Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 211952

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
252 PM EST Sat Jan 21 2017

Numerous waves of energy will bring rounds of heavy rain and
thunderstorms to the area through the remainder of the weekend. A
vigorous low pressure system will sweep towards the Carolinas
tomorrow and Monday, bringing abundant moisture and the potential
for storms. Some of the storms could be severe. Mild high pressure
will move in Tuesday and Wednesday until a cold front crosses the
region Wednesday night.


As of 235 PM EST: a meso-low type feature was working its way across
the western Carolinas, with an extensive area of precip in a frontal
wave-like configuration. The extensive precip was effectively
cooling the boundary layer, eliminating any surface-based
instability, which is good to keeping the immediate severe weather
threat to the south, but bad because this development has
necessitated yet another painful mid-course correction. Will
reconfigure categorical precip prob over the NC Piedmont, and will
have to significantly cut down on precip chances for late
afternoon/early eve over the west as the back end of the precip
moves in from the west over the next several hours. Meanwhile, the
rain-cooled air will be trapped east of the Blue Ridge in a
wedge-like configuration.

Once this mesolow/short wave cross the area, we enter another lull
phase that should last thru most of the evening hours. Will not rule
out pockets of dense fog developing once again. This may require
another dense fog advisory, although increasing advection could
result in a more variable visibility. Uncertainty about the severe
thunderstorm potential in the overnight hours is moderately high,
and depends greatly on the position of the remnant cool
pool/wedge-like boundary that will lurk somewhere over northeast
GA/Upstate SC/CLT metro. If the boundary is as far north as the
model guidance suggests, we retain over 500 J/kg of sbCAPE and
enough shear to support an organized storm. What we may lack is a
focus, while bad things happen closer to the Gulf Coast and closer
to the better upper support. Our threat increases after 06Z as the
next wave approaches, and some of the convection-allowing models
show clusters of showers/storms coming across northeast GA/Upstate
SC in the pre-dawn hours, similar to the GFS solution. Precip prob
ramps back up considerably to take this into account. Hard to say
for sure, but the combination of instability and shear suggests a
good chance of mini-supercells. At this point, best to check back in
on the fcst later tonight before turning in, just to make sure an
impending severe threat has not yet developed this far north.

This round should move through during the morning hours, and we ramp
precip chances back down briefly. Then it remains to be seen if the
timing is such that we are able to recover the air mass enough to
support yet another round of strong/severe storms that would move in
from the west in the late afternoon. Current thinking is...yes...
because the mid-level cool pool will move in and improve our
mid-level lapse rates. This final round could pose more of a large
hail threat with the expected temp profiles. The shear/CAPE
combination suggests an even greater chance of mini-supercells.
Precip probs ramp back up to categorical once again to cover for
this. High temp fcst should be well above normal.


As of 230 PM Fri: The occluding low pressure system will move over
the southern Appalachians Sunday night. While it is clear that
periods of rain will continue into the evening on the warm side of
this system, there is some indication that surface-based instability
will remain over parts of the area after sunset, prior to the cold
front pushing in. The opnl NAM is most dire, depicting SBCAPE
remaining nearly as high at the afternoon levels, still with the
remarkably strong shear; the 4km NAM and GFS are faster to diminish
the instability. We`ll have to take into account the stabilizing
effect of any earlier organized convection through the area.
Nonetheless a chance of thunder through the evening will be
advertised, and if there is thunder before the shear tapers off later
in the night, it would continue to pose at least a small severe risk.

Winds become northwesterly overnight into Monday morning, setting up
CAA. Winds accordingly will become quite gusty in the mountains. The
NAM does suggest higher elevations could see some snow mix in with
rain overnight--warm and wet ground should preclude any accumulation
except maybe on the very high peaks. East of the mountains,
downsloping is expected to offset the CAA and permit maxes Monday to
remain above normal, though cooler than those over the weekend. The
freezing level does drop nearer to the surface thru the day, but sfc
temps over the mountains should remain warm enough to allow an
all-rain forecast at that time. The column will be drying from the
top down and crystal growth becomes less likely except in upslope
areas, which leads me away from advertising any snow just based on
sfc temps in the mid-upper 30s.

Upslope-driven precip may continue Monday night and even Tuesday near
the Tennessee border, but PoPs taper off as that moisture is
depleted.  Cooler temps overnight could permit some minor accums
(generally under 1 inch) along the Tenn border, though the ground may
still be too warm. A shortwave ridge moves over the area as the upper
low moves away and before the next trough arrives. Clearing skies,
the ridge, and continued downsloping will keep maxes in the upper 50s
and low 60s in the mountain valleys and Piedmont.


AT 200 PM EST Saturday...On Tuesday evening an upper trough will
extend from FL to the Great Lakes. This feature progresses off the
East Coast on Wednesday, while a positively tilted upper trough
reaches the Great Lakes and southern Rockies. A robust shortwave
moving through the trough will cross the Southern Appalachians on
Thursday. By Friday the upper pattern deamplifies from NM to the
Carolinas, resulting in zonal flow, but positively tilted upper
troughing reappears by Saturday from ME to TX, with the models
disagreeing on the timing if vort lobes rounding the trough.

At the surface, an upper ridge will be crossing the Western
Carolinas and NE GA on Tuesday evening, while a cold front reaches
the MS River Valley. The boundary approaches the Southern
Appalachians toward dawn, but with somewhat limited moisture as the
pressure gradient weakens over the Gulf of Mexico. Moisture
increases along the front as it crosses our area on Wednesday,
persisting behind the boundary overnight, and perhaps into midday on
Thursday. The models differ on whether enough cooling will occur to
support snow on the back side of the front, however another round of
moisture from the Great Lakes is expected to reach the NC Mountains
sometime on Thursday, and cooling should be sufficient to support snow
at higher elevations. Precipitation should changeover to all snow in
the NC Mountains Thursday night, with the greatest coverage along
the TN border. Snow would persist into Friday, and perhaps into
Saturday of the GFS solution pans out. Temperatures will start out
above well normal, falling to near or slightly below normal in the
wake of the front.


At KCLT and Elsewhere: aviation weather looking rather grim over the
next 24 hours as waves of low pressure move up from the SW and
across the region, with several rounds of showers, separated by
periods of low stratus and fog. Conditions are likely to prevail in
the low IFR category, and occasional VLIFR conditions are possible.
Will not rule out a few MVFR/VFR sucker holes, particularly over the
NC mtns, but that will be left out of the TAF. As for the details,
in particular at KCLT, expect wind to remain NE for the next few
hours, with the warm-frontal-like precip arriving around 19Z. That
should start to bring the wind around to ESE thru the late
afternoon. There could be embedded, elevated thunderstorms that move
near the upstate/CLT TAF sites thru early evening, but will handle
that with amendments. This wave should pass by the early/mid
evening, at which point we have another lull in the precip, but do
not expect to see much improvement. Actually, it will be that time,
thru the early morning hours, that some of the model guidance takes
conditions back down into VLIFR. The next wave will move up and
across the region Sunday morning, but timing is uncertain and
details must be glossed over. Will add a PROB30 for thunder for the
morning hours as forcing/instability looks a bit better. Wind should
come around to S with the next wave, and then may back to SE by late
morning Sunday.

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...

            20-02Z        02-08Z        08-14Z        14-18Z
KCLT       High  81%     High  82%     High  87%     High  82%
KGSP       High  89%     Med   77%     High  80%     High  80%
KAVL       Med   78%     Med   78%     Med   65%     Med   78%
KHKY       Med   76%     High  83%     Med   76%     High  90%
KGMU       High  81%     High  86%     High  94%     High  82%
KAND       High  85%     High  83%     Med   75%     Med   68%

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:




SHORT TERM...Wimberley
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