Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 271743

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
143 PM EDT Sat May 27 2017

Warm and humid air will remain over our region 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 135 pm: Calm before the storm(s) this afternoon, as latest SPC
Mesoanalysis indicates respectable instability, but also a lot of
inhibition across the area early this afternoon. Little has changed
from earlier thoughts...

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 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 indicate
earlier convection dissipated as it encountered more stable/capped
air. The question is whether remnant outflow boundary 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: VFR and convection-free weather is expected
to continue through the afternoon. After that, convective trends
become very uncertain. The current expectation is that scattered
cells and clusters of storms should develop later this afternoon
across East TN and move fairly quickly into western NC during the
evening. Uncertainty regarding timing and overall coverage is such
that VCTS is limited to KHKY/KAVL from 22Z through 03Z, while a
PROB30 is reserved for the other terminals. Trends will be monitored
closely and amendments for convection should be expected, possibly
on relatively short notice. Otherwise, high/mid level clouds should
generally prevent much in the way of fog/low stratus development
late tonight, and other than MVFR conditions in the mtn valleys, VFR
will be forecast through the end of the period.

Another round of convection is possible prior to the end of the
period, as a decaying, organized convective system may impact the
area Sunday morning/early afternoon. Thus, PROB30s are included
everywhere except KCLT before the period ends.

Outlook: Thunderstorms and associated restrictions, mainly of the
afternoon/evening variety, as well as the potential for areas of
morning fog and/or low stratus will continue through at least the
middle of next week.

Confidence Table...

            17-23Z        23-05Z        05-11Z        11-12Z
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 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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