Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 211500

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1100 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 1050 AM: 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. Significant
capping continues to be indicated until late afternoon over most
of the area; interestingly several solutions feature a bullseye
of instability over the central Upstate leading to some discrete
development in that area a couple hours before any other initiation.

Consensus CAPE progs feature greatest values, over 2000 J/kg,
developing over the western Upstate and NE GA, but values over
1000 J/kg reaching as far north as Morganton and Salisbury. Bulk
shear of 25-35 kt within this unstable area will marginally support
organized convection. Latest Day 1 outlook from SPC now features
an enhanced risk area centered just to our west, with slight risk
very close to that on the previous issuance. The main threats will
be large hail of 1" diameter or larger and gusty winds.

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. The latest guidance has
trended a little faster with the progression of the front and
further south with the axis of shower and TSTM development. Storms
will now be mainly south of a line from KAVL to KCLT. There still
may be some convection in the vicinity; PROB30 or TEMPO handles
this possibility. A few storms may produce strong winds and large
hail. The best chances at the TAF sites still looks to be in the
22-04z 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 period.

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...

            15-21Z        21-03Z        03-09Z        09-12Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     Med   72%     High  86%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High  98%
KAND       High 100%     High 100%     High 100%     Low   28%

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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