Area Forecast Discussion
Issued by NWS Greer, SC

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099
FXUS62 KGSP 270003
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
803 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 750 PM, shower activity continues to diminish this evening, a
trend that should continue over the next couple of hours. However,
mesoscale model guidance depicts an increase in shower activity late
this evening into the early part of the overnight, as boundary layer
convergence increases near the entrance of a 30-35 kt LLJ.
Therefore, after a decreasing pop trend until late evening, pops
ramp up again to chance, esp across the eastern 2/3 of the forecast
area through the pre-dawn hours.

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 /00Z MONDAY THROUGH FRIDAY/...
At KCLT: Showers are dissipating near the terminal this evening, but
with additional activity about 40 miles upstream, decided to leave
the VCSH going to start the period. Meanwhile, high resolution
guidance suggests after an initial mid/late evening lull, shower
activity may increase again around midnight, so allowed the VCSH to
persist through much of the night. Plenty of low level moisture will
linger across the area through the overnight, so cig and visby
restrictions are in play, but confidence is pretty low as to how
poor conditions will become. Categorical MVFR conditions are
introduced at around 08Z, with tempo for 3SM/FEW003 BKN015 btw 10
and 12Z. Widely scattered showers are expected to develop across the
area Monday afternoon, and a VCSH is included throughout the
afternoon.

Elsewhere: Overall similar trends to KCLT, although chances for
restrictions appear much higher at KAVL and especially KHKY, where
temp/dewpoint spreads were already less than 5 degrees at forecast
time. Therefore, introduce tempos for MVFR at KHKY by midnight, with
categorical IFR conditions forecast during the pre-dawn hours.
Similar conditions at KAVL, albeit a bit later.

Outlook: moist pattern with sly low-lvl flow will persist, with
another frontal system impacting the area late Monday into Tuesday.
This system will likely create additional flight restrictions across
the region. Fair conditions should briefly return during mid-week
before another storm system begins affecting the region Thursday and
Friday.

Confidence Table...

            00-06Z        06-12Z        12-18Z        18-00Z
KCLT       High 100%     High  89%     High  83%     High  98%
KGSP       High 100%     Med   76%     High  85%     High 100%
KAVL       Med   74%     Med   68%     Med   71%     High 100%
KHKY       High  93%     Med   69%     High  82%     High  94%
KGMU       High 100%     Med   78%     High  86%     High 100%
KAND       High 100%     High  84%     Med   78%     High  96%

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...JDL



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