Area Forecast Discussion
Issued by NWS Greer, SC
FXUS62 KGSP 232055
Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
455 PM EDT Sun Apr 23 2017
A deep and vigorous low pressure system moves across Georgia to
the Carolina coast creating a lengthy period of moderate to heavy
rainfall and below-normal temperatures that persists through
Monday. Storm total rainfall accumulations of 2 to 5 inches are
likely. A drying and warming trend begins Tuesday and continues
through next weekend. A slight chance of diurnal mountain showers
and thunderstorms returns Thursday and persists through the weekend.
.NEAR TERM /THROUGH MONDAY/...
As of 445 PM EDT Sunday: The current forecast remains on track with
only very minor adjustments needed to PoP for current radar
placement of returns and to add slightly to sky cover. Concern
continues to grow for possibly excessive rainfall rates developing
later this evening as the deeper forcing arrives and precipitable
water values surge to +2 to +3 standard deviations. The current
Flood Watch remains well placed.
Otherwise, a closed upper low moving through the mid South into the
TN valley late this afternoon continues to drive deep/moist swly
flow aloft across northeast GA and the western Carolinas. At the
surface, high pressure anchored to the northeast continues to
promote CAD in the lee of the Appalachians, while a cold front moves
through the deep south ahead of the aforementioned upper low. The
wedge front remains generally along/near the I20 corridor, however
is expected to lift slightly to the north overnight potentially
reaching into the I85 corridor late tonight, into mid morning,
favoring elevated convection amidst non-zero MUCAPE. With that,
guidance favors quite a bit of additional precipitation across the
region, with the heaviest rainfall amounts oriented along the I77
corridor. The fcst will feature widespread categorical pops
through the period, with QPF totals across northeast GA and the
extreme western Carolinas reaching upward of 1-2 inches, increasing
to the east into the I-77 corridor where totals could top out in the
3-4 inch range through the period. Please refer to the latest
CAEFFAGSP for additional details. River levels will certainly rise
across the region, especially in/near the Catawba basin where the
highest QPF totals are favored. Conditions will slowly improve
during the day on Monday from the west as the entire system slides
east. However, expect little improvement across the NC/SC Piedmont
through the period. With that, pops will taper from the west,
however remaining in the likely/categorical range generally east of
.SHORT TERM /MONDAY NIGHT THROUGH WEDNESDAY/...
As of 205 PM Sunday: Obviously the main short term concern will be
timing the ending of the precip potential, especially across the
eastern part of the forecast area, as upper low continues to wobble
in the vicinity of the SC Coast. This will place the I-77 corridor
in a good position to receive continued bands of rainfall associated
with the deformation zone north and west of the cyclone. In fact,
there`s a decent consensus in deterministic guidance that another
.5-1 inch of rain will fall in this area Monday evening, and we will
forecast as much. This amount of qpf would certainly warrant an
extension of the Flood Watch, but that decision can be left to later
shifts and further collaboration with WFOs to our east and south.
Pops gradually taper off after midnight through Tuesday, really not
dropping below 20 percent until Tue afternoon, in line with
uncertainty regarding how quickly the upper low`s influence will end
for our area. Temps will also be a bit problematic on Tuesday, as
areas along the Upper Savannah River Valley will likely see clearing
skies by afternoon, while locations along the I-77 corridor should
underneath a cloud shield. Tuesday`s high temp forecast therefore
ranges from around 80 across the USRV to mid/upper 60s across the
northwest NC Piedmont.
The remainder of the short term will be decidedly tame, as upper
ridging builds along the Eastern Seaboard in the wake of the upper
low, and downstream of large scale central Conus height falls. This
will result in a return of much-welcomed dry conditions while Wed
max temps are expected to return to the 80s in most locations east
of the mtns.
.LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/...
As of 215 PM Sunday: The medium range will see the establishment of
a massive/broad upper trough encompassing much of the western 2/3 of
the Conus, with ridging expected to dominate much of the East Coast.
This will result in unseasonably warm temperatures continuing across
our area through at least the end of the weekend.
The first in a series of short waves ejecting from the western Conus
trough will ride along the western periphery of the Eastern ridge
Wed night through Thursday, passing well west and north of our area.
Nevertheless, a weakening/occluding cold front is forecast top
approach the southern Appalachians and vicinity by the end of
Thursday. There will be a deep fetch of Gulf of Mexico moisture
immediately ahead of this feature, resulting in a decent amount of
pre-frontal instability. Despite the weakening forcing, this should
provide decent chances for convection, mainly across our western
areas Thu afternoon/evening. Shear is forecast to be adequate (but
not much more than that) for organized convection, so the severe
threat will depend substantially on the amount of buoyancy that is
realized, and this is still quite uncertain.
Warm sector air mass remains pretty much ensconced across the area
late in the week through the weekend, with global model guidance
depicting a summer-like pattern, with anomalously strong Bermuda
high supporting plenty of heat/moisture/instability each afternoon.
Diurnal deep convective chances (mainly in the 20-40% range) will
therefore be advertised each day from Fri through the weekend.
.AVIATION /21Z 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 after
the current brief lull ends. 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 vsby/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 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.
21-03z 03-09Z 09-15Z 15-18Z
KCLT Low 46% High 89% Med 66% High 93%
KGSP Med 61% High 89% Low 55% Med 77%
KAVL Med 77% High 88% High 81% High 81%
KHKY High 85% High 85% Med 62% High 82%
KGMU Low 50% High 83% Med 66% Med 73%
KAND High 94% Med 66% Med 66% Med 69%
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:
No changes to the current Flood Watch are presently expected. None
of the river forecast points are forecast to go into flood
currently, but locations along the Catawba river chain will need
very close monitoring through Monday as the heavy rain axis could
pivot over the area.
NC...Flood Watch through Monday evening for NCZ033-035>037-048>053-
SC...Flood Watch through Monday evening for SCZ007>009-012>014.