Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 290639

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
239 AM EDT Wed Mar 29 2017

A cold front will push through this evening and settle southeast of
the area for Wednesday. Drier weather will develop as high pressure
builds southward from eastern Canada along the east coast in the
wake of the front. Moisture will gradually return on Thursday,
with abundant moisture in place by Friday as a strong low pressure
system moves east from the Mississippi River Valley. Drier weather
will return for the weekend, but unsettled weather could return
early next week.


As of 200 AM EDT: The departing surface cold front to the southeast
of the forecast area will continue to settle southward across
central Georgia and extreme southern SC today as sprawling surface
high pressure builds down from the eastern Great Lakes. Meanwhile, a
vigorous closed low pressure system over the southern plains will
deepen today and enhance downstream ridging over the southeastern
U.S. Good insolation and only limited thickness falls behind the
front will permit temperatures to once again reach into the lower
80s in much of the foothills and piedmont across northeast GA and
Upstate SC.

Under continued ridging aloft, 850 mb moisture will begin pooling
east of the Blue Ridge late today, and southerly upglide forcing and
moisture will develop tonight. This will permit light rain or
drizzle to begin breaking out, especialy over western NC east of the
Blue Ridge. This will herald the onset of cold air damming
conditions by daybreak Thursday.


As of 230 PM EDT Tuesday: A progressive long wave trough will cross
the mid and lower MS Valley, while a downstream ridge slides off the
East Coast thru the Short Term. As the upper trough approaches the
forecast area, southerly upglide flow will increase atop a wedge
that will set up east of the mountains by early Thursday. It still
looks like the models may be overdone on PoPs and QPF, as forecast
soundings show fairly shallow RH and low-level lift (while
persistent) is not supported much by mid or upper forcing thru at
least 00z Friday. In the very least, spotty showers and persistent
low clouds should keep temps down in the 50s to mid 60s across most
of the usual CAD areas, with upper 60s to mid 70s around the edges
in the southwest NC mountains and Upper Savannah Valley.

Thursday night thru Friday, a surface low will track from Ozark
Plateau to the southern Great Lakes. Increasing moisture and
deep-layer forcing should spread better shower coverage overnight,
especially along the south-facing slopes of the blue ridge
escarpment. Some areas may receive 1-2" of QPF, as mid-level flow
remains parallel to the mountains, helping train convection. Lows
will be well above normal. During the day on Friday, the wedge front
should begin to lift north and uncover a fairly unstable warm sector
across at least the I-85 corridor, if not further north. Models are
in good agreement on a slug of very strong deep-layer Q-vector
convergence around 18z Friday, in line with timing of a cold front.
Shear and helicity looks to support supercell or otherwise organized
convection within the warm sector, and confidence is increasing in a
severe weather threat. The SPC Day 4 convective outlook does have a
slight risk for the much of the Carolinas and GA. So I plan to add a
mention of severe tstms to the HWO. Temps should be able to get into
the upper 60s to mid 70s where the wedge erodes.


As of 200 PM EDT Tuesday:  The medium range fcst period kicks off
on Friday evening amidst passage of an upper shortwave, while an
upper ridge axis amplifies across the Plains.  At the surface,
a cold front will be approaching the region from the northwest
as broad high pressure digs out of the Upper Midwest into the
Ohio Valley.  Weak upper support associated with the passing
H5 wave will warrant at least mid chance pops initially, before
pops begin to taper from the west into Saturday morning, however
holding a bit longer along the TN line where weak nw upsloping is
possible.  By mid morning Saturday guidance favors rising heights
aloft as the surface ridge intrudes from the north with the cold
front clearing the I20 corridor, all leading to a drier fcst into
Saturday evening/night.  The surface anticyclone will shift sharply
to the southeast into/through Sunday while llv flow becomes more
veered, thus moist upglide atop the periphery of the wedge-like
pattern as well as weak upsloping potential will warrant low end
pops across the escarpment regions.  Meanwhile to the west, another
possibly closed H5 cyclone will be marching through the southern
Plains into the Midsouth region.  Pattern evolution from this
point forward is a bit unclear as operational guidance displays
some discontinuities regarding timing/placement of the Midsouth
cyclone, as well as the demise of the surface wedge pattern.
Given a more southern track of this system as displayed via the
ECMWF, the potential for strong/severe convection and heavy rainfall
is present via hslc fields as well as abundant upslope potential
along the Blue Ridge.  However at this point it is way to early
to lock in on such details, therefore for now the fcst will just
reflect increasing pops Monday night, holding at widespread chances
into/through Tuesday.  Temperatures through the medium range will
remain above normal levels.


At KCLT and elsewhere, surface high pressure will build down across
the forecast area from the north today and persist through tonight.
Expect generally NE winds east of the mountains, but with continued
NW upvalley flow at KAVL. Winds will gradually come around to a
southeasterly direction throughout by the midday hours and FEW to
SCT VFR clouds will form as 850 mb moisture starts to pool east of
the Blue Ridge under the ridging aloft. The main concern for any
restrictions through the period will be mountain valley fog,
primarily in valleys south of KAVL, around daybreak this morning.
VFR cigs will develop tonight as upglide forcing and moisture
improve, but restrictions look to be just beyond the current period.

Outlook: Moisture returning above a developing cool surface wedge
could result in low clouds and light rain across the area on
Thursday. The approach of a strong storm system will bring continued
chances of restrictions as well as a potential for heavy rain
showers and thunderstorms Thu night and Friday. Drier conditions
will return again for the weekend.

Confidence Table...

            06-12Z        12-18Z        18-24Z        00-06Z
KCLT       High 100%     High 100%     High 100%     High  94%
KGSP       High 100%     High 100%     High 100%     High  98%
KAVL       Med   62%     High  93%     High 100%     High 100%
KHKY       High 100%     High 100%     High 100%     High  96%
KGMU       High 100%     High 100%     High 100%     High  95%
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 and ensemble forecasts
are available at the following link:




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