Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 271530

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1130 AM EDT Sat May 27 2017

Warm and humid air will return to the region today through at least
Monday, as high pressure settles east of the Florida coast. A cold
front will slowly approach from the northwest, but will struggle to
push through the area during the middle of next week.


As of 1035 am: Flat ridge over the region will continue to gradually
break down through tonight. A series of weak vort maxes, some of
which are likely convectively-induced, will pass through the ridge
into the eastern Conus through the period, primarily from this
evening into the short term period. Obviously, convective trends
later today into tonight are THE forecast challenge for the next 24
hours, and uncertainty continues to abound. First of all, regional
12Z RAOBS are pretty astounding, capturing an expansive elevated
mixed layer, with near-dry adiabatic lapse rates from around 800 mb
to the tropopause shown on the FFC sounding. You just don`t see
soundings like this everyday in the Southeast coastal states.
However, as with any EML sounding, it is also capped, and quite
formidably so (low level moisture also needs some work, but that`s
another story). Surface heating should put a fairly decent dent in
the cap, but it`s highly unlikely that will be enough to remove it.
As such, we are not expecting much in the way of deep convection
through much (if not all) of the afternoon, but mid-level short
wave/MCV approaching the area from the TN Valley could provide
enough layer lift to remove the cap in some areas this evening, and
some short term guidance is suggesting this will be the case.

Looking upstream, regional radars over the TN Valley depict
isolated/generally waning convection across Middle Tennessee and
western Kentucky, with an associated outflow boundary. This activity
will continue to diminish as it encounters more stable/capped air to
its east. The question is whether remnant outflow will be able to
provide a focus for initiation of additional convection later today
across East TN and eastern Kentucky, and if so what form it will
take (i.e., will it remain scattered/discrete or organize into
something more significant). The most likely scenario is that
absence of strong forcing and the lack of a prominent surface
boundary to focus development, and somewhat limited (although still
significant) instability/low level moisture should limit the overall
coverage to scattered discrete cells and perhaps a few small
clusters developing to our west and moving into the area from late
afternoon through the evening and perhaps the early part of the
overnight. Long/straight hodographs may favor splitting supercells,
although the tornado threat should be very limited due to relatively
low RH in the boundary layer. Thus, damaging winds are the main
concern, and the dry BL should yield high DCAPE/intense microburst
potential. The steep mid-level lapse rates/relatively large CAPE in
the -10C to -30C layer will also yield a threat of large to very
large hail.

Tonight, an organized, likely severe MCS will be barreling through
the TN Valley, and this will obviously create some concern for our
area, although a consensus of high res and short term model guidance
suggests it will have a fairly strong SE trajectory toward the more
unstable/moisture rich environment across the Deep South.


As of 300 AM Saturday: the Short Term looks to be fairly active,
with a continued threat of severe weather. A flat upper ridge will
keep a deep layer westerly flow across the region. Embedded vort
maxes or MCVs will track thru this flow. The air mass within this
zone will be quite unstable, with guidance generally agreeing on an
axis of 2000-3000 J/kg of CAPE extending from the Mid-South east
thru the Carolinas by late Sunday afternoon. Bulk shear should more
than adequate for organized storms. The exact evolution of this
activity is still in question, as guidance cannot time individual
MCVs even that far, and subtle differences in the amount of buoyancy
and shear may change the primary storm mode. Some convection
allowing models are now going out thru Sunday, and they are not in
very good agreement. Based on the forecast soundings, it does look
like we should have a lot less CIN, and storms may start initiating
by midday, especially in the high terrain. Steering flow will take
this activity out across the Piedmont mid/late aftn thru the
evening. PoPs still range from chc across the GA/SC piedmont to
likely across most of the NC mountains. Temps will be one or two
categories above normal.

Sunday night thru Monday, an upper low will deepen and dig south
across the western Great Lakes, backing the mid and upper flow atop
the CWFA more out of the SW. A surface front will be approaching
from the west, but will slow as the flow becomes more parallel to
its orientation. So Monday is looking like another active day, with
the piedmont becoming the most unstable. Bulk shear looks about the
same magnitude as Sunday, but oriented more out of SW instead of
WNW. The new Day 3 SPC Outlook has just come in with a Slight Risk
from I-85 and southeast, with a Marginal Risk elsewhere, except the
TN border counties. So I will add a mention of severe weather
possible for Monday in the HWO. Temps remain above normal.


As of 330 AM Saturday: an upper low will wobble east and then north
across the Great Lakes, shifting a longwave trough axis slightly
east into the Ohio Valley. The 00z suite of guidance now all stall a
cold front over the southern Appalachians on Tuesday, with another
front pushing in from the NW early Wednesday, but also getting hung
up. So Pops were tweaked upward for both Tuesday and Wednesday, but
still mainly in the slight chc to low-end chc range. Fortunately,
mid level lapse rates will be weaker, and severe threat should be
muted. Temps will be around normal.

Thursday may be the quietest day, as some drier air finally pushes
in from the west, as the upper trough axis shifts east and flattens
out. I went with Superblend for the PoPs both Thursday and Friday,
but those may be too high, if the 00z model trends hold. Temps will
continue to be near normal.


At KCLT and elsewhere: A shallow upper ridge will keep VFR
conditions in place through the day for most of the area. Winds will
remain out of the SW (NW at KAVL) through the morning, with speeds
less than 10 kt throughout. High clouds will increase from the west
early with lower VFR moving in by afternoon with convection likely
developing upstream over TN. The latest convection allowing models
permit some of this activity to move across the NC mountains in the
afternoon and possibly survive into the foothills and piedmont
during the evening. Prob30 TSRA will be advertised throughout, with
low end afternoon gusts expected in the SW flow. Winds diminish
during the evening.

Outlook: Thunderstorms and associated restrictions will increase
through Sunday. Conditions may remain unsettled through early to
middle to next week as a train of upstream convection may continue
to ride through the area from the west from time to time.

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%     High 100%     High  88%
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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