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FXUS62 KGSP 120255

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
955 PM EST Wed Dec 11 2019

Dry but cold high pressure will persist through Thursday. A
brief period of freezing rain is possible early Friday as a
coastal storm develops, but the majority of the precipitation
will fall as a cold rain and persist through midday Saturday.
High pressure briefly returns late Saturday through early Monday,
but the next storm system will begin to impact our area late Monday.


As of 945 PM: With clear skies and relatively light winds, outside 
of the higher ridges, temperatures have fallen just a bit faster 
than guidance. Still anticipate slightly improving mixing on light 
northeasterly flow overnight as the surface high pressure center 
strengthens to the north of the forecast area. Will thus continue to 
skew the min temp forecast to the lower edge of guidance, but not 
reduce further at present. This will put most of the area well into 
the 20s overnight, with lower 30s mainly in the upper Savannah River 

Otherwise, the center of the sfc high to the north will drift east 
Thursday and is expected to be centered just off the New England 
Coast by early Thursday evening. This will establish a wedge-like 
pressure pattern over the Carolinas, though with no associated moist 
upglide yet, and help keep high temps well-below normal for mid-
December. Low-lvl winds will remain NELY through the day. The only 
exception will be the mountains, where winds will become more SELY 
for tomorrow. Expect a dry day, with dewpoints mixing out a fair 
amount again by the afternoon.


As of 245 pm Wednesday: The period begins Thursday night with
moisture beginning to advect northward in response to a shortwave
embedded in a broader low-amplitude longwave trough and an
attendant GoMex coastal low.  As long-advertised, this moisture
will encounter a cold-air wedge across our area associated with a
strong (1040mb) but transient high pressure system which is already
moving off the New England coast at the beginning of the period.
Light precipitation is expected to break out in response to upglide
over this near-surface airmass after midnight Friday while we
lose a direct connection to the cold airmass associated with the
parent high.  With our source of low-level air originating from
the central Atlantic coast, we will need to rely on nocturnal
cooling and evaporative cooling from the developing precipitation
to reinforce the cold-air damming (in-situ or in-place) and bring
low temperatures close to or just below freezing, as indicated by
current wet-bulb temperature forecasts.

Meanwhile, guidance continues to advertise, though to varying
degrees, a pronounced deep "nose" of warm air aloft (generally +3
to +7C from about 950mb to 700 mb) developing overnight Thursday.
For those cold enough, some sleet is possible at precip onset
when the warm nose is weakest and the low-level sub-freezing air
is deepest, but we still expect a quick transition to freezing
rain during the early morning hours on Friday.  Note that it will
take some time for the column to saturate and the coldest areas
(i.e., furthest north) will be the last to see precipitation
reach the surface, so this will narrow the "window of opportunity"
and work to further limit accumulations.  However, further south
along the Escarpment, saturation will occur earlier and overnight
precipitation forecasts are higher.  Therefore, there is a chance,
reflected a bit in the current ice accumulation forecast, that the
southern NC Escarpment may actually see some of the highest accums.

Nevertheless, the release of heat caused by rain freezing on
(primarily elevated) surfaces should limit the duration and
significance of any accumulation as we continue to expect surface
temperatures to quickly rise above freezing for most locations by
mid-morning on Friday absent any cold-air advection.  Therefore,
we continue to advertise accumulations less than 0.10" across the
western NC Piedmont, Foothills, Blue Ridge Escarpment, and central
and northern mountains.  If anything, the latest model guidance has
actually trended a tad warmer in the near-surface profile which,
at a minimum, suggests confidence is increasing that this event
will be of the nuisance variety.  At this time, we believe the
impacts will generally be to elevated surfaces (including trees,
powerlines, and bridges) since ground temperatures will start off
fairly warm; however, plan on altering your commute in the areas
of concern on Friday morning to account for patchy slick spots.
Though we will moderate above freezing on Friday morning and any
ice will quickly melt, expect much of the area to struggle getting
into the lower to mid-40s.

Unfortunately, the fun is only just beginning with this system as
model guidance remains confident that multiple shortwaves will
propagate across the Deep South Friday into early Saturday and
this portends multiple rounds of light to moderate rainfall during
this time.  There remains a non-zero chance that some of our area
will see totals in excess of 2" by the time all is said and done by
early Saturday afternoon, and general consensus is for the heaviest
totals to align closer to any remnant wedge boundary across the
southern NC and SC Piedmont.  We continue to expect the best chances
for heavier rainfall to occur Friday afternoon and early evening
in response to strong forcing associated with a negatively-titled
shortwave moving across the TN Valley.  The operational 06Z, 12Z,
and 18Z NAM reverted back to a solution of heavy upslope-induced
rainfall from the upper Savannah River Valley north and east along
the Blue Ridge Escarpment during this time and while this should
be monitored, this solution remains an outlier even amongst all
other SREF members.  Regardless, Saturday's high temperatures will
be a solid 10-12 degrees warmer than on Friday with westerly flow
eroding the wedge and perhaps inducing some downslope warming.


As of 245 pm EST Wednesday: By the beginning of the period Saturday
evening, conditions will be improving rapidly across much of the
area as the previously mentioned negatively-tilted shortwave pivots
over and east of the area.  However, there will be some moisture
associated with the trough axis as it pivots overhead Saturday
evening and with short-lived northwest flow behind the trough,
we continue to forecast continued chance PoPs along the NC/TN
state line resulting from upslope flow.  Without much CAA behind
this system, the forecast continues to call for mostly rain with
perhaps some insignificant light snow along the highest elevations.
Thereafter, generally zonal (west to east) flow aloft will set
the stage for a benign Sunday across the area while the next
upper-level trough begins to eject out of the Desert Southwest.

The guidance begins to activate the warm front associated with
the next system as early as Sunday evening from our western most
counties and points east across the TN Valley.  The ECMWF is the
most eastward with this warm front and induces modest upslope
showers in the southern mountains beginning Sunday evening whereas
the GFS is dry through Monday afternoon.  The forecast therefore
reflects increasing PoPs heading into Monday peaking with likely
to categorical PoPs overnight Monday into Tuesday morning as
a cold front enters the area.  This system will not present a
p-type concern for our area, save perhaps the highest elevations
at precip onset Monday night, as we are well south and east of
the surface low track and well within the deep-layer warm sector.
Guidance is still in good agreement that this front will clear the
area fairly quickly by Tuesday night setting up a quiet Wednesday.
As a result of the overall pattern, expect high temperatures to be
5-8 degrees above normal area-wide Sunday through Tuesday.  Expect a
return to near-normal temperatures behind the front for Wednesday.


At KCLT and Elsewhere: Surface high pressure will build over to the 
north of the region, with dry conditions and light NE flow 
prevailing, except for mainly NW flow early at KAVL becoming SE the 
latter half of the period. The northeast flow outside the mountains 
will increase slightly with mixing Thursday and low end gusts will 
be possible in a few locations during the afternoon. Gradually 
returning upglide moisture can be expected just beyond the current 
TAF period, with mainly lowering VFR cigs arriving near the end of 
the period from KAND to KCLT. Will omit any precip mention for now, 
but light precip will encroach from the south just beyond the 
current period Thursday night.

Outlook: A moist low pressure system approaching from the south is 
likely to bring back restrictions Thursday night into Friday. With 
cold air in place at the onset of precipitation, FZRA could result 
in some ice accumulation Thursday night and Fri morning, especially 
at KAVL and KHKY. Though precipitation will change back to rain 
during the day Friday, periodic restrictions are likely until the 
system departs the region on Saturday.

Confidence Table...

            03-09Z        09-15Z        15-21Z        21-00Z 
KCLT       High 100%     High 100%     High 100%     High 100%     
KGSP       High 100%     High 100%     High 100%     High 100%     
KAVL       High 100%     High 100%     High 100%     High 100%     
KHKY       High 100%     High 100%     High 100%     High 100%     
KGMU       High 100%     High 100%     High 100%     High 100%     
KAND       High 100%     High 100%     High 100%     High 100%     

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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