Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 270536

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

Weak, dry high pressure centered over Florida will allow a weak cold
front to slide south across Kentucky and the Virginias on Saturday.
Moisture and unstable conditions will return to the region over the
weekend, as this cold front slowly approaches from the northwest
into the start of the upcoming week.


As of 135 AM EDT: IR satellite imagery shows an MCS moving into
Ohio late this evening and Corfidi vector steering flow should send
this activity across the Virginias overnight if it survives. Little
or no risk is evident for convection reaching our area through
daybreak. High clouds increase across the area through the morning
with lower stratocu moving into the mountains.

Otherwise, the low amplitude ridge over the region will continue to
gradually break down tonight through Saturday. A series of weak vort
maxes, some of which are likely convectively-induced, will pass
through the ridge into the eastern Conus over the next 24-36 hours.
Meanwhile, significant thermodynamic changes are expected to occur
across our area over the next 24 hours, as low level moisture
increases on the heels of increasing W/SW flow, while expansive
elevated mixed layer air will yield very impressive mid-level lapse
rates (7-8 C/km) across the region through the day tomorrow. The
result is forecast sbCAPE values of 2000+ J/kg across the TN Valley
and southern Appalachians by late afternoon. Forecast soundings are
quite capped and characterized by high LFCs until about late
afternoon. 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. PoPs have been pushed up into a higher chance range
over western sections and increased a bit out east across the
foothills and into the piedmont through late Saturday afternoon
given the robust instability. Substantial low level shear and high
DCAPE could support cold pool organization, and some severe weather
remains possible.


As of 230 PM Friday: it looks like a busy Memorial Day weekend
with a continued threat of organized severe weather. The overall
pattern shows a flat upper ridge across the mid-MS Valley/TN
Valley/Srn Appalachians/Carolinas through the period. The model
guidance shows bits of upper divergence and vort maxes trucking
along periodically through this nearly-zonal flow. It appears as
though the numerical models are trying to tell us that we will
be in an MCS track through Monday. If that will indeed be the
case, the specifics of the model guidance, such as the convective
parameters and precip response, are perhaps less important than the
idea that we may see thunderstorm development and organization at
some point. The environment will probably develop at least moderate
instability with sfc-based CAPE in the 2000-3000 J/kg range along
with shear in the 20-30kt range at some point, mainly tied to the
diurnal cycle. This will spell the best chances in the afternoon
and evening, but not necessarily if an MCS develops upstream. At
this point, the convection-allowing model guidance does not go
out that far. What does all this mean? Upstream MCSs and leftover
MCVs are too difficult to time more than 24 hrs in advance, but
we expect something to be lurking out there Sunday and Monday,
which means we will have to be on the lookout for strong/severe
storms at least on Sunday. Precip probs will be kept mainly in
the chance range for now, owing to the uncertainty as to when the
most likely time for storms will be. Temps will be slightly above
normal through the period.


As of 210 PM EDT Friday: Starting with Tuesday morning after the
main cold front has passed just southeast of our area. As weak high
pressure will be in control of our weather Tuesday, there will be
very little convective activity. Expect slightly better instability
and convergence Wednesday.

A closed upper low located just north of the Great Lakes on Tuesday
creates a trough over the east. The closed upper low opens up late
Wednesday and moves east as another cold front drops SE in response
to the change in the upper pattern. This front approaching the NC
mountains late Wednesday should further enhance convection moving in
from TN. A frontal boundary is expected to lay out from west to east
from north TX to NC to start the weekend and give our region
scattered to numerous showers and storms. Some trofiness remains
over the East coast at the end of the week with upper ridging from
the Miss Valley to West Coast.

Temperatures will be near normal in this extended period.


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...

            05-11Z        11-17Z        17-23Z        23-00Z
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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