Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 220836

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
336 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 310 AM EST: First round of convection is approaching from the
SW. CAM guidance shows this convection moving in before daybreak
then sliding across the CWFA through the morning. While there looks
to be enough instability for thunderstorms mainly along and south of
the I-85 corridor this morning, the chance of severe storms looks
low. The best instability, shear, H85 jet, and severe parameters
slide south of the area along and south of a weak thermal moisture
boundary. Of course, will still have to keep an eye on the
convection as it slides through since instability and strong shear
will be nearby.

There will likely be lingering showers through the late morning and
early afternoon before the next system moves into the area during
the afternoon. Strong short waves and a deep upper low move into the
area from the SW and slide east through the evening. This brings
strong shear and an H85 jet across the CWFA. MUCAPE up to or greater
than 500 J/kg develops along and south of the I-85 corridor. A
surface low and frontal boundary cross the area keeping the low
level winds backed. This helps lead to significant 0-1 km helicity
values. The combination of these features had created an increase in
the Significant Tornado Parameter to above 1 along and south of the
I-85 corridor. All parameters show this area to have the best chance
of severe storms which is why SPC has put this area in a Slight and
Enhanced risk area. Could even see a large hail threat across the
area with the mid level cold pool with the upper low increasing
lapse rates. Greater threat of tornadoes will be along and south of
the I-85 corridor.

There will again be break late in the evening as the storms move
east of the area. However, showers move back in from the west as the
upper low slides across the area. Do not expect any severe storms
with the convection after midnight. Highs today and lows tonight
will be well above normal, but the lows will be cooler than the past
several days.


As of 200 AM EST Sunday:  The short term fcst period initializes on
Monday morning highlighted by the remnant closed H5 low migrating
slowly east atop the Southern Appalachians into Central NC/VA,
while broad ridging builds to the west across the Rockies.  At the
surface, a leading cold frontal axis pushes into Southern VA while
llv flow is slow to veer wly in its wake across Northeast GA and
the Western Carolinas, as surface ridging builds beneath the upper
ridge out west.  Large scale synoptic ascent directly beneath, and
on the eastern flank of the closed H5 cyclone will aid any shower
development through the morning hours, mainly across GA and the
extreme Western Carolinas.  Furthermore, guidance favors increasing
elevated instability into the afternoon hours eastward into the
Fthills/Piedmont region as the upper low transitions northeast and
temperatures cool aloft.  With that, the aforementioned showers are
expected to progress into the Piedmont by mid afternoon, possibly
with a few embedded thunderstorms.  By that time, NVA behind the
departing low will warrant lowering pops across Northeast GA and
the Upstate, while pops look to remain elevated across the high
terrain of GA/SC/NC given increasing CAA thanks to strong H85 winds
on the order of around 40-50kts promoting moist upsloping along
TN line.  Thermal profiles will cool from the top down, likely to
levels supportive of high elevation snowfall into Tuesday morning.
In addition, the above mentioned llv flow will transition from
south to north as the upper cyclone pulls away, thus expecting
rather gusty perhaps advisory level winds for the higher elevations,
while marginal downsloping of said flow into the low terrain will
aid drying and yield slightly weaker yet still gusty conditions into
midday Tuesday.  Beyond that point, the fcst will dry out as upper
ridging aids high pressure development/persistence at the surface,
lasting through the remainder of the period into Wednesday morning.
Overall any above noted snowfall accumulations via a brief nwfs
regime should remain at around an inch or less.  Lastly, as a
testament to how weak the surface cold front is expected to be,
temperatures on Monday are fcst to top out around a category above
normal, while downsloping flow on Tuesday will promote temperatures
around 2 categories above normal.


As of 255 AM Sunday...for the period Wednesday morning through
Saturday.  The upper level ridge moves off the Atlantic coast Wed
morning as the broad upper trough over the nations mid section moves
east across the Mississippi Valley. This upper trough slides east
through the end of the week with thickness values lowering each day.
This will coincide with temperatures going from roughly 15 degrees
above normal Wednesday to around 5 degrees below normal Saturday.
The broad trough will create a zonal west to east 500mb flow from
New mexico to the Carolinas Friday.

At the surface, the cold front will be crossing Tennessee Wednesday
morning, crossing our area late Wed and Wed night then reaching the
Carolina coast by Thursday morning. Moisture will be somewhat
limited along the front as flow off the Gulf will be rather weak.
Northwest flow will become persistent from near the Great Lakes
Thursday into the start of the weekend. This will result in mountain
snow showers mainly along the Tennessee and NC border areas for
roughly a couple days. Even the ECMWF is leaning more toward the GFS
forecast with this NW Flow. The GFS has the NW Flow snow showers
starting Thursday evening, continuing through Friday then breaking
down and ending Saturday. The EC has the NW Flow Friday but at least
it now strongly hints at this weather feature. Also of note, low
instability of 30 to 40 CAPE over eastern TN early Thursday night
would enhance snow rates a bit. Most of the snow will be late
Thursday night in the higher elevations along the TN and NC border.
Snow amounts were derived from QPF through Thursday night.
Confidence decreases by late Friday into Saturday as to whether it
will happen or dissipate earlier.


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...

            08-14Z        14-20Z        20-02Z        02-06Z
KCLT       High  88%     Med   75%     Med   64%     Med   67%
KGSP       High  89%     Med   72%     Med   76%     Med   61%
KAVL       Med   77%     High  83%     Med   74%     High 100%
KHKY       Med   79%     Low   55%     Med   72%     Med   74%
KGMU       Med   79%     Med   72%     High  81%     High  82%
KAND       High  84%     Med   67%     Med   75%     High  89%

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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