Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 130830

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
330 AM EST Wed Dec 13 2017

Dry high pressure will build over the region through tonight. A cold
front will cross the area from the northwest early Thursday morning.
Another dry high pressure will dominate our weather through Saturday
night. Low pressure and a cold front are expected to return from the
west on Sunday into Monday.


As of 300 AM EST Wednesday: A shortwave trough and associated
weak clipper low will dive southeast across the Midwest to the
Ohio Valley today, then cross the Central Appalachians tonight.
Meanwhile, gusty NW winds are still producing some advisory-level
gusts and dangerous wind chills in the NC mountains early this
morning. I have opted to keep the current hazards in place thru
12z. From there, winds should subside and back from NW to SW by
midday, then start to increase in the afternoon. Temps will be
chilly under mostly sunny skies with highs in the low to mid 40s,
except 30s above about 3500 ft.

Tonight, as the clipper low crosses the Ohio Valley,
west-southwesterly 850 mb flow ramps up to over 50 kts across
the southern Appalachians this evening. The flow will initially
be WAA flow, but will veer to west-northwest and become CAA
after midnight. This setup isn`t ideal for mountain wave strong
winds. However, elevations above 3500 ft will likely see some
advisory-level gusts. So will hoist another wind advisory for
the same areas that have one in effect attm from 00z to 12z
tonight. Some low-level moisture is progged to reach the TN/NC
line around 9z and may produce a few snow showers or flurries
before daybreak, mainly in the northern mountains. Temps will be
held up overnight due to intially WAA and overall mixing thru the
night. Lows in the 20s to mid 30s.


As of 245 AM EDT Wednesday: If not for the low probability of
two brief periods of light snow near the TN border of the nrn
mountains...the first being Thursday morning and the second being
Friday night...the forecast for the latter half of the week would be
fairly quiet as the upper pattern continues to deamplify. The first
"event" looks like it will end by noon as the better moisture pulls
out and the low level flow weakens and backs to something more
westerly. The probability is so low, perhaps 20 pct at best, of
seeing any measurable snow that one might wonder why we are talking
about it, but it is really the only interesting thing going on. The
westerly flow should allow temps to make it back to near normal,
maybe a bit above normal east of the Blue Ridge, in the afternoon
in spite of a passage of a weak cold front. High pressure builds
back in Thursday night and early Friday. Temps drop back below
normal Friday with more cloud cover expected and as thickness falls
behind the weak front. Late in the day, another short wave will
dive eastward, crossing the fcst area Friday evening. Mid-level
dpva looks ok, but the system is moisture-starved, so the passage
of the wave will barely register. Another brief period of low
level moisture will impact the nrn mountains late Friday night
as the flow veers around to NW for about six hours or so, but the
moisture pulls out quickly and any small chance of snow ends before
daybreak on Saturday.


As of 135 AM EST Wednesday: the extended forecast begins 12Z
Saturday with high pressure centered over the deep southeast but
transitioning to be off the East Coast by Sunday morning. Both the
GFS and ECMWF are now in agreement on initiating low pressure over
coastal Texas Saturday evening then moving it toward the Carolinas
Sunday and Sunday night. This will occur ahead of a 500mb trof which
will slowly progress east across the western Plains states late in
the weekend. The old EC is a bit faster in bringing moisture into
the far western mountains at 12Z Sunday. At that time, the new GFS
still has the leading moisture just entering western Alabama. The
GFS has convection firing along the Gulf coast and robbing moisture
transport to our region Sunday night. The old EC also shows the same
trend of weakening the northern area of rain from the Carolinas
north. Both models have the heaviest rain following the Gulf coast
to cross Florida and southern Georgia Sunday night into early
Monday. The front stalls out across Florida and the Carolina coast
on Monday as if waiting for the trof axis to arrive. The GFS brings
a wave of moisture up from the Eastern Gulf on Tuesday and Tuesday
evening mainly along and south of I-85. The old EC is more
progressive with less moisture for our region and by late Monday
night we are drying out which continues through Tuesday night.
During this forecast period, Max Temps should be near normal and Min
Temps above normal due to the additional cloud cover.


At KCLT and Elsewhere: Basically a wind forecast today thru tonight,
as a very fast progressing pattern will be on tap across the region.
Another clipper system is diving into the Midwest today, and as it
approaches, winds will switch from NW to SW. The winds are gusty in
the mountains early this morning, but should subside before the
shift around 17-18z. After the shift, the winds will increase, with
some gustiness expected by late aftn into early evening. KAVL will
be sheltered from the west-southwest flow this evening, with
forecast soundings showing 40-60 kts around 2000 ft AGL (depending
on the model), while sfc winds remain around 10-12 kts and turn to
NW. So will highlight LLWS there. Guidance is mixed on how much
gustiness there will be at the other sites. Given the strong LLJ,
there could be intermittent gusts over 20 kts at any of the sites,
but the highest chances look to be KCLT and KGSP/KGMU.

Outlook: Expect VFR conditions over the next several days, a series
of dry cold fronts bring only a little moisture to the TN/NC border.
Then, a low pressure system may try to organize over the Southern
Plains on Sunday, possibly bringing more abundant moisture atop the

Confidence Table...

            15-21Z        21-03Z        03-09Z        09-12Z
KCLT       High  81%     High  82%     Med   68%     Low   56%
KGSP       Med   70%     Med   75%     High  83%     Med   64%
KAVL       High  94%     High  85%     Med   70%     High  87%
KHKY       High 100%     High  83%     Med   66%     High 100%
KGMU       Med   76%     High  86%     High  85%     Med   63%
KAND       Med   76%     Med   65%     High  90%     Med   61%

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 until 7 AM EST this morning for NCZ033-
     Wind Advisory until 7 AM EST this morning for NCZ033-048>052-
     Wind Advisory from 7 PM this evening to 7 AM EST Thursday for
     Wind Chill Advisory until 7 AM EST this morning for NCZ033-


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