Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 240243

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1043 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 1030 PM EDT: The Flood Watch has been expanded to include all
of Upstate SC. Oconee and Pickens counties have been worked over
late today and latest HRRR/RAP runs bring another two inches to this
area overnight. Given the shower band feeding into the lower
piedmont and wrapping occurring east of the 850 mb low center, this
seems quite reasonable and could lead to flooding. Elbert County
remains the primary question for GA counties, but there should be
some additional eastward drift before the band becomes oriented more
NW to SE across SC. Will thus keep the GA counties out of the Watch.

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: 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 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...

            02-08Z        08-14Z        14-20Z        20-00Z
KCLT       Med   63%     Med   62%     Med   62%     Med   70%
KGSP       High  82%     Med   62%     Med   61%     Med   75%
KAVL       Med   72%     Med   76%     Med   65%     Med   66%
KHKY       High  80%     Med   70%     Med   69%     Med   70%
KGMU       Med   76%     Low   58%     Med   61%     High  85%
KAND       Low   51%     Low   54%     Low   58%     Med   66%

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 Monday evening for NCZ033-035>037-048>053-
SC...Flood Watch through Monday evening for SCZ001>014-019.


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