Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 191737

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
137 PM EDT Sat Aug 19 2017

High pressure will build across the forecast area and bring
relatively dry conditions through Sunday. Then a warming and
moistening airmass will build in, bringing hot and humid afternoons
Monday through Wednesday. A cold front is expected to pass through
Wednesday night and usher in cooler and drier air that will last
into next weekend.


As of 1030 AM EDT Saturday: Still have some mid-level cloudiness
over the area behind the front, and a little bit of Cu over the
mountains, but otherwise a very pleasant morning out there.
Dewpoints are dropping behind the front and the diurnal rise in
temperatures is a bit slower than forecast, so have made several
adjustments to hourly temperatures. Tweaked afternoon highs a bit
but result isn`t much different than what we had before. Pop
forecast (well, lack thereof) looks good. Should be a really nice
Saturday across the area.

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 noon Saturday: Consensus of GFS and EC continues to indicate
an amplified upper pattern developing in the middle of the week,
reaching its greatest magnitude around Thursday. As the component
trough digs over the East, a fairly strong cold front will push
into our region late Wednesday. At the present time it looks fairly
likely this front will focus deep convective development, given that
it will be moving into a hot and humid airmass associated with a
subtropical ridge. With 20-25 kt of 0-6 km shear we can`t rule out
persistent linear storm structures, though dry air aloft appears to
be lacking, which may mitigate the severe wx threat to some degree.

Given the amplified pattern, unsurprisingly a large sfc high
builds into the area following the front. While some guidance
depicts precip returning Thursday afternoon while the front is
still on our southern fringe, most indications are for subsidence
to have taken hold by that time, and therefore depict suppressed
convection. By Friday the high should have moved onto the Eastern
Seaboard, resulting in a cool wedge setup which persists into the
weekend, with deep convection looking even less likely. However,
easterly flow still warrants a low PoP in the high terrain. Max
temps will be 3-5 degrees below normal Thursday and into the weekend
on account of the wedge, with dewpoints in the upper 50s to lower
60s making for particularly pleasant conditions.


At KCLT and elsewhere: VFR through the period with the exception of
some mountain valley fog that may impact KAVL around sunrise. Post-
frontal NNE winds will dominate today, though may briefly veer
around to ESE with the weak pressure pattern. Speeds should continue
to diminish, generally <5kt. Direction will vary overnight, and
finally swing around E to SE by midday Sunday. Other than some low
VFR Cu this afternoon and again perhaps some low stratus with the
mountain valley fog at KAVL, generally just some passing high
clouds. Should see another round of Cu developing Sunday afternoon
but more likely after the end of the period.

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 mountain valleys.

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  83%     Med   66%
KHKY       High 100%     High 100%     High 100%     High  83%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     High 100%     High  83%

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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