Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 261853
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
253 PM EDT Sat May 26 2018

.SYNOPSIS...
Very moist conditions will persist over the region through
the holiday weekend as Alberto develops across the Gulf of
Mexico. Tropical moisture will likely affect our area through the
middle of next week, bringing the potential for continued heavy
rainfall and associated flooding to the region. A pattern change
might be in store for late next week.

&&

.NEAR TERM /THROUGH SUNDAY/...
As of 220 pm: Coverage and intensity of showers and storms continues
to steadily increase this afternoon within modestly unstable/high PWAT
environment. The most concentrated activity is currently located
across the northern mtns and foothills of NC, which is the remnants
of some morning convection that moved across the TN Valley/southern
Appalachians. Meanwhile, additional convection continues to fire
along the the ridge tops of the southern/central NC mtns. Likely
pops still appear warranted for the mtns and much of the NC
foothills this afternoon. It still appears that the atmosphere will
become weakly suppressed across the Piedmont into the evening as
upper ridging builds in from the south, while steering flow will
tend to take cells E/NE away from the I-85 corridor and points
south. Seeing anything more than spotty activity in these areas will
hinge upon outflow boundaries from the high terrain activity
meandering southeast into the evening. Weak downdraft CAPE, high
freezing levels and modest CAPE will limit the severe convective
potential, so heavy/perhaps locally excessive rainfall will continue
to be the main concern, especially if areas that have seen heavy
rain over the past 12-24 hours see a repeat this PM.

Convection will gradually diminish into the evening, with another
round of low stratus and patchy fog likely early Sunday. Min temps
will again be 5-10 degrees above climo.

It`s looking increasingly likely that tomorrow will be rather
inactive convectively (at least in comparison with recent days), as
weak/deep layer ridging will be in place. In fact, short term
guidance is in rather good agreement in wrapping more stable/lower
PWAT air into the forecast area (at least across the southeast half
or so) before the deeper moisture associated with Alberto begins
pushing into the area Sunday evening/night. As such, most of the
guidance has little to no qpf response east of the Blue Ridge
tomorrow afternoon. Pops have therefore been trended down ever so
slightly: ranging from 30-40 across much of the Piedmont/foothills,
to 50-60 across the high terrain. Max temps will be very close to
climo.

&&

.SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY/...
As of 230 PM EDT Saturday: At the start of the period the western
Atlantic high pressure system is locked in across our forecast area
(FA). However, conditions will be changing as Alberto makes a move
north/northwest in the Gulf of Mexico. This pushes the western
Atlantic high to the east and south of the CONUS. More importantly
this allows the flow of deep tropical moisture to make a move toward
our region.

There should be an initial band of precipitation, on the outer
fringes of Alberto, making a run toward our FA late Sunday night or
early Monday morning. The timing is critical, with the GFS on the
fast side, and the new European ( just arriving ) now a little
faster. Meanwhile the NAM and Canadian models not bringing the first
slug of rain until around Monday morning or so. No doubt there
should be heavy rain with PW`s running in the 1.5 to 2.0 inch range.

We plan to begin ramping up POPS from south to north late Sunday
night into Monday. Moisture will be plentiful with a diffluent
pattern aloft, therefore clusters of showers and thunderstorms will
continue through a good part of Monday. We have noticed a
bifurcation, or fork, developing in the precipitation swath, with
one area shifting to the east by Monday evening, and the start of an
upslope set-up for our western FA:  including the northeast Georgia,
western upstate of SC and the western mountains of NC.

This appears then to be a focus for late Monday night into at least
Tuesday. Very complicated during this period, as transitory issues
begin to work their way into the equation. Precipitable water values
remain elevated right into Tuesday and perhaps beyond. Overall we
may have multiple areas with water issues, shifting around our area.

As far as a potential issuance of a Flash Flood Watch, we have done
extensive coordination with our surrounding offices: CAE, RAH, FFC,
and our National Center (WPC) for details and support. At this point
we would be looking at a late 3rd period / or 4th period / watch...
then going forward in time. We feel we have time for another model
cycle to provide additional details, so timing and placement could
be realized.

Once again this soupy airmass will not allow for a large diurnal
swing in temperatures during this part of the forecast cycle.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
As of 235 PM EDT Saturday: Wetness continues thru the middle of
the week, but relief might be on the way. Our attention will
drawn to the passage of the remnant low of Alberto on Tuesday
night and Wednesday. The 12Z operational runs of the GFS and ECMWF
are in very good agreement with the track of the remnant low, and
maybe about 6 hrs faster than WPC, but on the same course. This
trend suggests an increase in confidence in the overall scenario,
but the details will remain murky as the track so far to the west
might just keep the bands of heavier rain to the west as well. As
the remnant low passes to the west on its northward track Tuesday
night, we should see the low level flow gradually veer around to
a direction more west of south. As a result, the focus for heavier
rain potential will shift from the Blue Ridge Escarpment toward the
Nantahala and Great Smoky Mtns heading into Wednesday. The location
of a deep moisture plume is uncertain and will affect who has the
higher potential for significant rain, while other locations will
have a diminished threat if a dry slot wraps around and into the
region from the SW. For the time being, the entire fcst area will
be kept in the likely category for rainfall, but eventually parts
of the region will probably need a categorical probability, with
the best potential being over the SW part of the fcst area. We
remain in the cyclonic flow aloft around the remnant low as it
lifts N/NE across the OH Valley/Midwest/Gt Lakes thru Thursday,
with an abundance of low level moisture, which suggests keeping
the precip prob in the likely range. An axis of vorticity is shown
moving across the region late Thursday which should be the beginning
of the end for our recent unpleasantness. After this feature moves
through, followed by the main upper trof axis Thursday night, we
should finally, at long last, get a change of air mass to which
we have been looking forward. The late part of the week should see
the development of a NW flow aloft in a relatively high amplitude
upper pattern, between the old upper trof moving off the east
coast and an upper ridge over the Plains. Friday may still be
fairly active over the mtns because of residual moisture in the
TN/OH valley moving in from the NW, but the developing downslope
east of the mtns should keep the precip chances in check east of
the mtns, with a return to a stronger modulation by the diurnal
cycle. On Saturday... gasp...we may even start to dry out as the
model guidance shows the development of an omega block with an
upper low forming over the nrn Mid-Atlantic, keeping us in the
cooler and drier part of that pattern.

&&

.AVIATION /19Z SATURDAY THROUGH THURSDAY/...
At KCLT and elsewhere: Significant convective coverage is currently
confined to the mountains and portions of the foothills. Tempos for
TSRA are warranted at KHKY and KAVL through 22/23Z, with VCTS and/or
prob30s warranted through the evening. Activity is currently more
spotty across the rest of the area, but at least VCSH/VCTS are
warranted at KCLT and the Upstate SC terminals, with a tempo still
carried at KCLT from 19-23Z. Convection should gradually diminish
through the evening. Otherwise, another round of low stratus and/or
fog is forecast to develop toward daybreak, then quickly improve
within a couple of hours after sunrise. Sunday afternoon convective
chances appear lower than in recent days, although at least isolated
coverage can still be expected.

Outlook: Tropical moisture will increase by Sunday night into
Memorial Day, increasing the frequency and intensity of SHRA and
associated restrictions through much of next week. Morning
stratus/fog are possible each day, especially following any heavy
rainfall the previous day.

Confidence Table...

            19-01Z        01-07Z        07-13Z        13-18Z
KCLT       High  91%     High 100%     Med   66%     Med   75%
KGSP       High 100%     High 100%     Med   78%     High  81%
KAVL       High 100%     High  98%     Low   58%     Med   73%
KHKY       High  92%     High 100%     Med   66%     High  81%
KGMU       High 100%     High 100%     High  87%     High  80%
KAND       High 100%     High 100%     High  86%     High  81%

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...PM
NEAR TERM...JDL
SHORT TERM...TS
LONG TERM...PM
AVIATION...JDL


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