Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 262353
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
753 PM EDT Fri May 26 2017

.SYNOPSIS...
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.

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
As of 730 pm EDT: The current forecast for the very near term
remains on track. Minor adjustments have beeen made to the Saturday
afternoon period to adjust PoPs up a bit as detailed below.

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.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH SUNDAY NIGHT/...
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.

&&

.LONG TERM /MONDAY THROUGH FRIDAY/...
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.

&&

.AVIATION /00Z SATURDAY THROUGH WEDNESDAY/...
At KCLT and elsewhere: A shallow upper ridge will keep VFR
conditions in place through Saturday morning 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 Saturday morning, with speeds
less than 10 kt throughout. High clouds will increase from the west
on Saturday with convection likely developing upstream over TN. The
latest convection allowing models permit some of this activity to
move across the NC mountains and possibly survive into the foothills
and piedmont late day. Prob30 TSRA will be advertised throughout,
with low end afternoon gusts expected in the SW flow.

Outlook: Thunderstorms and associated restrictions will increase
through the late weekend with coverage maximizing later 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...

            00-06Z        06-12Z        12-18Z        18-00Z
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:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

$$
SYNOPSIS...ARK
NEAR TERM...HG/JDL
SHORT TERM...ARK
LONG TERM...ARK
AVIATION...HG/JDL



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