Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 211804

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

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.


As of 1235 PM EST: warm-frontal-like feature continues to lift
north along with a large band of precip that covers srn half of
the fcst area at 17Z. Precip chances were adjusted a bit more on
the leading flank to deal with timing issues over the nrn half
of the fcst area. This should continue to lift nwd through
afternoon. Embedded thunderstorms may ride over the top of an
entrenched pool of cool air, at least over the srn zones. Temps
should stay rain-cooled for several hours, so current high fcst
might be too optimistic.

Otherwise, relatively strong short waves will cross the area today
in the SW flow aloft. An H85 low tracks along the I-85 corridor as
well. These features, along with the forcing, will lead to elevated
instability. The instability will be weak but bulk shear will be
high. Therefore, expect some TSRA will develop, but chance of severe
storms will be low this afternoon.

A negatively tilted upper low moves to the Mid MS River Valley
tonight. Short waves swinging around the upper low move into our
area. This pushes a complex frontal system into the area by morning.
A warm frontal boundary will set up somewhere near the I-85 corridor
by then with a wave of low pressure moving along it. Another H85 low
moves across this area as well, with a 50 kt low level southerly jet
ahead of it. The guidance agrees on precip chances ramping up again
after midnight. They also agree on elevated instability and strong
forcing moving in. However, they differ a little in the timing of
the low level jet and an any resulting severe storms. It is looking
more likely that severe storms will be moving into or across the
area late tonight depending on the timing of the jet. Therefore,
will have to keep a lookout as the event nears to try and button
down the timing. Lows will again be well above normal.


As of 330 AM EST Saturday:  The short term fcst period kicks off on
Sunday morning amidst a rather robust closed H5 cyclone moving into
the Midsouth region, while a series of shortwave impulses pass
through the mean flow to the east atop the southeast states into the
Mid Atlantic.  At the surface, primary cyclogenesis looks to prevail
across the Lower OH valley as a secondary meso low feature sweeps
along the I85 corridor out of North/Central GA across the SC Upstate
by around 12z, setting up the first of two rounds of potentially
strong/severe convection on Sunday.

Short term guidance at fcst initialization favors a broken line/qlcs
of potentially strong/severe convection moving northeast out of GA
into the Western Carolinas, eventually sweeping across the entire
fcst area through morning.  The mesoscale environment out ahead will
be increasingly favorable for rapid intensification by way of large
scale ascent beneath the left entrance region of an approaching
110kt swly H3 speed max as well an approaching shortwave ahead of
the primary H5 cyclone, along with a strengthening 45-50kt H85 speed
max providing increased shearing and llv moisture advection atop
backed surface flow.  With that, fcst soundings feature increasingly
deep cape profiles amidst long right turning hodographs, especially
ahead of any convective line where sfc flow remains backed.  The fly
in the ointment, at least initially, will be how unstable the atm
can get in the early morning hours.  Thus taking a gander at SPC`s
SREF Ensemble, 12z probability of sbcape >=250j/kg maximizes at
nearly 90% across the Lakelands region, tapering to near 70%
northward into extreme Western NC and the entire SC Upstate.
Furthermore, probabilities of sbcape >=500j/kg only lower to nearly
70% and 50% respectively, with both of these probability zones
spreading further up the I85 corridor through mid morning as further
destabilization occurs.  All in all, these conditions promote
increasing confidence in the potential for a severe weather event on
Sunday morning, with the best chances for shear/instability overlap
along the I85 corridor.

Moving on, the initial round of potentially severe convection is
expected to push through the I77 corridor into central NC by noon,
leading to a lull in the activity through early afternoon.
Meanwhile, the primary upper vortex will have advected further east
atop Middle TN, while the bulk of the aforementioned upper jet
dynamics remain at play.  However, by this point the low/mid lvl
lapse rates will have increased to near 6-7c/km thanks to cooling
aloft.  In addition, guidance favors another prefrontal surface wave
developing across North/Central Georgia, before rapidly intensifying
as it moves further to the northeast across the Western Carolinas
through the evening hours.  Given the continued upper support, and
development of this secondary meso wave working to back llv flow yet
again, another round of strong/severe convection looks possible
across the entire region.

All said, the latest SPC Day 2 Convective Outlook dissects the fcst
area with three risk categories.  The highest risk category is an
enhanced risk for locales across the NC/SC Piedmont south of the I85
corridor, likely in association with the eventual track of the
initial Sunday morning convective band which should encounter max
instability/shear across these areas.  Moving northward a slight
risk area encompasses the remainder of the Northeast GA, Upstate SC,
and portions of the Western NC FtHills/Piedmont along/south of I40,
while a marginal risk rounds out the remainder of the fcst area.
Will likely ramp up HWO wording a bit along the guidelines of SPC`s
Outlook, mainly to mention the threat for damaging winds and large
hail across the entire region, as well as the potential for isolated
tornadoes with the highest chances being within/near the enhanced
risk area.

Beyond that, the cold front looks to sweep into the region overnight
into Monday morning as the upper low slides northeast into the
Virginias.  This will allow for pops to finally taper down across
the low terrain, however likely not fully removed until the evening
hours as moisture wraps around said departing low yielding showers
across the mtns, possibly spilling into the Piedmont region.
Finally, the development of northwest flow into Tuesday morning
could warrant upslope showers along the TN line, with thermal
profiles cooling enough to support a transition to snow showers.
Currently the fcst only reflects minimal accums at the highest
elevations, generally less than 1 inch, while profiles in the mtn
valleys will likely still be too warm, and thus yield only rain.


AT 240 AM EST Saturday...On Tuesday an upper level ridge progresses
east from the Mississippi Valley as the deep trough moves NE from
the Carolinas.  Meanwhile in the lower levels, NW Flow snow comes to
an end Tuesday morning through midday as wind direction becomes more
westerly and moisture dries up. The 500mb ridge axis passes overhead
around 00Z Wed as the broad upper trough over the Rockies and Plains
builds east. The associated surface cold front will cross Tennessee
on Wed as its parent low pressure crosses the Great Lakes.
Temperatures 10 to 15 degrees above normal Tuesday and Wednesday
with the warmest day Wed will come to an end as the cold front
crosses NE GA and the western Carolinas Wed night. Some moisture is
tapped from the Gulf ahead of the front which slides east of the
foothills and piedmont by late Thursday evening depending on which
forecast model is more right. The GFS has the bulk of precip over
our area 18Z Thursday shifting it east at 00Z Friday. If the GFS
solution becomes reality late in the week, an extended NW Flow Snow
event will start Thursday evening and continue through the weekend.
On the GFS, as the upper ridge axis lingers over the Appalachians,
NW Flow will be persistent with low instability of 40 to 60 CAPE at
least for Thursday night and early Friday over TN and NC border
areas.  This would tend to enhance snowfall amounts as moisture is
lifted over the higher terrain. Temperatures go below normal going
into next weekend.


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...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       Med   75%     High  83%     High  88%     High  80%
KGSP       Med   76%     High  80%     Med   77%     High  89%
KAVL       Med   73%     Med   77%     Med   70%     Med   78%
KHKY       Med   76%     Med   78%     Med   69%     High  88%
KGMU       Med   79%     High  85%     High  90%     High  83%
KAND       High  84%     High  85%     High  80%     Med   77%

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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