Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 200552
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
152 AM EDT Sat May 20 2017

.SYNOPSIS...
The upper ridge moves east and off the East Coast this weekend as a
closed upper low swings northeast from Colorado. Expect a backdoor
cold front to drop south across the Carolinas Saturday and Saturday
night as a stronger cold front moves east across the Mississippi
Valley. As this cold front moves toward our region over the weekend,
deep moisture from the Gulf will bring an excellent chance
of rain late Sunday into Monday.  Expect another strong system in
mid week.

&&

.NEAR TERM /THROUGH TODAY/...
As of 1015 PM EDT: things have quieted down, with just a couple
stray showers left in the southwest NC mountains. I will keep a
slight chance pop along the TN border thru the overnight, but
otherwise, expect dry conditions with above normal temps. Some
valley fog will be possible, and some low stratus may develop across
the southern GA and SC piedmont before or around daybreak Saturday.
Temps will be above normal, similar to last night`s readings.

Amplification of the ridge between deep troughs/upper lows over the
Great Plains and the Canadian Maritimes is expected on Saturday,
which will occur more or less right over our forecast area.
Meanwhile, a back door cold front should be oozing SW across the
Virginia Piedmont when the day begins. Forecasting the SW penetration
of such boundaries is always a crapshoot, and the models have really
been struggling with it, even more so than usual in recent days.
Certainly part of the problem is the building ridge, which would
certainly act to limit the SW progress of the boundary. This isn`t
only problematic in terms of the temperature forecast, but also from
a pop standpoint. The warm sector air mass should be generally
capped, and guidance suggests the convective potential should be
limited to the immediate boundary. A consensus of the latest
guidance suggests the front will remain north of our area during
through the end of the day, and we will follow that solution in
terms of the max T forecast, which will be well above climo.
However, chance pops will be maintained across much of western NC
north of the I-85 corridor. Instability should be robust enough and
shear sufficient to promote an isolated severe weather threat with
any convection Saturday afternoon.

&&

.SHORT TERM /TONIGHT THROUGH MONDAY/...
As of 310 PM EDT Friday: The closed 500 mb low pressure center will
lift northeast across the western Great Lakes Saturday night through
Sunday, while the ridge axis along the southeast coastline moves
offshore. Moisture will deepen from the southwest late Saturday
night through Sunday as the low level flow backs and strengthens.
The backdoor boundary near the northeast part of the area should
slowly retreat northward through Sunday as flow increases in the
warm sector. The southern tier of the forecast area could get quite
unstable with any warming at all but shear should remain rather
modest at that time.

The deepest forcing and moisture will cross the region just ahead of
the cold front Sunday night through Monday morning. The 850 mb
southerly jet may briefly strengthen late Sunday through Sunday
night ahead of the approaching front, mainly over the piedmont, but
the nocturnal timing should dampen the severe weather potential. QPF
is slightly higher for the southern mountain upslope areas, and
rainfall in this area the next couple of days will need to be
watched very closely to assess moisture/stream levels. Shortwave
energy rounding the developing central/eastern CONUS trough will
likely cross the region on Monday. This will push the cold front
across the region, with gradual drying from the west through the
day. Temperatures will transition back below climo for maxes by
Monday afternoon.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
As of 210 PM Friday...The cold front will move to the Carolina coast
late Monday night taking the rain with it. This cold front is
expected to slow down and a wave of low pressure could develop along
the front over the eastern Gulf which would delay the exit of
moisture from our area before the next system arrives.  If the wave
does not develop, expect a brief period of drying between systems
Monday night into Tuesday morning. On Tuesday,a deep upper trough or
closed upper low will swing SE across the Mississippi Valley with
another period of deep moisture coming up from the Gulf Tuesday
night into Wednesday. The ECMWF is fastest in bringing moisture back
into our region with deep convection arriving from the north Gulf
coast late Tuesday. The GFS starts upslope showers over the southern
mountains before 00Z Wed then widespread showers and some stronger
convection Tuesday night into Wednesday. The GFS fires up deeper
convection as it moves all rain east of I-77 around 00Z Thursday.
The EC has the large band of convection arriving in NE GA by 18Z
with the main batch around 00Z Wednesday. The EC has more waves of
rain and convection passing over before moving it east Wed evening
similar in time to the GFS.  Height falls occur late Wed through
Thursday then after trough axis passes heights make good recovery
Friday and Friday night. Models are backing off precip late in the
week as EC continues the dry trend and GFS takes precip with
shortwave across Virginias late Friday.

Max Temps just below normal Tuesday and Wed then ten degrees below
on Thursday and 5 degrees below Friday.

&&

.AVIATION /06Z SATURDAY THROUGH WEDNESDAY/...
At KCLT and elsewhere: Except for KAVL where valley fog has
developed this morning, should see generally VFR conditions through
the day. KAVL will continue to see off-and-on LIFR conditions. Some
low-level moisture may move north and get close to KAND but have
continued previous trend of SCT MVFR. For now, convection chances
this afternoon too low to include, however, as a cold front
approaches from the west late tonight, should see increasing
activity at that point, and have introduced PROB30 SHRA for all but
KCLT and KHKY, with a PROB30 TSRA for KAND. Winds generally S
through the period, though backing to SE later this evening ahead of
the front.

Outlook: The approaching front is expected to bring more significant
chances of showers/storms/restrictions Sunday through Monday and
possibly into Tuesday.

Confidence Table...

            06-12Z        12-18Z        18-24Z        00-06Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High  98%     High 100%     High 100%
KAVL       Med   72%     High  91%     High 100%     High 100%
KHKY       High  94%     High  91%     High 100%     High 100%
KGMU       High 100%     High  97%     High 100%     High 100%
KAND       High  96%     High  88%     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...ARK/JDL
SHORT TERM...HG
LONG TERM...DEO
AVIATION...TDP



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