Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 220728

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
328 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 245 AM EDT Tuesday: 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: VFR to start the period, but with another
round of increasing low-level southerly moisture, expect MVFR
stratus to develop especially at KAND and attempt to work its way
north and east. Have kept trend of prevailing at KAND, tempo KAVL,
and just SCT MVFR elsewhere. Low-level moisture from  yesterday`s
rain may allow for some fog to form near KHKY so lowered vsby
accordingly. Expect higher chances for convection on Tuesday, and so
have introduced VCTS at most sites but kept the PROB30 as well, and
will have to narrow down timing for the 12z TAFs. 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...

            07-13Z        13-19Z        19-01Z        01-06Z
KCLT       High 100%     High  95%     High 100%     High 100%
KGSP       High  95%     High  87%     High  98%     High 100%
KAVL       High  81%     High  92%     High 100%     High  85%
KHKY       Med   72%     High  96%     High 100%     High  90%
KGMU       High  93%     High  87%     High  98%     High 100%
KAND       High  89%     High  85%     High  92%     High  97%

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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