Area Forecast Discussion
Issued by NWS Greer, SC

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000
FXUS62 KGSP 240736
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
336 AM EDT Mon Apr 24 2017

.SYNOPSIS...
A deep and vigorous low pressure system will slowly move across
Georgia today and reach the South Carolina coast by early Tuesday
morning. A drying and warming trend begins Tuesday and continues
through next weekend, as high pressure builds back in.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 330 AM EDT: Periods of heavy showers will continue across the
area as an upper low slowly moves from north Alabama this morning to
the SC/GA coast this evening. The keeps most of the area in a
southerly diffluent flow with a deformation zone pivoting across the
area. Low level flow will also remain favorable for heavy rain as
easterly H925 and H85 flow continues up to 4 standard deviations
above normal. This will keep strong upglide over the CAD cold dome
and upslope flow in place, especially over NC. The combined with
continued upper divergence leads to deep lift. In addition to the
forcing, PW values up to 2 SD remain continue as well. Some weak
elevated instability moves up to the I-85 corridor this afternoon
which could lead to some thunderstorms, only exacerbating the heavy
rainfall potential. Despite the lull in heavy rainfall across the
western CWFA, CAM guidance shows heavy showers redeveloping this
morning and moving north across this area before diminishing this
afternoon. Therefore, the threat of flooding continues across the
watch area, but the highest rainfall amounts will favor the I-77
corridor into the NC Foothills. In fact, the heavy rainfall could
linger into the evening across the I-77 corridor where the axis of
deep moisture and forcing rotate before moving east of the area late
tonight. Could see a situation where the watch is pared back across
the west this afternoon, but extended into the evening across the
east. As for additional QPF amounts, 1 to 4 inches is possible
across the area, with the highest amounts of 3 to 4 inches over the
I-77 corridor. This would certainly lead to significant stream level
rises over the Charlotte Metro Area with high water levels back to
the west as well.

Gusty winds will continue across the area given the strong barrier
jet in place in the CAD wedge and just enough mixing to bring these
winds to the surface. Even though wind gusts will be well below
advisory level, the combination of these winds and saturated soil
could lead to scattered downed trees and possibly power lines.

Precip tapers off from SW to NE overnight with showers lingering
across the northern and eastern CWFA through daybreak.

&&

.SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/...
As of 300 AM EDT Monday: the upper low should be centered near
Myrtle Beach around 12z Tuesday, then wobble toward the northeast
along the Mid-Atlantic coast thru the Short Term. The associated
surface low will be stacked underneath, so the CWFA should see
gradual drying from west to east as the low drifts away. It may take
the entire day Tuesday for showers and clouds to clear along the
I-77 corridor, while the Upper Savannah Valley should see clearing
by early afternoon. So a decent temp gradient from mid 60s east of
I-77 to upper 70s in the Upper Savannah Valley can be expected.
Additional QPF on Tuesday should be light, as heavy rain will be
well east of the area.

From Tuesday night thru Thursday morning, conditions should be
quiet, as upper ridge quickly builds in behind the departing low.
Wednesday should be a nice day with mostly sunny skies and temps
back above normal. Highs in the 70s to lower 80s and lows in the 50s
to around 60.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
As of 315 AM EDT Monday: A broad, massive trough will dominate the
CONUS in the upper levels to start the medium range, then will
retrograde west and allow a large ridge to build in across the
eastern states by the start of the weekend. A dying front will try
to reach the CWFA from the west on Thursday, and may bring enough
moisture to support a few showers and tstms in the mountains, but
otherwise, Thursday looks dry. Friday thru Sunday, the amplifying
ridge aloft should keep PoPs below climo, but a strong Bermuda High
in place will probably result in decent instability each afternoon
to support at least a few showers and storms, mainly in the
mountains. Highs will be about 10 degrees above normal, and lows
10-15 degrees above normal.

&&

.AVIATION /08Z MONDAY THROUGH FRIDAY/...
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...

            07-13Z        13-19Z        19-01Z        01-06Z
KCLT       Low   59%     High  80%     Med   78%     Med   77%
KGSP       Low   58%     High  84%     Med   79%     High  83%
KAVL       High  91%     High  90%     High 100%     High  94%
KHKY       High  85%     Med   73%     High  83%     High  85%
KGMU       Med   77%     Med   79%     Med   74%     High  80%
KAND       High  97%     High 100%     High 100%     High 100%

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:

www.weather.gov/gsp/aviation

&&

.HYDROLOGY...
Will leave the Flood Watch as is for now despite the limited
rainfall amounts currently across the western portions of the CWFA.
CAM guidance shows heavy showers will redevelop this morning and
spread north across the western portions of the CWFA. Also, high
water levels are continuing across the NC Mountains and any
additional rain will exacerbate the situation. Several rounds of
heavy showers will continue into the evening across the eastern
CWFA. Could see a situation where the watch is pared back over the
west today, but extended into the evening over the east. Additional
rainfall amounts of 1 to 4 inches are expected today and tonight,
with the highest amounts, 3 to 4 inches, over the I-77 corridor.

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...Flood Watch through this evening for NCZ033-035>037-048>053-
     056>059-062>065-068>072-082-501>510.
SC...Flood Watch through this evening for SCZ001>014-019.

&&

$$
SYNOPSIS...ARK
NEAR TERM...RWH
SHORT TERM...ARK
LONG TERM...ARK
AVIATION...RWH
HYDROLOGY...RWH



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