Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 231538

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1138 AM EDT Sun Apr 23 2017

Cool and moist high pressure will remain over the area through
Monday. Meanwhile, a deep and vigorous low pressure system moves
across Georgia to the Carolina coast creating a lengthy period of
moderate to heavy rainfall that persists through Monday. Dry high
pressure returns by Wednesday. Temperatures warm through next
weekend with a small chance of afternoon mountain thunderstorms.


As of 1130 AM EDT Sunday:  Guidance has trended higher as it
pertains to QPF across portions of the SC Upstate this evening and
tonight.  Specifically, cam guidance highlights enhanced elevated
convection sweeping across the region during this time frame as
upper divergence maximizes ahead of the H5 cyclone.  Therefore, with
upwards of 2-3, possibly 4 inches of QPF in the fcst, opted to
expand the Areal Flood Watch across all Upstate zones along/east of
I26.  While QPF remains high to the west along the southern I85
corridor, FFG values also remain high.  Thus, although an isolated
Flash Flood threat will exist across these areas, the spatial extent
of the threat doesn`t warrant watch coverage.  Otherwise, tweak
pops/t/td through the near term and left the remainder of the fcst
as it was for this update.

Previous Discussion: an upper closed low will continue to slowly
drift SE from the Mid-South to near Columbus, GA by 12z Monday. This
slow drift will place the CWFA within a region of very strong
deep-layer Q-vector convergence, especially tonight. At the surface,
an area of low pressure will stall over northern GA, while a
backdoor cold front clears our area to the south keeping a hybrid
wedge locked in across the entire CWFA. Persistent low level upglide
atop the wedge should bring rounds of showers thru the day. The CAMs
seem to agree on perhaps a couple "rounds" of categorical PoPs, one
this morning, then another this evening. Total QPF amounts today
thru tonight will range from generally 1.5 to 3", highest across the
SELY upslope areas of the Blue Ridge and adjacent foothills. So the
Flood Watch looks good for the NC zones. As for severe potential, it
really does look like the wedge front will make little progress
north as the low is slow to approach from the west, and ongoing rain
keeps it reinforced. I keep a mention of slight CHC to CHC for
thunder across the I-85 corridor and south/east, due to potential
elevated convection. There is still a marginal risk in the Day One
Convective Outlook for possible hail and wind, but the CAMs keep the
strongest convection south and east of the CWFA. Temps will be
nearly steady across most of the area, unless the wedge front does
nudge back into the southern tier of counties, where temps may bump
back up into the upper 60s to lower 70s. Lows tonight drop slightly
into the 40s in the mountains and lower to mid 50s elsewhere.


As of 355 AM Sunday: The heavy rain potential continues Monday as
our area will still be in the southerly diffluent flow/deformation
zone around the upper low moving east from west central GA to the
SC/GA coast. Upper divergence follows a similar pattern. Strong low
level easterly flow remains over the area as the low level low
pressure areas follow a similar pattern as well. In fact, the low
level easterly flow is up to 3 standard deviations above normal.
This will create strong isentropic lift over the cold dome from the
damming that remains in place. The moist air mass remains in place
as well with PW values 1 to 2 SD above normal. The heaviest rain
will likely fall across the NC Foothills and Piedmont, but could
shift south into the Eastern Upstate. An additional 1 to 2 inches of
rain could fall in these areas. Therefore, have added Union County
NC to the Flood Watch to cover the potential. Moderate to heavy rain
could linger Monday evening, but precip tapers off from SW to NE
overnight. Highs will increase a little but remain around 10 degrees
below normal. Lows will be up to 5 degrees above normal.

Precip continues tapering off from SW to NE Tuesday ending during
the afternoon. With precip ending and sunshine gradually returning,
highs will be near normal. Dry forecast continues Tuesday night with
lows around 5 degrees above normal.


As of 330 AM Sunday: Short wave ridge over the area Wednesday moves
east as a series of short waves move by to our north and west
Thursday and Friday. A stronger ridge develops over the SE CONUS
Saturday. Dry remains over the area Wednesday with temps above
normal. Moisture returns ahead of a weak cold front Thursday and
Friday. This will lead to diurnal convection each day favoring the
mountains. Temps remain above normal. Southerly flow around the
ridge surface and aloft will keep some moisture across the mountains
with diurnal convection once again. Temps remain above normal as


At KCLT and Elsewhere: As expected, IFR cigs have finally filled in
across all the TAF sites (except barely MVFR at KAVL), as showers
continue to develop atop a building cold air damming wedge. Guidance
is still on track for a period of heavier rain/greater coverage this
morning, then perhaps a relative lull this afternoon. Then another
round of rain tonight. Still may see some elevated thunder across
the upstate and at KCLT late this afternoon thru the evening. For
now, only will mention a PROB30 at KCLT where chance is greatest for
embedded thunder with the second round of rain. IFR cigs will likely
lower to LIFR across the piedmont sites tonight, as the cold dome of
the wedge starts to shrink with a passing area of low pressure to
the south. Showers will start to taper off in the west toward the
end of the TAF period.

Outlook: An area of low pressure will cross the southern CWFA thru
Monday while strong sfc ridging builds in from the north. Widespread
low clouds and precipitation are expected to persist thru this time.
Conditions should gradually improve Tuesday thru Wednesday morning
as dry air works in behind the departing low.

Confidence Table...

            15-21Z        21-03Z        03-09Z        09-12Z
KCLT       Low   55%     Low   53%     Low   57%     Med   75%
KGSP       Low   55%     Med   76%     High  89%     Med   66%
KAVL       Med   74%     High  85%     Med   77%     High  91%
KHKY       High  81%     Med   76%     Med   79%     Med   77%
KGMU       Med   61%     Med   76%     High  89%     Med   66%
KAND       Med   70%     High 100%     Med   72%     Med   79%

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:


NC...Flood Watch through Monday evening for NCZ033-035>037-048>053-
SC...Flood Watch through Monday evening for SCZ007>009-012>014.


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