Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 272033

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
433 PM EDT Thu Jul 27 2017

A strong cold front will cross the region from the west Friday
through Friday night. Cooler and drier air will arrive across the
western Carolinas and northeast Georgia through the weekend and
linger through the early part of next week. More typical summertime
conditions will return about the middle of next week.


As of 430 PM: Isolated/scattered convection will continue to
develop across the high terrain into late afternoon. Multiple
outflow boundaries associated with ongoing activity should continue
to propagate E/SE through the foothills and into the western/upper
Piedmont through early evening.  However, penetration of convection
into the Piedmont still looks to be limited by a general W->E
weakening of sbCAPE due to loss of heating and increasing cirrus
debris from upstream storms. PoPs have been adjusted to blend
in latest available HRRR, which reasonably analyzed the extant
convection. A few pulse severe storms (with microbursts as the
main threat) appear to be a good bet, with the foothills and far
northern/western Piedmont being the most likely areas to see a
brief severe storm. Convection should gradually diminish through
the evening.

There is a fairly strong consensus among high resolution/
convection-allowing guidance and operational short term models
that an area of pre-frontal widespread showers/embedded storms will
be approaching the southern Appalachians by sunrise Friday. It is
this probability that is providing the main heartburn regarding the
PoP/convective trends/ chances for severe weather on Friday. While
the NAM and the GFS more or less dissipate this area of precip
across the southern Appalachians Friday morning, they also sweep
the remnant cold pool into our forecast area such that they
depict virtually no sbCAPE across much of the forecast area by
18Z. While this is plausible, it`s by no means a given, and it
does create a considerable amount of uncertainty in tomorrow`s
forecast. This is a bit disconcerting considering the anomalously
strong mid/upper level flow, with attendant deep layer forcing and
low level & deep layer shear of 25-35 kts, as one would typically
expect a very active severe weather day with such ingredients
in late July. The combination of upper support, along with lee
side/pre-frontal trough and perhaps outflow boundary(s) associated
with the morning convection should support scattered to numerous
showers and storms across the area by afternoon. The threat for
severe storms in our area remains very much a point of contention,
and SPC has acknowledged this by shifting the Slight Risk primarily
to our east, where confidence is higher in significant afternoon
destabilization, although a marginal risk encompasses the entire
forecast area. While this seems very reasonable in light of the
inherent uncertainty, the potential for a few significant severe
storms still cannot be ruled out. Tonight`s min temps will be a bit
above climo, while tomorrow`s highs should be a couple of degrees
(if not more) below climo under expected widespread mid/high clouds.


As of 225 PM EDT Thursday: The best upper-level forcing rounding the
slowly deepening eastern trough will likely cross our piedmont
Friday evening before swinging rapidly east to the coast overnight.
An attendant sharp cold front will also cross the region through the
nighttime hours. PoPs will thus remain fairly high during the
evening hours before drier air starts arriving from the northwest
overnight post-fropa. However, the best severe convective threat
should have passed east of the area by evening - unless any strong
redevelopment occurs along the immediate frontal zone. Showers may
linger along the northern mountains in moist low level upslope flow
well into Saturday morning.

A closed, upper-level low pressure system will then migrate from the
central Appalachians to the Virginia Tidewater region Saturday
through Saturday night. The surface cold front will depart southeast
of the region and drier air will work in. The main exception is that
a trailing vorticity lobe rounding the upper low will swing down
across the western Carolinas Saturday night, reinforcing moisture in
the 850 to 700 mb layer. This will lead to some return cloudiness,
but probably no associated showers.

Deep layer northerly flow will continue Sunday through Sunday night
on the back side of the deep eastern coastal trough. Periods of mid
level moisture could permit slightly higher cloud cover at times,
but with no attendant showers expected. Anticipate maximum
temperatures about 1 to 2 categories below climo by Sunday


As of 225 PM EDT Thursday: Reinforcing shortwave energy arriving
from the upper Midwest will keep a mean trough axis positioned over
the eastern U.S. Monday through Wednesday. Mainly dry surface high
pressure will sprawl over to the north of the region through this
same period. The airmass will slowly modify, with isolated
convective coverage returning to the southeast mountains late Monday
and then more typical diurnal mountain ridge top coverage Tuesday
through Thursday. Some modest height recovery is expected on
Thursday with max temperatures finally returning to near seasonal


At KCLT and elsewhere: Coverage and intensity of convection should
continue to increase across the high terrain through mid-afternoon,
with outflow boundaries and mean winds carrying the convection E/SE
into the foothills during mid/late afternoon. Uncertainty persists
regarding the degree of coverage outside the mtns, so opted to limit
the convective mention to VCTS at KHKY and the Upstate SC terminals
beginning at 19-20Z. Uncertainty increases a bit more into the
Piedmont, but a VCSH/marginal tempo for TSRA appears warranted at
KCLT by 21Z. Tempos for TSRA may have to be added on relatively
short notice at the other terminals. Otherwise, convection should
begin to trend down by mid-evening. Can`t rule out development of
restrictions toward daybreak Friday, primarily in the form of low
clouds. However, confidence is low, and a clear consensus of
statistical guidance indicates that low clouds and/or fog should
remain more or less confined to the mtn valleys. Thunderstorm
chances get started earlier than usual on Friday, as an area of
showers and storms may approach the southern Appalachians during the
morning, with associated outflow boundaries possibly acting as a
focus for new convection by early afternoon.

Outlook: Scattered to numerous convective cells are expected to
impact the area Friday afternoon. Drier air will filter in from the
north behind a passing cold front this weekend. Chances for morning
fog and stratus will also be possible, Saturday morning, mainly in
the mountain valleys. A typical summer-time pattern is expected to
return by the middle of next week.

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  95%     High  94%     High  93%
KHKY       High 100%     High 100%     High  83%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High  98%     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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