Area Forecast Discussion
Issued by NWS Greer, SC

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058
FXUS62 KGSP 242035
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
435 PM EDT Thu May 24 2018

.SYNOPSIS...
Unsettled weather will persist over the next week, with chances for
showers and thunderstorms each afternoon. As low pressure develops
over the Gulf of Mexico, deep tropical moisture will overspread the
area over the weekend and persist into next week, bringing
potentially heavy rainfall for Memorial Day weekend and beyond.

&&

.NEAR TERM /THROUGH FRIDAY/...
As of 430 PM: Most of the CWFA remains under a low-level easterly
flow regime fostered by high pressure centered over the Virginias.
Instability is increasing over the mountains, NE GA, and the upstate
as the low clouds scatter out. Instability is weaker over the NC
Foothills and Piedmont where clouds are taking longer to scatter.
That said, they may scatter soon enough for instability to increase,
especially over the Southern Foothills and Piedmont. DCAPE values
are quite high, but the most robust storms are over the Lakelands.
Can`t rule out a strong storm over this area or the Southwestern
mountains. However, heavy rainfall continues to be the main threat
with very high PW values and storms exhibiting only a slow eastward
drift. Training of echoes and/or anchoring of cells along higher
terrain could create enough rainfall for flooding to develop.

Tonight, the upper pattern progresses slightly and the sfc high
to our north will begin to shift off the East Coast. In response
low-level flow will veer, returning us to a more typical regime,
albeit still resulting in terrain-induced lift. Above the boundary
layer, thermodynamic profiles essentially do not change, and with
temps/dewpoints remaining elevated, sfc-based (or near-sfc based)
instability will linger and the aforementioned mechanical lift may
be enough to maintain some convection thru the night. Forcing may
also be enhanced by a lobe of vorticity rotating around the weak
upper low centered over Georgia. Thus, we will retain a high chance
to likely range PoP over the western half of the area, though the
focus shifts back to the south-facing Escarpment. Any cells that
develop will still be possible of intense rainfall rates and will
move very slowly...so the heavy rain remains our biggest concern.

Realistically there is little change in the expectations for
tomorrow versus today. Though the flow will be southerly and not
easterly, lapse rates remain mediocre, and we will again deal with
low clouds that will have to dissipate during the morning, before
we can really destabilize. PWATs are expected to be higher overall,
given that we will see less influence from the continental high
currently to our north. Heavy rain may be slightly easier to come
by as a result.

&&

.SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY/...
As of 230 PM EDT Thursday: In this part of the forecast we start
with trouble. The upper level ridge from the eastern Great Lakes to
the Caribbean either breaks down or shifts slightly east. This
allows the old upper level low pressure system in GA to shift
northeast into our forecast area (FA) Friday night. This feature has
had a history of nocturnal heavy rain. Indeed all guidance is
suggestive that Friday night may become active with a locally heavy
rainfall potential. Precipitable waters rise to between 1.7 and 1.9
inches, which is about the 90th percentile. Very soupy. We
entertained a Flash Flood Watch for Friday night, but will wait one
more cycle before pulling that trigger. Overall the model QPF does
not appear drastic, however other players at work may have this
underdone. We would like to see an uptick before moving forward, and
we have a bit more time.

As the upper level low moves northeast we remain in the soupy like
atmosphere Saturday. Meanwhile the ridge from the Caribbean starts
to reestablish itself back to the west, into our FA. Nevertheless,
an unstable atmosphere, meso scale boundaries and some mid level
support, should lead to scattered or numerous showers and
thunderstorms. Heavy rain possible.

Somewhat of a lull (relatively speaking ) for Saturday night as
upper level ridging, and nearby confluence zone, may allow for
lowering POPS until better forcing arrives.

As precipitable water values Sunday shift into the 1.5 to 2.00 inch
range, and the initial arrival of some forcing ( outer banding )
from the system in the Gulf of Mexico, it would be seem prudent to
ramp POPS back up.

Temperatures should exhibit a minimum diurnal trend in this type of
atmospheric structure.

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
As of 230 PM EDT Thursday: The medium range forecast is made very
interesting by the presence of what is looking more and more likely
to be a tropical/subtropical low pressure system moving inland from
the Gulf of Mexico. Guidance is coming into better agreement on the
track of the system, though a word of of warning...all models are
being initialized on a system that has not yet been able to form
even a weak closed center of circulation. This renders any model
output on the track/intensity of the system especially uncertain.
Going off what we have to work with, though, it appears that this
very wet low pressure system will be steered northward into the
central Gulf coast between upper subtropical ridges to its east and
west, transporting with it a plume of deep tropical moisture that
will swing northward towards the Carolinas on Monday. This will be
aided by southerly flow off the Atlantic induced by the surface
Bermuda high, resulting in PW values that well exceed the 90th
percentile in the CFSR climatology across the entire area, with some
zones approaching record max values Monday and Tuesday.

To further complicate matters, a sideways omega-type blocking
pattern will persist across the central CONUS in the upper levels,
morphing into more of a Rex block by Tuesday. The tropical low,
which will likely be inland over the Gulf states by this point, will
therefore make little progress through the southeastern CONUS
through Wednesday until an upper trough is able to absorb it on
Thursday. This means that there will be a relatively uninterrupted
flow of deep tropical moisture into our area through Thursday before
the upper trough is able to hopefully clear the system remnants from
our area. At this point, it should be noted that regardless of
development of this system, "spaghetti model" type plots and others
that emphasize just the track of the system`s center will do a very
poor job of demonstrating where the heavy rain threat will occur.
With the likely asymmetrical nature of the system after it moves
inland early next week, it looks increaingly likely that very heavy
rainfall will occur over portions of our area at times despite the
center of the system remaining hundereds of miles to our southeast.
Wind impacts in our area seem unlikely at this point with the heavy
rainfall threat the most concerning potential impact, especially
along the Blue Ridge zones. It is currently almost impossible to pin
down an exact number as far as QPF goes through the next week (we
are currently forecasting 3-5" across the area, though that will
likely change going forward as more banding features may develop),
though with the heavy rainfall we`ve received over the previous week
or so creating very wet antecedent conditions, interests across the
entire area are advised to stay tuned to forecast updates.

&&

.AVIATION /21Z THURSDAY THROUGH TUESDAY/...
At KCLT and elsewhere: Low clouds slowly scattering out with
instability increasing. That said, most of the sfc instability is
south of I-85 attm, with isolated SHRA ongoing at the start of the
TAF cycle. The extant clouds will tend to lift and scatter somewhat,
though bases will remain near the VFR threshold. All sites have a
chance of precip by mid-aftn. As low-level flow turns southerly
tonight, and possibly with the aid of a weak upper low drifting
north over GA, precip chances will persist particularly south of the
NC mountains. PROB30s have been used in some cases to handle this
ongoing chance. Heavy rain rates and vsby restrictions are likely
during any SHRA/TSRA. With sfc moisture so plentiful it looks likely
widespread IFR cigs will develop overnight, with some fog especially
where rain occurs this aftn and evening. After slow improvement to
MVFR or low VFR in late morning, convection should redevelop
diurnally Friday.

Outlook: The unsettled pattern will continue into next week,
with flight restrictions possible each day under diurnal showers
and thunderstorms. Tropical moisture will increase by the late
weekend, further increasing frequency/intensity of SHRA. Morning
stratus/fog are possible each day, especially following heavy rain
the previous day.

Confidence Table...

            20-02Z        02-08Z        08-14Z        14-18Z
KCLT       High 100%     High 100%     Med   72%     Med   74%
KGSP       Med   63%     Med   62%     Med   75%     High  88%
KAVL       High  89%     Med   75%     Med   70%     High  94%
KHKY       High  95%     High  82%     Med   64%     Med   78%
KGMU       Med   70%     Med   70%     Med   72%     High  88%
KAND       High  94%     High  83%     Med   70%     High  84%

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...Carroll
NEAR TERM...RWH/Wimberley
SHORT TERM...TS
LONG TERM...Carroll
AVIATION...RWH/Wimberley



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