Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 230705

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
305 AM EDT Wed May 23 2018

A moist air mass will remain over the area through today, with good
chances for showers and thunderstorms in the afternoon and evening.
A weak backdoor cold front will then move into the forecast area from
the north later tonight and into Thursday and allow for some brief
drying across North Carolina. As a tropical low develops over the Gulf
of Mexico, deep tropical moisture will overspread the area over the
weekend and persist into early next week.


As of 230 AM EDT Wednesday: A broad upper trough is pushing across
eastern Canada and New England this morning, with a shortwave
rotating toward the Appalachians at AFD time. The former subtropical
upper low, now just a bit of a weakness, remains in place over GA,
as upper ridging continues to build over the center of the country.
The eastern trough is pushing the western Atlantic upper high
farther south toward the Bahamas, and the center of the surface high
off to the east, though westward ridging toward the South Atlantic
Bight remains in place. Surface front associated with the upper
trough is making its way down the Mid-Atlantic states, with a bit of
pre-frontal troughing remaining in place across the NC/SC/GA
Piedmonts which will be absorbed by the incoming front today. The
"front" is really just a wind shift as no airmass change will be
expected behind it as we push toward the short term.  Highs pretty
close to seasonal normals today, and lows still 8-10 degrees above.

The big change for today will be the intrusion for dry air aloft,
generally 500mb and above but some creeping as far down as 700mb.
This will help to limit the efficiency of the rainfall processes,
and of course PWs will be lower today (more like 1.5" vs 1.75-1.8"
that we`ve been seeing). However, with the triggering mechanism of
the front, a little upper support from the weak dPVA, we could see a
little more intensity to the storms. Nothing significant; deep-layer
shear is at best 20kt, but the mid-level dry air entrainment could
lead to some gusty winds especially with the steep low-level lapse
rates and improving mid-level lapse rates. And even with the lower
PWs (just one ingredient of many for potential flash flooding),
soils remain very moist and it will not take much to cause
additional problems. CAMs seem to be pointing to a round of
convection initiating over the mountains and pushing off the
escarpment late this morning, drifting southeast and slowly
dissipating, with another round of convection triggering in the NW
Piedmont along the front late this afternoon and evening. It would
really be this second round that would carry the minimal severe
weather concern. SPC has us in General Thunder for Day 1 which seems
perfectly fine given the very low-end concerns. As of this writing,
the WPC Excessive Rainfall Outlook hasn`t been updated yet, but
areas along the escarpment that may see the initial convection later
this morning will need to be watched closely given previous days`


As of 255 AM EDT Wednesday: The short-term fcst picks up at 12z on
Thursday with steep upper ridging spreading over the Eastern CONUS
from the west and deamplifying in the process. Although upper ridging
will remain over the region thru the rest of the period, fairly stout
shortwave activity moving into the Great Lakes region will act to
suppress the ridging to some degree. At the sfc, a fairly robust
Canadian High will move south and over the Great Lakes before the
start of the period. On Thursday, the center of the high will move
SE of the Great Lakes and try to push some of the moist SLY flow
south of the fcst area. The result is that much of the deeper moisture
is pushed just south of NC, while SC remains under the deeper moisture.
By early Friday, the high begins to drift offshore as low-lvl flow
remains out of the east becoming more southeasterly later in the day.
As this happens, the deeper moisture begins to overspread Western
and Central NC again. By early Sat, a tropical low is deepening over
the Northern Gulf of Mexico and bringing another surge of deeper
moisture over the entire fcst area. As for the sensible fcst, no
major changes were needed. Moderate instability and a weakly sheared
environment will support pulse-type convection, with a minimal severe
threat, each afternoon/evening. Also, the risk remains high for
additional flooding thru the period, especially for areas that have
wet antecedent conditions. High temps are expected to be a degree or
2 above normal, while low temps will be about 6 to 8 degrees above
normal each day.


As of 245 AM EDT Wednesday: The extended fcst picks up at 12z on
Saturday with the persistent upper ridge being flattened as a broad
upper trof axis moves off the coast of Nova Scotia. By late Sat/early
Sunday, an upper low develops over the Northern Gulf of Mexico and
slowly drifts northward over the Gulf Coast. On Monday, the latest
model guidance has the low moving inland, however with the weak steering
flow aloft the models vary quite a bit with the track. By the end
of the period late Tuesday, the upper low is still expected to be
centered somewhere over the Southeast. At the sfc, a tropical low
associated with the above mentioned upper low, will deepen over
the Gulf of Mexico and move northward and towards the Gulf Coast.
Quite a bit of uncertainty remains with the track of this system
as it moves onshore. The ECMWF and Canadian models remain more in
synch with each other. They have the low gradually moving onshore
over the Gulf Shores/New Orleans region and then essentially stalling
out just north of there thru day 7. The GFS, on other hand, moves
the low northward over Florida on Sunday and then has the system
slowly track farther north and become less organized as it does.
At this point, the GFS solution still appears the least likely,
but it has been varying quite a bit from run to run. Regardless,
we can expect more deep moisture and nearly saturated profiles
over the fcst area from early Sat onward. High temps are expected
to remain near normal, if not slightly above, while low temps will
remain well above normal.


At KCLT and elsewhere: Still some VCSH around the Upstate TAFs but
diminishing. MVFR cigs already in place across most of the Upstate
and should expand across the area overnight, with pockets of IFR as
well. Slow improvement after daybreak with plentiful low-level
moisture in place. Expect another round of convection this afternoon
and converted some of the PROB30s to VCTS or TEMPO TSRA; timing a
little uncertain so hopefully can narrow that down with the 12z set.
Winds remain light SW today, though with the front coming down
expect a wind shift overnight with winds on the NE side for KCLT by
daybreak Thursday. Guidance hinting at another round of MVFR cigs
Thursday morning as well.

Outlook: The unsettled pattern will continue the rest of the week,
with flight restrictions possible each day under diurnal showers and
thunderstorms. Tropical moisture may increase through the late
weekend. Morning stratus/fog are possible each day - especially
following heavy rain the previous day.

Confidence Table...

            07-13Z        13-19Z        19-01Z        01-06Z
KCLT       High  81%     High  93%     High 100%     High 100%
KGSP       Med   68%     High  92%     High 100%     High 100%
KAVL       High  80%     High  96%     High 100%     High 100%
KHKY       High  86%     High  98%     High 100%     High 100%
KGMU       Med   75%     High  91%     High 100%     High 100%
KAND       Med   75%     High  80%     High 100%     High  96%

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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