Area Forecast Discussion
Issued by NWS Greer, SC

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106
FXUS62 KGSP 201757
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1257 PM 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 1245 PM EST: radar clear at this time, so dry fcst for the
balance of the afternoon looks good. The satellite imagery is the
real bummer...with low stratus deck seen over the western Piedmont
and Foothills of NC. This will be very difficult to dislodge before
we get past peak heating, so it is increasingly likely that we will
not get out of the mid 50s at best in the I-40 corridor. Will make
some adjustments to the high temps in that area. Otherwise,
satellite imagery shows plenty of breaks over the mtns/western
Upstate/northeast GA as drier air mixes in. Might have to bump highs
upward there.

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 /18Z FRIDAY THROUGH WEDNESDAY/...
At KCLT...looks like a busy period of weather has begun. Satellite
imagery shows an extensive deck of IFR stratus covering the western
Piedmont. This will take some time to scatter/lift around the edges.
Conditional climo does not have a large enough database of previous
events to be of much help with confidence, so the idea put forth by
the LAMP guidance of that ceiling lifting to MVFR around 20Z seems
reasonable and was followed. Once that happens, expect a few hours
of SCT/BKN ceiling in the MVFR range before clouds scatter out for a
time and we go VFR for several hours this evening with the absence
of deeper moisture and forcing. Things get more tricky overnight.
Nearly all the guidance keeps plentiful low level moisture,
light/variable wind, and dry air aloft, which is a good setup for
widespread IFR low stratus/fog. Much of the guidance takes KCLT down
to VLIFR conditions, and that is very much within the realm of
possibilities. At this time, prefer to go with an LIFR fcst this far
out, but know to expect many locations to be VLIFR around daybreak.
It may take until the arrival of shower activity from the SW ahead
of the next upper wave to bring some improvement to flight category.
The arrival time is uncertain, but could be as early as 15Z, thus
the beginning time of a PROB30 group. Precip is much more likely
beyond the end of the TAF period. Wind will be light, but will favor
a southerly component during the period.

Elsewhere...seeing clearing from the west as drier air mixes in.
This should take care of lingering IFR at KGSP/KGMU in short order,
leaving most sites with SCT/BKN low end MVFR thru the rest of the
day. The exception is KHKY where low IFR stratus may hold on and not
burn off at all. The rest of the period should behave similar to
what was outlined for KCLT, with earlier timing on precip chance
arrival on Saturday morning.

Outlook: The very unsettled pattern will continue through Sunday,
bringing periods of rain and/or restrictions. Heavy rain showers and
perhaps thunderstorms are possible Sunday. Expect improvement early
next week.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       Med   78%     Med   77%     Med   65%     Med   71%
KGSP       High  87%     High 100%     Med   73%     High  82%
KAVL       Med   71%     Med   70%     Low   55%     Med   75%
KHKY       Low   47%     Med   63%     Med   64%     Med   70%
KGMU       High  90%     High 100%     Med   65%     Med   68%
KAND       High  83%     Med   79%     Med   60%     Med   63%

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...PM/RWH
SHORT TERM...CDG
LONG TERM...DEO
AVIATION...PM



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