Area Forecast Discussion
Issued by NWS Greer, SC

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000
FXUS62 KGSP 211747
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
147 PM EDT Fri Apr 21 2017

.SYNOPSIS...
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.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 145 PM EDT Friday:  Updated discussion for taf issuance.
Recent radar analysis continues to indicate convective development
along/north of I40.  Cam guidance seems to have a good handle on the
coverage therefore pops were updated yet again with most recent
runs.  Also tweaked t/td and sky in the immediate near term, with no
other changes made at this time.  Otherwise, full fcst discussion to
follow within the hour.

Previous Discussion: 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.

&&

.SHORT TERM /SATURDAY THROUGH MONDAY/...
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.

&&

.LONG TERM /MONDAY NIGHT THROUGH THURSDAY/...
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
period.

&&

.AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
At KCLT and Elsewhere:  VFR through the period at all sites as high
pressure dominates across the region, aside for at KAVL/KHKY where
MVFR/IFR cigs and visb are featured by mid morning.  Otherwise,
diurnal instability continues to build regionwide, with nearly 1-1.5
j/kg sbcape in the vicinity of the I85 corridor.  Cam guidance
favors little/no convection in this region however, likely due to
ample warm air aloft, as well as sfc mixing.  With that, only
carried vcsh at KCLT given proximity of greatest pops, with no
mentionable precip at the SC Sites this taf cycle.  VCTS with tsra
tempos where featured at KAVL/KHKY where best overlap of max
instability and min CIN are expected.  Any convection will erode
across the area the with heating loss this evening, thus allowing
any cu/stratocu to slowly sct, revealing high level cirrus into
morning.  Meanwhile, a cold front continues to move through the OH
valley, yet not expected to push into the region, stalling just to
the north.  Thus, all sites will remain firmly in the warm sector on
Saturday therefore favoring diurnal cu/convection yet again with
winds increasing out of the sw with low end gusting possible.

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 precipitation expected.

Confidence Table...

            17-23Z        23-05Z        05-11Z        11-12Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     High 100%     High  83%
KHKY       High 100%     High 100%     High 100%     Med   62%
KGMU       High 100%     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:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

$$
SYNOPSIS...RWH
NEAR TERM...ARK/CDG
SHORT TERM...RWH
LONG TERM...RWH
AVIATION...CDG


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