above the cap, CAPE values above 2000 J/kg are progged over most of
the area. Where this instability is realized, vigorous tstms are
expected. Shear is expected to be too strong for pulse storms but too
weak for supercells, so in light of the dry mid to upper levels seen
on profiles, a multicell cluster producing damaging winds is not out
of the question.

&&

.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
As of 230 PM EDT Tuesday: Agree with the previous assessment that
our weekend will start out feeling very summerlike under a Bermuda
high pattern. Diurnal convection will gradually diminish Friday
night and have tried to indicate a lull overnight and early
Saturday, before scattered mainly afternoon/evening convection fires
up again on Saturday. Temps will be well above normal and may push
the upper 80s or near 90 over the Piedmont. Most of the convection
will die off Saturday evening, but a developing upslope E/SE flow
may keep some shower activity going overnight near the Blue Ridge
Escarpment.

The forecast gets a bit more tricky on Sunday as the upper pattern
amplifies with a deep upper low coming out over the Srn Plains and
an upper ridge off the East Coast. Upslope flow continues to
strengthen on Sunday and the model guidance responds with more
precip development emanating from the Blue Ridge. Precip probability
ramps up into the likely range in the upslope areas. Temps are
problematic. If the clouds and precip develop as in the model
guidance, the max temp will be something on the order of 5-10
degrees cooler than Saturday, but if not, we will be well above
normal again.

The most interesting day looks like Monday, as the progression of
the upper low lifting to the upper Midwest drives a cold front
eastward. Of note is the possibility of organized severe weather
along the front to our west as it approaches. There is good upper
support with this strong front, although a weakening trend is noted
in the guidance as it moves across our region. Timing is the key.
The operational GFS suggests that some strong storms could move in
from the west late Sunday night, but then redevelopment would happen
over the wrn Piedmont of the Carolinas Monday afternoon. The
environment would be supportive of more strong storms with
sufficient shear but marginal CAPE. Will have to keep an eye on this
one. If the system slows down as suggested by the latest ECMWF, our
threat will be greater.

The front should blast across the region late Monday, clearing us
out for Monday night and Tuesday, with temps cooling back close to
normal.

&&

.AVIATION /00Z WEDNESDAY THROUGH SUNDAY/...
At KCLT: Abundant piedmont moisture is keeping MVFR cigs in place
and these will be slow to lift and scatter through the evening
hours. No further shower development is expected. The main concern
will be the potential for overnight fog or low stratus given the
very moist surface layer. There is some potential for stratus to
reduce the fog potential, or for light NW winds to provide some
downslope and limit both stratus and fog. So, will keep developing
low stratus scattered and feature nothing worse than 2SM vsby in fog
at this point. Winds will adjust to SSW through Wed with scattered
cumulus with heating.

Elsewhere: Ceilings across the mountains and foothills have lifted
from MVFR to VFR. Convection allowing models depict isolated showers
around the region late this afternoon, but confidence is far too low
to mention it at any TAF sites. Abundant moisture will create the
potential for stratus/fog overnight but will remain conservative in
the forecast for now given MOS uncertainty and the potential for
weak downsloping. Cumulus should develop with heating through the
late morning hours along with light SSW flow.

Outlook: Return flow moisture will develop ahead of the next system
Thursday into Friday with the potential for thunderstorms
increasing. Scattered, diurnal thunderstorms will then continue over
the weekend.

Confidence Table...

            23-05Z        05-11Z        11-17Z        17-18Z
KCLT       High 100%     High 100%     Med   65%     High 100%
KGSP       High 100%     High 100%     High  85%     High 100%
KAVL       High 100%     High 100%     Low   31%     High 100%
KHKY       Med   66%     High  85%     High  80%     High 100%
KGMU       High 100%     High 100%     High  85%     High 100%
KAND       High 100%     High 100%     High  90%     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:

www.weather.gov/gsp/aviation

&&

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

&&

$$
SYNOPSIS...DEO
NEAR TERM...CSH/HG
SHORT TERM...Wimberley
LONG TERM...PM
AVIATION...CSH/HG



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