Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 201928

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
328 PM EDT Tue Jun 20 2017

A remnant frontal boundary lifts back north this evening to focus
yet another round of showers and storms across the area before drier
high pressure builds in for Wednesday.  Rainfall chances increase
yet again Thursday and Friday as tropical moisture spreads across
the region.  A cold front approaches the area Sunday and moves
through the area early next week with drier weather to follow.


As of 230 PM EDT Tuesday:  Overall the synoptic pattern is fairly
active this afternoon as a newly named Tropical Storm Cindy
highlights the wx in the GOM, while a deep/broad H5 trof slides
eastward across the Upper Great Lakes, and a quasistationary frontal
axis remains draped from southwest to northeast along/south of
the I85 corridor.  Currently across northeast GA and the western
Carolinas, the aforementioned front is the dominant feature
as it provides focus for convection.  Modest lapse rates amidst
continually moistening profiles will present a non-zero chance for
thunder in association with some of these cells this afternoon
and early evening, however with little/no threat of any strong
convection.  As stated, profiles adjacent to the front are moist
with pwats approaching/exceeding 2 inches today/tonight, which
combined with the frontal convection will present an elevated
threat for heavy rainfall.  With that, the primary concern will
be the prolonged nature of said rainfall as shra/tsra remains near
the stalled frontal axis itself.  Given that Flash Flood guidance
regionally is lowest across the Lakelands region of SC as well
as northward into the SC Piedmont and lower NC Piedmont, think
these areas will have the greatest threats for isolated flooding.
Fortunately model QPF remains rather low, generally under 1
inch over the next 24 hours, however isolated higher amounts are
certainly possible.  Overall, think the threat will be isolated and
best handled on a case by case basis, therefore no flash flood watch
is planned at this time.  As for pops, likely/categorical levels are
featured furthest south through the afternoon and early evening,
generally tapering to slight/chance levels along/north of I85.
Otherwise, temperatures across the region reflect the coverage of
convection with locales across western NC near normal, with levels
being near to well below normal further south of I85 thanks to
abundant sky cover and precip.  Guidance favors lowering of rain
rates overnight as any minimal instability wains, therefore so
should the isolated flash flooding threat with the only possible
exception being across the southern most zones.

Moving into Wednesday, moisture will continue to advect northward
from the Cindy as she slides northward approaching the Gulf States.
However as the upper aforementioned upper trof ejects northeast,
a subtropical ridge will wedge itself back in across the southeast
CONUS on the eastern flank of Cindy.  Thus, think convective
chances actually diminish a bit on Wednesday as profiles dry out
and increased mid level temperatures lower lapse rates aloft.
That said, any subsidence associated with this subtropical ridge
will be weakest across the western half of the fcst area, therefore
a few shra/tsra are certainly possible if CIN becomes weak enough.
Therefore the fcst will feature slight/chance pops regionwide with
the highest chances being over northeast GA, the western Upstate,
and extreme southwest NC.  Temperatures on Wednesday will return
to near/above normal levels.


As of 230 PM EDT Tuesday: This looks to be a wet period with models
showing tropical moisture lifting gradually northward. Instability
will be limited but precipitable water values will be around or even
exceed two inches at times. At least locally heavy rain will be
possible each period especially Thursday night.

With the increased cloud cover and rainfall, the diurnal temperature
range will be smaller than normal leading to cool days and warm
nights. The high moisture content will make for muggy conditions.


Overall there isn`t too much change in the forecast during the
extended period. TS Cindy still gets absorbed into the westerlies
while an open shortwave trough and attendant cold front dives
southeast into the mid-Mississippi River Valley Saturday morning.
Whatever low-level circulation that remains from Cindy will
interact with the approaching cold front and propagate across the
TN Valley and towards the central Appalachians on Saturday.  By the
time the front reaches the southern Appalachians, PWs remain high
between 1.75-2 inches, but lower than the peak values forecasted
during the short-term.  Therefore, the forecast maintains high-end
chance to likely pops across the mountains where the forcing is
best and chance pops elsewhere.  Rainfall will be efficient, but
widespread heavy rainfall is not expected with the frontal passage
and the heaviest totals associated with the remnants of Cindy are
currently expected to stay further west and north towards TN/KY/WV.
Of course, these details will need to be fine-tuned as guidance
gets a better handle on the timing of the front and Cindy.

The front is still expected to move through the area early Monday
morning which will usher in a cooler and drier continental
airmass across the area, with the low-70 dewpoints from the
weekend replaced by dewpoints in the 50s and lower 60s by late
Monday through early Wednesday.  High temperatures may run 5-10
degrees below normal for Monday and Tuesday.  There is disagreement
amongst the guidance about the return of light precip associated
with another shortwave trough propagating through the westerlies.
The ECMWF keeps the area completely dry while the Canadian is the
most aggressive with widespread rainfall, and the GFS is a fair
compromise between the two.  As a result, the forecast maintains
slight chance pops as the shortwave approaches Tuesday evening
into Wednesday morning with only light rainfall expected.


At KCLT and the SC Sites:  A mixture of low VFR, MVFR, and
eventually IFR through this period.  A stalled frontal axis just
south of the I85 corridor will continue to be a focus for iso/sct
shra and possibly tsra this afternoon/evening.  With that said,
confidence in tsra at the terminals is fairly low, thus will only
include vcsh at the SC sites while a tempo for shra is favored at
KCLT through the afternoon hours amidst low VFR/MVFR stratocu.
Will continue with a lowering cig trend into the evening and
overnight hours with IFR becoming more likely as shra potential
slips slightly.  IFR cigs amidst MVFR visb will remain in the fcst
through daybreak before both improve by one category to round out
the taf cycle.  All precip timing in tafs was based upon a CAM
guidance consensus.  Winds will remain light sly through the period.

Elsewhere:  Lower chances for restrictions at KAVL/KHKY given
the proximity of the stalled frontal axis being well to the south.
That said, ample BL moisture will allow for at least low end chances
for shra later this evening and overnight amidst development of
MVFR and possible IFR cigs/visb.  Conditions will improve at both
sites into the post dawn hours on Wednesday.  Winds will remain
generally sly through the period.

Outlook: Could have MVFR/IFR/LIFR restrictions across the region
persisting early Wednesday morning due to leftover low moisture.
Drier conditions may finally return during the latter part of
the week if the cold front can push south of the area, however
confidence remains low as possible remnants of a tropical system
may impact the area late in the week.

Confidence Table...

            19-01Z        01-07Z        07-13Z        13-18Z
KCLT       Med   78%     Low   58%     High  86%     High  88%
KGSP       High  80%     Med   67%     High  80%     High  84%
KAVL       High 100%     High  89%     Med   69%     High  87%
KHKY       High 100%     Med   69%     Low   56%     Med   66%
KGMU       Med   75%     Med   66%     High  87%     High  88%
KAND       Med   66%     Med   74%     High  82%     High  80%

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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