Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 231513

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1113 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 1030 AM EDT Wednesday: A few patches of light rain/showers,
mainly over the nrn Foothills and the upper Savannah R basin as
of mid-morning. Suspect that much of this initial activity will
choke and die with the late morning lull, during which we will wait
for destabilization and new storm development elsewhere, based on
satellite imagery. This could take awhile across northeast GA and
the wrn Upstate where cloud cover was more extensive. However, the
mtns, foothills, and wrn Piedmont of NC are free to convect. The
convection-allowing models still appear to be somewhat vexed by
this air mass as each one develops showers at different times and
different areas. Some minor changes were made to precip chances
mainly to agree with neighboring fcsts.

Otherwise, a broad upper trough is pushing across eastern Canada
and New England this morning, with a shortwave rotating toward the
Appalachians. 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. The
WPC Excessive Rainfall Outlook does not actually outline any of
our areas today, but areas along the escarpment that may see the
initial convection later this morning will need to be watched
closely given previous days` rainfall.


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: MVFR to patchy IFR (and isolated lower) cigs
are in place across the area this morning, with a few holes here and
there. Guidance trend is to continue delaying improvement until noon
or possibly shortly thereafter, and did delay the improving trend in
the TAF but not quite as much as new guidance is indicating; will
have to monitor through the morning. Expect another round of
convection this afternoon though coverage looks less than earlier
anticipated; have continued VCTS everywhere with a TEMPO TSRA at
KCLT, trending to PROB30s later in the afternoon that will have to
be updated with later AMDs. 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 and have
included this for all sites.

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...

            15-21Z        21-03Z        03-09Z        09-12Z
KCLT       High 100%     High 100%     High 100%     High  86%
KGSP       High  88%     High 100%     High 100%     High  92%
KAVL       High 100%     High 100%     High  98%     High  88%
KHKY       High  98%     High 100%     High 100%     High  90%
KGMU       High  90%     High 100%     High 100%     High  87%
KAND       High  86%     High 100%     High 100%     Med   76%

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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