Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 282358
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
758 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 745 PM EDT Saturday...satellite imagery shows a thickening
veil of high and mid clouds streaming northward across the fcst area
from the circulation around Bonnie. KGSP radar shows the vanguard of
light precip spreading up from the coast into the Charlotte metro
area. Think it may be several hours before any of this reaches the
ground as more than just sprinkles. Temps will be adjusted based on
recent observations. Otherwise...no major changes suggested thus
far.

Overall synoptic setup has a shortwave trough moving across the
northern tier of the country with the upper weakness associated with
Bonnie 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 Bonnie approaching the SC coast 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, subsidence around the periphery of
the system is suppressing the typical summertime-like convection
that we might otherwise be seeing across the mountains 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 /00Z SUNDAY THROUGH THURSDAY/...
At KCLT: VFR this evening with thickening mid/upper clouds and an
ESE wind...but lower cloud decks already showing up in observations
near the coast...moving toward the NW.  Model guidance has the wind
coming around to N of E in the mid evening...and some light precip
may spread in from the east before midnight as Bonnie approaches the
coast. The guidance has been consistent enough with introducing an
MVFR level ceiling in the 06Z to 08Z time frame...so the TAF keeps
this change at 07Z. Precip should develop/move in from the SE in the
pre-dawn hours...what remains to be seen is how much visibility will
be affected. The trend in the guidance is toward a more optimistic
visibility...so the TAF follows that trend. Think the main precip
area will move out in the early afternoon and chances of thunder are
too low in the afternoon to warrant a mention in the new TAF.

Elsewhere: Other TAF sites will be farther away from impacts from
Bonnie, but overall should see a similar pattern with increasing mid
and then low clouds through the period. KAVL will have less of a
chance of seeing IFR fog early Sunday because of thickening cloud
cover. The least chance for precip will be over the west...so KAND/KAVL
never get more than a PROB30.

Outlook: Uncertainty remains over the westward reach of tropical
moisture associated with Bonnie Sunday night 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...

            00-06Z        06-12Z        12-18Z        18-00Z
KCLT       High 100%     Med   71%     High  87%     High  90%
KGSP       High 100%     High 100%     High  97%     High 100%
KAVL       High 100%     High 100%     High 100%     High 100%
KHKY       High 100%     High 100%     High  85%     High 100%
KGMU       High 100%     High 100%     High  99%     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...PM/TDP
SHORT TERM...JAT
LONG TERM...JAT
AVIATION...PM


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