Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 201458

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

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.


As of 945 AM EST: radar trends show lingering shower activity
diminishing...mainly confined to the nrn mountains/foothills, and a
smaller patch moving along the I-85 corridor between the GSP and CLT
metro areas. This downward trend should continue thru midday. Think
it safe to re-evaluate the low precip chances and effectively remove
them for the afternoon. Satellite imagery shows some breaks
developing over the west, which should translate eastward with time.
This makes the high temp tricky. Suspect we may have to bump temps
up over the SW where it is already clearing out.

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.


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.


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

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


At KCLT and Elsewhere: Generally IFR cigs will continue through the
morning, lifting to MVFR during the afternoon then low VFR before
daybreak. Expect VSBY restrictions to be limited to any heavier
showers this morning. S to SW winds continue or develop this morning
and continue through the day. With lingering low level moisture in
place, cigs fall through MVFR this evening to IFR or even LIFR
overnight. Better chances for VSBY restrictions overnight with MVFR
likely and IFR to LIFR possible. Light SW winds expected overnight.
KAVL the exception where S wind this morning becomes NNW by
afternoon and continues overnight.

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...

            15-21Z        21-03Z        03-09Z        09-12Z
KCLT       Med   74%     High  90%     Med   63%     Med   79%
KGSP       High  83%     High 100%     High  96%     Low   58%
KAVL       Med   72%     High  86%     High  87%     Low   55%
KHKY       High  80%     Med   72%     Med   75%     Med   65%
KGMU       High  83%     High 100%     High  98%     Med   63%
KAND       High  81%     High  98%     Med   75%     Med   64%

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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