Area Forecast Discussion
Issued by NWS Greer, SC

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171
FXUS62 KGSP 260229
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1029 PM EDT Sat Mar 25 2017

.SYNOPSIS...
Low pressure will lift from the southern plains to the Midwest and
push a band of moisture toward the southern Appalachians tonight and
Sunday. Deep moisture and persistent southwesterly flow will set up
over the southeast Sunday through Tuesday with several rounds of
showers and thunderstorms expected. Drier air should return by the
middle of next week.

&&

.NEAR TERM /THROUGH SUNDAY/...
As of 1025 PM EDT Saturday:  A line of convection continues
to roll slowly eastward across AL and middle TN in association
with the large rotating upper cyclone centered over MO.  Heights
continue to fall out ahead over the southern Appalachians into
the Carolina/Georgia coastline as deep swly flow dominates.
With that, abundant stratocu spread across the vast majority
region highlights the mostly cloudy to overcast skies tonight,
with the only exception being some thinning of low clouds furthest
east along the I77 corridor given proximity of deeper low/mid
level moisture to the west.  That said, guidance still favors
increasing sky cover overnight as the entire synoptic complex
moves eastward spreading remnant shra across northeast GA and the
extreme western Carolinas.  Fortunately, deep convection is not
expected through dawn, with destabilization favored beyond that
point leading to increased chances for tsra on Sunday.  However,
for tonight, trimmed back pops once again over the next few hours
as cam trends have been a bit to aggressive with any leading shra.
Also made a few minor tweaks to t/td and sky, which yielded no
sig changes.  Think patchy fog could develop in association with
any overnight shra, but outside of that chances look slim given
current td depressions and guidance.

Previous Discussion:  The upper lvl ridge axis will slowly shift
east of the area thru the period as H5 heights begin to fall across
the western zones. An active occluded system has developed across
the southern plains and this will track to the NE...reaching the
Midwest by 12z Sun. Meanwhile...good diffluent flow aloft will
spread over the CWFA and continue generating thick cirrus over
the fcst area. Precip coverage should be on a downward trend by
the end of the fcst period at 00z Monday with the most favorable
dynamics shifting northward.

&&

.SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY NIGHT/...
As of 230 PM EDT Saturday: the early part of the next week still
looks unsettled. One wave will have already lifted to the NE at
00Z Monday. In its wake, a short wave ridge will move past Sunday
night, but with the negative tilt, we should maintain a southerly
component to the flow. The models suggest that any remaining precip
production will retreat back toward the Blue Ridge, so the highest
precip probability was kept there and precip chances were nudged
downward over the ern part of the fcst area. The short wave ridge
axis moves past by Monday afternoon and then the next short wave
trof approaches from the west. This will see the precip chances
start to trend back upward Monday night and Tuesday as the short
wave passes. Once the wave moves past, precip chances should
end east of the mtns and pare back to the upslope areas on the
TN border. Think we should see some kind of break in the action
early Wednesday morning. Severe weather and heavy rain chances
look minimal through the period. Temps should be on the warm side
of normal.

&&

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
As of 225 PM Saturday: Fairly complex, split-flow pattern will be in
place across the Conus through the medium range. In the southern
stream, the tendency for short wave troughs to dig into the Four
Corners and close off across the southern Rockies/southern Great
Plains, only to get kicked out a day or two later by the next wave
making landfall on the West Coast will continue. Meanwhile,
northern stream activity will actually be the primary driver of the
weather across our area early in the period. This is the result of
sprawling surface high pressure spilling east of the Appalachians,
in the wake of a deep area of low pressure pulling away from the New
England Coast. Global models depict cooler and drier air funneling
down the Eastern Seaboard toward our forecast area sometime in the
Thursday/Thu night time frame. Due to timing uncertainty (with the
ECMWF pushing the back door cold front into the area 6-12 hours
slower than the GFS), huge temperature bust potential exists, as
areas ahead of the front could well see maxes in the 80s on
Thursday.

In terms of the precip potential, moist isentropic lift developing
over the post-frontal air mass may result in expansion of light
precip Thu night into Friday, and pops will gradually ramp up during
that time. By Friday/Fri night, global models indicate the southern
Plains upper low will be kicked out over the Ohio/TN Valley.
However, the GFS and ECMWF differ widely regarding their handling of
this feature, with the GFS maintaining a closed circulation in split
flow, while the ECMWF opens the wave and phases the northern and
southern stream. Interestingly enough, these differences don`t
result in a great deal of discrepancy in terms of the sensible
weather regime, as both solutions depict a bit of an evolving
Miller-B scenario (the intensity of surface features/forcing being
the primary differences), and likely pops appear warranted for
upglide/upslope precip Friday through at least early Saturday.

The severe weather potential for late week appears to be
underwhelming but non-zero, mainly due to the specter of
in-situ/hybrid cold air damming to limit surface-based buoyancy.
Nevertheless, some instability will be present around the periphery
of the cold air damming air mass, while wind shear should at least
be moderately strong.

&&

.AVIATION /03Z SUNDAY THROUGH THURSDAY/...
At KCLT:  VFR conditions expected to prevail through the evening
as low llv cu cigs sct out beneath high level cigs.  The upstream
H5 cyclone and consequent flow through the profile will lead to
further moistening of the llvs into daybreak leading to development
of MVFR stratus.  Therefore taf features all of the above with
a prob30 for shra and possible IFR with timing based on a cam
guidance consensus.  Precip chances will increase into the late
morning to early/mid afternoon hours as the atm destabilizes and
an upper wave passes through.  Therefore opted to carry a final
prob30 at 18z to highlight low VFR recovery of prevailing cigs
however mainly to mention vcsh and  4sm shra.

Elsewhere:  Trends similar to that of KCLT above, however with
confidence on chances/timing being higher, especially further west.
Thus, all tafs feature increasing/lowering of cigs into the predawn
hours with either vcsh and/or prob30s for shra with timing based
on a west/east progression of current ongoing AL/GA convection.
This convective band will intrude, however should weaken as it
encounters more stabilization therefore pops are highest at KAVL
and KAND, which also have the best chances for ts.

Outlook: areas of precip will continue into Sunday evening
likely creating additional flight restrictions across the area. An
unsettled and moist pattern will persist thru next week and maintain
chances for low CIGS and VSBY.

Confidence Table...

            02-08Z        08-14Z        14-20Z        20-00Z
KCLT       High 100%     High  95%     Med   68%     High 100%
KGSP       High 100%     Med   70%     Med   62%     High  81%
KAVL       High 100%     Med   65%     Med   66%     High  80%
KHKY       High 100%     Med   73%     Med   69%     Med   74%
KGMU       High 100%     Med   66%     Med   63%     High  81%
KAND       High 100%     Med   77%     Med   62%     High  86%

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...SW
NEAR TERM...CDG/JPT
SHORT TERM...PM
LONG TERM...JDL
AVIATION...CDG



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