Area Forecast Discussion
Issued by NWS Greer, SC

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000
FXUS62 KGSP 271903
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
303 PM EDT Sun May 27 2018

.SYNOPSIS...
Deep moisture will persist over the area through Memorial Day as
tropical cyclone Alberto moves inland along the gulf coast. Abundant
amounts of moisture will remain in place through the week as the
remnants of Alberto move north through Alabama and then over middle
Tennessee. More seasonal and drier air will mix in from the west
over the weekend.

&&

.NEAR TERM /THROUGH MONDAY/...
As of 245 PM: Scattered showers and a few thunderstorms continue
to develop over the mtns where thinner high overcast allowed
better heating earlier in the day. Meanwhile...thicker cloud cover
was gradually spreading northward from the outer reaches of the
circulation around Alberto. Think the CAMs were onto something
when they suggested a relatively quiet day convectively across the
area to the east of the mtns because radar trends are bearing that
out. Will keep the higher precip probs over the higher terrain and
will follow the CAMs in delaying the more meaningful chances over
the Piedmont/Upstate.

The rest of the fcst naturally follows in line with the model/NHC
consensus. After a relative lull during the late evening hours,
our attention will turn toward the arrival of the deeper plume of
tropical moisture fcst to spread up from the S and into the region
around daybreak. Model guidance has shown some variations on the
focus for the location of heavier precip as the coverage increases
around daybreak. Some of the guidance sources focus on the upslope
areas near/over the Blue Ridge, while some of the guidance seems to
zero in on the CLT metro area/wrn Piedmont. Either way, our threat
for flooding rain goes up around daybreak over some part of the fcst
area. First main concern will be the Blue Ridge because of recent
rainfall and the possibility of slope failures if we get excessive
rain with embedded higher rates. Have issued the Landslide PNS to
raise awareness in light of recent landslide activity from earlier
rains in the past week. Second main concern will be the possibility
of urban flooding from CLT toward the Piedmont Triad Monday morning
if the better activity is directed there. Will expand the Flash
Flood Watch over the NW Piedmont to take this into account. See
the hydrology section for more information. Needless to say, precip
prob ramps up to likely/categorical across the entire region Monday
morning and stays there until midday, with a decreasing trend in
the afternoon. Another concern will be severe weather. If we move
out the numerous/widespread precip coverage in the afternoon,
we could get enough breaks in the cloud cover to destabilize to
the tune of 1000/1500 J/kg of sbCAPE, which the models indicate
might happen to the S/E of I-85, while low level shear begins
to increase from the S. There is the concern that some of the
stronger storms with some tornado potential could make a run at
the southern zones in the late afternoon/early evening before
the boundary layer stabilizes. Temps will remain seasonally cool
because of the morning precip and the extensive clouds.

&&

.SHORT TERM /MONDAY NIGHT THROUGH WEDNESDAY/...
As of 2pm EDT Sunday:  Forecast is little changed with periods of
heavy rain expected Tuesday and Wednesday.  Subtropical Storm
Alberto tracks through AL and GA and then northeastward, well
outside of the GSP area.  While there will be minimal wind effects
from Alberto in this area, the GSP area will experience moist
southerly flow on the east side of the subtropical system, leading
to periods of moderate to heavy rain showers, with 2 to 3 inches of
rain expected in the Tuesday-Wednesday part of the forecast alone.
The outer circulation of Alberto will give southerly to
southeasterly surface winds, which will enhance precipitation amounts
over the Southern Appalachians, where flooding and debris flow issues
will be most acute.

While excessive rainfall will be the greatest concern, some elevated
low-level vertical wind shear, along with some CAPE in piedmont
areas, give some possibility for severe weather Monday night,
especially an isolated and relatively weak tropical tornado.

Current flash flood watch looks good, and may well end-up being
extended in time as hydrology issues will continue into midweek at
least.

The temperature range will be subdued due to cloudy and moist
conditions, with lows around 10 degrees above normal and highs near
normal.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/...
As of 140 PM Sunday...The remnants of TC Alberto will continue to
get pulled north Thu as it becomes more under the influence of the
westerlies and a developing nrn stream trof. The FA will remain in a
general mean trof...however...thru the period as the combined/hybrid
h5 low deepens and drops into the Carolinas and NE GA Sun.
Meanwhile...the sfc flow will remain quite moist Thu/Fri and PWATS
remaining above 1.5 inches. There will be high theta/e adv per h85/h92
streamline analysis all the while the atmos remains marginally
unstable. Thus...will expect decent coverage of showers will some
thunder Thu continuing into the overnight. The srn escarpment will
once again be the focusing zone for higher precip totals...so with
high antecedent conds flooding concerns will continue.

The flow becomes a little more continental during the day Fri so
believe the better chance precip will be confined moreso across the
NC/GA mtns with isol flooding rain possible yet again. On Sat...some
drier air will work in as the h5 trof axis shifts east a bit and
PWATS will lower below 1 inch...so PoPs will be lowered into the
slight to low chance range with less of a flooding threat. By Sun
the llvl cP airmass mixes in better and sfc td/s drop by 5 degrees F
or so...thus will keep slight to low-end chance -shra PoPs mainly
due to ulvl forcing. Sun shud be an overall pleasant day and it will
feel less muggy than the past couple of weeks. Max temps will begin
the period arnd normal with high thick cloud cover. On Fri/Sat...max
temps will reach abt 5 degrees abv normal and then back to near
normal levels Sun with the anticipated airmass mix.

&&

.AVIATION /19Z SUNDAY THROUGH FRIDAY/...
At KCLT and elsewhere: Lower cloud deck finally scattering out
across most of the region with higher clouds spreading overhead
from the upper level exhaust of Alberto, so think most locations
will be in and out of VFR/MVFR through the afternoon outside of
deep convection. Wind will be light S/SSW. Most places will get a
VCSH this afternoon, but CAM guidance not keen on much coverage,
nor indicative of thunderstorms. This may improve during the evening
hours, so the PROB30 has been pushed back until late evening, when
it is used as a ramp up to the increasing threat/likelihood of a
band of more numerous showers expected to arrive around the start
of morning ops Monday along with the deep tropical moisture. That
said, most locations will stay in/out of MVFR/low VFR ceiling thru
the early morning hours, then restrictions arrive in the 09Z-12Z
time frame. Details were omitted after 12Z Monday, but suffice to
say that numerous showers and a few thunderstorms will spread up
from the south on the outer periphery of the circulation around
Alberto. Restrictions...at least MVFR...are expected...and likely
this will take the form of an IFR ceiling and MVFR visibility. Once
established, we will keep the restrictions through the rest of
the TAF period with an increasing S/SE wind.

Outlook: Tropical moisture will linger through at least
mid-week. Showers and embedded thunderstorms will become
widespread and increase restrictions across the region Monday
through Wednesday. Overall improvement is not expected until
Thursday, perhaps.

Confidence Table...

            19-01Z        01-07Z        07-13Z        13-18Z
KCLT       High 100%     High  98%     Med   71%     High  87%
KGSP       High  85%     Med   76%     Med   76%     Med   76%
KAVL       Low   57%     Med   64%     High  85%     High  92%
KHKY       Low   56%     Med   78%     High  82%     High  85%
KGMU       High  83%     Med   67%     High  89%     High  87%
KAND       High  91%     High  81%     Med   78%     High  87%

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

&&

.HYDROLOGY...
Heavy rainfall over the past 7 to 10 days across much of the
forecast area, particularly the mountains, has set the stage for
serious flooding and landslide issues to possibly develop with any
additional tropical rainfall through mid-week. Numerical model
profiles exhibiting precipitable water values over two inches and
deep warm cloud layers in excess of 12 kft will make for highly
efficient heavy rainfall processes and further increase the threat
of flash flooding and river flooding. The latest 3/6 hour FFG
guidance values are generally running 2.5 to 3.5 inches and these
will be quite achievable in the expected waves of heavy tropical
rain showers.

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...Flash Flood Watch from 8 PM EDT this evening through Tuesday
     morning for GAZ010-017-018-026-028-029.
NC...Flash Flood Watch from 8 PM EDT this evening through Tuesday
     morning for NCZ033-048>053-058-059-062>065-068>072-082-
     501>510.
     Flash Flood Watch from 2 AM EDT Monday through Tuesday morning
     for NCZ035>037-056-057.
SC...Flash Flood Watch from 8 PM EDT this evening through Tuesday
     morning for SCZ001>014-019.

&&

$$
SYNOPSIS...SBK
NEAR TERM...PM
SHORT TERM...WJM
LONG TERM...SBK
AVIATION...PM
HYDROLOGY...SBK



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