Flash Flood and Damaging Wind in North Carolina, August 21, 2018

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
155 PM EDT Tue Aug 21 2018

.SYNOPSIS...
A cold front will approach from the west today and is forecast
to move across the region this evening. In the front's wake, dry
high pressure will spread over the area for the latter half of the
week, making it feel more like mid-September. The warm and humid
air will return for the weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 155 PM EDT Tuesday: Main update for the 18Z TAF issuance
along with adjustments made to temperatures/PoPs to account for
trends as the rest of the near term forecast remains on track.

Latest radar imagery depicts isolated showers and thunderstorms
continuing to move across the western portions of the FA. Given
lingering widespread cloud cover through this morning, temperatures
have been slow to warm up, which has kept the area a tad more stable
than previously anticipated, thus less coverage of convection than
suggested by CAMs. However, satellite imagery is showing a few more
breaks in the clouds, which may help destabilize the area a bit more
ahead of the approaching cold front from the west, and allow for
increasing coverage of convection this afternoon and this evening.
Current temperatures remain in the upper 70s/low 80s.

Otherwise, upper low continues to push eastward, gradually opening
up as it approaches the Appalachians later today. The associated sfc
cold front will reach the Foothills by roughly 00Z Wed. Ahead of the
front is ample moisture with precip water amounts just over 2''. The
front will be accompanied by some upper support, and widespread
showers and thunderstorms are likely this afternoon and evening with
activity progressing from the mountains to the southeast. Wind
profiles will strengthen today due to the influence of the frontal
system, with bulk shear peaking in the 25 to 30 knot range this
afternoon. This will increase the chance for severe weather;
primarily severe wind gusts from storm outflows. This is represented
by a Marginal Risk area from SPC. Instability amounts will be
similar to the past few days, with lapse rates and sfc dewpts
similar to yesterday. More rapid storm motion will limit excessive
rain potential, though some flooding potential exists if training or
repeated rounds of precip occur over locations. Rain and clouds will
limit highs today to the low to mid 80s. Winds will remain
persistently SWLY ahead of the front with some speed increases and
gusts later this afternoon as the front approaches.

&&

.SHORT TERM /WEDNESDAY THROUGH FRIDAY NIGHT/...
As of 235 AM EDT Tuesday: The models are still on board with giving
us a brief respite from summer during the latter half of the week. A
cold front should be well to our east by daybreak on Wednesday,
with a relatively strong WNW flow aloft for this time of year as an
upper trof axis lingers overhead. Lingering low level moisture will
fuel the development of upslope showers near the TN border through
the day, so a chance of precip was kept in that location. Although
the flow will be generally inhospitable east of the Blue Ridge,
some bands of showers are possible in axes of low level convergence
downwind of some of the higher terrain, such as the Balsams. A
trailing short wave is expected to dive down into the upper trof
during the afternoon, which should enhance the precip chances
late in the day. Thus, have generally accepted the slight chance
seen in the NAM across Upstate SC and the wrn Piedmont of NC. The
loss of heating will take care of this activity Wednesday evening,
while the upslope moisture will taper off near the TN border. The
downslope east of the mtns should keep temps a bit closer to normal
than otherwise expected on Wednesday, but that will not be the case
for the rest of the week. The rest of the fcst is dry with temps
about five degrees below normal courtesy of sfc high pressure moving
past to the north on Thursday and lingering off the New England
coast on Friday. Might even get cool enough to open the windows
Thursday night and Friday night. The difference in humidity should
be notable, with dewpts down in the mid/upper 50s east of the mtns.

&&

.LONG TERM /SATURDAY THROUGH MONDAY/...
As of 320 AM EDT Tuesday: Alas, summer will return over the weekend,
not quite with a vengeance, but the models suggest the upper pattern
will evolve toward one that usually spells above normal heat for
August during the early to middle part of next week. In the mean
time, a progressive upper trof moving past to the north on Saturday
will push a dying front toward the region. An easterly low level
flow ahead of the trof will fuel a return to diurnally-driven
showers and storms on Saturday with temperatures still about
five degrees below normal. That changes on Sunday as an upper
anticyclone over the srn Plains plods eastward thru Monday. This
feature will bring a warming trend, with temps going up a category
for Sunday and another one again on Monday. Precip chances favor
diurnal showers over the higher terrain.

&&

.AVIATION /18Z TUESDAY THROUGH SUNDAY/...
At KCLT and elsewhere: With an approaching cold front currently west
of the area, mainly MVFR prevails this afternoon as low cigs
continue to linger much longer than expected, though a few areas are
beginning to scatter out to VFR, while others have briefly
deteriorated to IFR given a SHRA/TSRA moving over the TAF site.
Expect showers and thunderstorms to increase in coverage this
afternoon and tonight as the cold front moves into the area,
bringing periods of IFR heavier downpours/lower cigs and gusts up to
30 kts possible in stronger storms. Per latest guidance, anticipate
coverage of SHRA/TSRA to become isolated in nature overnight,
diminishing with the cold front. Otherwise, SW winds AOB 12 kts will
prevail through the rest of today, gradually veering with the cold
front, beginning across the mountains later tonight, and eventually
at KCLT towards Wednesday morning. Isolated showers will be possible
across the mountains Wednesday morning, which could bring lingering
MVFR/IFR to KAVL. Otherwise, expect VFR.

Outlook: Quiet weather conditions will be in place behind the cold
front as drier air infiltrates in. Thus, expect VFR to prevail
through the end of the week.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High  99%     High 100%     High 100%     High 100%
KGSP       High  99%     High 100%     High 100%     High 100%
KAVL       High 100%     High  97%     Med   61%     High  82%
KHKY       High  99%     High 100%     High 100%     High 100%
KGMU       High  98%     High 100%     High 100%     High 100%
KAND       High  97%     High  99%     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...PM
NEAR TERM...JPT/SGL
SHORT TERM...PM
LONG TERM...PM
AVIATION...SGL

Hazardous Weather Outlook
National Weather Service Greenville-Spartanburg SC
618 AM EDT Tue Aug 21 2018

GAZ010-017-018-026-028-029-NCZ033-035>037-048>053-056>059-062>065-
068>072-082-501>510-SCZ001>014-019-221030-
Rabun-Habersham-Stephens-Franklin-Hart-Elbert-Avery-Alexander-
Iredell-Davie-Madison-Yancey-Mitchell-Swain-Haywood-Buncombe-Catawba-
Rowan-Graham-Northern Jackson-Macon-Southern Jackson-Transylvania-
Henderson-Cleveland-Lincoln-Gaston-Mecklenburg-Cabarrus-Union-
Caldwell Mountains-Greater Caldwell-Burke Mountains-Greater Burke-
McDowell Mountains-Eastern McDowell-Rutherford Mountains-
Greater Rutherford-Polk Mountains-Eastern Polk-Oconee Mountains-
Pickens Mountains-Greenville Mountains-Greater Oconee-
Greater Pickens-Greater Greenville-Spartanburg-Cherokee-York-
Anderson-Abbeville-Laurens-Chester-Greenwood-
618 AM EDT Tue Aug 21 2018

This Hazardous Weather Outlook is for northeast Georgia, piedmont
North Carolina, western North Carolina and upstate South Carolina.

.DAY ONE...Today and tonight.

A robust cold front will approach the forecast area from the west
and help to generate numerous showers and thunderstorms. Some of
these storms could become severe and produce large hail and damaging
wind gusts. Isolated urban and small stream flooding is also a threat
with these storms. The flooding threat will be greatest over the
higher terrain.

.DAYS TWO THROUGH SEVEN...Wednesday through Monday.

No hazardous weather is expected.

.SPOTTER INFORMATION STATEMENT...

Spotters are requested to forward any reports of large hail,
damaging winds, tornadoes, or flash flooding to our office.

JPT