Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 240017

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
817 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.


As of 750 PM EDT: Water vapor imagery shows the cutoff upper low
spinning slowly southeast across northern Alabama early this
evening, and the RAP height fields (which have handled this fairly
well) bring the center of circulation slowly to the central GA/AL
border by daybreak Monday. Strong upper divergence is already in
place atop the region this evening and the low track will bring
strengthening southeasterly 850 mb flow to the western Carolinas and
yield continued isentropic upglide and upslope terrain flow into the
Blue Ridge. Given the precipitable water values surging to +2 to +3
standard deviation, the afore mentioned forcing, and the steepening
850 to 500 mb lapse rates overnight, anticipate improved
precipitation rates through the nighttime hours and well into
Monday. Hydro problems are likely to worsen quickly late this
evening and overnight. No changes to the existing flood watch will
be made, but the hydro focus will increasingly shift northward into
the NC foothills/piedmont and the I-77 corridor over time.
Anticipate fairly solid 2 to 4 inch QPF amounts going forward, with
localized totals near 6 inches possible with any training.
Conditions will slowly improve from the southwest during the day on
Monday as the entire system slides slowly east. Deformation banding
north and northwest of the 850 mb circulation could keep localized
maximum QPF stripes in our piedmont through late Monday.


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.


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.


At KCLT: Several cells moving northward from the Midlands have
brought the potential for occasional cloud to ground lightning
strikes despite the wedge layer in place. Still anticipate primarily
heavy rain showers to grow and expand across the region as the
backing 850 mb jet strengthens late this evening and overnight.
Anticipate mainly IFR cigs early this evening, transitioning down
into the LIFR range with the heavier showers overnight. Very little
variation is then expected through much of the rest of the period
with gusty NE winds continuing along with IFR vsby and LIFR cigs.
There should be a shear layer atop the wedge, but probably not low
enough for LLWS conditions at present.

Elsewhere: the expansive precipitation shield spreading into the
terminal forecast area from the southwest this evening should
stabilize ceiling conditions in the IFR range early, with LIFR
ceilings likely overnight. The surface wedge appears deep enough to
keep thunder out of the TAFs at present. Gusty northeast winds will
continue across the foothills, but with more of a northerly
component at KAVL. Some modest recovery is possible from KAND to
KGMU late Monday as the deeper moisture pivots northward.

Outlook: An area of low pressure will move slowly across the
Carolinas toward the coast Monday night through Tuesday. Low clouds
and precipitation will be slow to dissipate behind the departing
system. Conditions should gradually improve later Tuesday through
Wednesday morning as dry air works in behind the departing low.
Return flow moisture ahead of the next system will develop late in
the week.

Confidence Table...

            00-06Z        06-12Z        12-18Z        18-00Z
KCLT       Med   72%     Med   64%     Med   66%     Med   68%
KGSP       High  81%     Med   69%     Med   63%     Med   74%
KAVL       Med   72%     Med   79%     Med   73%     Med   77%
KHKY       Med   77%     Med   69%     Med   70%     Med   68%
KGMU       High  81%     Low   59%     Med   64%     High  85%
KAND       Med   67%     Med   70%     Low   52%     Med   77%

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.


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