Area Forecast Discussion
Issued by NWS Greer, SC

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000
FXUS62 KGSP 222101
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
501 PM EDT Tue Aug 22 2017

.SYNOPSIS...
Summer like conditions will prevail into Wednesday when a cold front
sweeps through the region from the west allowing for increasing
precipitation chances.  High pressure will build into the region
late in the week yielding drier and cooler conditions through the
weekend.  Ongoing tropical activity will make for a variable
forecast leading into next week.

&&

.NEAR TERM /THROUGH WEDNESDAY/...
As of 445 PM EDT: Fairly dry profiles continue to limit convective
coverage across the region late this afternoon and PoPs have been
trended down through the evening hours, with the lower piedmont
seeing the best lingering chance.

Meanwhile, a vort lobe will shear out as it tries to lift thru the
axis of a broad upper trough over the Ohio Valley tonight. An
associated cold front pushing through the Ohio valley won`t reach
the NC mountains until Wednesday afternoon. Guidance generally
agrees on a pre-frontal line of convection approaching the NC
mountains from the NW. Shear looks fairly weak upstream with veered
850 mb flow around 20 kts over TN. The CAMs make sense in their
depiction of weakening convection. Will keep a CHC pop along the
border, but not expecting anything too strong (with whatever does
arrive late tonight.) Lows will be a couple categories above normal.

Wednesday, it`s tough to tell exactly when the front will "jump"
from the Blue Ridge to the usual lee trough position. It looks like
around 00z Thu. Ahead of the front, there may be a fair amount of
cloud cover from overnight convection that wafts into the area as
the flow veers more out of the west. The operational models do not
show anything more than 1000 J/kg in the mountains, while the
piedmont gets into 1500-2000 J/kg range. Shear will be remain weak.
So overall, PoPs were lowered slightly. Severe threat looks low, as
all the better ingredients remain north and east of the area. Highs
will be near to slightly above normal.

&&

.SHORT TERM /WEDNESDAY NIGHT THROUGH FRIDAY/...
As of 2:30 PM EDT Tuesday:  A cold front will ease through our
forecast area (FA) Wednesday night into early Thursday morning.
Meanwhile the upper air pattern has become more northwesterly,
therefore we have sped up the FROPA just a tad.

It still looks like parts of the I77 corridor, and down into the
upstate of South Carolina, will have the best shot of showers and
thunderstorms Wednesday night (mainly first half). Depending if
these areas receive prior storms, lingering instability Wednesday
evening, along with precipitable water values between 1.75 and 2.00
inches, and convergence along the frontal boundary, could lead to
some strong storms. Shear values however remain rather marginal.

Interestingly an inverted trough pokes up from northeast Georgia and
the mountains of western SC, into the southern mountains of western
NC Thursday into Thursday evening. Along this inverted trough we
expect an axis of low level convergence. Model guidance was showing
enough moisture for some random showers. A quick look at instability
also shows a corridor of elevated values, but for the most part we
plan to leave any thunder out of the equation.

Elsewhere dry conditions should prevail, although we will watch our
far southern piedmont counties closely, for any precipitation trying
to sneak back in.

High Pressure should build into much of the region Friday with dry
conditions, although a sliver of moisture remains over the
mountains.

&&

.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
As of 145 PM EDT Tuesday: The medium range fcst period kicks off on
Friday evening amidst a broad/deep yet departing longwave upper trof
across the eastern CONUS, while a sharp ridge builds westward atop
the Plains.  At the surface, high pressure centered beneath max
confluence on the descending branch of the aforementioned upper trof
will extend across much of the OH/TN Valleys, eastward atop the
Appalachians and into the mid Atlantic seaboard.  With that,
cooler/drier air looks to slide into the region by way of a damming
wedge on Saturday morning with below climo pops favored, the highest
of which residing over the terrain where subsidence is weakest.  At
that point much uncertainty arises in the fcst given two distinct
systems of tropical origin.  As of now, the first of these systems
is the remnants of TC Harley which is expected to rapidly intensify
over the western Gulf, possibly regaining TS/HC status by Saturday
with various model sources favoring landfall along the TX coast.
The second system is currently a weak area of low pressure off the
southeastern FL shores, which is also expected to slowly gain
strength with time, possibly moving northward and approaching
northeast FL by Saturday.

Sunday into midweek, the latest GFS seems to have come into form
with the 00z ECMWF as 06z GFS runs favored additional tropical
moisture advection into the region leading to broad scale isentropic
lift and thus showers, which combined with improving easterly llv
could lead to upsloping as well.  However, these two solutions now
tend to favor intensification of the Atlantic system as it slides
northward along the GA and Carolinas coastlines Monday through
Tuesday.  Meanwhile, the TC Harvey looks to have slowed along the
TX/LA coastline all the while potentially producing significant
rainfall, however with little/no impact across CWFA through the
period.  All said, given the expected track of the Atlantic system,
would expect the high pressure wedge to be reinforced during this
time frame keeping PWATs rather low, generally under 1.5 inches.
Thus, will carry token slight/chance pops through the remainder of
the period, increasing to widespread chances Tuesday evening into
Wednesday.  Temperatures through the period will remain generally a
few degrees below normal across the high terrain, possibly as much
as 6-10 degrees below normal over the low terrain where wedging and
thus abundant sky cover could prevail.

&&

.AVIATION /21Z TUESDAY THROUGH SUNDAY/...
At KCLT and Elsewhere: Isolated showers continue to develop mainly
south of KCLT, with little to no coverage elsewhere. Will keep VCSH
going at KCLT but remove elsewhere this evening. Later tonight, a
line of showers and tstms may reach the the NC/TN border before
daybreak Wed, but is expected to weaken as it approaches. Confidence
is too low to have an early morning TS/SH mention at KAVL. KAVL
could also see another round of fog and/or low stratus late tonight,
so have some restrictions in the 9-13z. Winds will favor a SW
direction this AFTN, becoming light this evening. As the front
approaches, they will veer slightly to WSW, but a shift to NW is
expected to be after 18z.

Outlook: The highest chances for convection will be on Wednesday as
a cold front pushes into the area. The front should push thru and
allow drier air to work in from the north. So the probability of
restrictions and diurnal convection should diminish Thursday and
Friday.

Confidence Table...

            21-03z        03-09Z        09-15Z        15-18Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     Low   36%     High  94%
KHKY       High 100%     High 100%     High 100%     High 100%
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...CDG
NEAR TERM...ARK/HG
SHORT TERM...TS
LONG TERM...CDG
AVIATION...ARK/HG



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