Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 171917

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
317 PM EDT Sat Mar 17 2018

Somewhat unsettled weather with normal to above normal temperatures
and chances for rain and/or thunder tonight and Monday night as
region experiences the passage of a number of shortwaves, along with
moisture increases.


As of 235 PM: We are now officially in our convective lull, which
should last for the better part of the rest of the afternoon,
so precip chances were cut back across all but the TN border/nrn
mountains. The biggest discrepancy with the fcst continues to
be the dewpoint over the nrn foothills, where westerly flow
was contributing to downslope drying and dewpts much lower than
expected. An adjustment was made using the new guidance. Elsewhere,
temp trends were recovering, but still problematic in a few spots
where westerly downslope was helping to warm the air mass much
more than expected, such as Asheville.

The potential for thunderstorms, possibly severe across the nrn
zones, remains the primary concern for the near term period. The
latest Day 1 Convective Outlook has kept the risk across the
nrn zones for later in the day, with a Slight Risk along our
nrn border. We will toward the Cumberland Plateau for signs of
thunderstorm development late this afternoon, where the environment
has sbCAPE on the order of 1000-1500 J/kg and effective bulk
shear of 50-55kt. The environment across that area would support
supercells and expect that if convection fires that eventually
some of it will organize as such and make a run at the mtns. What
remains to be seen is if the motion would take it past just to our N
across NC, or if it would scrape across the nrn tier of zones. The
greatest threat still looks like it will be mainly large hail and
damaging wind, with the timing after 00Z. Hard to say how well it
could survive a trip over the mtns, unless the air mass over the
NC foothills acquires a great deal more low level moisture than
it has currently. We will monitor.

Otherwise, the main short wave driving the storms will move
quickly SE overnight across the mtns and then nearly to the coast
by 12Z Sunday, carrying sfc low pressure across NC and dragging a
front down across the region. Precip prob looks greatest on the
TN border thru the early morning hours, with a chance possible
mainly along/NE of I-26 in the mid/late evening. The passage of
the wave should take most of the precip chances with it, except
for a small chance in the pre-dawn hours on the TN border owing to
some lingering moisture. Min temps will remain well above freezing
so that would be all liquid. Expect clearing sky around daybreak,
with high pressure building in from the NW. Temps are expected to
be a bit tricky as we maintain a gradient across the fcst area
in the afternoon...above normal over northeast GA/upstate SC,
but below normal across the NW Piedmont.


As of 2PM EDT Saturday:  Fairly flat mid-level ridge is progressing
eastward across the region Sunday evening, with precipitation
associated with next upstream wave well to the west and southwest.
Fairly progressive upstream system dives into the south-central
CONUS with the precipitation ahead of it reaching the western parts
of the CWA early Monday morning.  At 12Z Monday, triple-point
surface low is near northeast Oklahoma with a warm front extending
southeast down the NC/GA stateline.  This warm front lifts northward
across the CWA on Monday, bringing an increase in BL moisture and
increasing chances for showers.  Cold front passes through the area
Monday night on the back side of the system.  Frontal passage will
be associated with the strongest chance for precipitation for this
event.  FROPA Tuesday morning will begin drying the BL and end that
round of precipitation, with dry northwesterly advection on Tuesday.
Main upper forcing from upper shortwave holds-off until Tuesday
evening, which will give some chance for light showers later on
Tuesday, despite low-level dry advection, especially over higher

Warm frontal passage on Monday will increase dewpoints enough to
create some CAPE Monday afternoon/evening, which along with good low-
level vertical wind shear, will creating some marginal chance for
strong thunderstorms, per the SPC outlook for day 3.

Temperatures will be seasonally warm on Tuesday, behind the warm
front and ahead of the cold front.  Once the cold/dry advection
kicks-in behind the front Tuesday night, temperatures will begin to


As of 235 PM EDT Saturday: the extended fcst picks up at 00z on
Wednesday with a deepening southern stream upper trof moving over
the fcst area while upper ridging builds over the Western CONUS.
As we move into Thursday, the upper trof appears to be reinforced
from the backside as it gradually drifts off the Southeast Coast.
The latest long-range model guidance has the trof axis lingering
just off the coast well into Fri and possibly into Sat, as the
upper ridge is flattened just to our west. At the sfc, a complex low
is expected to be centered over the region to start the period. This
system is expected to move off the Southeast Coast during the day as
Canadian high pressure slides in behind it from the north. The
models suggest that a decent amount of deeper-lyr moisture could
linger over part of the CWFA well into Thurs, as the low moves
farther offshore. By early Friday, things should be dry with high
pressure squarely over the region. The next low pressure system is
expected to move eastward out of the plains by the end of the period
and bring another cold front towards the fcst. The latest 12z GFS is
about 24 hrs faster than the ECMWF and Canadian with the approach of
the front, and has it moving thru the CWFA next Sat, whereas the
other 2 models have the fropa on Sunday.


At KCLT and Elsewhere: The back edge of an MVFR-level cloud deck was
moving off to the east and scattering out at issuance time. Altho
KCLT had last reported an MVFR ceiling, satellite imagery suggests
this will be gone by the time the next observation. Most of
the region will have only scattered clouds with bases 035-045
once heating takes over. Deeper mixing should allow for some
gusts to develop thru the afternoon hours, with a SW direction
prevailing. Gustiness should go away with sunset. The main weather
concern will be this evening as showers/storms could develop
along/S of a boundary that sets up from northeast TN and nrn
NC. Guidance suggests a chance of precip generally in the 01Z to 06Z
time frame. Have opted for a TEMPO instead of a PROB30 in order to
better highlight the threat of thunder at KCLT. The threat should
end by 06Z or so as an upper disturbance passes, thereafter the wind
will come around to NW for the remainder of the overnight hours,
and then N/NNE at daybreak as high pressure builds in from the N.

Outlook: Drier conditions return briefly on Sunday before moisture
returns again from the west with a complex low pressure system
on Monday and Tuesday. Colder air may wrap south into the region
behind the system on Wednesday and some mixed precipitation cannot
be ruled out. Restrictions will be likely in the Mon-Wed period.

Confidence Table...

            19-01Z        01-07Z        07-13Z        13-18Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     High  93%     High  96%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High  99%     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:




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