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fxus62 kgsp 120903 

Area forecast discussion
National Weather Service Greenville-Spartanburg SC
403 am EST Thu Dec 12 2019

a developing coastal storm will bring rain to much of the southeast
tonight and Saturday. Across portions of western North Carolina,
with temperatures being especially cold tonight, freezing rain
will bring light accumulation of ice to some areas. Precipitation
will continue during the day Friday as as a cold rain and persist
into Saturday, before dry high pressure returns to the area.


Near term /through Friday/...
as of 320 am: strong high pressure, presently centered between Lake
Ontario and Chesapeake Bay, will dominate our weather today. A dry
northeasterly flow will continue, maintaining Max temps several
degrees below normal, despite mainly clear skies. The high will
form a quasi-wedge over the region that will persist into tonight.

An upper trough seemingly comprised of phased northern and southern
stream shortwaves will shift east out of the High Plains early this
morning, with a sfc low developing over the Midwest. Cyclogenesis
eventually will occur ahead of the southern wave near the eastern
Gulf Coast tonight, but warm upglide will develop over the cold
high as early as this afternoon. The associated moistening will
quickly reintroduce cloud cover to our County warning forecast area from south to north,
followed by precip. With parts of the area very near or below
freezing, this will have notable impacts.

Pops reach likely range over the GA, western SC and NC Escarpment
zones around midnight. The cooling of temps will be tricky, with
the increasing cloud cover but also evaporative cooling as precip
gets underway. The fact that the global models generally show less
precip but cooler temps during latter part of the night is a little
concerning. The NAM is characteristically bullish with precip rates
and the related wet-bulbing, but also is quick to warm temps in
the early morning hours as the "parent high" migrates offshore;
this is important as this is when the guidance consensus on quantitative precipitation forecast is
greatest. Looking at vertical profiles from the available models,
even the cooler guidance still features enough of a warm nose
to expect freezing rain where sfc temps are below 0c; even if
the warm nose is not as strong as progged, the layer under the
inversion will be shallow and it will be hard to imagine much
refreezing. Altogether, most parts of our wnc zones between the
Blue Ridge Escarpment and I-85, excluding the Charlotte Metro
heat island, will be at or below freezing for at least some of
tonight's precip. Accumulation looks unlikely to exceed a tenth
of an inch in most areas, and unlikely to exceed a quarter inch
throughout. We will hoist a Winter Weather Advisory for 02z to
17z Friday mainly for impacted Road conditions in the early morning.

Though precip most likely will continue into Friday afternoon, temps
will warm enough for ice accumulation to end in the morning. It
will remain a cold and "wedgy" day with breezy northeast winds. Max
temps will be near normal in the far west, but as much as 15 below
normal in the wetter east.


Short term /Friday night through Saturday night/...
as of 315 am thursday: although coastal cyclone is expected to be
deepening near Cape Hatteras btw 00-06z Saturday, high pops are
maintained through Fri evening, as additional upstream height falls
will support a persistent warm air advection regime across the region until the
upper trough axis begins crossing the area early Saturday. Residual
cold pool will begin to drain Friday night, with temps likely slowly
warming into Saturday, likely culminating in Max temps a solid 15
degrees warmer than friday's readings. A moist west/northwest flow is expected
to develop in the wake of the trough axis late Sat into early
Sunday, with attendant upslope flow likely resulting in scattered
shower development near the Tennessee border. However, cold advection will
be muted due to yet more height falls impacting much of the western
and central conus, which results in a quick return to a zonal flow
regime by early Sunday. Thus, northwest flow snow is expected to be
primarily of the high elevation variety (with mostly rain in the
valleys), where some light accums are possible Sat night.


Long term /Sunday through Wednesday/...
as of 330 am thursday: continued backing of the upper flow over our
region in response to central conus height falls will support above
normal temps during the early part of the medium range, with
generally dry conditions expected until at least Monday. Precip
chances then increase by the end of Monday, and esp into Mon night
in Tue, as a cyclone tracks from the southern Great Plains into the
Ohio Valley. Pops initially increase in response to expected warm
frontal activation possibly bringing showers mainly to western areas
on Monday. Chances increase to likely Monday night as global model
guidance agrees in principle that a well-forced cold front will
cross the forecast area. The forecast cyclone track well west and
north of the area would not favor any p-type concerns, but would
suggest some high shear/Low Cape convective and heavy rainfall
potential, and indeed guidance seems to be trending ever-so-slightly
toward depicting some degree of instability drawing up close to our
area in advance of the front. Drying conditions are expected for day
7, with temps likely returning to, if not slightly below climo in
the wake of the front Wed afternoon.


Aviation /09z Thursday through Monday/...
strong dry high pressure north of the region will keep skies clear
early this morning. Good radiational cooling has allowed most areas
to decouple and winds thus are generally calm, but otherwise southeast
at kavl and north to NE at all other sites, reflecting The Wedge-like
pattern. Aloft, a shortwave trough will induce formation of a
low pressure system near the Gulf Coast starting late today and
continuing into Friday morning. Moist isentropic lift over the
cold sfc high will introduce low VFR cigs this evening, lowering
to MVFR and eventually IFR, though not until after 06z. The other
complication will be development of a wintry mix as precip falls
into the cold air. Such precip should be relatively short-lived,
most likely occurring between about 06z and 15z Fri. The most
difficult aspect of this event will be determining the sfc
temperature trends. Most guidance suggests temperatures aloft
will be warm enough to expect mainly freezing rain or pl when forcing for
precip will be strongest. For now, not having the opportunity to
fully assess the situation in light of the latest guidance, all
mention of wintry precip is in prob30 groups. Planning to have a
better handle on this part of the fcst for 12z tafs.

precipitation will change back to rain during the day Friday,
but periodic restrictions are likely until the system departs
the region on Saturday. Dry/VFR conditions will return until the
next front arrives Monday night or Tuesday.

Confidence table...

09-15z 15-21z 21-03z 03-06z
kclt high 100% high 100% high 100% high 98%
kgsp high 100% high 100% high 100% high 95%
kavl high 100% high 100% high 100% high 98%
khky high 100% high 100% high 100% high 100%
kgmu high 100% high 100% high 100% high 86%
kand high 100% high 100% high 100% high 94%

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:



Gsp watches/warnings/advisories...
NC...Winter Weather Advisory from 9 PM this evening to noon EST
Friday for ncz033-035>037-049-050-053-056-057-063>065-



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