Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 151511

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1111 AM EDT Tue Aug 15 2017

A stationary front is expected to remain across the region through
Tuesday while a second front approaches from the west.  This next
front will move into the region Tuesday night into Wednesday. Drier
high pressure is expected for the end of the week leading to hotter
conditions.  Moist and unsettled weather gradually returns through
the weekend into the start of the new work week.


As of 1100 AM EDT Tuesday: Have made several changes to hourly
trends including sky cover, to clear things out in the SC Upstate
early. For now have only tweaked hourly temperatures with no major
changes to afternoon highs, but if the clearing continues across the
Upstate may need to adjust some more. CAM guidance not as excited
about afternoon pops as earlier, but not ready to bite off on this
lower trend, so overall kept pops similar, but with drastic
reductions for the late morning timeframe.

Otherwise, increasingly zonal flow aloft through the day today will
drive a more potent shortwave across western NC during peak heating.
In addition to the upper forcing, a surface boundary will remain
draped through the region to provide a low level focus. Deep warm
cloud layers and continued high precipitable water values in
profiles will elevate the risk of flash flooding in any training
cells that develop. However, there should continue to be about 20 kt
of mean cloud layer flow to keep any flooding isolated in nature.
The deep moisture in profiles should once again serve to limit
instability and severe weather seems unlikely despite the high
coverage of decently strong convection this afternoon. Will feature
a rather sharp gradient in PoP, with higher likely values over
western NC, tapering down to solid chance in the southern tier.
Warmer maxes nearer 90 in SC/GA, along with lower 70s dewpoints,
will push heat index values into the 95 to 100 range there.

A shortwave ridge will develop over the southeast tonight, with any
lingering precipitation dwindling quickly overnight. Meanwhile,
Hurricane Gert will remain well out of the picture off the east
coast. With no real airmass change, another muggy night of lower 70s
mins is expected east of the mountains, with plenty of low clouds
and patchy fog throughout.


As of 300 AM EDT Tuesday:  The short term fcst kicks off on
Wednesday morning leading to into a relatively drier period as
compared to recent days.  Broad upper ridging will be building
across the southeast states while a weak surface wave slides east
across the Carolinas, likely setting up along the NC coastline at
fcst initialization.  Said surface wave looks to help pull a weak
frontal boundary into the region for Wednesday with slightly drier
air to accompany.  This drier air combined with building heights
aloft are expected to keep convection rather limited over the lower
terrain, with sct showers and thunderstorms favored over the mtns.

Moving into Thursday, a potent H5 trof looks to dig across the upper
Midwest states leading to sfc cyclogenesis beneath, and a cold front
stretching southward along the MS river.  However, closer to home
the Bermuda high looks to regain control of the flow across the
southeast allowing for gradually improving moisture across northeast
GA and the western Carolinas.  With that, profiles on Thursday
exhibit higher lfcs with improved lapse rates and weak steering
flow.  Therefore am expecting an uptick in coverage as compared to
Wednesday, thus the fcst will feature iso/sct pops over the Piedmont
with sct/num pops in the mtns.  Temperatures will gradually moderate
through the period from normal levels on Wednesday to a few degrees
above normal on Thursday when ample moisture could lead to heat
indices in excess of 100 degrees, possibly reaching near 105 along
the Midlands border zones.


As of 315 AM EDT Tuesday:  The medium range fcst period initializes
on Friday morning as a broad H5 anticyclone dominates the pattern
across the Gulf States, while an upper trof moves through the Great
Lakes and OH Valley regions.  At the surface, Bermuda high pressure
will control the flow across the southeast, while a sfc cyclone
beneath the aforementioned Lakes trof drives a cold front into the
OH/TN valleys.  Pattern evolution through the medium range will
feature the intrusion/passage of the above noted cold front on
Friday with some slight discontinuity between the ECMWF/GFS with
regards to timing as the ECMWF is a bit faster bringing the front in
during the afternoon hours, with the GFS in the evening/overnight.
Although convection is expected either way, the strength of said
convection will hinge on that timing.  As for now the fcst will
feature increasing pops to chance/likely levels for the afternoon,
holding in the chance range through the evening, slowly lowering
into daybreak Saturday.  The front looks to push into the
Midlands/Low Country before stalling out with slightly cooler/drier
high pressure building in behind the front across the CWFA.
Therefore pops on Saturday will be maximized in the chance range
along/south of I85 in closer proximity to the stalled boundary,
tapering to slight chances elsewhere.  Conditions look increasingly
wet on Sunday as another upper trof swings through the region, and a
back door front surges in from the north leading to another round of
sct convection.

As for Monday (Eclipse day), the passage of the upper trof will
bring in a period of quasizonal flow aloft, while the upper ridge
holds in the GOM.  At the surface, the aforementioned back door
front looks to slow/stall in an east/west orientation parallel to
the mean steering flow aloft.  Thus, chances for precipitation and
thus cloud cover look to be on the upswing as the ECMWF locates the
front nearly overhead on Monday afternoon as sct/num convection is
favored.  The GFS isn`t much different as far as placement, yet does
favor slightly less convection.  All said, the fcst reflects a
slight increase in sky cover (approx 50%-60%) on Monday afternoon,
however keep in mind that at this range much could change, and
likely will.  Temperatures through the medium range period will kick
off on Friday at above normal levels with excessive heat indices
possible south of I85, then gradually cooling to just below normal
into/through the weekend given multiple fropas and possible
convection.  Temperatures on Monday are currently fcst a few degrees
below normal.


At KCLT: Held onto IFR cigs for another hour for the KCLT 15z AMD,
then trended slowly back up MVFR to VFR by 17z. Winds remain WNW and
kept these as well, delaying the WSW shift until 17z. Otherwise, a
stronger shortwave should drive convection through the Piedmont once
again late this afternoon and this evening, but with a quicker end
to the showers tonight. Winds will toggle back toward WSW through
the day.

Elsewhere: Scattered shower development this morning should pass
mainly north of KAVL to KHKY, but could brush the KHKY vicinity
through 14Z. Anticipate cigs in the IFR to LIFR range for an hour or
so past daybreak, with brief IFR fog possible at times as well.
Anticipate slow but steady improvement across the mountains and
foothills through 16Z. Light winds will come up from the NW at KAVL
and from the WSW at the foothills sites as mixing develops today.
Expect showers and thunderstorms to return from the west through the
afternoon - especially from KAVL to KHKY. Another round of low
stratus and patchy fog is likely tonight.

Outlook: Chances for restrictions will remain elevated through late
week as a moist pattern persists. Scattered afternoon and evening
showers and thunderstorms are favored each day, with morning low
stratus and patchy fog likely each night, especially in the mountain
valleys and in locations seeing appreciable rainfall.

Confidence Table...

            15-21Z        21-03Z        03-09Z        09-12Z
KCLT       High  93%     High 100%     High 100%     High  94%
KGSP       High  96%     High 100%     High 100%     High  94%
KAVL       High  90%     High 100%     Low   52%     Med   73%
KHKY       High  88%     High 100%     High 100%     Med   64%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High  86%     High 100%     High 100%     High  94%

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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