Area Forecast Discussion
Issued by NWS Greer, SC

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529
FXUS62 KGSP 262054
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
454 PM EDT Sun Mar 26 2017

.SYNOPSIS...
Moist southerly flow will linger over the southeast through Tuesday,
with several rounds of showers and thunderstorms possible. Drier air
will briefly return by the middle of the week as a cold front
settles south of the area. Another low pressure system will bring
moisture back to the southeast Thursday, increasing rain chances
through late Friday.

&&

.NEAR TERM /THROUGH MONDAY/...
As of 435 PM, Scattered showers, along with a thunderstorm or two
continue to develop along an axis roughly aligned with the I-85
corridor, with significantly less coverage noted elsewhere across
the forecast area. Based upon the latest mesoanalysis from SPC, it
appears the I-85 corridor will remain the most likely area for
convection to initiate, as LCLs and LFCs are relatively low there,
while an area of surface convergence can be inferred from Metars
along that axis. 50 pops will be carried there for the next couple
of hours, while 30-40 percent chances appear warranted elsewhere.
The main limiting factor for severe convection is buoyancy, which is
generally in the 500-1000 J/kg range across the western half of the
area (and weaker elsewhere). Considering this degree of instability,
shear parameters leave something to be desired, but we`ve already
seen one freak, weak severe wx event today, so obviously cannot rule
out another one.

Otherwise, we will remain in a moist pattern with sly low-lvl flow
thru the near term. The next sfc low will be moving up to our NW by
the end of the period 00z Tues, bringing another round of deep-lyr
moisture and sct to widespread showers and ts to the region.

&&

.SHORT TERM /MONDAY NIGHT THROUGH WEDNESDAY/...
As of 235 PM EDT Sunday: it looks like the next in a series of upper
trofs will be on our doorstep Monday evening, with a lead vort max
already moving eastward across the region at 00Z Tuesday. The models
are a bit at odds as to the coverage and probability of precip,
but generally agree that we should have at least a chance across
the region. We lose the better low level upslope flow in advance of
the wave, but we make up for it with mid/upper forcing as we work
thru the night. The upper trof axis is fcst to cross the region
in the 09Z to 18Z Tuesday time frame, which should lay a cold
front down across the region during the day, and then push it off
to the south by the end of the day. This ought to give us another
non-zero chance of strong to severe thunderstorms, but this set-up
will have more of the same problems as recent days, that being poor
lapse rates and a tall thin CAPE profile. Shear will also not be
quite as good, but we might have more of a trigger. Have to think
right now our chances are low. Precip prob should diminish in the
afternoon as downslope flow develops east of the Blue Ridge. We end
the day with lingering W/NW flow upslope showers on the TN border,
but that too should die off Tuesday night.

Wednesday looks fairly benign, yet somewhat problematic. There are
indications that a reinforcing back-door cold front will push down
from the N/NE in the afternoon, which could have an affect on the
high temp fcst, especially over the NW Piedmont. Have stuck close
to the raw blend of the guidance for now, but this could easily
have to be lowered a few categories if the back door arrives early.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/...
As of 200 PM EDT Sunday:  The medium range fcst period initializes
on Wednesday evening amidst an upper ridge axis atop the eastern
CONUS, while a rather robust closed H5 cyclone deepens across the
southern Plains.  As a result of the highly amplified pattern aloft,
the surface field will also be rather interesting as cyclogenesis
beneath the aforementioned upper cyclone lifts a warm front across
the deep south setting up an increasingly large baroclinic
zone. In addition to that, high pressure over the Great Lakes
will slide east before extending southward in the lee of the
Appalachians setting up CAD.

Pattern evolution through the period will be highlighted by the
interactions of the above mentioned features Thursday/Friday
as the MS Valley centered upper system advects east, into the
destabilized warm sector over Dixie Alley possibly leading to
widespread strong/severe convection on Thursday.  At the same time
warm/moist GOM air will spread northward along a warm front into
north GA and the western Carolinas, with said moisture lifting
isentropically to yield iso/sct ra atop the western periphery of
the ridge Thursday night into Friday morning.  By that time the
entire convective complex will have shifted east into the OH/TN
valleys as the associated cold front looks to cross the MS river.
At that point the details become a bit unclear as to how the
approaching wave will interact with the stubborn CAD wedge given
that the op models feature some discontinuities.  Specifically the
GFS is a bit more progressive however less amplified overall,
while the ECMWF is slower, which if perfect progged would lead to
better chances for widespread strong/severe convection on Friday
given impressive shear and improving lapse rates thanks to the
approaching negatively tilted H5 trof axis.  The faster and less
amplified GFS looks relatively unstable at this range, however
the flow is fcst to be more veered overall as the upper wave has
passed and any CAD TMB has retreated further north.  All said, these
details will likely modify with time given additional guidance and
narrowing of scope.  Beyond that, the cold front will sweep across
the fcst are on Friday night into Saturday as deep ridging builds
to the west.  Guidance does hint at some remnant upslope showers
behind the front on Saturday morning, eventually drying into the
afternoon hours.  Said ridging will dominate on Sunday before
another deepening Plains system approaches to round out the period.

As for the fcst, pops will ramp up across the CWFA on Thursday as
the wedge settles atop the Piedmont/Fthills thus providing enhanced
focus for upglide ra, then increasing/spreading atop the entire
region to feature high end chances to low end likely levels into
Friday morning.  Pops will hold at these levels through much of
the day Friday before tapering overnight, eventually leading to
slight chances north of I85 by daybreak Saturday.  Beyond that,
mentionable pops will retreat to the central/northern mtns
Saturday afternoon before drying out completely Saturday night,
lasting through periods end.  Temperatures will be a bit tricky
early in the period as the wedge and associated upglide stratiform
precipitation work to keep highs below normal on Thursday, before
warming back to near normal levels to round out the period.

&&

.AVIATION /21Z SUNDAY THROUGH FRIDAY/...
At KCLT: VFR conditions prevail thru the first part of the TAF
period with MVFR cigs from roughly 10z onward. I included a TEMPO
group for the first couple of hours for reduced visby and cigs
associated the rain showers currently moving over the area. I also
included a TEMPO group from 10 to 12z for early morning IFR visby
and cigs with MVFR cigs otherwise prevailing. Winds should remain
SLY thru the period before becoming more SWLY by the end of the
period.

Elsewhere: I kept cigs at MVFR for most of the period with periods
of IFR cigs during the early to mid morning hours. I also included
PROB30 groups for IFR and MVFR visby from roughly 08z onward but
these will likely be very hit and miss during that time. Otherwise,
I don`t expect conditions to improve to VFR again until after 18z.

Outlook: moist pattern with sly low-lvl flow will persist into mid-
week with another frontal system impacting the area late Monday into
Tuesday. This system will likely creating additional flight
restrictions across the region.

Confidence Table...

            21-03z        03-09Z        09-15Z        15-18Z
KCLT       High 100%     High 100%     Med   72%     High  82%
KGSP       High 100%     High  98%     Low   53%     Med   64%
KAVL       High  98%     Low   55%     Med   68%     High  89%
KHKY       High  96%     High  83%     Med   72%     Med   79%
KGMU       High 100%     High  96%     Low   59%     Med   75%
KAND       High 100%     High 100%     Low   48%     Med   78%

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:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

$$
SYNOPSIS...LEV
NEAR TERM...JDL/JPT
SHORT TERM...PM
LONG TERM...CDG
AVIATION...JPT



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