Area Forecast Discussion
Issued by NWS Greer, SC

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000
FXUS62 KGSP 221523
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1123 AM EDT Sun Apr 22 2018

.SYNOPSIS...
Dry high pressure over the eastern United States today will
gradually give way to abundant moisture arriving with a low pressure
system from the west tonight through Monday. Heavy rainfall ahead of
the low will likely continue through Monday night before tapering to
showers and a few thunderstorms Tuesday into Tuesday night. The mid
to late week period may remain unsettled as additional cold fronts
and upper level disturbances move through the region.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 1100 AM Sunday: Forecast is still generally on track. Dew
points and sky cover were updated to account for earlier moistening
and faster filling in of sky cover, especially across the western
parts half of the FA. QPF forecast still in good shape based on
latest guidance, and the Flood Watch that will go into effect later
tonight still looks well-placed.

With sfc high pressure still to the ENE of the area, low level
moisture continues to rapidly infiltrate into the Southeast from
both the GOM and Atlantic as increasing mid to high level clouds
stream overhead, ahead of the approaching system progged to bring
heavy rainfall to the area beginning later tonight. Per latest
guidance, the upper low moving eastward from the southern plains
into the lower MS Valley throughout the day will continue to become
better organized as it and it`s parent sfc low pressure system
approach northeast GA and the Carolinas late tonight. At the same
time, the aforementioned sfc high pressure is beginning to build
back along the southern Appalachians allowing for a cold air damming
event to evolve.

As the atmospheric column continues to saturate, mid to high level
clouds will continue to become more opaque and widespread in nature
today (from west to east) as the nearly cloud-less skies we observed
this past Friday and Saturday become only but a distant memory for
the next several days. Some radar returns are already apparent in
the best upslope areas of the southern NC/SC/GA mountains, and
rainfall will gradually expand in coverage as well, though
precipitation will likely not reach the Charlotte-Metro area until
later tonight. Did slightly delay PoP onset in the eastern zones
this evening into tonight while keeping the higher higher QPF
amounts confined to the upslope areas overnight - with 1 inch to 1.5
inches of rain possible in these areas. Heavy rainfall and runoff
soon thereafter will lead to the beginning of the flooding potential
that will spill over into the short term forecast period, thus
warranting the issuance of the Flood Watch for portions of the area.

High temperatures will remain slightly below normal today, with mid
to upper 60s across the Upstate and NW Piedmont, cooler into the
40s/50s across the mountains. Overnight lows will be around normal.

&&

.SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/...
As of 325 AM Sunday: The timing of heavy rainfall and associated
flood problems appears to be coming a bit more into focus early this
morning. The numerical models continue to feature excellent
agreement in locating the closed, strong 500 mb low center over the
TN/MS/AL border region around daybreak on Monday. The band of
deepest moisture and forcing east of this feature should pivot
northeast across the forecast area on Monday with the associated
heaviest rainfall rates of the event. Meanwhile, hybrid or in-situ
cold air damming will continue across the region, which will promote
continued upglide forcing as the southeasterly low level jet
continues at 40 to 50 kt above the wedge layer. The east coast upper
blocking ridge will permit only slow eastward movement of the system
toward the Appalachians through Monday night, with the system
finally phasing and lifting over the central Appalachians Tuesday
through Tuesday night. Some mid level drying will wrap in from the
southwest on Monday night, but strong upglide and upslope forcing
with the robust southeasterly low level flow will continue to wring
out rainfall. All told, the storm total QPF will now feature some 3
to 4 inches across the foothills and the eastern upslope areas, with
localized 4 to 6 inches possible in the most upslope preferred
locations. This will warrant posting a Flood Watch for the easterly
upslope mountain areas and all of the foothills - which will start
tonight and last at least until Tuesday morning.

As the phasing trough lifts northeast, the arrival of the upper
trough axis will steepen lapse rates for Tuesday afternoon. In
addition, while the main surface low moves north across eastern KY,
a secondary surface low will likely transition through our lower
piedmont - possibly helping to move the wedge front northwest. These
features may conspire to increase thunder chances across our
southeast sections Tuesday afternoon. Fortunately, shear values will
decrease by then. Shower chances will continue through Tuesday
night, mainly for the NC mountains in NW flow moisture. Temperatures
should recover a few categories Tuesday over the chilly wedge
temperatures on Monday.

&&

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
As of 330 AM Sunday: The mid to late week period may remain a bit
unsettled as a sequence of upper waves and associated frontal
systems cross the eastern U.S. Shortwave energy will likely dig into
the mean eastern trough on Wednesday, but with limited model
agreement on the location and timing of the next forcing feature. At
least scattered PoPs will be required with a frontal zone arriving
from the northwest sometime in the late Wednesday to Wednesday night
period. Max temps will warm to the warmest values of the week ahead
of this front.

More prominent height falls appear slated for late week into the
weekend, but with the GFS/GEFS remaining faster/farther east than
the ECMWF camp. Will feature a consensus blend that places higher
shower PoPs over mountain locations than the southeast piedmont
through the period. Conditions should dry out with deep layer
northwest flow in place by at least the middle of next weekend.

&&

.AVIATION /16Z SUNDAY THROUGH THURSDAY/...
At KCLT and elsewhere: VFR still hanging on across the area this
morning with mid to high level clouds streaming in overhead and
gradually becoming more opaque in nature. With the exception of KAND
where MVFR cigs and -SHRA could move in as early as 15Z (initially
as -DZ/-RA), expect VFR cigs to become widespread this morning with
-SHRA and the deterioration to MVFR cigs becoming widespread by
later this afternoon/evening. Once conditions deteriorate,
anticipate MVFR/IFR cigs will persist through the end of the valid
TAF period, with ongoing -SHRA. Winds are continuing to increase
gradually to around 10kt at most sites, becoming NE at
KGSP/KGMU/KAND due to developing cold air damming, and remaining ESE
at the other TAF sites. Intermittent gusts up to 20 kts could not be
entirely ruled out this evening.

Outlook: Conditions will deteriorate tonight with widespread rain
overspreading the area. IFR/MVFR conditions will be likely thru the
day on Monday, with some improvement on Tuesday, as precip largely
shifts NE of the area. Some moisture will linger, however, and
unsettled weather will continue on Wednesday and Thursday as a
trough of low pressure persists across the region.

Confidence Table...

            15-21Z        21-03Z        03-09Z        09-12Z
KCLT       High 100%     High 100%     High  96%     High  81%
KGSP       High  96%     High  85%     Med   63%     High  94%
KAVL       Med   68%     High  91%     Med   78%     High 100%
KHKY       High  98%     High 100%     Med   70%     High  87%
KGMU       High  95%     Med   78%     Med   63%     High  89%
KAND       High  96%     Med   72%     Med   71%     High  94%

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...
Storm total QPF values have been increased slightly for the upcoming
event. Although rates do not appear sufficient in any 3 to 6 hour
windows to produce flash flooding, duration flooding is looking more
likely - especially in the most upslope preferred locations and over
the adjacent foothills. Storm totals of 3 to 4 inches are now
expected, with localized 4 to 6 inch amounts in the most upslope
preferred areas.

Flooding of the most common flood-prone and low-lying areas may
steadily develop from the southwest on Monday, with main stem
flooding of the Upper French Broad River and associated tributaries
quite possible. Flooding of main stems and streams in the Upstate
may also develop, especially along the Broad and Saluda rivers, with
crests not occurring until Tuesday and perhaps early Wednesday.
Additional significant rises may develop along the Catawba River
chain in western NC during and immediately after the heavy rainfall.
Water level forecasts for these rivers will incorporate the total
forecasted rainfall later this morning and will be accessible at
http:/water.weather.gov/ahps2/index.php?wfo=gsp.

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...Flood Watch from late tonight through Tuesday morning for
     GAZ010-017-018-026-028.
NC...Flood Watch from late tonight through Tuesday morning for
     NCZ033-049-050-053-059-062>065-501>510.
SC...Flood Watch from late tonight through Tuesday morning for
     SCZ001>007-010.

&&

$$
SYNOPSIS...HG
NEAR TERM...Carroll/SGL
SHORT TERM...HG
LONG TERM...HG
AVIATION...Carroll/SGL
HYDROLOGY...JMP


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