Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 281903
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
303 PM EDT SAT MAY 28 2016

.SYNOPSIS...
After a dry day today, a period of cooler and wetter weather may
develop late in the weekend through early next week as Atlantic low
pressure moves toward the South Carolina coast.

&&

.NEAR TERM /THROUGH SUNDAY/...
As of 245 PM EDT Saturday: Complicated near-term forecast this
afternoon mainly with effects from TD2. Overall synoptic setup has a
shortwave trough moving across the northern tier of the country with
the upper weakness associated with TD2 trying to phase with this
shortwave but generally remaining its own entity. Luckily the GFS
and ECWMF are starting to line up a little more and this is
confirmed by guidance from national centers, so have generally taken
a blend through the near-term. So what we have is TD2 - or Bonnie or
whatever it will be - approaching the SC coast somewhere just SW of
Charleston overnight. Guidance differs on timing of landfall with
NAM the fastest but looks like it should be right around 12z Sunday.
By this time, the northern tier shortwave has progressed east enough
that it will halt the NWward progression of the tropical system, so
the surface low will sort of sit and spin around the SC coast,
slowly shift NEward, through the rest of the period. The GFS has the
strongest surface reflection but all operational models are within a
few mb of each other.

As for sensible weather impacts, we are already seeing an increase
in high clouds as the system approaches the coast. Subsidence around
the periphery of the system is keeping a good chunk of the forecast
area clearer than earlier anticipated and also suppressing the
typical summertime-like convection that we might otherwise be seeing
across the mountains, so have pulled back on afternoon/evening PoPs
over the higher terrain. CAM guidance starts bringing some showery
activity associated with rain bands into the area by around
midnight, with coverage increasing through the day on Sunday. Even
though we`re talking the 1st- into 2nd-period forecast, still pretty
much impossible to nail down details regarding timing and intensity
of any potential rain bands to move in, but in general Sunday looks
like a fairly rainy day with "occasional" rain being a good
descriptor. Have introduced categorical pops for eastern zones
generally between 12z-21z Sunday; bufr soundings for KCLT are
basically saturated all but the very lowest layer by around 5am.
Expect some tapering off toward the end of the period. Basin-average
QPF remains less than 1/2" but we all know that these tropical rain
showers can be fairly efficient rainfall producers, but luckily
looks like we should not have any significant hydro impacts (at
least with this forecast package).

Thunder potential looks overall fairly low with really not much in
the way of sbCAPE available, but enough elevated/muCAPE to keep a
mention of slight chance thunder in the grids through the afternoon
Sunday. Mesohigh along the eastern slopes of the Appalachians may
keep CAPE at a minimum for the central portion of the forecast area.
Severe threat is minimal (and indeed only General Thunder for the
Day2 outlook). Surface winds will increase out of the north tonight
and Sunday to between 10-15kt or so with low-end gusts but otherwise
impacts from winds should be minimal.

Overnight low temperatures tonight will continue to be on a bit of
an increasing trend with lows in the mid 50s across most of the
Piedmont and at best 10 degrees cooler at the higher elevations of
the mountains. Should see much cooler highs tomorrow with widespread
cloud cover and rain...with highs in the mid to upper 70s over the
NC Piedmont and maybe mid 80s over the Upper Savannah Valley.

&&

.SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY/...
At 230 PM Saturday: On Sunday evening an upper ridge will extend
from the vicinity of Bermuda to New England, while a low amplitude
upper ridge will extend from the Great Lakes to the Southern
Appalachians. The pattern deamplifies over the eastern USA,
resulting in zonal upper level flow by Monday. By Tuesday very weak
upper troughing sets up over FL, while week upper ridging sets up
over the western Great Lakes, with the Southern Appalachians in
between.

At the surface, on Sunday evening a tropical system is expected to
be near the SC coast, with substantial associated moisture spreading
inland over the NC piedmont and northern SC piedmont. More limited
moisture will be present over the foothills and mountains. The
system moves slowly to the NC coast by Monday, with abundant
moisture remaining over the piedmont until late in the day. Although
the bulk of the moisture moves east of the piedmont by Tuesday, the
models show enough moisture remaining over our area to support
modest precipitation.

At this time, precipitation amounts are expected to be limited over
our area, however the Interstate 77 corridor may be subject to
substantial rainfall if greater moisture associated with the
tropical system spreads slightly farther inland. Instability both
Sunday and Monday appears sufficient to support modest convection.
Temperatures are expected to run slightly above normal.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
At 230 PM Saturday: On Tuesday night the models close off an upper
low near the NC coast, associated with the tropical system moving
slowly up the NC coast. Meanwhile, week upper ridging will be
present over the gulf states, and an upper trough will extend from
the northern plains to northwest Mexico. The upper low fills and
moves northeast by Thursday, while the upper trough upstream slowly
progresses. By Friday the upper trough reached the MS River Valley,
and by Saturday it reaches the OH River Valley and the Gulf States.

At the surface, a cold front will cross our area on Tuesday night,
stalling near the coast by Wednesday, where it remains into
Thursday. Moisture associated with the front may extend far enough
inland to support lingering precipitation into our eastern zones.
Another cold front approaches from the west on Thursday night,
moving slowly over our area on Friday ands Saturday. Temperatures
will fall from slightly above normal to near normal.

&&

.AVIATION /19Z SATURDAY THROUGH THURSDAY/...
At KCLT: VFR this afternoon and the first part of the overnight
period but with increasing high clouds as the cirrus from TD2
continues to spread inland. Winds will flirt around E but in general
should be mainly ESE until close to 06z, when approaching surface
low swings wind around to NE. Expect MVFR cigs to build in about
this time with increasing chances for -SHRA so have introduced VCSH.
Went prevailing -SHRA by 09z with solid MVFR cigs and in-and-out
MVFR vsbys, depending on where the rain is at the time. Winds
continue backing to the N and pick up after 12z to 10-12kt or so as
-SHRA chances increase. Introduced PROB30 TSRA after 15z on Sunday.

Elsewhere: Other TAF sites will be farther away from impacts from
TD2 or whatever it will be at the time, but overall should see a
similar pattern with increasing high clouds through the period. Did
introduce MVFR cigs and -SHRA at 15z for KHKY, and included PROB30
TSRA everywhere by 15z Sunday for all but KAND. KAVL might well see
another period of crashing vsby/cigs in the morning, but with high
clouds over the area have only reduced vsby to 2SM.

Outlook: Uncertainty remains over the westward reach of tropical
moisture associated with TD2 through the day Sunday and into early
next week as the surface low spins slowly up the SC and then NC
coast. KCLT will continue to have the best chance of lingering
SHRA/TSRA. Otherwise, patchy fog chances continue at KAVL each
morning, with scattered afternoon/evening SHRA/TSRA increasing into
early next week.

Confidence Table...

            19-01Z        01-07Z        07-13Z        13-18Z
KCLT       High 100%     High 100%     Med   71%     Med   78%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     Med   69%     High  91%
KHKY       High 100%     High 100%     Med   79%     Med   70%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     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...HG
NEAR TERM...TDP
SHORT TERM...JAT
LONG TERM...JAT
AVIATION...TDP



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