Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 210023

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
723 PM EST Fri Jan 20 2017

Numerous waves of energy will bring rounds of rain and
thunderstorms to the area on Saturday. 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.


As of 700 PM EST: Water vapor imagery shows a central plains low
pressure system that is sharpening up the deep layer southwesterly
flow from TX to the TN River Valley this evening. In the absence of
deep forcing east of this system, expect plenty of clouds over the
southeast but very small precipitation chances. Anticipate the quick
development of low stratus overnight in the moist boundary layer,
with very light flow yielding some dense fog in a few locations -
especially east of the mountains. The current forecast has this
fairly well in hand and confidence remains too low for any fog
products yet.

Otherwise, the model guidance remains in fairly good agreement with
lifting the next in the 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.


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

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.


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.


At KCLT: Low clouds managed to scatter before sunset this evening,
but abundant low level moisture should lead to a quick return to
MVFR cigs and then lowering IFR cigs and visibility overnight. In
fact, several MOS sources are coming in line with 1/2SM or less vsby
in fog by daybreak with cigs 200 feet or less, so the forecast has
been adjusted one tick more pessimistic with the latest issuance.
Anticipate light winds throughout becoming a bit more southerly over
time. Any improvement through Saturday will be limited and slow as
modest warming occurs but as deeper moisture and better rain shower
chances spread back in from the southwest through the day. Will thus
keep ceilings IFR through late afternoon during the period of best
shower activity. Will hold off on any thunderstorm mention since the
low stratus should help stabilize the piedmont.

Elsewhere: Low clouds scattered all but KHKY late today, which
should now be locked in with IFR to LIFR restrictions through the
terminal forecast period. The Upstate SC TAF sites should see a
fairly quick return to MVFR then IFR to LIFR cigs overnight with
areas of LIFR fog quite possible as well. The mountain valleys may
stay just a bit more mixed overnight, but with IFR to LIFR still
likely at times. Anticipate little to no improvement through the day
on Saturday as the boundary layer gets very little heating and
deeper moisture and showers return from the southwest. Anticipate
mainly light southerly winds but with foothill winds toggling
easterly at times as the showers arrive. Will keep any thunder out
of the forecast given the likely lack of surface based stability.

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...

            00-06Z        06-12Z        12-18Z        18-00Z
KCLT       Med   66%     Low   55%     Med   68%     Med   72%
KGSP       High 100%     Low   55%     Med   66%     Med   77%
KAVL       High  97%     High  91%     High  92%     High  92%
KHKY       Med   70%     Med   64%     Med   67%     Med   76%
KGMU       High 100%     Med   68%     Med   66%     Med   75%
KAND       High  94%     Med   67%     Med   63%     High  88%

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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