Area Forecast Discussion
Issued by NWS Greer, SC

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683
FXUS62 KGSP 251107
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
707 AM EDT Fri May 25 2018

.SYNOPSIS...
Unsettled weather will persist over the next week. Expect
mainly afternoon and evening showers and thunderstorms today and
Saturday. As low pressure develops over the Gulf of Mexico, deep
tropical moisture will then overspread the area the latter half of
the weekend and persist into next week, bringing potentially heavy
rainfall for the Memorial Day weekend and beyond.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 7 AM: Expect surface high pressure center over the North
Carolina outer banks early this morning to slip southward through
the day, with a very weak remnant surface boundary remaining in
place across the western Carolinas. Weak vorticity lobes aloft will
continue to move northward across the region today to provide some
modest forcing, while low-level southeast winds early today will
veer slightly to the southwest through tonight - keeping upslope
forcing going along the southern escarpment areas. Although profiles
appear a bit too warm and moist for much of a severe thunderstorm
chance, very deep warm cloud layers of 10 to 12 kft along with
precipitable water values two standard deviations or more above
climatology will keep a localized heavy rain/flooding threat going
this afternoon and evening during peak heating. Anticipate some
diminution in coverage tonight, but will keep PoPs highest along the
eastern and southern upslope areas. Maximum temperatures this
afternoon will be near climo, with min temps tonight remaining about
10 degrees above climo.

&&

.SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
As of 330 AM EDT Friday: A meandering upper low that was able to
propagate from around GA up into the western Carolinas given the
breaking down of the upper ridge across the Great Lakes will
continue to bring isolated to scattered showers and thunderstorms at
the beginning of the short term forecast period which begins
Saturday morning. Per latest guidance, it`s suggested that the upper
ridge will return Saturday night, bringing a lull in precip coverage
across the Upstate and NW Piedmont Saturday night into early Sunday
morning, as showers and isolated thunderstorms continue across the
upslope areas. On Sunday, attention turns towards the Gulf of Mexico
as both the ECMWF, GFS, and other models continue to develop a
tropical/subtropical low pressure system, which is highlighted in
the latest Tropical Weather Outlook from NHC, noting that the
formation chance through 48 hours is 70% and through 5 days is 90%
(as of 105 AM EDT Friday). As noted with the previous forecast, it`s
important to keep in mind that all models are tracking a system
which has yet to evolve yet, so nothing it set in stone. As of now,
the system isn`t progged to move onshore until Monday night, which
will be discussed in the extended forecast period discussion below.
However, general consensus between the guidance continues to suggest
plenty of moisture will be infiltrated into the FA Sunday night,
with PWATs 2 standard deviations above climatology (1.5 to 2
inches). With this deep moisture plume set for northeast GA and the
Carolinas, we could see QPF up to near one inch to even higher in
some areas in total Sunday morning through Monday morning. Given
recent rainfall, it will not take much rain at all to cause (flash)
flooding issues if these trends continue. With this forecast update,
have held off on any issuance of a Flash Flood Watch, but will
likely be issued within the next run or two, if trends continue.
Temperatures this weekend will settle just below normal.

&&

.LONG TERM /MONDAY THROUGH THURSDAY/...
As of 400 AM EDT Friday: With ongoing showers and isolated
thunderstorms expected, the medium range forecast period picks up
Monday morning, where we`re continuing to discuss the future track
of the tropical/subtropical low out of the Gulf of Mexico and it`s
affect that it`ll have across our area through the work week.
Despite the specific track, the weather pattern looks to remain
unsettled through the period with plenty of cloud cover and isolated
to scattered showers and thunderstorms. Per latest guidance, some of
the heaviest precip is expected to fall overnight Sunday into Monday
morning. As mentioned above, both the ECMWF and GFS continue to
slowly come into better agreement regarding the timing of the
tropical/subtropical low moving onshore, but it`s track not as much.
With the latest model runs, the ECMWF depicts the system moving
onshore around Mobile AL Sunday night where it slowly meanders up
through the TN Valley before propagating into the Northeast by the
end of next week. The GFS remains just a tad faster with the
progression of the system, bringing it into the panhandle of FL
Sunday evening, where it then meanders up through the FL panhandle
into the Carolinas where it potentially redevelops along the
Carolina coast. The overall track will be important for the amount
of precip that can be expected across our area, but even then, it`s
slow progression given a developing Omega/Rex Block across the CONUS
will allow for the continuous plume of moisture into the area.
Looking at QPF amounts from the near term forecast period through
the end of the medium range, upslope areas could see up to 5 inches
of precip with 2 to 4 inches elsewhere - though keep in mind, any
change in the pattern could easily change these amounts. Again, with
recent rainfall, it won`t take much precipitation to create flooding
issues.

&&

.AVIATION /11Z FRIDAY THROUGH TUESDAY/...
At KCLT and elsewhere: Scattered to numerous showers in the
deeper moisture over the southern part of the terminal area will
impact mainly the Upstate sites through the morning hours, with more
isolated to scattered activity across the NC sites - including CLT.
Mainly MVFR stratus, with tempo IFR, is expected throughout this
morning, with a slow improvement in the ceilings through MVFR
through late morning. Anticipate lower VFR cigs for the bulk of the
afternoon hours. Instability during the peak heating period should
permit at least scattered thunderstorms throughout, and this will be
handled via PROB30 or TEMPO at all sites for the late afternoon
hours. Coverage should then dwindle again to mainly scattered
showers for tonight.

Outlook: The unsettled pattern will continue through the period.
Flight restrictions will be possible again on Saturday, with morning
stratus and fog in the moist airmass and diurnal showers and
thunderstorms in the afternoon/evening. Tropical moisture will
increase by the late weekend, further increasing the frequency and
intensity of SHRA and associated restrictions. Morning stratus/fog
are possible each day, especially following heavy rain the previous
day.

Confidence Table...

            11-17Z        17-23Z        23-05Z        05-06Z
KCLT       High  81%     High  94%     High 100%     High 100%
KGSP       High  94%     High  83%     High  94%     High  85%
KAVL       High  81%     High  83%     High  87%     Med   78%
KHKY       Med   75%     Med   78%     High 100%     Med   78%
KGMU       High  94%     High  83%     High  87%     High  85%
KAND       Med   75%     High  89%     High  81%     Med   64%

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...HG
NEAR TERM...HG
SHORT TERM...SGL
LONG TERM...Carroll/SGL
AVIATION...HG



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