Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 250226

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1026 PM EDT Thu May 24 2018

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.


As of 1025 PM: Instability is diminishing and cloud tops are
cooling indicating weakening thunderstorms across the area. Showers
linger across the mountains, behind the outflow boundary moving
north across the NC Foothills and Piedmont, and along a weak
boundary moving west across the Eastern Upstate. Can`t rule out a
rumble of thunder overnight, but most of the convection should
remain as showers. Don`t expect any additional heavy rainfall but
moderate amounts are possible in the heavier showers. Winds along
the gust front continue to weaken as well, but low end gusts remain
possible until it dissipates or moves north of the area. Otherwise,
going forecast is on track with developing low clouds and patchy fog
and lows 5 to 10 degrees above normal.

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 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.


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.


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.


At KCLT and elsewhere: Line of showers and scattered thunderstorms
is drifting WNW across the area. The SC sites look to be the most
likely to see TSRA, so have included a TEMPO there. KCLT the next
likely, but winds likely not as strong there. Have a TEMPO there but
without the gusty winds. Convection least likely at KAVL/KHKY but
SHRA will likely develop overnight in the moist southeasterly flow.
Expect MVFR then IFR cigs overnight in that moist flow, with MVFR
vsby. IFR vsby possible if heavier showers develop. Vsby
restrictions end by mid morning MVFR cigs linger into early
afternoon. Expect another round of showers and thunderstorms, so
PROB30s are in place. Showers and low VFR will linger into Friday
evening at KCLT with restrictions possible late in the period. Winds
already ESE at KCLT and they will continue to slowly move through SE
to S through the period. Elsewhere outside of the mountains NE wind
becomes SE after daybreak and SSE during the afternoon. KAVL sees S
to SSE wind through the period.

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...

            02-08Z        08-14Z        14-20Z        20-00Z
KCLT       High  90%     High  90%     High  84%     High 100%
KGSP       Med   78%     Med   75%     High  80%     High 100%
KAVL       High  83%     High  92%     High  89%     High  92%
KHKY       High  88%     Low   58%     Med   77%     High 100%
KGMU       High  80%     Med   77%     High  82%     High 100%
KAND       High  85%     High  89%     Med   77%     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:




NEAR TERM...RWH/Wimberley
LONG TERM...Carroll
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