Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 220557

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1257 AM EST Sun Jan 22 2017

Numerous waves of energy will bring rounds of heavy rain and
thunderstorms to the area today and tonight. A vigorous low pressure
system will sweep towards the Carolinas today and move northeast of
the Carolinas on Monday, bringing abundant moisture and the
potential for storms today and tonight. 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 1255 AM EST: The forecast remains on track across the western
Carolinas and northeast Georgia late this evening as a 539 dm H5 low
center spins eastward across OK and widespread strong to severe
convection continues to break out across southern MS/AL east of the
translating upper system. Little more than spotty showers will
continue over our region for the next few hours, with areas of fog
likely and embedded patchy dense fog possible in a few locales.
Then, the next shortwave lifting from the Gulf coast toward the
western Carolinas should bring the upstream convection toward or
into the area before daybreak. The activity will spread quickly
northeast through daybreak with categorical PoP surging northward in
the process. Shear will ramp up substantially during this time as
well, with the latest NAM operational run featuring a 50 kt 850 mb
jet crossing the Savannah river around 12Z and transiting the lower
piedmont through late morning. Instability remains the primary
question at this hour given the current lingering NE surface flow
late this evening. However, the dewpoint contrast continues to
weaken - with 55 to 60 degree surface dewpoints likely as the
convection arrives. Thus, strong to severe thunderstorms remain
possible prior to daybreak across southern sections.

The daybreak round should move through the region during the morning
hours, with a brief midday lull in coverage. 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 and early evening
hours. The 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 temperatures should be well above


As of 230 PM Sat: 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: Abundant low level moisture will keep mainly
IFR to LIFR ceilings in place through most of the period, with
visibility likely trending down into LIFR this morning as well. The
next round of showers and embedded thunderstorms moves into the
lower piedmont ahead of the deep southern stream low pressure system
early this morning. Expect two separate rounds of convection to
impact the terminal sites, with morning activity starting before
daybreak near SW TAF sites and then moving quickly northeast through
late morning. After a mid day lull, another round of showers and
possibly strong thunderstorms will arrive with the upper level
system later in the afternoon and during the evening hours. IFR
conditions, heavy rainfall, and possibly strong to severe
thunderstorms may impact the terminals - particularly the Upstate
and especially KCLT. NE winds this evening and early overnight will
gradually adjust to the SE and southerly and become gusty as a
thermal moisture boundary slides northwest into the forecast area.
The location of the boundary through Sunday will largely determine
the areas of greatest threat for severe weather - with KCLT most
likely under the gun late in the day.

Outlook: The very unsettled pattern will continue through Monday as
the upper low pressure system moves slowly east of the area. This
may allow periods of rain and/or restrictions to continue. Expect
gradual improvement by Tuesday but with moisture returning ahead of
the next cold front Wednesday into Thursday.

Confidence Table...

            06-12Z        12-18Z        18-24Z        00-06Z
KCLT       Med   72%     Med   73%     Med   65%     Med   70%
KGSP       Low   56%     Med   62%     Med   68%     Low   58%
KAVL       Low   44%     High  91%     Med   77%     High  99%
KHKY       Med   62%     Low   53%     Med   66%     Med   74%
KGMU       Med   61%     Med   69%     Med   78%     Med   77%
KAND       Med   62%     Low   59%     Med   73%     High  80%

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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