Area Forecast Discussion
Issued by NWS Greer, SC

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539
FXUS62 KGSP 200810
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
310 AM EST Fri Jan 20 2017

.SYNOPSIS...
A vigorous low pressure moves from Texas Sunday to the Carolinas
Monday bringing abundant moisture and the potential for storms.
Mild high pressure will be in control Tuesday and Wednesday until a
cold front crosses our region Wednesday night.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 205 AM EST: Short wave rotating around an upper low over the
mid-west will cross the area this morning. This will push the
showers currently over the western CWFA east of the area early this
morning. Another round of showers will move across at least the
mountains later this morning as a secondary short wave moves
through. Upper ridging builds in during the afternoon. Have timed
the PoP to follow these trends, but did leave isolated showers in
for the afternoon with the lingering low level moisture. Cannot rule
out an isolated thunderstorm with either round of showers given the
forcing and weak elevated instability. Highs today will be tricky
with early showers, lingering clouds, and developing southwesterly
flow. For now, have gone with a guidance blend which is around 15
degrees above normal.

The upper ridge axis slides east of the area tonight with weak short
waves moving toward the area in the developing southwesterly flow.
Lingering low level moisture continues, but deep moisture remains to
the west. There will be enough forcing or upslope flow for some
isolated showers to develop along and south of the I-85 corridor
after midnight. However, any precip will be very light. Lows tonight
will be around 20 degrees above normal.

&&

.SHORT TERM /SATURDAY THROUGH MONDAY NIGHT/...
As of 300 AM EST Friday:  The short range fcst period initializes on
Saturday morning amidst an increasingly active pattern that is
highlighted by broad cyclonic H5 flow across the western CONUS and a
series of shortwaves impulses passing through the mean flow, while a
shortwave ridge axis slides off the east coast as height falls
approach from the west.  At the surface, very weak high pressure
will be under retreat over the Outer Banks, while a slow moving warm
front slides north into the I20/I85 corridor region.  Modest swly
flow through the profile will yield increasing instability as waa
dominates, which combined with weak vort energy aloft, will
promote/enhance convection along the aforementioned warm frontal
axis as it sweeps across Northeast GA and the Western Carolinas into
Saturday evening/overnight.  With that, fcst soundings become very
saturated amidst narrow/deep Cape profiles, sufficient to support
scattered/numerous showers and thunderstorms.  Fortunately H85 flow
remains fairly swly veered during this time frame therefore any
upsloping potential would be contained to the southwest facing
slopes alone, which is looking less likely than in days past as QPF
guidance remains focused along the lifting frontal convection
along/south of I85.  As for the convection, the new Day 2 Convective
Outlook from SPC features all of Northeast GA, and the extreme
westernmost portions of the Carolinas in the Marginal risk area for
severe convection.  Model instability/dynamics fields do feature a
few hundred joules sbcape on Saturday afternoon, while the potential
for mesolow development along the front could back sfc flow beneath
a 40-50kt llv jet, thus really increasing llv helicity which the Nam
indicates in the 4-5 hundred m^2/s^2 range.  Thus, given weak
mid/upper level lapse rates think the primary severe threats would
be damaging winds and perhaps an isolated lewp/qlcs spin-up tornado,
however confidence is fairly low given the bleak instability
profiles.

Moving on into Sunday, the pattern becomes highly amplified as a
deepening H5 low ejects from the Four Corners states into the
Southern Plains, all while the warm sector continues to moisten with
dewpoints reaching into the upper 50s to lower 60s by daybreak
across the Southern Appalachians.  Increasing upper lift aided by
the approaching aforementioned H5 low looks to promote mesoscale
surface cyclogenesis in the warm sector across North/Central Georgia
around sunrise, with decent model agreement favoring a track of said
mesolow along the I85 corridor across Northeast GA and the Upstate,
moving into the lower NC and upper SC Piedmont by mid/late morning.
Although the surface pattern is fairly uniform across various
guidance sources, the upper pattern does feature at least some
discontinuity as the Nam is much more progressive and further north
with the upper low and any associated synoptic scale lift, while the
ECMWF/GFS and to some extent the CMC feature a slower and more
southern track.  The latest SPC Convective Outlook for Day 3 favors
the later solution, which would place the best overlap of lift,
instability, and wind dynamics favorable for damaging winds and
tornadic potential to the south across the Low Country/Midlands.
That said, scattered to numerous showers/thunderstorms are expected
to move out of North/Central Georgia into the daybreak hours likely
as a remnant/broken squall line, and given any northward deviation
from this now preferred solution, could enhance this threat into the
I85/I77 corridor region into midday.  As if that wasn`t enough,
another round of convection look plausible late in the day as the
upper low migrates through the region yielding steepened mid lvl
lapse rates thanks to cooling aloft, thereby introducing an
additional threat of hail with any of the strongest storms mainly
for locales south of I40.  Lastly, as with Saturday above localized
moderate/heavy rainfall due to efficient and deep convection will
once again be a threat.

On Monday, chances for additional showers remain elevated as the
meandering upper low struggles to slide eastward and thus fully exit
the region.  There doesn`t look to be much of cold front associated
with this system, at least not via any temperature fields.  Winds
will veer wnwly on Monday as brief upper ridging moves in behind the
departing upper low, however t/td doesn`t look to suffer all that
much.  Given the newly developed wnwly flow, upslope showers along
the western slopes of the NC mtns look like a good bet to round out
the remainder of the period into Monday night, while pops are
finally allowed to lower to non mentionable levels across the rest
of the fcst area.

&&

.LONG TERM /TUESDAY THROUGH THURSDAY/...
As of 230 AM Friday,  for Tuesday morning through Thursday night. As
strong low pressure moves off the mid Atlantic coast Tuesday
morning, remaining NW flow precip dissipates along the NC and TN
border. Tropical high pressure ridges northward across much of the
SE states on Tuesday but then is suppressed south and east away from
our area with large mid nation trough expanding east. Low pressure
crosses the Great Lakes Wednesday while dragging a cold front across
the southern Appalachians by Wednesday night.  The front slows down
as it crosses the Carolinas Thursday. Low pressure moves from the
northern Gulf coast to the Carolina coast from Thursday to Thursday
night. Most of the precip will be in the form of rain but for the NC
Mountains transitioning to snow Thursday night as colder air filters
into the area. The GFS delays the low along the front and the bulk
of the precip by several hours compared to the ECMWF. The GFS has
the bulk of the precip crossing over NE GA and the western Carolinas
around 00Z Friday. This would bring a 6 hour total rain of a third
of an inch to the area. The EC has a tenth of an inch or less mainly
south of I-85 in the 06Z to 12Z Thursday timeframe. At this point, I
do not want to make much change to the current forecast as this 7th
Day is a week away and we have plenty of time to nail the details
down.

Temps will be 10 to 15 degrees above normal Tuesday and Wednesday
then cooling closer to normal as the front moves east of our area
Thursday.

&&

.AVIATION /08Z FRIDAY THROUGH TUESDAY/...
At KCLT and Elsewhere: Conditions will gradually deteriorate from
the southwest early this morning with the approach of a strong upper
level shortwave and associated deeper moisture. Should see a
transition down into MVFR then IFR as showers move in and intensify.
Cannot rule out an isolated TSRA, but chance very low with little in
the way of instability. Expect winds to become SE before daybreak
then SW after as showers taper off behind the wave. Although mid and
upper levels will dry quickly behind the departing wave this
morning, low levels will remain moist. Thus, expect slow improvement
from MVFR early to VFR by mid to late afternoon in most places.

Outlook: The very unsettled pattern will continue through the
weekend, bringing periods of rain and/or restrictions. Heavy rain
showers and perhaps thunderstorms are possible Sunday.

Confidence Table...

            08-14Z        14-20Z        20-02Z        02-06Z
KCLT       High  91%     Med   73%     Med   68%     Med   73%
KGSP       High  84%     High  90%     High 100%     High 100%
KAVL       High  82%     Med   75%     High 100%     High  93%
KHKY       Med   70%     Med   66%     High  82%     Med   68%
KGMU       Med   78%     High  83%     High 100%     High 100%
KAND       High  83%     High  87%     High  98%     High  85%

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...DEO
NEAR TERM...RWH
SHORT TERM...CDG
LONG TERM...DEO
AVIATION...RWH



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