Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 181045

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
645 AM EDT Fri Jun 18 2021

High pressure will remain in place today into Saturday morning
before dissipating. A potential tropical low will track north out of
the Gulf of Mexico this weekend into early next week increasing
shower and thunderstorm chances over our area. A cold front
approaches from the west early next week as the low departs the
forecast area keeping shower and thunderstorm chances around.


As of 640 AM...No big changes to the forecast with this update. A
little valley fog showing up on the latest nighttime RGB product,
but that should dissipate quickly after sunrise. Temps are running
a little cooler than forecast thanks to calm wind and clear skies,
but don`t think that will have any impact on the max temps today.

Otherwise, dry sfc high pressure will linger over the area today
under weak NWLY flow aloft. Sfc flow will begin to shift out of
the SW as the center of high pressure shifts east off the Carolina
coast. Thicknesses continue to rebound, and highs are expected to
top out a category or two above normal this aftn. Fortunately,
humidity will remain a little below normal for mid-June, with
dewpts staying in the 50s to perhaps lower 60s. The dry air will
result in sunny to mostly sunny skies again today, with just a
few high-based cu and wisps of cirrus today.

Tonight, a tropical disturbance is expected to become a named
storm (Claudette) and make landfall along the LA coast. The system
will be fairly compact and little moisture will reach this far NE
thru daybreak Saturday. Dewpts will begin to tick upward within
a SWLY flow, and low temps will be near to slightly above normal,
mainly in the 60s to around 70. Skies will start out mostly clear,
with some increase in clouds toward sunrise.


As of 330 am Friday: Tropical system is expected to make landfall
near the start of the period over southeast Louisiana, with a jog to
the northeast expected thereafter. Associated moisture is unlikely
to impact our forecast area until Saturday night at the earliest,
but short term guidance depicts sufficient instability across
western areas Sat afternoon to advertise 20-30 PoPs for diurnal
convection. The remainder of the short term is rife with
uncertainty, due to low confidence in the eventual track/intensity
of the tropical system and its impact on the PoP/QPF forecast.
Eventually, the system will likely make a jog more or less due east,
as large scale height falls emanate from the central Conus toward
the end of the short term. For now...almost all major deterministic
model guidance depicts the swath of heavy rainfall across our
extreme southern forecast zones...or just south of the CWA. (It
should be noted that the ECMWF depicts an interesting jog to the
north into the NC Piedmont late Sunday, possibly as the system is
undergoing some sort of extra-tropical transition within that
model.) Despite that relative agreement, confidence will remain low
until the system becomes better organized, which may well not happen
for another 24 hours or so. All that is to say that the potential
still exists for heavy rainfall Sunday into early Monday, especially
across the southern half of the area, but uncertainty abounds
regarding where/when/how much.


As of 345 am Friday: Global models generally agree that tropical
remnants should be lifting east and well away from the forecast area
at the start of the period...just as a frontal zone associated with
large scale height falls overspreading the East approaches the area.
As such, after a bit of a lull in precip probabilities early Monday,
PoPs ramp back up into the likely range from late Monday into
Tuesday. The GFS actually depicts some respectable deep layer shear
considering the time of year(30-40 kts), thus some degree of severe
weather threat will likely exist, depending upon timing/the degree
of instability. Locally excessive rainfall will also be possible,
although that will probably depend heavily on the state of
antecedent hydrologic conditions in the wake of the tropical system.
Anomalously low heights over the East will result in the resumption
of an unseasonably dry air mass with cooler-than-normal temps during
the middle part of next week.


At KCLT and Elsewhere: VFR conditions expected thru the period. Calm
or LGT/VRB to start the 12z TAFs, then winds should pick up
from the SW in the morning as sfc high pressure center shifts
east. Despite this flow, moisture return will not really occur
until after this period, so just FEW high-based cu are expected
to develop by afternoon. Guidance not showing much fog in the
mountain valleys tonight, so keeping KAVL VFR.

Outlook: Dry high pressure will continue into Saturday, before
tropical moisture arrives Sunday. Considerable uncertainty
exists with the tropical potential from the Gulf of Mexico, but
rainfall and restrictions will be possible Sunday through Monday,
and possibly on Tuesday with the arrival of a cold front from
the northwest.

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 100%     High 100%     High 100%     High 100%
KHKY       High 100%     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 are available at
the following link:




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