Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 210751

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
351 AM EDT Tue Mar 21 2017

A cold front will approach the forecast area today into Wednesday,
increasing chances for rain and thunderstorms. A few of the
thunderstorms could be strong this evening. Cooler air will return
behind the front to end the work week before another frontal system
approaches the area next weekend.


As of 300 AM EDT...a broad upper trough will dig and sharpen up
somewhat over the Great Lakes today thru tonight. A vort max
embedded in the increasing NWLY flow will track thru the Ohio Valley
and will help support convection along/ahead of an approaching cold
front. The zonally oriented cold front is expected to slowly sag
south into NC late this afternoon and progress thru the rest of the
CWFA this evening. Temperatures are expected to warm into the upper
70s to mid 80s across the piedmont, and with steep mid level lapse
rates in place, will result in moderate SBCAPE. The NAM and GFS
agree on a pocket of CAPE values of 1500-2500 J/KG extending from
the southern TN border to the Upstate. Bulk shear of 25-35 kt within
this unstable area will marginally support organized convection. The
new Day 1 SPC Convective Outlook is very similar to the old Day 2,
with a slight risk across southern TN to our SW corner of the CWFA.
A marginal risk encompasses the rest of the FA. The main threats
will be large hail of 1" diameter or larger and gusty winds. The 00z
suite of CAMs generally agree on scattered discrete cells developing
within the slight risk area, then morph into loosely organized
multicell clusters as they track ESE into eastern GA and SC. So I
have shifted the axis of highest PoP south, favoring the Upstate and
the southern NC piedmont.

Tonight, mid and upper support will actually increase behind the
front, as the aforementioned trough digs. So both the NAM and GFS
keep lingering QPF response thru the overnight, especially across
the southern half of the CWFA, with lingering elevated CAPE atop the
frontal zone. I think that overall, the low levels will become
unsupportive of new convection due to CAA and downslope NWLY behind
the front. So while I have PoPs tapering off to slight chance
overnight, showers may end a little quicker than that. Temps will be
well above normal under mostly cloudy skies.


As of 240 AM EDT Tuesday: Guidance in good agreement on the overall
pattern, but some significant detail differences show up in the
forecast sensible weather. Some weak short waves move through the NW
flow over the area Wednesday and Wednesday night. Additional weak
short waves move over or near the area Thursday and Thursday night
even as heights rise as a ridge axis moves west toward the area
reaching a line from Lake Michigan to Nashville to Mobile by the end
of the period.

At the surface, high pressure begins to nose into the area from the
NE in a cold air damming pattern on Wednesday behind the departing
cold front. The high strengthens and the ridging remains in place
through the period even as the center of the high slides into the
Atlantic. The GFS has plenty of low level moisture with widespread
cloudiness returning north over the developing cold dome but little
in the way of isentropic lift and resulting precip. The NAM and SREF
have much more in the way isentropic lift and precip. Given the
differences, have gone with a guidance blend which has diminishing
precip Wednesday but brings chance to slight chance PoP back into
the area Wednesday night before diminishing again on Thursday.
Should this precip develop and move into the NC Mountains Wednesday
night temps would be cold enough for a wintry mix. P-type would most
likely be freezing rain given the forecast warm nose, but sleet or
even snow would be possible across the northern mountains. Luckily
QPF would be very light so no significant accumulations would be

Highs will be near normal Wednesday then fall to 10 to 15 degrees
below normal Thursday with the cool high pressure, clouds, and
possibly precip moving in. Lows around 5 degrees below normal
Wednesday night rise a couple of degrees Thursday night putting
frost/freeze concerns back in play.


As of 345 AM EDT Tuesday: Guidance in pretty good agreement through
the medium range. An upper ridge axis moves into the area Friday
then east of the area Saturday and an upper low moves to near the
confluence of the OH and MS rivers. The low then moves to the
eastern Great Lakes or Mid-Atlantic on Sunday as short waves
rotating around the low move over the area. A surface high pressure
ridge remains across the area Friday then weakens Saturday as the
center of the high moves east into the Atlantic. This allows a moist
southerly flow to develop as a cold front moves into the MS valley.
The flow and moisture increase on Sunday as the front continues to
move east. Friday should be dry with precip chances increasing
Saturday and maximizing on Sunday. Will see some thunderstorms
Sunday as instability increases, especially outside of the
mountains. Temps start a little below normal Friday rising above
normal for Saturday and Sunday.

Short wave ridging moves through Monday as a weaker upper low takes
a similar track to the weekend low. The cold front weakens as it
moves into the area Monday. However, enough moisture and instability
remain for a slight chance of showers and storms. Temps rise about 5
degrees from Sunday.


At KCLT and elsewhere: a cold front will push thru the area from the
north this afternoon and evening. Scattered to numerous showers and
thunderstorms are expected to develop along and south of the front
and track SE thru the area, with a few storms possibly producing
strong winds and hail. The best chances at the TAF sites will be in
the 21-03z time frame. Showers and possibly a few storms will linger
thru late evening. Winds will be out of the SW ahead of the front,
then switch to north as it passes this evening. The exception is
KAVL, where all the guidance keeps winds out of the north thru the

Outlook: Low level moisture may not completely dry out on Wednesday
(especially in the southern part of the area), before starting to
lift back north and deepen by Thursday, keeping chances of showers
and low cigs into the weekend.

Confidence Table...

            08-14Z        14-20Z        20-02Z        02-06Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     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:




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