Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 141059

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
659 AM EDT Mon Aug 14 2017

A stationary front is expected to lift northward into the region on
Monday, possibly washing out on Tuesday while another weak front
approaches from the west.  This next front will move into the region
Tuesday night into Wednesday with a brief round of drier high
pressure favored through Thursday.  Another cold front will move
into the area on Friday warranting increased chances for
precipitation, with drier high pressure expected in its wake for
Saturday into Sunday.


As of 650 AM EDT: Isolated showers continue to redevelop in the
southerly upslope zone of the mountains around daybreak this
morning. This coverage should wane as 950 to 850 mb flow slowly
veers more WSW or westerly through the morning hours.

Back to the west, water vapor satellite imagery continues to depict
a series of shortwaves embedded in the central conus trough. These
waves will eject east from the middle to lower MS River Valley
today. They will dampen slightly as they move east, but still
provide several rounds of upper forcing over the western Carolinas
and northeast GA today through tonight. At the surface, weakening
1015 mb high pressure to the north may permit the stalled piedmont
surface frontal boundary to undulate northward at times today,
bringing both more instability and additional triggering to the
region this afternoon and evening. Profiles remain quite moist, with
precipitable water values near two inches likely to continue across
much of the region. Heavy rain and isolated flooding will thus
continue to be a threat with any convection, although westerly mean
cloud layer winds should continue to move things along at 20 kt or
so. Surface  to 6 km bulk shear values will remain a decent 30 kt or
so across the region. The main question mark for any severe weather
will be the instability, which will depend highly on the boundary
position and breaks in the clouds for heating. Afternoon sbCAPE
values in the 1500 to 2500 J/kg range could be sufficient for some
multi-cell organization and a low end, isolated wind gust threat to
accompany the heavy rain threat. Mostly cloudy conditions should
keep maximum temperatures a touch below climo throughout.

The diurnal convective activity will likely sweep eastward across
the piedmont late today, with the best coverage in the western NC
piedmont through the evening hours. Lingering isolated to scattered
mountain showers are then possible overnight with additional weak
upper vort lobe energy passing through. Meanwhile, Tropical System
Gert will pass well east of the eastern seaboard and not impact the
area. Expect another night of very warm minimum temperatures some 5
to 7 degrees above climo.


As of 300 AM EDT Monday:  The short term fcst kicks off on Tuesday
morning amidst quasizonal flow aloft at the base of a
broad/departing H5 trof, while ridging builds westward over the
Plains.  At the surface, TC Gert will be approx 300 miles off the
Carolina coastline, with the fcst track driving the storm northeast
further out to sea.  Otherwise, weak surface ridging will be in
place across the southeast CONUS thanks the Bermuda high, however
with some ambiguity in pressure/wind field thanks to Gert.
Shortwave energy looks to pass through the mean flow aloft on
Tuesday while profiles remain rather moist and unstable ahead across
northeast GA and the Carolinas as a weak cold front approaches out
of the OH/TN valley.  With that, think sct/num showers and
thunderstorms are favored given ample instability and continually
elevated PWATS, therefore pops will be reflective of such.

Moving into Wednesday, the aforementioned front is progged to surge
southward into/across the SC Midlands by sunrise, while at the same
time Gert begins to move so far northward that the southeast becomes
under control of the elongated/migrating Bermuda high once again.
The airmass behind the front will lead to slightly lower PWATS on
Wednesday, however weak return flow could lift the front back along
the I85 corridor, which combined with modest diurnally favored
instability will warrant at least climo pops region wide.  All in
all, think the heavy rainfall threat will have diminished somewhat
on Wednesday, at least on a broader scale.  Temperatures on Tuesday
will be at normal levels, warming a few degrees on Wednesday to
above normal.


As of 315 AM EDT Monday:  The medium range fcst period kicks off on
Thursday morning amidst broad ridging centered over the GOM, however
extending northward into the southern Appalachians, while a
shortwave trof digs across the Great Lakes region.  At the surface
high pressure both over Bermuda and the GOM will dominate the
pattern over the southeast states, while a surface trof/front
extends southward beneath the aforementioned upper trof into the
upper/middle MS River Valley.  Pattern evolution will favor lower
precipitation chances on Thursday given proximity of the deep
ridging and only marginal diurnally favored cape, thus near climo
pops were highlighted.  The aforementioned front does look to move
into/through the TN valley Friday morning possibly arriving in
western NC by early afternoon as the upper ridge retrogrades
westward a bit.  As a result, lightly higher pops were included on
Friday as the front slowly moves into the region.  Guidance favors
poor shear in association with this front, thus overall think
convection on Friday will be rather unorganized.

As for the weekend, the weak frontal boundary looks to push south of
the region for Saturday however yielding only marginal, if any
airmass change.  At that point some discontinuity in the op guidance
becomes ever more evident as the ECMWF is a bit more progressive
with the next wave moving in on Sunday while the first frontal axis
lifts back north into the I85 corridor, thereby keeping pops
elevated on Sunday.  The GFS other hand is about 12 hours slower and
yielding a drier Sunday.  Temperatures through the end of the week
will moderate and remain a few degrees above normal, topping out on
Friday before cooling slightly on Saturday and Sunday.


At KCLT: Showers continue to gradually weaken and dissipate near the
airfield around daybreak this morning. Will thus hold onto only a
VCSH mention through 15Z. Otherwise, expect IFR cigs to develop
quickly around sunrise given the abundant moisture, and these
restrictions will be very slow to lift through MVFR through late
morning and the early afternoon. Will feature TEMPO conditions for
afternoon/evening convection as models agree fairly well that robust
convection should develop off the Blue Ridge later this afternoon.
Winds will be mainly N of E and fairly light through the current
period, but brief southeast flow at times cannot be ruled out at.
Expect another light of MVFR to IFR low stratus.

Elsewhere: Upslope showers developing along the SC mountains this
morning should generally not impact the Upstate TAF sites or KAVL.
IFR to MVFR cigs will be the main aviation concern, being slow to
lift through the higher MVFR range through late morning. Anticipate
mainly lower VFR cigs with showers and thunderstorms redeveloping
from the west this afternoon ahead of an approaching shortwave. KAVL
should see TSRA the earliest, by 19Z, with activity spreading to the
foothills from 20Z through 00Z. Winds will be NE early and become
southeast to southerly later in the day. Another night of low
stratus is expected tonight as the precipitation winds down.

Confidence Table...

            11-17Z        17-23Z        23-05Z        05-06Z
KCLT       High  89%     High  85%     High 100%     High 100%
KGSP       High  94%     High  90%     High 100%     High 100%
KAVL       Med   66%     High  85%     High  95%     Low   52%
KHKY       High  89%     High  85%     High 100%     Low   52%
KGMU       Med   66%     High  90%     High 100%     High 100%
KAND       Med   72%     High  95%     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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