Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 301454

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1054 AM EDT Sun Apr 30 2017

High pressure will weaken and move east today before a cold front
approaches eastern Tennessee overnight. This front will move across
the forecast area Monday with strong to severe thunderstorms
possible in the afternoon. Weak yet dry high pressure builds across
the southeast Tuesday through Wednesday. By Thursday...a moist area
of low pressure will work into area from the southwest.


As of 10:30am EDT Sunday: Currently, radar is clear, but conditions
are mostly cloudy with some gradually thinning of clouds allowing
some sunshine through.  Cloud cover, both mid-level and upper, are
expected to keep a bit of a lid on temperatures today, with highs
running around 5 degrees below yesterday.  Reduced temperatures will
also reduce CAPE and the chances for any significant precip today,
but precip. becomes likely as major front approaches late tonight
and tomorrow from the west.

Chances for precipitation today are limited by noticeable cap in
soundings. There may be a period from 18 to 21Z where the cap erodes
enough to allow surface-based convection, especially in upslope
areas, but that is the only immediate window.  Precip chances
improve as front approaches the area Monday morning, with attendant
upper support and low-level focus.

Area is currently in a well-defined and large warm sector ahead of
major frontal system currently stacked over the south-central
CONUS.  Cold front with line of convection is currently moving
through LA into MS.  Latest timing has front into the GSP CWA early
Monday morning, passing through by late Monday afternoon.
Significant precipitation/thunder is expected near and along the
front.  Passage of cold front will cool temps and scour moisture,
ending immediate chances for precip. Main question with this event
will be the chances for severe weather.  Instability and wind shear
are both in the moderate range with 30 to 40 kts of bulk shear and
LI to -5 Monday afternoon.  Shear is much better further north, but
combination of ingredients should be enough for at least some severe
weather on Monday, with wind gusts in bowing segments of expected
QLCS-type system.  An isolated, brief tornado is not out of the
question, but alignment of shear direction with front orientation
argues for QLCS and non-super-cellular storms.

850mb flow will also be quite strong with passage of the front, with
southerly to southwesterly winds of 40 to 55 kts from tonight
through Monday, which could produce the need for a wind advisory at
higher elevations.


As of 240 AM EDT Sunday: The short term begins with a bang as a well
advertised cold front pushes into ern TN right arnd 12z Mon.  A
pre-frontal trof will be the main focus area for deep convection as
it crosses east of the FA by 00z Tue. With a deep upper cyclone
aiding the llvl pattern...very good shear will develop mainly below
18 Kft where 50+ kts are noted at h8. Instability will be a limiting
factor for svr tstms across the mtns...however as the system moves
east across the fthills and piedmont an uptick in sbCAPE to moderate
levels will combine with bulk shear on the order of 45 kts or so.
Upper forcing will be relatively weak however as the stacked low
traverses northwest of FA and develops difl well north of the area.
In any looks like there will be enuf llvl forcings and
instability for organized multi-cell tstms forming along a line thru
the afternoon and early evening. With the fairly quick speed of the
system and only modest accelerations noted thru the HGZ...will expect
more of a isol damaging wind event with outflows/shear possibly
producing/maintaining strong cold pools ahead of the line. The
majority of activity will be east of FA area by 00z as llvl td/s
lower quickly behind trof axis. Max temps should be right arnd
normal with widespread cloudiness maintained thru the day.

Tues looks to be a very nice day. A dry srn stream sfc high will
build in and slowly track east. Sfc td/s will fall into the 40s
while max temps reach seasonal levels. Mins overnight will also be
very comfortable with near normal lows expected.


As of 305 AM EDT Sunday: Another nice day is in store for Wed. The
FA will remain under flat ridging while weak sfc high pressure
tracks off to the east. Max temps will be a little warmer than
Tue...with a highs reaching a cat or so above normal. However sfc
td/s will still be rather low making for another comfortable day.
The NC mtns will begin to see -shra and possibly a few tstms late in
the day as ulvl difl increases ahead of a strong h5 trof diving into
the srn plains. This trof and associated sfc reflections will become
the main synoptic feature affecting our area for the remainder of
the ext range.

By early Thu...the ulvl trof will be instigating cyclogenesis across
the MS river valley. The models have varying ideas as to the actual
amount of deepening and placement of the sfc low...with the GFS/CMC
being much quicker and stronger than the lagging ECMWF. The evolution
of this system will be interesting to watch as the models are going
back and forth with cutting off the upper low or progressing the
whole system quickly NE of the FA by Fri. The 00z ECMWF has just
displayed a 180 from it/s previous soln and is now the faster and
less wet outlier than the other op models. One thing that looks
fairly confident is Thu night thru Fri morning shud see decent
coverage of precip with moderate totals...likely arnd an inch or so
most locales. There will likely be embedded tstms...but soundings
are not conducive for anything stg to svr due to the timing. After
Thu...the fcst is more uncertain and pops/qpf have been adj down
accordingly. For looks like the period Fri thru Sat will
see moist wrap around -shra/tstms affecting the mtns as the sfc
cyclone will probably be somewhere from the OH Valley to the
Mid-Atl region. With good cloud cover expected max temps will be
held a cat below normal...however Fri/Sat highs could end up much
warmer depending on the eventual speed of the system.


At KCLT and elsewhere: Isolated shower activity is at a minimum this
morning, even generally staying away from KAVL in the most upslope
prone areas. Abundant boundary layer moisture is the bigger concern
throughout as MVFR to IFR cigs have developed and continue to
blossom around the region in moist S to SE flow. IFR to MVFR
conditions may continue through much of the morning, with slow
recovery to the lower end of VFR expected by afternoon. Isolated to
scattered showers will develop this afternoon, but given the
cloudiness and weak triggering, VCSH should suffice most areas.
Deeper moisture will then start building from the west late in the
period, with a return to MVFR cigs from 00Z to 06Z and shower
coverage increasing overnight. Anticipate southerly winds around 10
kt or less, but with a few low end gusts possible late in the day,
and becoming gustier overnight.

Outlook: Widespread showers and thunderstorms are expected
throughout on Monday as a cold front crosses the area. Drier weather
is expected Tuesday. A return of afternoon/evening thunderstorms is
possible Wed, while another cold front may bring another round of
widespread showers and storms by Thursday into Friday.

Confidence Table...

            15-21Z        21-03Z        03-09Z        09-12Z
KCLT       High  93%     High 100%     High  80%     High  84%
KGSP       High  91%     High  91%     High  86%     High  94%
KAVL       High  81%     High  98%     High  95%     High  83%
KHKY       High  91%     High 100%     High  81%     High  92%
KGMU       High  91%     High  93%     High  85%     High  96%
KAND       High  88%     High 100%     High  93%     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:




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