Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 272105

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
505 PM EDT Tue Sep 27 2016

A stationary front will remain over the Midlands of South Carolina
and the Sandhills of North Carolina through mid week. An upper level
low will drop south into the Ohio Valley on Wednesday and linger
there through the end of the week.


As of 455 PM, widely sct/discrete convection firing off the high
terrain encountered some moderately unstable air, moderate shear,
and rather strong dCAPE across the foothills, and some of the cells
have made the most of it, producing brief large hail and damaging
winds. This threat will continue as convection moves across the
Piedmont over the next few hours. Meanwhile, additional activity was
developing off the Blue Ridge in the wake of the initial round.
Outflow and anvil canopy has cooled the air mass downstream of the
Blue Ridge convection, so this probably doesn`t pose as high of a
severe weather threat, esp since the air mass wasn`t all that
unstable in the first place. Fortunately, the improved mid-level
flow is allowing cells to move along at a pretty good clip (20-30
mph). This along with the (thus-far) lack of any multicell
organization suggests the chances for excessive rainfall are small
in the next 2-4 hours. However, enough shear is present such that
future organization is a definite possibility, most likely over the
I-77 corridor. That being said, based upon the currently widely sct
nature of the activity, pops were scaled back a bit in most areas
through early evening, although a likely was maintained along/east
of I-77.

In the big picture, the western Carolinas and north Georgia should
remain in the cyclonic flow aloft through Wednesday as a large upper
low over L Superior today drops almost due S over the next 24 hours,
keeping the trof axis to our west. This development should keep the
front moving only slowly east across the fcst area overnight.
Convection should wane in the late evening, with only widely
scattered precip expected over the Piedmont in the early morning
hours. The front will probably still be hung up across the
foothills/wrn Piedmont on Wednesday. Synoptically, we should retain
much of what we have today, that is weak upper divergence and a few
weak spokes of vorticity rotating around the upper low. Combined
with the low level focus of the sfc boundary, expect that new storms
will fire over the Piedmont/Upstate during the late morning and
afternoon. There should be enough shear and buoyancy along/east of
the front to provide for a few strong/severe storms, although the
shear/instability combo will be better just to the east. The Day 2
has the wrn Piedmont in the Marginal Risk and again this will not be
debated. Will not be surprised to see it expand west or get an
upgrade in later issuances. Leaned toward the warmer guidance for
temps, especially for highs on Wednesday with the front not all the
way through the area.


As of 100 PM Tue: The region will still be in the process of pattern
change Wednesday night, as a cold front pushes into central NC and
the SC Midlands, and a deep cutoff low moves south into the lower
Ohio Valley. Consensus of NAM, GFS and SREF suggests convective
activity will linger over our lower Piedmont zones thru the evening
along/ahead of the front. Meanwhile, CAA sets up over East Tennessee
overnight into Thursday morning, concurrent with enough moisture to
warrant steady PoPs in our Tenn border zones. Min temps Thu mrng will
be back around normal.

The proximity of the upper low will permit deep mixing and sustain
good lapse rates aloft over much of the CWFA thru Thursday.
Deterministic models do not respond to this pattern with appreciable
QPF, so the higher PoP output from some MOS is assumed to reflect the
association of the pattern with abundant if not shallow showers.
Though the low remains centered in essentially the same position
Friday, heights are higher and profiles more stable over our area, so
we can advertise a dry forecast then. Temps will be refreshingly
cooler Thursday, with maxes near normal. These trend still cooler for
Thu night and Fri over most of the area, with the additional sunshine
seemingly offsetting the cooler air in the north and mountains. They
should top out generally 2-4 degrees below normal.


As of 155 PM EDT Tuesday...Starting at 00Z Saturday the cut off
upper low will be centered near Louisville KY. This low will slowly
lift north and fill as it becomes absorbed in the westerly upper
flow near the eastern Great Lakes Sunday night into Monday.  Models
still have a surface high pressure over eastern Canada to start the
week ridging southward to the Carolinas. A high amplitude upper
ridge will be over the East during the first of the week as a trough
sweeps across the northern Rockies and plains states.  Ridgetop
convection becoming more favorable in the PM of Day 7 for the NC

Concern will be building next week as a pending hurricane moving
north out of the Caribbean.  The 06Z GFS has it crossing Haiti at
12Z Monday then just east of Cape Hatteras as a powerful hurricane
at 18Z Wednesday as it heads toward Long Island. The old ECMWF has
it crossing eastern Cuba at 18Z Wednesday so this model is much
slower.  The new 12Z GFS has a track farther to the east with still
a powerful hurricane at 12Z Thursday halfway between NC and Bermuda.
All progression of this tropical system must be watched very closely.

Temperatures are expected to be a few degrees above normal from
Saturday through the first half of next week after the cut off low
has departed and we are under the upper ridge. Although Min Temps at
first will be near or a little below Saturday.


At KCLT...we await the development of deep convection near the Blue
Ridge, which should approach from the west in the late afternoon. In
the mean the mid-level ceiling moves east, we should see
additional low cloud development that should be in the MVFR range.
Cannot rule out a brief MVFR broken ceiling, but this was not
included in the TAF for brevity. Instead, think it more important to
indicate the possibility of a line/area of showers and storms moving
across metro Charlotte in the evening hours. The CAM guidance is in
relatively good agreement that convection will impact KCLT, perhaps
as early as 21Z and as late as 03Z. The TEMPO will fit in the middle
of that time frame. Once the convection moves east, expect a few
hours of convective debris, eventually clearing out enough to cool
off to allow fog to develop. Some of the guidance hits this very
hard with VLIFR possible. Prefer for now to indicate the development
of an MVFR restriction (which will be widespread across the wrn
Piedmont) and let the next shift include more IFR or lower details
after the storm threat subsides. Any morning restriction should lift
by 14Z or so. The front will remain close by on Wednesday morning.
Could see redevelopment right on top of KCLT during midday. This
also will not be included for brevity at this time. Winds will be SW
today and should veer a bit more toward WSW on Wednesday, with gusty
and variable winds near storms.

Elsewhere: Should see deep convection initiating near the Blue Ridge
by 19Z or so. Have timed this across the fcst area in a 4-hr window
with a TEMPO for TSRA at all TAF sites, followed by a VCSH for
lingering showers. Similar evolution to KCLT after that, with
widespread MVFR/IFR fog/stratus developing in the early morning
hours on Wednesday. Again, details had to be omitted. Think best
chance of new storms on Wednesday will be to the east.

Outlook: Cold front will slowly push out of the area Wednesday
afternoon. Dry high pressure will build in slowly behind the front
Wednesday into Thursday, which should bring VFR conditions in most
places through the end of the week, although some instability could
lead to showers over the NC part of the area on Thursday afternoon.

Confidence Table...

            21-03z        03-09Z        09-15Z        15-18Z
KCLT       High 100%     High 100%     High  92%     High  96%
KGSP       High 100%     High 100%     Med   77%     High 100%
KAVL       High  85%     Med   79%     Med   78%     High 100%
KHKY       High 100%     High  94%     High  90%     High  96%
KGMU       High 100%     High 100%     High  83%     High 100%
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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