Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 201841

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
241 PM EDT Thu Apr 20 2017

An unsettled pattern will continue as a front stalls over our area,
with showers and thunderstorms possible each afternoon through the
weekend. A low pressure system will exit the area Sunday, with the
area drying out next week.


As of 230 PM EDT Thursday:  The pattern aloft features weak ridging
across the southeast while split flow yields a closed northern
stream low over the Great Lakes, while another trof moves through
the southern stream atop the Desert Southwest.  At the surface,
subtropical ridging dominates over the southeast leading to deep
ridging, while a cold front drives eastward out of the Plains into
the mid MS and OH River Valleys.  Closer to home, ample diurnal
heating has allowed for modest instability, upwards of 1000j/kg
over the NC/GA and SC mtn regions, while tapering down to around
500j/kg eastward over the Piedmont.  With that, any weaker CIN has
eroded, or continues to erode atop the mtns/escarpment as convection
prevails, while any CIN continues to hold for points east.  Guidance
tends to favor continued growth of coverage through the afternoon as
heating peaks, with any mtn convection propagating slowly eastward
along the mean steering flow aloft.  This warrants low likely to
high chances pops for the mtns through the afternoon, topping out at
high chances eastward across the northern NC Piedmont.  Points south
into CLT metro as well as the I85 corridor regions of SC/GA will
be under the influence of the strongest CIN as mentioned above,
thus only slight chance pops were featured.  Any convection should
remain below severe limits as mid lvl lapse rates are rather poor,
therefore only garden variety tsra is expected.

Pop will lower into the overnight hours as diurnal heating induced
instability wains, thus leading to non mentionable pops by NLT
midnight.  Convective debris spread about the region will erode
with time yielding mostly clear skies, however with the exception
of the mtn valleys where low stratus is possible.  Overall, not
expecting widespread fog issues tonight as Td depressions should
remain large enough to negate saturation, yet locations that
experience rainfall today/tonight will have a non zero chance of
patchy fog, especially in the aforementioned mtn valleys.

Otherwise for Friday, despite height falls aloft, H5 flow will
develop a more swly pattern therefore yielding warmer temperatures
through a deep layer.  With that, guidance favors even poorer lapse
rates and thus less instability on Friday in the warm sector ahead
of the front, which should remain north of the fcst area through
the remainder of the the period.  However, the fcst will still
feature healthy pops across the mtns, with near climo pops over
the lower terrain.  Temperatures on Friday will be quite warm,
generally upper 60s to mid/upper 70s over the mtns, while reaching
into the mid 80s across the Piedmont amidst partly sunny skies.


As of 130 PM Thursday: A split flow regime will be in place across
the eastern Conus through the short term period, with an upper low
expected to drift from the mid-Mississippi Valley into the southeast
coastal states from Fri evening into Monday. There is some
controversy as to the location of an associated frontal zone at the
start of the period, as the NAM and a majority of mesoscale guidance
tends to keep the boundary north and west of our forecast area,
while the global models generally depict it as having moved into the
area by Saturday evening. Considering the orientation of the upper
flow pattern, the NAM and its cohorts probably have the right idea,
as I don`t see any mechanism, other than perhaps organized
convective outflow that would give the boundary a southward push
prior to Saturday night. (In fact, even the global models tend to
retreat the boundary north of our on Saturday). As such, we expect
convection to remain scattered and to be primarily driven by the
diurnal heating cycle through the day Saturday. Instability will be
adequate for strong updrafts Sat afternoon/evening, but shear won`t
support anything much more organized than perhaps a couple of
mesoscale clusters, so a severe weather threat exists, but appears
to be marginal.

The forecast gets a bit more interesting (and uncertain) beginning
Sat night and continuing through Sunday, as deep layer forcing
increases across the region, and the front begins sweeping across
the TN Valley and the southern Appalachians. Meanwhile, high
pressure building into the northeast Conus in the wake of a
transient northern stream short wave trough is expected to establish
cold air damming across the Eastern Seaboard Sat morning through
Sunday. As usual, the NAM is by far the most aggressive in the
timing and southward extent of the cool air wedge, although the
global models are also trending in the direction of developing CAD
over our area, albeit keeping it primarily confined to western NC.
Pops increase to likely to categorical across the area during this
time, but uncertainty regarding the surface pattern (including the
location of potential surface cyclogenesis) means there is also
considerable uncertainty regarding the degree of any severe weather
threat for the late weekend. It`s also worth noting that the NAM is
much more intense and slower with the upper low track, digging it
farther to the south than the global models. The result is a strong
upslope/ upglide response/heavy rain signal over the cold pool by
the end of the day Sunday. The NAM does have a habit of over-
intensifying upper lows, so this evolution seems dubious, but that`s
one more potential scenario to ponder.

Temps will remain much above normal through at least Saturday, then
are expected to fall to near or even a few degrees below climo
Sunday, as cold air damming will likely become established across at
least part of the forecast area.


As of 200 PM EDT Thursday: Medium range picks up 12Z Monday AM just
as the the surface low is sliding off the Atlantic coast to exit our
area. The upper low may linger over the Carolinas for another 24
hours or so with Atlantic moisture wrapping around the back side in
the cyclonic pops were tapered off gradually through
Monday PM. A wedge of surface high pressure will build over the
Appalachians in the wake of the surface low late Monday and through
Tuesday. The parent high is progressive and will shift off the
northeast coast by late Tuesday, allowing the surface ridge to break
down as another low pressure system sweeps across the central
plains. Global models are showing different solutions as far as the
track of this low, though they both come to a similar conclusion as
far as sensible weather. The ECMWF, with the more southerly low
track, brings a surface cold front through the area Wednesday
afternoon, though anemic deep layer moisture greatly limits any QPF
response. The more amplified GFS swings the low up towards the
northeast, sweeping the front northward before it can make it into
our area. Though the upper flow amplifies considerably towards the
end of next week, any issues should remain past the end of the
current extended forecast. A slight increase in cloud cover is the
only sensible weather advertised through the end of the medium range
on Thursday. Temperatures will begin the period near average and
gradually rise to above average through Thursday.


At KCLT and Elsewhere:  VFR through the period at all sites,
aside for possible convection induced restrictions at KAVL/KHKY
this afternoon, as well as morning fog at these locations.
Initialized all tafs with either vcsh/vcts, with chances being
greatest along/north of I40 therefore tsra tempos were included
this afternoon at KAVL/KHKY based upon current trends and cam
guide timing.  Otherwise, diurnal heating will lead to increasing
cu across the NC/SC Piedmont, with low vfr to mvfr cigs possible
adjacent to any heavier convection.  Winds will remain sly/swly,
with some low end gusting possible for the Piedmont sites.
Conditions will improve into the evening hours as heating subsides
and low cu gives way to only high cirrus through the night, with
the exception of at KAVL and possibly KHKY given abundant sfc
moisture favoring fog.  Friday will be much of the same as a cold
front approaches from the west, however look for winds to remain
swly amidst building cu into the afternoon, while prob30s where
included at KAVL/KHKY given prefrontal shra/tsra chances.

Outlook: Diurnally driven TSRA are expected again Friday. Another
backdoor cold front is expected to settle over the fcst area this
weekend, with restrictions and precipitation likely.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     Low   41%     High  92%
KHKY       High 100%     High 100%     High  94%     High 100%
KGMU       High 100%     High 100%     High 100%     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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