Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 212339
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
739 PM EDT Wed Jun 21 2017

.SYNOPSIS...
A chance of showers continues through tonight as Tropical Storm
Cindy approaches the Louisiana coast. As Cindy lifts north into
extreme western Louisiana, tropical moisture spreads across the
region Thursday and Friday.  A cold front approaches the area
early Saturday and lingers across the area through Sunday keeping
a chance of showers in the forecast.  Cooler weather follows the
front with perhaps another round of light rainfall by mid-week.

&&

.NEAR TERM /THROUGH THURSDAY/...
As of 735 PM EDT: Satellite imagery this evening shows the
circulation of Tropical System Cindy just south of the western
Louisiana Gulf Coast. A broad expanse of tropical rainfall extends
well northeast of this system across the Deep South, but KGSP radar
continues to show only low end scattered to isolated showers across
the area - forming mainly in the deformation zone overhead. A
vorticity lobe will lift northward across the Deep South overnight
and likely reach the southwest part of our forecast area around
daybreak Thursday. Light southwesterly upslope flow in the surface
to 850 mb layer will continue through the night, keeping the PoP
focus on the southern mountains. Anticipate mild overnight mins as
clouds again thicken and lower.

The upper vorticity axis will lift northward across our region
through the day on Thursday, with the best forcing and deepest
tropical moisture likely impacting the mountains - with a continued
southern and southwestern mountain QPF focus. Precipitable water
values will be very high, above 2 inches in many areas, but cloudy
conditions and very moist profiles will keep instability rather
limited on Thursday. The lack of better convective rates along with
low stream and river levels will help curtail flooding problems
through the near-term. Max temps should be capped in the 70s most
areas.

&&

.SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/...
As of 230 PM EDT Wednesday:  The short term fcst period kicks off
on Thursday evening amidst a rather active pattern thanks to TC
Cindy moving deeper into the Arkaltex region, while subtropical
ridging dominates over the western Atlantic.  The primary fcst
challenge during this period will be the eventual progression/track
of Cindy`s remnant low/moisture across the TN Valley into the
southern/central Appalachians.  At fcst initialization moisture
advection thanks to Cindy will have pushed pwats well above 2 inches
across much of region per both the NAM/GFS solutions, while a band
of channeled vorticity associated with an H5 wave moves northward
through west/central NC.  Given this ample moisture and sufficient
synoptic lift, think above normal pops are warranted through the
evening.  With that said, improved sly H85 flow will warrant the
highest pops across the southern facing slopes of the escarpment,
at least into Friday morning before said flow begins to veer wly
as Cindy slides into the MidSouth.  Although the overall flooding
threat will be diminishing as the plume of higher pwats advects
northward, think at least a non-zero flooding threat will persist
along the Blue Ridge into early morning Saturday.

As the initial swath of above normal pwats advects well to the north
and the subtropical ridge slides slightly west and strengthens,
think chances for precipitation will lower a bit on Friday with
the highest pops on the periphery of the ridge across the western
NC and GA mtns.  Meanwhile, Cindy is fcst to get swept up by a
deepening H5 trof that will dive through the Upper Midwest into
the Great Lake and Ohio Valley by 00z Saturday, providing focus
for increasing low/mid level flow and thus shear.  In fact, as
Cindy approaches through the day the 12z Nam favors increased H85
flow likely leading to breezy conditions across the area, while
increasing further into the evening hours when nearly 50-60kt flow
spreads through northeast GA and the western Carolinas ahead of
the low circulation effectively increasing 0-6km bulk shear in
the vicinity.  Fortunately model instability fields are less than
impressive, however with ample dynamics at play a marginal risk
of severe tsra is logical, thus the Day 3 SPC outlook.

With the remnants of Cindy ejecting further east on Saturday
becoming less of a focus for the southern Appalachians, a weak
"cold" front will move through the high terrain, likely stalling out
along/near the I85 corridor on Saturday afternoon.  Guidance tends
diverge a bit at this point with the primary uncertainty lying with
the timing of the weak frontal passage, and any aforementioned
stalling location.  Ahead of the front the environment will be
relatively unstable with guidance favoring upwards of 1-2k j/kg
sbcape suggesting a favorable environment for convection with the
best chances lying along/south of I85 per the latest guidance.

&&

.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
The long-advertised cold front is forecast to move through most of
the area Sunday morning and as it does so, rain chances diminish
significantly with only lingering light showers expected to
start the day.  With the parent low and upper-level support far
removed from the surface front, the front will struggle to reach
the Midlands.  High pressure behind the front will be centered
across the southern Plains so the overall surface wind field will
be northerly, advecting modified continental air into the region
to start the work week.

Subtle shortwave energy will cross the Appalachians during the
day on Monday while a stronger, reinforcing shortwave trough over
the Great Lakes will amplify the broad longwave trough over the
eastern CONUS.  As a result, at this time I cannot completely remove
slight chance to chance pops across the area given the proximity
of the remnant front to our south and east and weak forcing aloft.
Regardless, Monday will be cooler and any showers should not ruin
the pleasant airmass.

The stronger shortwave trough will finally push the longwave trough
axis to the east of our area late Tuesday and surface high pressure
builds underneath the height rises occurring over the Ohio Valley.
This will further lower our dewpoints as a true continental
airmass takes hold across the region and maintains control
heading into Thursday.  Therefore, work week high temperatures
will run about 5-7 degrees below normal with low temps near normal
given low humidity and dewpoints in the upper 50s and lower 60s.
The weather will not get much better for late June!

&&

.AVIATION /00Z THURSDAY THROUGH MONDAY/...
At KCLT: Showers have stayed away from the airfield but have been in
the vicinity. Very limited forcing, despite the moist profiles,
should keep coverage to a minimum with little more than VCSH
warranted through the evening. Anticipate a return to MVFR
conditions through the overnight hours, with showers starting to
break out again from the southwest after daybreak Thursday. A period
of morning IFR cigs will be possible with any showers that arrive.
MVFR cigs should linger through the day with showers lingering.
Expect light southwest flow through the period.

Elsewhere: Showers have been near or over all but KHKY. Isolated
showers will continue through the evening. Anticipate better
tropical moisture and forcing arriving from the southwest overnight,
first affecting KAND, then KGSP to KAVL by daybreak, spreading to
KHKY through the later morning hours. An extended period of MVFR to
IFR ceilings will be likely as the better moisture arrives. Expect
light south to southwest winds through daybreak. However, winds turn
easterly for the SC sites for the afternoon.

Outlook: Abundant moisture will remain over the area through
Saturday, enhanced by remnants of Cindy which will approach from the
west on Saturday. An uptick in diurnal convection and associated
restrictions can be expected, as well as the potential for morning
fog/low stratus. Some drying may occur on Sunday.

Confidence Table...

            23-05Z        05-11Z        11-17Z        17-18Z
KCLT       High 100%     High 100%     Med   60%     High  94%
KGSP       High 100%     Low   55%     Low   43%     Med   65%
KAVL       High  89%     Low   48%     Low   43%     Med   75%
KHKY       High 100%     High 100%     Low   53%     Low   46%
KGMU       High 100%     High 100%     Low   48%     Med   65%
KAND       High 100%     Med   75%     Med   65%     Med   69%

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:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

$$
SYNOPSIS...JMP
NEAR TERM...HG/RWH
SHORT TERM...CDG
LONG TERM...JMP
AVIATION...RWH


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