Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 270740

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
340 AM EDT Thu Apr 27 2017

A dissipating cold front will cross our region today, following by
warm and humid high pressure building back in for Friday and
Saturday. Another front approaches from the west on Sunday, and
crosses the area on Monday.


As of 335 AM EDT: Guidance is coming together depicting an active
period of weather this afternoon and evening. Clouds will spread
across the area this morning as moist southerly flow increases ahead
of an approaching cold front. Convection is already ongoing ahead of
this front and moves into the area from the west this morning.
Synoptic scale and convection allowing models show this convection
weakening and possibly diminishing as it moves in. However, strong
short waves associated with this system move across the area through
the afternoon into early evening. Upper divergence increases as well
with strong synoptic scale forcing crossing the area. Despite the
clouds, enough heating develops, especially along and south of the
I-85 corridor, to combine with the anomalously high dew points for
moderate CAPE values. The wind field strengthens as well with 30 to
as much as 50 knots of 0-6 km Effective Bulk Shear spreading across
the area. This creates the potential for an organized squall line to
develop across the western CWFA late this morning and move east
reaching the CLT Metro Area by late afternoon. Damaging winds would
be the main threat, but an isolated tornado cannot be ruled out
along and south of the I-85 corridor as helicity values increase
creating strongly curved hodographs suggesting rotating updrafts.
The storms should be fast moving enough to preclude any widespread
flooding potential. However, given the wet antecedent conditions,
isolated flooding cannot be ruled out, especially along and south of
the I-85 corridor where one half to one inch of QPF is possible.

Have blended in the CAM guidance to show PoP trends. Cat PoP
increases across the mountains through the morning, moving east to
the NC Foothills, western Upstate, and NE GA around 18Z. These PoPs
move to the NC Piedmont and eastern Upstate around 21Z, then east of
the area around 00Z. Showers quickly taper off from west to east
during the evening ending by midnight. With the increasing wind
fields, expect gusty winds to develop even outside of storms ahead
of the front. With these PoP trends, highs will be near normal
across the west and about 5 degrees above normal east.

Skies clear overnight across much of the area, but clouds may linger
across the Piedmont of all 3 states. Forecast soundings suggest fog
is likely after midnight with the wet ground, lingering moist low
levels and diminishing winds. Lows will range from 5 to 10 degrees
above normal, with 15 degrees above normal possible where clouds


As of 300 AM Thursday: An upper ridge will build along the East
Coast, while a deep upper low closes off into a large low over the
Four Corners region during the Short Term. At the surface a cold
front will wash out over the Carolinas, leaving just a tongue of
higher surface dewpts just to our south. Return flow will start to
advect this moisture back north, but the building ridge should keep
a strong capping inversion just above the mixed layer. So I will go
with a completely dry forecast for Friday. Temps will rebound back
to above normal readings with low to mid 80s in the Piedmont.

On Saturday, the dewpts should increase and strong diurnal heating
will result in stronger instability and weaker cap. So isolated to
widely scattered deep convection is expected, especially along the
Blue Ridge where southerly upslope flow enhances lift. Shear will be
weak, so pulse storm mode is expected, a few of which may produce
large hail and/or microbursts. It will feel like early summer, with
highs in the upper 80s to near 90 across the Piedmont with dewpts in
the mid to upper 60s. Diurnal convection should wind down Saturday
night, but it will remain muggy with lows mainly in the upper 60s.


As of 330 AM EDT Thursday...An upper low will lift northeast across
the Central Plains to the Upper Midwest Sunday thru Monday.
Meanwhile, the large upper ridge near the East Coast will drift
east, allowing decent height falls atop the CWFA. An associated low
pressure system will track northeast across the Midwest and drag a
cold front across the Mid-South on Sunday. The 00z suite of medium
range guidance shows good agreement on the timing of the front,
pushing it across the CWFA on Monday. The front should be
accompanied by a solid band of precip, likely strong storms Sunday,
weakening by the time it reaches the CWFA on Monday. Given the
expected timing, the line may re-intensify Monday afternoon,
especially across the eastern half of the area, with daytime heating
to the east. The GFS and ECMWF show about 800-1200 J/kg of sbCAPE
with 0-6 km bulk shear ramping up to 35-55 kt in the vicinity of the
line. A severe threat seems possible, but confidence is probably not
high enough this far out to get a Day 5 outlook area by SPC. Anyway,
PoPs ramp up to likely by Monday. The models still seem overdone on
PoPs for Sunday within the moist return flow ahead of the front. but
will go with a CHC PoP, favoring the Blue Ridge. Temps will be a
little cooler with more clouds and precip potential on Sunday, and
even cooler on Monday with the fropa.

Behind the front, expect conditions to dry out and temps to return
to near or even slightly below normal for Tuesday. A broad upper
trough remains anchored over the western CONUS, with more energy
bringing another low pressure system toward the area by Wednesday.
So PoPs start to increase into the CHC range with temps warming back
above normal.


At KCLT and Elsewhere: Low VFR clouds have developed in the moist
southerly flow across the area. These clouds will continue through
the morning even as mid and high clouds move in from the west. This
should keep fog to a minimum, but some can`t be ruled out over the
more foggy locations or where the clouds are delayed. Guidance now
in good agreement that MVFR clouds develop through the morning in
the continued moist southerly flow into the Blue Ridge ahead of the
approaching cold front. Convection moves in with the front by early
afternoon across the west and late afternoon at KCLT. Have retained
the PROB30 to indicate the best timing for convection. Low VFR cigs
linger behind the convection before scattering out over the west
during the evening. S to SW winds this morning become gusty for the
afternoon ahead of the front. Speeds diminish some behind the front
but remain S to SW, except at KAVL where they turn NNW.

Outlook: More in the way of limited diurnal tstm chances are
expected Friday with the potential for thunderstorms increasing
progressively over the weekend.

Confidence Table...

            07-13Z        13-19Z        19-01Z        01-06Z
KCLT       High 100%     High 100%     High  98%     High  93%
KGSP       High 100%     High  94%     Med   70%     High  95%
KAVL       High  99%     High  88%     High  85%     High 100%
KHKY       High 100%     High  98%     High  81%     High  90%
KGMU       High 100%     High  94%     Med   73%     High  93%
KAND       High 100%     High  85%     Med   79%     High  94%

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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