Area Forecast Discussion
Issued by NWS Greer, SC

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074
FXUS62 KGSP 210550
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1250 AM EST Sat Jan 21 2017

.SYNOPSIS...
Numerous waves of energy will bring rounds of rain and
thunderstorms to the area today and tonight. A vigorous low pressure
system will sweep towards the Carolinas Sunday and Monday, bringing
abundant moisture and the potential for storms. Mild high pressure
will be in control Tuesday and Wednesday until a cold front crosses
the region Wednesday night.

&&

.NEAR TERM /THROUGH TODAY/...
As of 1250 AM EST: Fog and low clouds are spreading across the area
with areas of dense fog developing. Will likely need a dense fog
advisory at some point this morning, but will wait a little longer
to determine the best location.

Otherwise, water vapor imagery continues to show a central plains
low pressure system sharpening up the deep layer southwesterly flow
from TX to the TN River Valley late this evening. Precipitation
chances remain small for overnight hours given the lack of deep
forcing, although low end PoP may encroach on the southern tier as a
warm front lifts northward overnight. The model guidance remains in
fairly good agreement in lifting a series of short waves out of the
upper trof to the west and across the western Carolinas during the
daytime on Saturday. At the same time, weakening upper divergence
moves across the region and isentropic upglide improves. Add the
arrival of more deep moisture and we should expect another wave of
precip to cross the region. Arrival will probably not be until after
daybreak if the model timing is correct, with the rain spreading
northeast across the region through the late afternoon. Will ramp
precip probability up into the likely range accordingly. Precip amts
do not appear that impressive as the flow remains relatively weak.
The big question is whether any strong thunderstorms that will
probably develop to our southwest and near the Gulf Coast can reach
this far northeast during the day. The current thinking is probably
not, and the expected low stratus/dense fog might have a lot to do
with it. If the morning is as murky as expected, we would have a
significant delay to any warmup, which would ruin our chances for
destabilizing. That would mean any stronger storms that form to our
SW would not make it before running out of gas. Will not entirely
turn our backs on the possibility given the strong shear that is
expected, but for now we think any Marginal Risk will not be a
threat until after sunset. The forecast will include some thunder
possibilities because of elevated instability over the srn half of
the fcst area in the afternoon. Temps will remain on the order of 15
degrees above normal, but there is a significant bust potential.

&&

.SHORT TERM /TONIGHT THROUGH MONDAY/...
As of 230 PM Fri: Upper heights continue to fall Saturday night into
early Sunday as shortwave cuts off over the southern Plains, all the
while moving eastward. In the lower levels, WAA and moisture
advection continue overnight. The combination leads to increasing
lapse rates aloft and therefore increasing buoyancy from atop the
inversion. Effective shear from the NAM and GFS exceeds 30 kt by
daybreak Sunday, closer to 50 kt in our srn NE GA and Upstate zones.
Lift will be enhanced by upper divergence and possibly MCVs.  This
could lead to strong thunderstorms, perhaps producing marginally
severe hail; damaging wind threat will be limited by the moist
profiles.

Models agree reasonably well on a compact, open shortwave ejecting
from the Gulf Coast up into the Carolinas Sunday morning. GFS and EC
have this entering our area around daybreak whereas the NAM is a
little slower, closer to 15z. This feature should supply a more
coherent shot of forcing. It also is depicted as backing low-level
flow and increasing shear and helicity. These parameters may peak
before SBCAPE increases in the afternoon. NAM/GFS would suggest some
surface-based instability in the morning, but not reaching
unseasonably high levels until afternoon. The midday overlap is when
we are currently most concerned about severe weather. Damaging winds
or even a tornado would be possible, but large hail might also be a
concern if the instability and shear are as great as the NAM depicts
(I`ll go so far as to say its prog soundings are a bit scary early
Sun aftn). It is important to note, however, that the EC features
much less CAPE than the others, though otherwise it is not all that
different from the GFS. 4km NAM, which now extends through 00z Mon,
does not develop much convection either Saturday night or Sunday.
These notwithstanding, given the primed environment and fairly good
agreement on dynamic lift mechanisms, we will continue to advertise
severe weather in the HWO more or less as the previous shift did.

In addition to any severe threat, we are anticipating at least a
small possibility of flash flooding. Area streamflows are below
normal, so rainfall totals for the whole event are not expected to
bring them even to action stage. Late Saturday night and Sunday
morning we have good overlap of deep moisture and forcing, and this
occurs again Sunday evening (when strong sfc convergence may develop
near the deep sfc reflection of the upper low). Any t-storms that
develop in those periods might cause localized flash flooding, but
the threat of severe wind/hail outweighs the flood threat.

Late Sunday night into Monday our area will be on the back side of
the low, with winds becoming northwesterly in the morning. Colder air
will begin to return to the area as a result, and winds accordingly
will become quite gusty in the mountains. The NAM does suggest some
high elevations could see some snow mix in with rain if sfc temps
turn out cold enough; however, the best-performing guidance blend for
that period keeps mins too warm Monday morning for anything but rain.
Downsloping should offset the CAA and permit maxes to remain above
normal, though cooler than those over the weekend.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
AT 200 PM EST Friday...On Monday evening an upper trough will be
extend from the Great Lakes to the Bahamas, while an upper ridge
will be over the Eastern Great Plains. The pattern progresses such
that by Wednesday the ridge reaches the East Coast, while a
positively tilted upper trough extends from Central Canada to AZ. By
Friday the trough moves to Atlantic Canada and TX, with strong
shortwave energy reaching the Southern Appalachians.

At the surface, on Monday evening an upper low and associated
moisture will be departing Northeast GA and the Western Carolinas,
however some northwest flow moisture may linger over the Western NC
Mountains into Tuesday, with light rainfall resulting. A cold front
will approach from the west on Tuesday night, with precipitation
reaching the NC mountains before the front arrives on Wednesday
evening. The front crosses the area by Thursday morning, although
moisture lingering behind the boundary does not depart until late in
the day. There is some potential for a changeover to snow in the NC
Mountains before the front departs, if the cooler GFS solution if
right. There is better potential for snowfall in the NC mountains
from Thursday evening into Friday as a persistent round of northwest
flow moister arrives from the northwest. Temperatures will run above
normal until frontal passage, after which they will fall to near or
below normal.

&&

.AVIATION /06Z SATURDAY THROUGH WEDNESDAY/...
At KCLT and Elsewhere: LIFR and VLIFR conditions have developed at
all but KAVL and KAND this morning. Expect restrictions to develop
there as well, but not as certain how low they will go. VLIFR more
probable at KAND. Conditions will only slowly improve but may remain
IFR through the day with weak wedge in place and precip moving in.
Winds will remain generally NE to SE through the period, with SE at
KAVL.

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

Confidence Table...

            06-12Z        12-18Z        18-24Z        00-06Z
KCLT       Low   56%     Med   66%     Med   67%     Med   73%
KGSP       Low   53%     Med   66%     Med   68%     Med   63%
KAVL       Med   76%     Med   71%     High  88%     High  84%
KHKY       Med   69%     Med   67%     Med   63%     Med   65%
KGMU       Low   36%     Low   51%     Med   62%     Med   65%
KAND       Med   66%     Low   45%     Med   68%     Low   58%

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...HG/PM/RWH
SHORT TERM...Wimberley
LONG TERM...JAT
AVIATION...RWH



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