Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 231914

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
314 PM EDT Fri Mar 23 2018

Low-pressure from the west will move along a warm front draped
across the western Carolinas Saturday into Sunday bringing a wintry
mix for the northern mountains and cold light rain elsewhere.
Well below-normal temperatures will persist through Monday as strong
high pressure builds down the Appalachians. Temperatures rebound
back to near normal for mid- and late week as high pressure moves
offshore and moisture increases ahead of the next system.


As of 230 PM: Developing low pressure in the lee of the Rockies will
begin to move through the Plains late this aftn and evening. A warm
front will activate ahead of it, initially over the mid-Mississippi
Valley. The axis of a midlevel ridge will shift overhead tonight
as the warm front edges its way eastward, setting up from eastern
KY to the NC Piedmont by Saturday morning. However, weak upglide
will develop over our mountains well before dawn, and that is when
precip chances are expected to quickly increase. Unfortunately
it`s not looking like the event will unfold in a straightforward
manner from that point. It looks likely that temperature profiles
will feature non-freezing values only in the sfc layer, supporting
generally rain or snow dependent on sfc temp at the start of the
event. The arrival of the warm front will gradually change that,
introducing a warm nose in the morning.  While warm advection
generally should continue through the day, 850mb plots from
the NAM/GFS seem to reflect wet-bulb cooling for a time after
the onset of precip. Even the development of the warm nose thus
looks difficult to time. Furthermore, as CAD begins to develop, sfc
temps will remain cool across the northern mtns and NW NC Piedmont,
which will help maintain the possibility of wintry wx. As you can
see the p-type fcst is complicated.

For surface temps through the period, verification statistics from
the last 2 wedge events were used to choose which guidance members
to include in a blend. A non-diurnal trend is expected in parts
of the area due to wet-bulbing and the onset of CAD. Overall mins
tonight will be in the upper 20s to low 30s over higher elevations
of the mtns, with mid-upper 30s in the NC Piedmont, and lower 40s in
the Upstate. Most of WNC north of US 74 will see only a few degrees
of warming tomorrow, but areas south (including the SW NC valleys)
will warm comparatively well. However, only the very far southern
zones will see values even close to normal.

Initially deep saturation, the aforementioned isentropic forcing,
and any DPVA that results from a shortwave rounding the upper ridge
should combine to produce reasonably good QPF over the northern
mountains after midnight tonight. Snow ratios are expected to be
at or above normal at this time, though the midlevels will dry
during the day and ratios will taper off. Of course, by that time
snow is no longer likely to occur in many areas due to the warming
aloft. Using WPC QPF and the Top-Down technique to account for the
complex thermal profiles, warning criteria snowfall is expected to
occur between midnight and noon in Avery-Mitchell-Yancey and the
high elevations in NE Buncombe; a Winter Storm Warning will begin at
midnight in these areas. Some of the advertised accumulations will
include sleet that develops during the day. By afternoon nearly
all areas still with sufficiently cold temps will see IP/ZR. Ice
accumulation will then become the primary concern. A portion of
the area adjoining the warning will receive a Winter Wx Advisory
for lesser accumulations.


As of 215 PM EDT Friday: The forecast remains very complicated for
the short term, with continued wintry precip concerns especially for
the northern mountains. We begin the period at 00z Sunday with an
area of surface low pressure over the OH/MS River confluence, with a
warm front extending east and then wrapping around the mountains
with a bit of insitu damming against the Southern Appalachians. The
shortwave driving the surface low will continue to slowly be
absorbed by the longwave trough over New England and SE Canada as we
push into early Sun day, with the bulk of the moisture pushing
toward the coast. Upper ridging will develop up the MS Valley
resulting in an unusually strong area of upper confluence (between
it and the NE upper low) over the northern Mid-Atlantic states, and
the damming should morph into a very strong classical signature,
which should serve to push any lingering precip out of the way as
the wedge strengthens down the eastern slopes of the Appalachians
with very strong low-level dry air advection. Some lingering light
precip will remain across the SW mountains, NE GA, and extreme
southern Upstate but only very light at this time.

As always, however the devil is in the details. As we start the
period, very cold low-level temperatures can be expected, but with
strengthening WAA at 700mb, this leads to an impressive warm nose
across the area. The surface temperatures will then be the deciding
factor on rain vs. freezing rain overnight Saturday night. NAM
continues to be on the colder side with some very scary looking
soundings for KTNB, whereas the GFS keeps surface temperatures at or
just slightly above freezing. Near-term hires guidance that`s coming
in for the beginning of the period however seems to also be on the
cool side, so have taken a general blend but the result is slightly
more in line with the cooler NAM than the warmer GFS. With the warm
nose in place aloft, that starts us out with some freezing rain at
the beginning of the period, and since this lines up with the
heavier QPF before the surface low pushes east, the 00z-06z period
Sunday could be dicey up in the northern mountains with a quick shot
of 1/4" ice. Pops will be on a rapid decline after 06z Sunday so do
not expect much in the way of additional accumulation thereafter,
but as temperatures drop, the wintry precip should spread SW into
the rest of the mountains with a complicated wintry mix of sleet and
even snow building in as the warm nose erodes. Of course, the
eroding warm nose (which in and of itself would lead to more of a
snow forecast) will compete with the drying aloft, which means we
lose moisture in the dendritic snow growth zone with no ice nuclei.
With only supercooled liquid to work with, light freezing drizzle or
ice pellets will likely be how the event ends as we push into Sunday
morning. With this, and in collaboration with the near-term
forecast, the upgraded Winter Storm Warning and Advisory will be
allowed to expire at the previous watch ending time of 15z Sunday,
but accumulating precip it may very well be over by sunrise.

With the very strong damming building in on Sunday, expect a very
cold and miserable day, a good 20 degrees below seasonal normals and
frankly even below normal for January (so much for astronomical
spring, much less meteorological...). Highs in the northern
mountains will be barely above freezing (so may or may not be able
to melt any wintry precip that falls), and expect a hard freeze in
the northern mountains Sunday night with at least 30s everywhere.
With lingering precip in the SW mountains, could see some continued
light wintry mix, and can`t rule out that some trace accumulations
will be possible, but since Sunday night is 5th period, certainly
not worthy of any products at this time, though will keep (or add if
necessary) the winter weather wording in the HWO.

Expect a little recovery in temperatures on Monday (maybe getting
into the lower 50s across portions of the Piedmont), but still a
good 15 or so degrees below normal. GFS remains a little more robust
with the slug of moisture continuing along the warm/wedge front in
Georgia and have allowed pops to linger across the Upper Savannah
Valley, but nothing significant.


As of 245 PM Friday: The model guidance is still at odds early
next week with how much low level moisture will be entrained
from the Atlantic and moved over the top of the cold air damming
wedge across the forecast area Monday night and Tuesday, with the
GFS remaining the wetter solution. Still feel the need to remain
consistent with the dry ECMWF this far out in time, especially given
the expected strength of the sfc high to our NE and the expected
cold temps. Confidence doesn`t get any better from mid-week onward
as the models also do not agree on where a significant slow moving
cold front will be strung out SW-to-NE as it runs up against the
upper ridge holding position along the southeast coast. The GFS is
just a bit more progressive with the upper pattern so naturally it
brings the leading edge of the wide precip band associated with
the front over the mtns on Wednesday, then laying it down across
the region on Thursday, and moving it east on Friday later in the
day. Meanwhile, the ECMWF keeps the boundary farther off to the
west. The fcst reflects a blend of the two with a trend toward the
GFS because of the expected moisture transport from a wide-open
Gulf of Mexico after Wednesday. Confidence sort of goes up for the
end of the week as the ECMWF also shows the front coming thru on
Friday, so the fairly high chance late Thursday night and Friday
feels more certain. The trend of the GFS is for less in the way
of instability, so thoughts of severe weather with this system
are put on a back burner for the time being. Excessive rain would
appear to be the bigger threat, but until more of an indication of
a tropical connection is seen in the guidance, we will not raise
any expectations. Temps were nudged downward about a category or
so mid-week owing to the greater cloud cover and precip coverage,
but still come back above normal for the end of the period.


At KCLT and elsewhere: Conditions will be VFR until tomorrow
morning, when warm frontal activation over the area will permit
precip to develop overnight, and ceilings to subsequently
lower. None of the TAF sites are expected to see wintry wx, but
areas of the mtns and NC Piedmont (particularly north of KAVL/KHKY)
are not out of the woods. Cigs are progged to continue lowering
as precip continues into Saturday. Development of CAD Sat aftn
looks to lock in LIFR over KHKY and other northern Piedmont sites,
though the chance of this occurring before the end of the TAF
period precludes its inclusion in this set. Right now it appears
the IFR/LIFR will remain north of KCLT until after 00z Sun.

Outlook: Elevated precipitation chances and periodic flight
restrictions will occur this weekend with another approaching low
pressure system. Cool and dry high pressure will try to build in
from the north early next week.

Confidence Table...

            19-01Z        01-07Z        07-13Z        13-18Z
KCLT       High 100%     High 100%     High 100%     High  85%
KGSP       High 100%     High 100%     High 100%     High  84%
KAVL       High 100%     High 100%     High  83%     High  82%
KHKY       High 100%     High 100%     High 100%     Med   76%
KGMU       High 100%     High 100%     High 100%     High  80%
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:


NC...Winter Weather Advisory from midnight tonight to noon EDT
     Sunday for NCZ048-501-503-505.
     Winter Storm Warning from midnight tonight to noon EDT Sunday
     for NCZ033-049-050.


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