Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 211051

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
651 AM EDT Fri Apr 21 2017

An unsettled pattern will continue as a front stalls over our area,
with showers and thunderstorms possible today and Saturday. A low
pressure system will move into the area Sunday bringing widespread
showers and some thunderstorms. The low slowly moves east of the
area Monday bringing an end to the rain. Drying is expected through
the rest of the week.


As of 645 AM EDT: Not much change to the forecast with this update.
I did introduce a little higher PoP in the Northern Mountains early
this morning, as area radars show scattered showers brushing the
area from the TN Valley. Otherwise, just minor tweaks to the fog in
the grids per latest obs.

As of 315 AM EDT: An upper ridge will shift east off the East Coast
today, as a pair of troughs approach from the west. The northern
stream trough will cross the Great Lakes, while the southern stream
trough dives into the Central Plains. At the surface, a cold front
cross the Ohio Valley today, then push into the NC mountains this
evening. Guidance is in pretty good agreement on a little bit more
moisture and instability for greater coverage of convection than
last couple days. PoPs will range from likely northwest to slight
chance along the SE fringe of the CWFA. SBCAPE should approach
1000-1500 J/kg with 0-6 km bulk shear in the 25-35 kt range in the
mountains (<25 kts in the piedmont). So a marginal risk of severe
storms is forecast north of I-85 and general thunder south. The main
threats will be hail up to quarter-sized and damaging wind gusts.
Highs will be about 10 degrees above normal.

Tonight, the models have the cold front pushing to about the I-40
corridor, then start lifting it back north by daybreak. Deep-layer
west-southwesterly flow will stream plenty of lingering moisture
from earlier convection across the area, but we should stabilize by
late evening. Min temps will be held up well above normal under the
clouds and moist BL, about 10-15 degrees above normal.


As of 250 AM Friday: A split flow regime will be in place across
the eastern CONUS through the short term period, with an upper low
expected to drift from the mid-Mississippi Valley into the southeast
coastal states from Saturday into Monday. There is good agreement on
the location of an associated frontal zone on Saturday. The front
remains just north and west of the forecast area. Better instability
and shear will be across NC closer to the front. This is where the
highest PoP and severe storm chance will be. That said, the shear
isn`t very strong, so organized severe storms don`t look likely.

Precip chances increase Saturday night and Sunday as the upper low
moves in pushing the associated surface low into the area. As the
low moves in from the west, high pressure builds in from the north
in a cold air damming pattern pushing the front south into the area.
The low then moves into SC during the day. There is disagreement on
the track of the low, the strength of the damming, and the location
of the boundary. The GFS seems to keep the front and surface low
farther north, keeping more of the CWFA in the warm and unstable air
mass. The NAM is farther south with only a small portion of the
southern CWFA in the warm and unstable air mass. The other guidance
is between these 2. Of course, the location of these features will
have a major impact on the potential for the development of any
severe storms. There will be plenty of shear, helicity, and forcing.
Even the NAM shows elevated instability, but with a relatively deep
cold air dome, it is questionable any strong winds would be able to
reach the surface. Have gone with a blend of to indicate
thunderstorm potential. This keeps them limited to the western
mountains and mainly south of the I-85 corridor. Continued with the
high PoP and at least moderate QPF as all the guidance agrees on
this. The axis of best precip slowly moves east Sunday night and
east of the area Monday. PoP trends follow this as well.

Temps will remain much above normal Saturday, then fall below normal
Sunday and Monday.


As of 330 AM EDT Thursday: Guidance in good agreement that the upper
low along the SE Atlantic coast at the start of the period will
slowly move up the Carolina coast Monday night and Tuesday. The
surface low follows similar pattern keeping a moist easterly low
level flow over the area. This keeps precip over the area, favoring
the eastern CWFA, Tuesday morning which tapers off during the
afternoon and evening. Temps climb back to near or a little above
normal for Tuesday.

Upper ridging slides over the area through the rest of the period.
Weak surface high pressure moves across the area as well. This keeps
any frontal systems at bay to our west. Therefore, a dry forecast is
expected with temps rising back to well above normal through the


At KCLT and elsewhere, except KAVL: VFR conditions expected thru the
period. Isolated to scattered showers and thunderstorms will try to
survive off the mountains late this afternoon and evening with
steering flow out of the west. Winds will pick up to around 8-10 kts
this afternoon out of the southwest.

At KAVL, valley fog as developed, producing variable vsby and cigs,
but occasionally VLIFR in dense fog. The fog should lift/scatter out
by 13z and the stratus by 14z. This afternoon, scattered to numerous
showers and thunderstorms are expected across the NC mountains. Will
go with VCTS this afternoon, then PROB30 for TSRA this evening. With
the expected precip coverage, and light winds tonight, fog will
likely develop again late tonight, so will put IFR conditions after

Outlook: Another backdoor cold front is expected to settle over the
fcst area this weekend, with restrictions and precipitation likely.
A strong area of low pressure will cross the region Sunday into
Monday, with widespread low clouds and precip expected.

Confidence Table...

            11-17Z        17-23Z        23-05Z        05-06Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High  94%     High 100%     High 100%     High 100%
KAVL       Med   62%     High 100%     High 100%     High 100%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High  94%     High 100%     High 100%     High 100%
KAND       High 100%     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:




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