Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 221100

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
700 AM EDT Tue May 22 2018

A moist air mass will remain in control of our weather through
midweek, with good chances for showers and thunderstorms each
afternoon and evening. A weak backdoor cold front will move into
the forecast area from the north on Thursday and should allow
for some brief drying. Another round of deep tropical moisture
is expected to lift across the area over the weekend.


As of 630 AM EDT Tuesday: Very little change to the forecast this
morning, with only some refinements to pops. Increased pops across
the mountains; CAMs continue to look a little worrisome with
convection in areas that have had a bit too much rain lately, so
this will have to be monitored closely. Guidance is wanting to keep
temperatures down across the western Upstate/Upper Savannah Valley a
little lower than the current high temperature forecast. Have
started trending this way with hourly temperatures but no changes to
afternoon highs just yet until we see trends start to line up

Otherwise, the upper pattern is kind of complicated as we start the
period, with ridging over the western Atlantic, a cutoff low over
southern CA, a broad upper trough over eastern Canada, a shortwave
pushing down the Upper Midwest, and the pesky persistent subtropical
weakness still over the Deep South. Low-level flow remains warm and
quite moist, with weak westerly steering flow. The Upper Midwest
shortwave will push a front toward the area toward the end of the
period, with some pre-frontal troughing taking shape down the lee of
the Appalachians this afternoon. Lapse rates across the area remain
fairly low with a nearly tropical-looking profile, but will begin to
increase somewhat ahead of the approaching shortwave. Minimal to
perhaps moderate CAPE (no more than 1500J/kg) will develop across
the area today and could certainly see some gusty winds out of that,
but the main story remains the rain. PW values remain very high, 1.7-
1.8, with 925-850mb moisture flux vectors hitting right up against
the SW mountains. CAMs all seem to point to some showery activity in
the typical upslope areas later this morning, but as the slug of
moisture lifts north toward the mountains, convection should begin
to bubble up in weakly organized clusters across the Upstate just
before noon, lifting northeast with the weak flow around the Deep
South upper weakness, and expanding as they spread across the
Piedmont. A secondary round of storms should trigger in the higher
elevations of the mountains and spread east with the weak westerly
steering flow from the longwave pattern to our north. As with
previous days, perhaps not everyone will get rain, but those who do
could get it pretty heavy, as warm rain processes will dominate. The
I-77 corridor seems least likely to get rain, while the Upper
Savannah Valley/western Upstate and of course the mountains could
easily get a quick 0.5-1" basin average. Localized flash flooding
will remain a concern, especially in areas that had problems over
the past few days.

Temperatures today will be slightly above normal in the east where
some sun may break out this morning before clouds/convection push
in, and at to just below normal to the west. Soupy overnight lows
(again) and depending on how widespread the rain is this afternoon,
could see some patchy dense fog toward the end of the period.


As of 310 AM EDT Tuesday: The short-term fcst picks up at 12z on
Wednesday with a very broad upper trof moving across Southern
Ontario. This trof, although very far to our north, will act to
suppress the persistent upper ridge that has been over Southeast
for several days. At the same time, upper ridging will amplify
significantly over the Central CONUS. Thru the rest of the period,
heights will gradually recover as the upper ridge slides slowly
eastward and deamplifies in the process. At the sfc, a fairly
robust Canadian High will move south and over the Great Lakes
by the start of the period early Wed. This will put the CWFA
between moist SWLY flow from the south and drier NLY flow,
associated with the high, just to our north. On Thursday, the
center of the high moves SE of the Great Lakes and pushes the
moist SLY flow mostly south of the fcst area. By early Friday,
the high begins to drift offshore as low-lvl flow remains out
of the east and the bulk of the deeper moisture remains along
our southern fringe. As for the sensible fcst, moderate instability
and a weakly sheared environment will support pulse-type convection
with a minimal severe threat each afternoon/evening. Also, the
risk remains for additional hydrologic issues especially for
areas that have favorable antecedent conditions. High temps are
expected to be a few degrees above normal, while low temps will
be about 4 to 6 degrees above normal each day.


As of 245 AM EDT Tuesday: The extended fcst picks up at 12z on
Friday with broad upper ridging centered over the region. As
we move into the weekend, heights begin to gradually fall as
broad upper trofing digs down across Eastern Canada. By early
Sunday, the long range models develop an upper low over the
Northern Gulf of Mexico and slowly move it northward over the
Gulf Coast. By the end of the period early next week, model
guidance has the low centered over the Alabama/Mississippi/
Louisiana region. At the sfc, Canadian high pressure will be
moving offshore by early Friday as a tropical low deepens over
the Gulf of Mexico. Over the weekend, the low is expected to
strengthen as it moves northward and towards the Gulf Coast.
There is still quite a bit of uncertainty wrt the track of
this system as it moves onshore. The ECMWF has it moving
onshore by early Sunday and then tracking up the Mississippi
River thru day 7, while the Canadian is about a day slower
but has a very similar track. The GFS, on the other hand,
develops a weak low over Western Cuba and moves it northward
over Florida on Sunday. It then has the system make an abrupt
turn to the left and moves it across Alabama and Mississippi
early next week. At this point, the GFS solution 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 our area from early Sat onward. High
temps are expected to remain slightly above normal through the
period while low temps will be well above normal.


At KCLT and elsewhere: Low-level moisture in place from
yesterday`s convection has led to VLIFR conditions at KHKY; another
round of incoming moisture from the south has brought patchy MVFR to
the Upstate TAFs and KAVL. Expect higher chances for convection
today than what we saw yesterday, and with the previous introduction
of VCTS have added TEMPO TSRA for most sites as well with this TAF
set. Expect at least MVFR conditions to be possible with convection
and possibly gusty winds, but bigger issue will be heavy rain. After
brief improvement after the convection begins to wane, plentiful low-
level moisture will bring at least MVFR cigs/vsby to the area,
especially the mountains and adjacent TAFs, for Wednesday morning.
Winds will remain out of the S/SW generally below 5kt, though may
creek above during the afternoon.

Outlook: The unsettled pattern will continue the rest of the week,
with flight restrictions possible each day under periodic showers
and thunderstorms. Morning stratus/fog are possible especially
following heavy rain the previous day.

Confidence Table...

            11-17Z        17-23Z        23-05Z        05-06Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High  94%     High  89%     High 100%     High 100%
KAVL       High  87%     High  94%     High 100%     Med   71%
KHKY       Med   69%     High  94%     High 100%     Med   75%
KGMU       High 100%     High  89%     High 100%     High 100%
KAND       Med   75%     High  89%     High  94%     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 and ensemble forecasts
are available at the following link:




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