Area Forecast Discussion
Issued by NWS Greer, SC
FXUS62 KGSP 231754
Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
154 PM 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.
.NEAR TERM /THROUGH TONIGHT/...
As of 150 PM EDT Sunday: Updated discussion for taf issuance.
Tweaked pops/t/td through the immediate near term to align with
recent radar/guid trends, and left the remainder of the fcst as it
was. Full fcst discussion to follow within the hour.
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.
.SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/...
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.
.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
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
.AVIATION /18Z SUNDAY THROUGH FRIDAY/...
At KCLT and Elsewhere: Vast IFR/LIFR restrictions expected through
this period as a CAD wedge remains draped atop the region.
Meanwhile, a slow moving upper low continues to slide east across
the MS/TN valleys, moving through the fcst area overnight into
Monday. With that, ample upglide as well as upper divergence will
yield increasing precipitation coverage through the evening, however
with a brief lull in the immediate near term for KCLT and the SC
Sites. Steepening mid level lapse rates atop the wedge in
association with the approaching upper low will lead to at least a
non zero chance for ts, however think probs are too low to include
at this time. Overall, expect multiple round of shra/+shra
tonight/overnight with variable visbs/cigs, both in the IFR/LIFR
range. Nely winds will remain quite elevated beneath the wedge due
to a barrier jet, thus did keep such in at all sites through the
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.
18-24Z 00-06Z 06-12Z 12-18Z
KCLT Low 52% High 84% Med 67% High 85%
KGSP Low 55% High 85% Med 62% High 80%
KAVL High 85% High 96% High 82% High 86%
KHKY Med 78% High 87% Med 71% High 80%
KGMU Low 59% High 84% Med 65% High 80%
KAND Med 70% Med 71% Med 77% Med 68%
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.