Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 262031
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
431 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/...
of 430 pm EDT: The current forecast is on track with only minor
adjustments up on hourly temp trends in the warmer upper Savannah
river region. Sky cover has been lowered as well for late
afternoon/early evening.

Otherwise, mean troughs will persist across the central Conus and
just off the East Coast through the period, with a mean/low
amplitude ridge in between. However, model guidance indicates a
series of weak vort maxes, some of which are likely
convectively-induced, passing through the ridge into the eastern
Conus over the next 24-36 hours.

Meanwhile, significant thermodynamic changes are expected 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. In light of this,
it`s not surprising that guidance is trending toward convection
initiating across the TN Valley mid-late afternoon, then moving into
western NC late afternoon/early evening. By that time, shear will be
quite respectable, especially considering the degree of instability,
and at least isolated severe convection would be likely over our
area if this scenario were to unfold. Substantial low level shear
and high DCAPE could support cold pool organization, but this is
always a tough call, especially 24+ hours in advance. Pops have been
increased to 20-40 percent across much of western NC by late
afternoon.

&&

.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 /21Z FRIDAY THROUGH WEDNESDAY/...
At KCLT and elsewhere: VFR conditions will continue across much of
the area through the period. The only exception will be in the mtn
valleys west and SW of KAVL, where another round of morning fog/low
stratus is expected on Saturday. However, the main TAF airfields
will not be affected. Otherwise, winds will remain out of the W/SW
(NW at KAVL) through the period, with speeds of 5-10kts expected
during the daylight hours, and less than 5 kts at night.

Outlook: Thunderstorms and associated restrictions may develop
across NC Saturday afternoon. Scattered afternoon/evening
thunderstorms and associated restrictions are expected in all areas
Sunday through and Wednesday.

Confidence Table...

            20-02Z        02-08Z        08-14Z        14-18Z
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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