340
FXUS62 KGSP 181323
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
923 AM EDT Wed Oct 18 2017

.SYNOPSIS...
Cool high pressure will move off the East Coast by the end of the
week providing a light south to southwest flow of milder air.
Temperatures will warm a little to a few degrees above normal
through the weekend. A strong cold front will arrive next week
bringing precipitation and potentially the coldest air so far this
season.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 915 AM: Fog in the major mountain valleys has crashed at
sunrise, and is still dense in some of the obs. I think the coverage
is fairly localized and should lift very soon, so I opted not to
issue a Dense Fog Advisory. Also, allowed frost advisory to expire
as temps warm this morning. The rest of the forecast is on track.

Otherwise, high pressure is centered over the Mid-Atlantic states and
will force a light northeasterly flow into the area today and
tonight. The atmospheric column will remain quite dry above the PBL
so skies will be largely free of clouds. That said, a shortwave
trough is moving in from the west and will bring some bands of
cirrus across this morning.

The pattern changes little over the next 24 hours; the core
of the sfc high will push offshore tonight in response to the
advance of the aforementioned trough. Winds will continue to be
light and temperatures below normal, though modification of the
airmass will result in maxes/mins a little warmer than the previous
day. Some frost is expected, though not likely as widespread as this
morning. Rather than issue another advisory with this package, we
will let day shift evaluate based on next guidance cycle as well as
on coop reports this morning, i.e., how much frost actually develops
this morning. Following persistence within this quiescent pattern,
I added some mtn/river valley fog in the wee hours again tonight.

&&

.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
As of 135 AM EDT Wednesday: Upper ridging builds across the eastern
CONUS Thursday and Friday. Surface high pressure remains across the
area as well. Thicknesses rise each day leading to a warming trend.
There may be some cirrus from time to time, especially on Friday,
but mostly clear skies will prevail. Highs a couple of degrees above
normal Thursday rise to around 5 degrees above normal Friday. Should
be good radiational conditions each night leading to inverse lapse
rates across the mountains and colder conditions outside of the
normally warmer thermal belt and heat island areas. Lows will be a
couple of degrees above normal each day however, keeping any frost
chances to a minimum.

&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
As of 345 AM EDT: Guidance in good agreement through the weekend.
Ridging surface and aloft slide off the east coast allowing a moist
southerly flow to develop. Isolated showers may develop Sunday
afternoon along and west of the I-26 corridor, but most locations
will remain dry. Temps will be up to 5 degrees above normal both
days.

Guidance, which showed an uncanny resemblance last night, have begun
to diverge slightly regarding the disposition of the upper trough
developing over the central CONUS. The ECMWF and GFS both show
southern stream energy splitting off from the trough as the upper
stream moves quickly east. However, the GFS shows stronger deepening
of the developing southern stream closed low and slows it down while
the ECMWF keeps the wave open and faster. The GFS shows a stronger
surface low pressure forming along the Gulf Coast only slowly moving
it and an associated cold front into the area by Tuesday. The ECMWF
has the cold front but a weaker low and a much faster movement with
the cold front moving east of the area by Tuesday. Given these
differences have used a model blend increasing PoP Monday and Monday
night with a slower decrease on Tuesday. Temps remain nearly steady
into Monday then fall to a little below normal on Tuesday.

There is the potential for a very cold air mass to move in behind
the cold front. However, given the significant strength and timing
differences will wait to show these trends until better agreement
develops in the guidance.

&&

.AVIATION /14Z WEDNESDAY THROUGH SUNDAY/...
At KCLT and Elsewhere: Cool and dry high pressure will remain
the dominant feature thru the period. That said, with abundant
near-sfc moisture, fog will plague the mtn valleys (and KAVL)
for the first couple hrs of the TAF period. Winds are expected
to prevail NE today, backing slightly toward N tonight if not
going calm. With so little change in the pattern through tonight,
persistence appears most reasonable. Hence fog is being reintroduced
similar to how restrictions unfolded this morning.

Outlook: Seasonably cool and overall VFR conditions will persist
through the work week with the only possible exceptions being
mtn valley fog/stratus restrictions in the morning. Moisture will
begin to return to the area Saturday in advance of a cold front
early next week.

Confidence Table...

            13-19Z        19-01Z        01-07Z        07-12Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       Med   77%     High 100%     High 100%     Low   47%
KHKY       High  95%     High 100%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     High 100%     High  96%

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:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

$$
SYNOPSIS...RWH
NEAR TERM...ARK/Wimberley
SHORT TERM...RWH
LONG TERM...RWH
AVIATION...Wimberley