Area Forecast Discussion
Issued by NWS Greer, SC

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572
FXUS62 KGSP 260550
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
150 AM EDT Tue Sep 26 2017

.SYNOPSIS...
Dry and warm high pressure will remain across the region through the
middle of week...while Hurricane Maria is forecast to remain off the
East Coast. A cold front will sweep across the area Thursday while
pushing Maria out to sea. A secondary cold front crosses Friday
night bringing much cooler air for the weekend into early next week.

&&

.NEAR TERM /THROUGH TODAY/...
As of 145 AM: Cirrus emanating from Hurricane Maria, which will
continue to drift northward off the coastal Carolinas through the
near-term period, continues to decrease across the forecast area,
and is currently confined to only the eastern 1/3 of the forecast
area. Meanwhile, scattered stratocumulus has developed across the
Blue Ridge and portions of the SC Piedmont in association with
shallow moisture trapped beneath a subsidence-induced stable later.
Meanwhile, low stratus and fog will continue to expand in the
southwest mountain valleys, and possibly develop in the French Broad
Valley for a brief window around daybreak.

Otherwise, the low level ridge axis will persist along the
Appalachian mountain chain through Tuesday, with low level flow
steadily backing more northerly with time. Expect less cirrus on
Tuesday but slightly more stratocumulus with daytime heating -with
max temps a tick above Monday`s values. A few of the mesoscale
models have isolated ridge top showers in the afternoon but this
looks unlikely given the inversion aloft and lack of upslope flow.

&&

.SHORT TERM /TONIGHT THROUGH THURSDAY/...
As of 2:30 PM EDT Monday: Our Forecast Area (FA) will initially be
pinched between Tropical Cyclone Maria and an upper level ridge
extending from southern New England, across the Ohio and Tennessee
Valleys, to the western Gulf of Mexico. The outer fringe`s of
cirrus, associated with Maria over our eastern FA at the start, will
thin and gradually pull off to the east by Wednesday morning.
Elsewhere clear skies are expected.

A strong subsidence pattern across the area Wednesday, combined with
very favorable compressional effects, and abundant sunshine, should
push temperatures up some 5 to 10 degrees above normal.

Meanwhile, we left Wednesday night quiet, with ridging to hold sway
for one more period. Overall no rain and a mild night.

Although winds remain predominately northerly, it appears the cold
front will reach parts of our area late Thursday afternoon in the
north. The GFS offers an increase in clouds, with some spotty light
showers. Those showers should be few and far between, however we
kept a small POP, for a short period late Thursday afternoon, in
those upslope locations in the mountains of western North Carolina.
Elsewhere, more dry conditions on tap.

We have knocked temperatures back a few degrees on Thursday, but
more substantial changes will take place beyond this forecast
period.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
As of 145 PM Monday: The medium range models are in good agreement
to start the period Thursday night then not as good agreement toward
Day 7.  Maria will be pushed rapidly east across the Atlantic as the
cold front interacts with it.  With the passage of the dry cold
front and falling heights with the strong shortwave moving SE from
the Great Lakes, cooler air will be on the way for the weekend. The
GFS is more progressive with the trough axis crossing early Saturday
while the EC waits for Saturday evening to pass the trough axis.
Cool high pressure centered west of the Great Lakes late Friday
should be centered over Michigan late Saturday and shifting off the
East Coast Sunday evening.  Any development of a low along the
stalled out front deep over Florida on Saturday should be far enough
away from our region to have any real affects.

Monday will be a day with increasing cloudiness and appreciable rain
will depend on how the weather pattern sets up with the approach of
a strong shortwave. The old 06Z GFS is progressive with the trough
passing from the Mississippi Valley late Monday morning to the Great
Lakes and Ohio Monday night.  Perhaps a brief shot at rain late
Monday and Monday night if this occurs.  The EC cuts off a 500mb low
having it centered over Memphis early Tuesday. This would create a
strong flow of Gulf moisture into our area in the middle of next
week. The new 12Z GFS made a radical departure from the previous
run, building a ridge over the East at the start of next week and
keeping us dry. The trough does not get anywhere close to us with
the new GFS. If this is a new trend, we may not get a drop of rain
for a few weeks.

Temperatures will be at or just above normal Friday then fall to 5
to 8 degrees below normal by Sunday then rebound a little on Monday.

&&

.AVIATION /06Z TUESDAY THROUGH SATURDAY/...
At KCLT and elsewhere: SCT-BKN VFR stratocu is expected over the
first couple of hours of the forecast period at KAVL and the Upstate
SC terminals. However, the main concern (as usual of late) is the
potential for low stratus and/or fog in the mountain valleys later
this morning. The potential appears to be higher than yesterday
morning, due to the decrease in high clouds that has occurred in the
past 24 hours. Statistical guidance is also hinting at a higher
potential. Therefore, went ahead and included a tempo for 2SM/SCT002
btw 10-12Z. While uncertainty abounds, the probability for
conditions to be worse than forecast is probably slightly higher
than the probability for them to be better than forecast. Otherwise,
other than a tempo for MVFR visby at KHKY, VFR conditions are
forecast through the period. Some high resolution model guidance
suggests an area of IFR/low MVFR cigs may make a run at the western
Piedmont from the east by sunrise, but this should remain east of
KCLT if it does indeed develop. Winds will remain light NE or
calm/light & vrbl overnight, increasing to around 10 kts late Tue
morning.

Outlook: Dry conditions are expected most of the week. Under mostly
clear skies and calm conditions, fog and/or low stratus are possible
in the mountain river valleys each of the next few mornings. A
mostly dry cold front will cross the region late in the week.

Confidence Table...

            05-11Z        11-17Z        17-23Z        23-00Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       Low   57%     Med   61%     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:

www.weather.gov/gsp/aviation

&&

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

&&

$$
SYNOPSIS...DEO
NEAR TERM...HG/JDL/PM
SHORT TERM...TS
LONG TERM...DEO
AVIATION...JDL



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