Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 270745

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
345 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 320 AM EDT: Flat ridge over the region will continue to
gradually break down today and 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. Meanwhile, a
significant increase in low level moisture develop in the increasing
WSW flow. In addition, an expansive elevated mixed layer air will
yield very impressive mid-level lapse rates (7-8 C/km) across the
region through the day and into the evening. The result is forecast
sbCAPE values of 2000+ J/kg across much of the area by late
afternoon. However, forecast soundings show deep layer capping with
high LFCs until late afternoon or early evening. This will be around
the time that some of the higher resolution guidance depicts a
mid-level vort max moving across the TN Valley, where the atmosphere
will be less capped and more unstable. The very latest hi-res
convection allowing models continue to trend toward better
convective coverage surviving into the region from the west. That
said, the convection over our area is delayed unto late afternoon
and early evening when the cap erodes and forcing from the western
convection moves in. Therefore, PoPs have been delayed but increased
to likely across the NC mountains. PoP has been increased to the
good chance range across the NC Foothills and Piedmont for the
evening. NE GA and the Upstate has been limited to slight chance or
low end chance as the forcing only brushes these areas. The very
unstable air mass, strong shear, and dry mid levels indicate severe
storms with large hails and damaging winds may develop along and
north of I-85 late this afternoon into the evening. With lower
coverage expected south of the I-85 corridor, severe storms are less
likely but still possible.


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. The main
exception will likely be another round of LIFR to IFR low clouds and
fog in the southwest mountain valleys. 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

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...

            07-13Z        13-19Z        19-01Z        01-06Z
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  97%
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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