Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 180558

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
158 AM EDT Fri Aug 18 2017

Moist and unsettled weather will return late week and into the early
weekend, with hot and muggy conditions remaining over the area. Some
drying is favored by Sunday with a return to moist conditions by the
middle of next week.


As of 150 am: Warm/muggy night is resulting in an expansion of fog
and low stratus across the mtn valleys early this morning, and this
trend should continue through daybreak. Fog/low stratus is also
expected to develop across the foothills and Piedmont, but should be
much patchier, primarily confined to cool spots and areas that saw
rain Thu afternoon/evening. A band of pre-frontal showers over
Middle/East TN may make a run at far western NC by daybreak,
assuming they hold together.

Heights fall from the west Friday as an upper trough approaches.
This will push a cold front into the area. The synoptic scale
guidance is not overly impressed with precip chances, but the CAM
guidance is. They do agree that the best chances for convection will
be across NC with lower chances across the Upstate and NE GA. Expect
a line of convection to develop across the mountains and spread SE
across the area during the afternoon. Instability and DCAPE will
again be moderate. Shear, while not strong, will be higher than
previous days. Therefore, isolated strong to severe storms will
again be possible. Have followed these trends for the PoP forecast.
Highs should be a couple of degrees cooler, but dew points will
remain high. This should keep heat index values below 105, but they
may go above 100 along and south of the I-85 corridor.


As of midday Thursday: Cold front should exit the CWFA to the
east Friday night as its parent low works its way northward
thru Quebec. At 00z the wind-shift line should be across the
upper Piedmont, and both NAM and GFS show as much as 2000 J/kg of
MUCAPE persisting ahead of it as low-level convergence peaks during
the evening. Both models develop some convection in the I-77 and
eastern I-85 corridors after 00z as a result. Deep layer shear
is expected to be less than 20 kt, suggesting little organization
along the front, though dry vertical profiles imply some damaging
wind risk may linger. Furthermore the NAMNest and HiRes Windows
are a bit earlier with the fropa and would lend confidence to an
evening forecast free of convection. At any rate, chance PoPs at
the start of the period will taper off as the front departs.

A weak surface high will fill in behind the front and remain
overhead through Sunday. A shortwave swinging across the Ohio Valley
Saturday will briefly maintain the mean eastern trough, but heights
rise again that night, leaving nearly zonal flow overhead. While the
US operational models keep us dry Saturday, a few SREF members as
well as last night`s run of the EC permit some convection over the
Blue Ridge as well as the lower Piedmont. While subsidence should
be enough to preclude the former, it seems plausible to expect the
front will stall close enough to our southeast border to include
a small diurnal PoP there. The case is more or less the same on
Sunday, though with low-level flow having veered to the south again,
I will allow PoPs to return to the northern mountains/foothills.

Max temps only look to drop a degree or two following the front,
though dewpoints will return to about normal, keeping heat
indices below 100. For Sunday the temps will rise a bit further
under rebounding thicknesses but excessive heat concerns are not
anticipated then either.


As of 200 PM EDT Thursday: Subtle changes in the medium range
with this forecast package that continue a slight drying trend. The
extended period picks up Sunday night with a subtropical ridge
building into the southeast, both at the surface and aloft. Major
models are now coming into better agreement that this ridge will be
able to spread well into the southeastern CONUS, and the
intensity of the ridge (as well as possible subsidence and
convective suppression) has been trending slightly stronger over
the past few model cycles. Subtle H5 shortwave ridging will
also be in place over the Carolinas Monday afternoon (there is a
surprising amount of model agreement on this), which would also
bode well for some convective suppression. Southerly surface
flow with a generally Gulf fetch will encourage a slight
increase in moisture Monday, but model RHs have come down
slightly since the last forecast package, again encouraging for
the possibility of slightly less cloud cover. The last (and
probably most significant) factor will likely be the surface
stationary front stalled over the lowcountry of SC, and whether
or not it is able to pivot northwards enough to provide an axis
for moisture convergence in our area, and therefore increased
rain chances and cloud cover. The trend is toward a more
optimistic solution for Monday with the front getting hung up in
the Midlands, but it should again be noted that there is still
a good amount of uncertainty with the forecast for Monday at
this point.

An upper shortwave and surface front will begin to approach the
forecast area Tuesday afternoon, increasing chances of shower and
thunderstorm activity with a diurnal peak. Longwave troughing
deepens over the eastern CONUS as a surface front continues its
advance through the forecast area. Profiles will be quite moist
Wednesday and Thursday, so while there doesn`t appear to be much of
a severe threat, heavy rainfall may be an issue that bears watching.
Temperatures will be just a few degrees above average through the


At KCLT and elsewhere: Main concern in the very near term is the
potential for fog and/or low stratus, mainly at KAVL and KHKY, with
KAVL already reporting LIFR conditions as of 0530Z. Expect mainly
VLIFR, with periodic VLIFR conditions to persist at KAVL through at
least 12Z, while tempo LIFR conditions are forecast at KHKY from
10-12Z. VFR is forecast to persist through the period at the other
terminals, but would not rule out some patchy low stratus briefly
impacting the Piedmont terminals around sunrise. A weak cold front
is expected to bring scattered convection to the area during the
afternoon. Although KAVL and KHKY have the best chance of seeing TS
in their vicinity, chances are high enough that a PROB30 is
warranted at all terminals during the afternoon and early evening.
Winds should generally be light SW through much of the period,
before turning to the NW by Friday evening in the wave of the front.

Outlook: Drier air is expected to result in an inactive weekend,
with very little chance of diurnal convection, and probably only
patchy early AM mtn valley fog/low stratus Sat and Sunday. A more
typical late summer pattern returns next week, with isolated/
scattered afternoon/evening storms expected, along with better
chances for patchy fog/low stratus, especially in the mtn valleys.

Confidence Table...

            06-12Z        12-18Z        18-24Z        00-06Z
KCLT       High  97%     High  94%     High 100%     High 100%
KGSP       High  97%     High  94%     High 100%     High 100%
KAVL       Low   43%     High  90%     High 100%     High  97%
KHKY       Med   69%     High  89%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High  97%     High  94%     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:




SHORT TERM...Wimberley
LONG TERM...Carroll
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