Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 191041

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
641 AM EDT Sat Aug 19 2017

High pressure will build across the forecast area and bring
relatively drier conditions over the weekend. A warming and
moistening airmass will return Monday before cool dry air works in
behind a frontal passage Wed night.


As of 630 am: Latest GOES-16 imagery indicates plenty of fog and low
stratus across the mtn valleys this morning, which should lift/burn
off by 10 AM. Otherwise, heights will fall slightly today, as a
short wave trough passes north of the region. Associated weak
frontal zone will continue to ooze southeast of the forecast area
today, allowing lower surface dewpoints to continue filtering into
the forecast area. Aided by boundary layer mixing, dewpoints in the
lower/mid 60s, with perhaps some 50s here and there are expected
this afternoon. Combining this with mid-level lapse rates of less
than 6 C/km yields PM forecast soundings with minimal buoyancy and a
decent amount of CIN. That being the case, deep convective
development appears unlikely this afternoon, and pops will be
advertised at less than 20% in all areas. Temps will also be a
couple of degrees cooler than on Friday, which along with the lower
dewpoints will make for a comparatively pleasant day after the
mugginess of the past week. The "not-too-bad for mid/late August"
weather persists into tonight, with the drier air expected to allow
min temps to settle very close to climo, if not a degree or two less
in some spots.


As of 200 AM EDT Saturday: Upper level heights will begin to rise
and broaden rather quickly Sun as an h5 trof lifts over New Eng.
Sub/trop ridging will nudge in from the east as well and maintain a
1020 mb high across the ern CONUS. Thus...with good insol thru a
deep layer...convec and shra will be hard to come by. Model
soundings indicate below normal pwats and a pronounced subs inver
arnd h6. Think the best shot of thunder will be across the NC mtns
in ridge top convg where perhaps a couple short lived storms may
develop during the afternoon. Deep sfc-based mixing will tap into
dry air aloft and continue the relatively low td/s making for a hot
but not unbearable feeling day for mid Aug. A quiet overnight period
is in store and weak llvl flow will gradually shift se/ly as the sfc
high centers off the mid/Atl coast.

This flow will allow an increase in sfc tds...but not enough to
overcome the deep layered negative forcing to produce a sigfnt
convec threat outside the higher terrain. With some mech lift in
play...a few stronger thermally driven pulse storms will be
possible across the NC mtns thru the afternoon. Most areas outside
the mtns will see little if any storms as llvl forcing will remain
south of the FA and strong ridging dominates. The models agree
fairly well with keeping a stationary front moisture axis situated
across the Low Country with a little movement to the NW during the
day likely due to the sea breeze. The latest NAM is more aggressive
with llvl moisture flux than the GFS...but as far as the impact on
cloud cover during the shouldn/t make a noticeable
difference. Basically...will expect fair weather or slightly enhance
Cu to the south covering about 30-40% of the sky during the early
afternoon and rather good eclipse viewing with this pattern. Things
could change depending on the strength and alignment of a building
Atl sfc ridge...but for now things look to remain on track similar
to the previous few fcsts. Max temps Mon shud still reach the u80s
or l90s non/mtns even with a 5-7 degree or so drop off during the
eclipse. Sfc td/s will rebound to arnd 70 F however making for
muggy conds and elevated heat index values.


As of 245 AM EDT Saturday: A period of transition is in store over
the medium range. Strong ulvl ridging will continue to hold a
stagnant warm and moist airmass over the FA with little forcing to
speak of outside of llvl thermal lift mainly across the mtns. Low
end PoPs have been maintained with the favored locale being the SW
NC mtns in modest upslope but moist flow. Heights begin to fall Wed
ahead of an incoming h5 wave and an attendant sfc front. Strong
energy rounding a closed low over ern Canada will push this front
into and thru the area late Wed with prefrontal lift becoming a
focusing zone for stg/svr storms by mid afternoon. The front looks
to remain pretty much intact as it crosses the high terrain and it
will benefit from continued GOM moisture flux early on. Thus...some
organization is possible with this system as deep layered shear
reaches 30 kts or so. The shear may not be a huge factor however as
relatively stronger winds remain above h75. But nonetheless...good
llvl forcing and an unstable airmass will be in play. A strong cP
high will be forced south across the Midwest Thu as deep h5 ridging
develops into wrn Canada. This will bring a lowering of td/s and
max temps by a couple cats or so...which shud be reinforced Fri as a
broad sfc high situates across the srn Glakes region. There is still
a shot at some afternoon mtn top convec...mainly showers...Thu and
Fri afternoon...but the latter half of the fcst period will see
unseasonably pleasant feeling weather compared the recent hot
and muggy conds.


At KCLT and elsewhere: Per the latest GOES-16 satellite imagery,
there is plenty of fog and/or low stratus within the mtn valley this
morning, with some of this trying to creep its way toward KAVL.
Included a SCT003 in a tempo for the first hour of the period, but
this may need to be upgraded to a cig and possibly visby restriction
before the forecast is issued. Otherwise, VFR along with a
convection-free afternoon is expected through at least this evening.
Another round of fog/low stratus is possible in the mtn valleys
tonight/early, although chance are considerably lower in light of
the drier air. Therefore, kept restrictions out of KAVL as well as
all others during the out period. Winds will generally favor light
NW through NE through the morning hours, but should become more
variable in direction this afternoon.

Outlook: The inactive pattern is expected to continue into early
next week, although patchy morning fog/stratus will remain possible,
mainly in the mountain valleys. A more typical late summer pattern
returns by mid-week, with isolated/scattered afternoon/evening
storms expected, along with better chances for patchy fog/low
stratus, especially in the mtn valleys.

Confidence Table...

            10-16Z        16-22Z        22-04Z        04-06Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High  86%     High 100%     High 100%     High 100%
KHKY       High  91%     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:




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