Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 240545

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
145 AM EDT Mon Apr 24 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 145 AM EDT: Forecast generally on track with widespread
moderate to heavy showers moving north across the area. Have updated
PoP trends to cover current conditions and the latest CAM guidance
which features a brief lull in precip across the west before
returning toward morning. Lows will be a little cooler given the
current temps.

Otherwise, water vapor imagery shows the cutoff upper low spinning
slowly southeast across northern Alabama 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 deviations above climo, 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 overnight.
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: Expect generally MVFR VSBY and IFR cigs through the
morning as rounds of showers move across the airfield from the
south. Could see some IFR VSBY in the heavier showers and brief MVFR
cigs. Otherwise...these trends should continue through the day with
a moist wedge and strong isentropic lift. Expect Gusty NE winds to
continue through the day as well. Heavy rain tapers off this evening
but periods of showers will continue. MVFR VSBY and IFR cigs should
continue as well, along with the gusty winds.

Elsewhere: Similar conditions to KCLT except with generally worse
conditions at KHKY and better at KAND. A slight improvement may
develop this evening, especially with VSBY as rain tapers off. Gusty
winds expected at all locations except KAVL, where they will be
brief this afternoon.

Outlook: An area of low pressure will move slowly up the Carolina
Coast on Tuesday. 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...

            05-11Z        11-17Z        17-23Z        23-00Z
KCLT       Low   57%     Low   38%     Med   66%     Med   62%
KGSP       Med   71%     Med   66%     Med   61%     High  87%
KAVL       Med   64%     Med   66%     Low   50%     Low   35%
KHKY       Low   57%     Med   66%     Med   61%     Med   62%
KGMU       Low   57%     Low   55%     Med   61%     High  81%
KAND       Low   57%     Low   38%     Low   33%     High  83%

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:


The Flood Watch has been expanded to all of Upstate SC as feeder
bands of heavier showers continue to develop upstream through GA and
back into FL. Hydro conditions will steadily worsen overnight, with
Flood Advisories becoming more common and Flood Warnings likely
being needed late tonight through Monday, expanding in area and
duration through the day on Monday as additional precip amounts of 2
to 4 inches, with isolated 6+, add up. 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 this evening for NCZ033-035>037-048>053-
SC...Flood Watch through this evening for SCZ001>014-019.


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