Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 172153

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
553 PM EDT Thu Aug 17 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 530 PM EDT: Looks like we have topped out for the day, and
with temps dropping off a few degrees we should be out of danger for
the Heat Index reaching 105 the rest of the day. Convection was
doing its best to move off the Blue Ridge Escarpment, mainly over
the NC Foothills. The fairly high precip prob in that area looks
good for the very near term. CAPE values are well into the moderate
range and remain very unstable in some locations. DCAPE values have
been slowly increasing and would support strong downbursts, so a few
strong to severe storms are possible into early evening. PW values
are very high, but cells are moving. Excessive rain potential
remains low, but could increase if training cells develop. Although
convection dissipates this evening, showers may return to the
mountains ahead of an approaching front. Debris clouds and clouds
associated with the approaching front may help limit fog overnight,
but some fog should develop again in the mountain valleys and I-40
corridor. Lows will be around 5 degrees above normal.

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 wel 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: Low VFR cumulus and developing convection this afternoon.
Have kept the timing for the TEMPO as is but added variable gusty
winds with the storms. Winds turn S this evening with ending
convection but lingering low VFR clouds. Winds become light S
overnight with only a low chance of any fog. Scattered cumulus
develops early Friday with increasing SW wind. Convection may start
early as well, but left out of this TAF period for now.

Elsewhere: Cumulus increasing and convection developing across the
area. Have included TEMPO at KAVL/KHKY where convection more likely
and VCTS for the SC sites. Convection ends and low clouds scatter
out this evening with fog developing at KAVL and KHKY. Could be
dense but there may be some lingering convective debris limiting
severity. Early cumulus and convection expected Friday but left
convection out for now. S to SW wind this afternoon becomes light
overnight. SW wind picks back up on Friday, NW for KAVL.

Outlook: The transition to a more typical late summer pattern
continues through early next week. Expect scattered afternoon
convection and morning low stratus and patchy fog each day,
especially in the mountain valleys and in locations seeing
appreciable rainfall.

Confidence Table...

            22-04Z        04-10Z        10-16Z        16-18Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High  91%     Med   67%     Med   77%
KHKY       High 100%     High 100%     High  91%     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:




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