Area Forecast Discussion
Issued by NWS Greer, SC

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854
FXUS62 KGSP 272117
AFDGSP

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

.SYNOPSIS...
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.

&&

.NEAR TERM /THROUGH SUNDAY/...
As of 500 PM EDT: The latest two runs of the HRRR have convective
coverage increasing over the mountains and along the I-40 corridor
this evening and this matches up fairly well with the latest radar
trends in upstream areas. PoPs will be boosted in these areas
through 00Z, but with continued questions marks on how the
convective is able to survive downstream into piedmont sections
where convective inhibition is much stronger.

Otherwise, the forecast remains on track with a flat ridge over the
region continuing 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 early part of the short
term. The coverage and mode of convection remain uncertain. Despite
unusually steep mid-level lapse rates associated with expansive
elevated mixed layer, sbCAPE remains somewhat muted over the area,
generally in the 1500-2000 J/kg range (per SPC mesoanalysis) owing
to a) lack of rich low level moisture, b) stubborn broken clouds
over the region, and c) considerable CIN in the lowest 800 mb or so.

Mesoscale/high res models depict some measure of involve upscale
organization into an MCS impacting part of our area during mid/late
evening, so that possibility is still on the table. However, our
anticipated scenario since this morning has been that discrete cells
and/or small clusters of convection would develop in association
with a remnant outflow boundary across eastern Kentucky and
northeast TN by late afternoon and drop SE or E/SE into our area
through the evening. 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 continue to yield high
DCAPE/intense microburst potential through the late evening. The
steep mid-level lapse rates/relatively large CAPE in the -10C to
-30C layer will also yield a threat of 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.
Nevertheless, after a bit of an early overnight lull, pops ramp back
up to solid chance across the southwest third of the area by sunrise
Sunday. Impacts of this feature on our area will be critical to
evolution of convective trends Sunday. Mid-level lapse rates will
remain steep at 7.5-8 C/km while low level moisture will continue to
improve. Thus, the potential exists for very strong destabilization,
as long as early day convective debris clouds and/or lingering
convection do not muck up surface heating significantly. Short term
guidance depict the next mid-level short wave trough/MCV approaching
the area by the end of the day, while pseudo warm-frontal boundary
could focus convection, especially across northern areas. Shear will
remain more than adequate for organized/severe convection, with
perhaps a better potential for rotating cells in areas of enhanced
low level helicity near the boundary. Thus, all manner of severe
weather is possible on Sunday.

&&

.SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY/...
As of 230 PM EDT...This portion of the forecast becomes very fuzzy
with multiple embedded ripples of vorticity (some convective feed-
back) moving through our region. Although the models offer various
output in this section of the forecast, we will play one outgoing
shot of convection Sunday evening, then lower POPS, with another
shot of energy perhaps reaching our western forecast area (FA)
Monday morning with another uptick in POPS western sections.

The evolution of additional thunderstorms Monday will be somewhat
dependent on whether the early morning convection (if it happens),
tempers the atmosphere. On the other hand we still have a broad
southwesterly flow aloft in place, due to the large upper low north
of the Great Lakes, and spokes of energy rotating through. Also a
weak surface wind shift dropping south into the area, helping to
provide a focus. Considering the unstable nature of the atmosphere
we will continue with medium range chances, and future forecasts may
be able to focus in on additional details.

For Monday night and Tuesday, with the flow aloft southwesterly it
is hard to imagine the front/boundary pushing through our FA, unless
cold pool driven. Therefore we will bring POPS down Monday night,
and then run lower chances on Tuesday, especially south of I40.
There actually may be a series of wind shift lines to cross the area
through Tuesday.

Since we note a downward nudge in the thermal profile Tuesday, we
will back readings off a few degrees compared to Monday.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
As of 210 PM Saturday: A closed upper low located just north of the
Great Lakes will rotate east but then return to just north of the
Great Lakes next weekend from Hudson Bay. This broad feature creates
a big trough over the east.  Meanwhile, at the surface, a stalled
frontal boundary stretches from Texas to the Carolinas through the
second half of the week except for Thursday when high pressure
coming down from near the Great Lakes pushes the front south of our
area but only for about a day. Shortwave energy and waves of low
pressure will pass across the deep south increasing the chance of
showers and storms Friday and Saturday.  Severe threat should be
quite low.

Expect temperatures to be near normal through the medium range
period.

&&

.AVIATION /21Z SATURDAY THROUGH THURSDAY/...
KCLT and elsewhere: Convective trends will become very uncertain
through late evening. The current expectation is that scattered
cells and clusters of storms should develop across East TN and move
fairly quickly into western NC during the early evening hours.
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, except for a new TEMPO on the
latest update at KCLT for 02Z to 06Z. 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...

            21-03z        03-09Z        09-15Z        15-18Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     High  94%     High  98%
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  98%

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...DEO
NEAR TERM...HG/JDL
SHORT TERM...TS
LONG TERM...DEO
AVIATION...HG/JDL



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