Area Forecast Discussion
Issued by NWS Greer, SC

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000
FXUS62 KGSP 220806
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
406 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 355 AM Sunday: With this fcst package, a Flood Watch has been
issued for portions of the FA, which will go into effect later
tonight.

With sfc high pressure well ENE of the area this morning, low level
moisture continues to infiltrate into the Southeast from both the
GOM and Atlantic as increasing cirrus streams overhead ahead of the
approaching system progged to bring heavy rainfall to the area
beginning later tonight. Per latest guidance, am expecting that 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 will begin 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 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. Rainfall will gradually expand in coverage as well,
beginning as early as later this morning across the southwestern
portions of the FA, with precipitation likely not reaching the
Charlotte-Metro area until later tonight. Did slightly increase the
progression of PoPs 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 /08Z SUNDAY THROUGH THURSDAY/...
At KCLT and elsewhere: VFR prevails across the area this morning
with cirrus infiltrating in and becoming more opaque ahead of an
approaching system. With the exception of KAND where MVFR cigs and -
SHRA could move in as early as 15Z, expect VFR cigs to become
widespread through early this afternoon with -SHRA and the
deterioration to MVFR cigs becoming widespread by this evening.
MVFR/IFR cigs will persist through the end of the valid TAF period,
with ongoing -SHRA. Light winds this morning are expected to
increase to 5 to 10 kts, 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 Sunday night 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, but with some moisture lingering.
Unsettled weather will continue on Wednesday and Thursday, as a
trough of low pressure persists across the region.

Confidence Table...

            08-14Z        14-20Z        20-02Z        02-06Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High  82%
KAVL       High  98%     High  85%     High 100%     High  95%
KHKY       High 100%     High 100%     High 100%     High  83%
KGMU       High 100%     High 100%     High  91%     High  81%
KAND       High  98%     High  91%     High  82%     Med   73%

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...SGL
SHORT TERM...HG
LONG TERM...HG
AVIATION...SGL
HYDROLOGY...HG



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