Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 180817

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
317 AM EST Thu Jan 18 2018

High pressure will overspread the region and drift offshore by early
Saturday, with temperatures increasing to above average through the
weekend. Another cold front will bring precipitation to the
Carolinas early next week.


As of 245 AM EST: All quiet again across the western Carolinas
and northeast Georgia, save the widespread black ice across much
of the region. Temps running colder in some locations, probably
because of the fresh snow cover.

Looks quiet and continued cold over the next 24 hours as the
upper low moves away to our east and is replaced by short wave
ridging aloft today and tonight. This will allow high pressure to
continue building in from the W and SW today. The pressure gradient
relaxes early this morning allowing the wind to diminish, which
should in turn alleviate the wind chill concern over the most of
the mtns. The fcst is right on track with allowing the advisory
for the central and southern mtns to expire at 12Z, and the one
for the nrn mountains to expire at 15Z. In spite of full sun,
temperatures will remain on the cool side of guidance because of the
new snow cover, and will also be slower to warm up than previously
thought. It doesn`t seem to make much sense allowing an Advisory
for black ice to expire when temperatures remain below freezing,
so the Advisory will have to be extended. For that, we propose to
break it into three large chunks. The Upstate and northeast GA go
above freezing at 17Z so that will be one segment, followed by the
Charlotte metro area and the rest of the wrn Piedmont/foothills of
NC at 18Z, then the mtns with the exception of the high elevations
along the TN border at 19Z. Temps will fall well below freezing
again tonight with excellent radiational cooling potential, no
doubt requiring another product to deal with another round of black
ice. Prefer to see how much melting we get through mid afternoon
once we get the sun involved. This will allow the day shift to
see some satellite imagery that might suggest where another round
of black ice might be widespread later tonight/early Friday, thus
suggesting another Advisory, versus where it would be more patchy
and could be handled with a Special Weather Statement.


As of 230 AM EST Thursday: The short term forecast looks quiet, as
an upper ridge builds over the eastern states in an amplifying
pattern. There is an interesting southern stream compact low that
lifts thru the building ridge, but has little moisture to work with.
The models are still in disagreement on exactly how the low will
track and eventually get sheared out. The GFS is the fastest and
keeps a closed low the longest. It brings some mid and high clouds
thru the area on Saturday, but otherwise, no sensible wx impacts.
The NAM and ECMWF are slower lifting the low out and keep it south
of the area near the Gulf Coast. So other than perhaps a little
increasing clouds on Saturday, it should be mostly clear thru the
period. Temps will be on a warming trends, as the ridge builds.
Highs near normal Friday, and a couple categories warmer on
Saturday. Lows slightly below normal Friday night, but a couple
categories warmer Saturday night.


As of 200 AM EST: The area will be under the influence of surface
high pressure and an upper ridge to start the medium range Sunday
morning, with relatively warm and dry conditions persisting through
Sunday and into early Monday morning. The next system will approach
the area Monday afternoon and evening, with a strong upper and
surface low propagating across the Great Lakes. Pre-frontal moisture
ahead of a cold front trailing the surface low will increase on
Monday, with the QG-induced lift from the upper low inducing QPF
response in our forecast area Monday some time around 18Z. The
system looks quite dynamic in global models and produces shear
profiles that would be quite conducive to organized severe weather
if the instability were not severely limited by the "wet noodle"
soundings at all sites in our CWA. The bulk of the precip and the
best forcing associated with the front will likely arrive overnight
Monday through early Tuesday morning, a timeframe that should also
prevent any diurnally-enhanced instability. Model agreement on the
lack of instability above 100 J/kg is striking, though this could
change if the timing of the front changes. The remains slight
disagreement in the 00Z model suite over when the precip will come to
an end Tuesday, though all global models have clearing with drier
air arriving by Tuesday afternoon. As some cold air arrives behind
the front in the NC mountains Tuesday morning, expect a transition
to rain/snow mix of briefly all snow in the higher terrain of NC.
All other locations will receive liquid precip for the duration of
the event.

The balance of the medium range remains quiet, with a dry
continental airmass moving into the area as a surface high over the
OH River Valley expands towards the northeast. Unusually warm
temperatures ahead of the front will moderate back towards average
by the end of the next work week.


At KCLT and Elsewhere: VFR through the period. Wind will be N to
NW overnight. KAVL will have frequent gusts for the first several
hours, but the gustiness should diminish by daybreak. A lee trof
is expected to develop during the middle of the day, which should
bring the wind direction around to the WSW east of the mtns. Wind
will go light SW at sunset. Expect sky to remain essentially clear
through the period.

Outlook: Dry/VFR condition are expected to continue through the
week and into the weekend.

Confidence Table...

            08-14Z        14-20Z        20-02Z        02-06Z
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:


GA...Winter Weather Advisory until noon EST today for GAZ017-018-
     Winter Weather Advisory until 2 PM EST this afternoon for
NC...Winter Weather Advisory until 1 PM EST this afternoon for
     Winter Weather Advisory until 2 PM EST this afternoon for
     Wind Chill Advisory until 7 AM EST this morning for NCZ048-
     Wind Chill Advisory until 10 AM EST this morning for NCZ033-
SC...Winter Weather Advisory until 1 PM EST this afternoon for
     Winter Weather Advisory until noon EST today for SCZ001>007-


LONG TERM...Carroll/Wimberley
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