Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 242153

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
453 PM EST Sat Feb 24 2018

A cold front will approach from the west tonight and then slowly
cross the region Sunday into Monday, with improved chances of
showers and thunderstorms through the period. Cooler and drier air
will settle southward into the region behind the front on Tuesday
before moisture rapidly returns to the area mid to late week.


As of 440 PM: Updated Sky/PoP thru daybreak and touched up T/Td
through mid-evening.

The synoptic pattern will finally experience some changes over
the next 24 hours as the persistent upper anticyclone over the
western Atl weakens as trofing move in from the west.  The same
can be said for the surface features as high pressure retreats
ahead of an approaching surface MLC.  As such, ample instability
across the MS river valley has setup a fairly convectively active
day to our west as prefrontal convection gets going over the
MidSouth.  This complex will work its way eastward through the
evening/overnight hours. Hi-res model runs continue to depict it
not reaching our westernmost mountain zones until after dawn. With
instability now declining diurnally, the most appreciable forcing
in play is upsloping, and models do not have enough response to
that to warrant any more than a chance mention, and only near the
Escarpment at that.

Moving on, the aforementioned prefrontal convection will work east
as the front itself lags behind by 50-75 miles or so.  With that,
the primary upper jet/vort will lift northeast into the mid/upper OH
valley by daybreak Sunday, as the decaying line of convection moves
into northeast GA and the extreme western Carolinas.  At this time
it looks as if instability will be lacking, possibly improving into
the late morning to early afternoon hours.  Improving instability
will be highly dependent on surface heating and thus sky cover
(given lack of steepening lapse rates aloft), which is progged to
be fairly high through much of Sunday.  All said, it does looks as
if the general consensus yields a decaying convective line moving
into the western tier of the fcst area just after sunrise, sliding
east through morning before possibly strengthening along/east of
the I26 corridor to round out the afternoon.  Fortunately even the
most robust guidance keeps cape levels sub 500j/kg, therefore would
expect low/mid topped convection which could tap into some elevated
winds aloft.  Thus, the fcst will feature increasing pops through
morning with likely pops to prevail at a minimum regionwide at some
point during the day, with iso/sct thunderstorms favored, a few
of which could be strong with gusty winds and frequent lightning.
Otherwise, brief heavy rainfall is possible, with the highest QPF
progged across the western tier of the fcst area where upward of
1-1.5 inches is possible.  Temperatures on Sunday will remain well
above normal, however likely not reaching record levels.


As of 200 PM EST Saturday: The intial band of pre-frontal convection
will have weakened and/or moved east of the forecast area by Sunday
night. However, models indicate that another shortwave will move
through the Ohio and Tennessee Valleys on Sunday night and Monday.
This feature will induce a weak frontal wave and act to push an area
of showers across the forecast area Sunday night and Monday. It
looks like most areas will receive half an inch to an inch from this
wave. Will carry categorical pops for most of the forecast area
Sunday night and mention a slight chance of thunder in spots.

The wave and associated precip will move east of the area through
the day on Monday as a diffuse cold front slowly clears the area to
the east. Drier area will push into the area on Monday night as high
pressure moves across the mid-Atlanic region. Expect a mainly sunny
day on Tuesday although clouds will be on the increase in the
afternoon as the next system approaches from the west. Expect highs
in the 60s outside of the higher peaks.


As of 150 pm Saturday: A welcome pattern change (at least for those
who don`t appreciate the prematurely warm and humid conditions) is
in store for the medium range, as the complete break down of the
eastern ridge is expected by the end of next week. In the interim,
the primary feature responsible for this pattern change: a potent
northern and southern stream phasing/ developing upper low will
bring a round of significant weather to the Southeast during
mid-week. Precip chances associated with increasing isentropic
lift/warm front activation E/SE of rapidly deepening Great Plains
cyclone will begin to increase across our area by daybreak
Wednesday, with chances increasing to likely/categorical across much
of the area by afternoon. Pops remain elevated into Wed night/early
Thu, although in reality, developing warm sector may yield a bit of
a lull at some point during this time frame, before frontal band of
showers and perhaps embedded thunderstorms sweeps through the area,
likely by Thu afternoon. As one would expect in such a dynamic
scenario, forecast wind shear parameters are very high, while the
instability (as usual) is a subject of much uncertainty. However,
global model sbCAPE values have been slowly creeping up from
run-to-run, and based upon pattern recognition alone, there will
likely be some degree of a high shear/low CAPE severe convective
threat, esp if the afternoon timing holds.

Much drier and cooler air should begin advecting into the area by
Friday, with pop-less/mostly sunny conditions and almost seasonable
temps forecast on Days 6/7.


At KCLT and Elsewhere:  Conditions continue to improve across the
region from earlier low stratus and/or fog restrictions.  With that,
moisture in the llvs will remain sufficient for redevelopment of cu
this afternoon beneath mid level cigs, thus all sites favor such.
Winds will remain sly with some low end gusting possible across
the NC piedmont, thus KCLT/KHKY tafs feature such this afternoon.
Otherwise, low stratus and/or fog will return tonight amidst a
decoupling BL, all ahead of cold front that is progged to move into
the region late Sunday morning.  With that, MVFR/IFR restrictions
are expected by around daybreak, lasting a few hours before sct`ing,
then returning in association with any frontal convection toward
the end of the taf cycle.  Did include an afternoon prob30 at KCLT
for tsra.  Sly winds will increase ahead of the front on Sunday,
thus all sites feature low end gusting.

Outlook: Moist conditions will keep chances for restrictions
elevated through the period, with the best chances for low cigs/vsby
to occur each morning.  Precipitation chances are also slightly
elevated, mainly across the high terrain.

Confidence Table...

            22-04Z        04-10Z        10-16Z        16-18Z
KCLT       High 100%     High 100%     High  94%     High  89%
KGSP       High 100%     High 100%     High  91%     High 100%
KAVL       High 100%     Low   57%     Med   66%     High 100%
KHKY       High 100%     High 100%     High  91%     High 100%
KGMU       High 100%     High 100%     High  89%     High 100%
KAND       High 100%     High 100%     High  94%     High  92%

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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