Flash Flood in North Carolina, August 24, 2020

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
524 PM EDT Mon Aug 24 2020

.SYNOPSIS...
Abundant tropical moisture flowing into the area around a deep ridge
of high pressure will maintain precipitation chances through
Tuesday. An upper level high will strengthen Wednesday and
temperatures will rise above normal into the weekend. The remnants
of tropical cyclones Laura and Marco will bring higher levels of
moisture and increased precipitation chances especially in the
mountains for Friday and Saturday.

&&

.NEAR TERM /THROUGH TUESDAY/...
As of 453 PM Monday: Watching a broad band of showers with embedded
thunderstorms over the SC Midlands with some interest...as it is
moving/propagating gradually toward the NW late this afternoon. The
CAMs have not had an especially good handle on this activity. The
environment out ahead of the convection was characterized by
1500-2500 J/kg of sfc-based CAPE. Having a hard time figuring out
why it should fall apart before reaching the southeastern fringe
of the fcst area in the early evening, so have started the trend
toward bringing higher precip probs into the nrn Upstate/ern
Piedmont of SC and the Charlotte metro area this evening. The
dCAPE over metro Charlotte is about 800 J/kg, so we'll call this
a non-zero chance of some strong/gusty winds if a thunderstorm
manages to keep going that far north. With the loss of heating
this evening the activity should start to wane. Temp/dewpt trends
look good.  Over the mtns...we are starting to see the initiation
of some deeper convection, but it should remain scattered at best,
with most of the fcst area still under the SW/NE plume of thicker
clouds that would act to keep the convection at bay.

Otherwise, the eastward movement of the trof toward the NE Coast
will shift the convergence axis south tonight. Another shot of
tropical moisture arrives on slightly stronger 850mb winds, which in
the vicinity of the convergence axis leads several guidance sources
to produce an uptick in QPF overnight. Elevated instability may
be released as midlevel lapse rates improve. Following an evening
lull, PoPs rebound late tonight in our southern zones as a result,
though coverage remains in the sct range. Min temps will be similar
to this morning if not slightly warmer.  Areas of fog and low
stratus are expected to form overnight over the southern zones,
and the usual mtn valleys.

&&

.SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY/...
As of 230 PM Monday...a 500 mb ridge axis will strengthen a bit
along a SE-NW axis across the Southeast, pushing Laura toward the LA
or SE TX coast on Wednesday. Meanwhile, a northern stream trough
will cross the Northeast. This will shunt a quasi-stationary front
to the south, and bring in a light downsloping NW wind atop the
forecast area for most of the day. Convection looks more suppressed,
with isolated to widely scattered activity in the NC mountains, but
generally dry. Temps will warm above normal, with low to mid 90s
across the Piedmont. Heat index values may top out in the 99-104
range across most of the Piedmont, as dewpoints remain in the lower
70s.

The upper ridge will begin to weaken slightly atop the region on
Thursday, but a pronounced area of NVA is shown in the synoptic
models rotating over around the ridge axis Thursday aftn, as Laura
lifts north into ArkLaTex region. So convection looks relatively
suppressed again, with just isolated coverage in the high terrain.
Highs will be similar to Wednesday's readings. And dewpts might not
mix out as much, so heat index values may be a deg or two higher.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
As of 200 PM Mon: A subtropical ridge will continue to hold
steady off the Southeast coast as we head into the beginning
portions of the medium range. Model guidance does not have
the ridge retrograding as fast towards land as it had done so
previously. Tropical cyclone Laura will have made landfall during
this time frame and will be on the western fringes of the lower
Mississippi Valley on Friday. An upper trough begins to dig from
the Canadian Provinces and Upper Great Lakes, while the remnants
of Laura tracks eastward through the prevailing westerlies from the
subtropical ridge and digging motion from the northern trough. Since
the Bermuda high is going to gradually approach Florida, the
steering flow will be influenced slightly more by the digging
trough. This will push the remnants of Laura farther south into
Virgina and into the northern portions of North Carolina. In this
case, more tropical moisture will enter the area, especially in
the mountains where hydro issues will arise in this scenario. As
of now, impacts directly from Laura will not be a major concern
for much of the cwa, but changes will definitely occur between
now and then. Expect daily diurnal convection and above-climo
temperatures as higher heights and warmer air aloft builds in from
the subtropical high through the weekend. One thing to note, model
guidance show a decent looking surface boundary associated with
the upper trough and tropical moisture Saturday and could potential
kick-off a severe threat for parts of the area as an uptick in wind
shear will be available. Moving towards the end of the period, the
subtropical high is expected to move across the Gulf early next
week as upper ridging begins to build over the East Coast. This
will lower PoPs as the tropical moisture from Laura moves out to
sea and more subsidence aloft builds into the region. Temperatures
will mainly stay above-climo for much of the period.

&&

.AVIATION /22Z MONDAY THROUGH SATURDAY/...
Overview: We're still observing bkn to ovc high clouds with few to
sct mid and low-lvl clouds early this aftn. This cirrus deck that is
associated with TS Marco will likely remain over much of the area
for the remainder of today and keep convection isolated to widely sct
thru the aftn/evening. KAVL (and to a lesser degree KHKY) still have
the best chance of destabilizing over the next few hours. This was
handled with a TEMPO for TSRA from roughly 20 to 24z. The other taf
sites just carry a VCSH with no mention of thunder at this time.
The weak boundary between subtropical high pressure and the trof
passing to the north will remain over the area into tonight and
shift south later tonight and tomorrow.

At KCLT: As mentioned above, TS chances still look minimal for the
rest of the aftn/evening, with just a VCSH carried beginning at 21z.
I pushed the PROB30 for SHRA back a few hours and have it beginning
at 03z and ending at 08z. I lowered cigs to MVFR at 08z overnight,
as I still anticipate any IFR cigs to remain south of the site. If
the sfc boundary ends up drifting farther north, they could certainly
see some IFR cigs. Otherwise, another round of sct convection is
expected tomorrow aftn/evening across the area. Thus, I included a
PROB30 for TSRA beginning around 19z.

At KAVL: Overall, conditions appear less likely to support thunder
this aftn, with cloud cover remaining bkn over the area. Nonetheless,
enough chance remains for convective initiation to support a TEMPO
TSRA near the diurnal peak. Shower coverage has decreased enough
across the area to warrant just a VCSH in the current prevailing
group. Otherwise, KAVL will likely see another round of IFR visby
and cigs before sunrise tomorrow morning. They remain MVFR after
14z with a PROB30 for SHRA for the last few hours of the period.

At KGSP: Very similar to the KCLT fcst, with just a VCSH carried for
the remainder of the aftn/evening and a PROB30 for SHRA overnight.
I introduce MVFR cigs and visby at 08z, and maintain the MVFR cigs
thru the end of the taf period with a PROB30 for SHRA beginning at
15z tomorrow.

Outlook: Afternoon/evening showers and thunderstorms are expected
again for Tuesday. Fog and low stratus restrictions will be possible
both Tue and Wed mornings, especially over the usual mountain valleys.
Convection will be less likely for the middle part of the week as a
fairly strong ridge of high pressure builds over the area.

Confidence Table...

            21-03z        03-09Z        09-15Z        15-18Z
KCLT       High 100%     High  97%     Med   63%     High  82%
KGSP       High 100%     High  91%     Med   61%     Med   79%
KAVL       High  98%     High  93%     Med   62%     High  86%
KHKY       High 100%     High 100%     High  82%     High  96%
KGMU       High 100%     Med   73%     Low   59%     Med   73%
KAND       High 100%     Med   72%     Low   54%     Med   78%

The percentage reflects the number of guidance members agreeing
with the scheduled TAF issuance flight rule category. Complete hourly
experimental aviation forecast consistency tables are available at
the following link:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

SYNOPSIS...CAC
NEAR TERM...JPT/PM
SHORT TERM...ARK
LONG TERM...CAC
AVIATION...JPT

Hazardous Weather Outlook
National Weather Service Greenville-Spartanburg SC
527 AM EDT Mon Aug 24 2020

NCZ033-048>053-058-059-062>065-501-503-505-507-509-250930-
Avery-Madison-Yancey-Mitchell-Swain-Haywood-Buncombe-Graham-
Northern Jackson-Macon-Southern Jackson-Transylvania-Henderson-
Caldwell Mountains-Burke Mountains-McDowell Mountains-
Rutherford Mountains-Polk Mountains-
527 AM EDT Mon Aug 24 2020

This Hazardous Weather Outlook is for western North Carolina.

.DAY ONE...Today and tonight.

Scattered thunderstorms will develop across the mountains of North
Carolina this afternoon. The main threats will be gusty winds, small
hail, and cloud to ground lightning. Additionally, heavy rainfall
combined with slow moving storms may produce localized minor flooding.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday.

No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...

None.

GAZ010-017-018-026-028-029-NCZ035>037-056-057-068>072-082-502-504-
506-508-510-SCZ001>014-019-250930-
Rabun-Habersham-Stephens-Franklin-Hart-Elbert-Alexander-Iredell-
Davie-Catawba-Rowan-Cleveland-Lincoln-Gaston-Mecklenburg-Cabarrus-
Union NC-Greater Caldwell-Greater Burke-Eastern McDowell-
Greater Rutherford-Eastern Polk-Oconee Mountains-Pickens Mountains-
Greenville Mountains-Greater Oconee-Greater Pickens-
Greater Greenville-Spartanburg-Cherokee-York-Anderson-Abbeville-
Laurens-Union SC-Chester-Greenwood-
527 AM EDT Mon Aug 24 2020

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

.DAY ONE...Today and tonight.

Scattered thunderstorms will develop across the western Carolinas and
northeast Georgia this afternoon. The main threats will be gusty winds,
small hail, and cloud to ground lightning.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday.

No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...

None.

Krentz

Special Weather Statement
National Weather Service Greenville-Spartanburg SC
736 PM EDT Mon Aug 24 2020

NCZ035-056-502-250015-
Greater Caldwell NC-Alexander NC-Catawba NC-
736 PM EDT Mon Aug 24 2020

...A HEAVY RAIN SHOWER WILL IMPACT NORTH CENTRAL CATAWBA...WESTERN
ALEXANDER AND EASTERN CALDWELL COUNTIES UNTIL 815 PM EDT...

At 736 PM EDT, Doppler radar was tracking a heavy rain shower 6 miles
northeast of Lenoir, or near Cedar Rock, moving southeast at 10 mph.

Locations to be impacted include...
Lenoir, Taylorsville, Hickory, St. Stephens, Bethlehem, Cedar Rock,
Lake Hickory, Hudson, Granite Falls and Northlakes.

Very heavy rainfall rates up to 2 inches per hour will be possible in
these areas.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Torrential rainfall may flood areas with poor drainage, such as
ditches and underpasses. Avoid these areas and do not cross flooded
roads. Water levels of small streams may also rise rapidly. Seek
higher ground if threatened by flood waters.

&&

LAT...LON 3602 8134 3600 8133 3601 8131 3594 8110
      3571 8127 3590 8156 3604 8145 3605 8145
TIME...MOT...LOC 2336Z 300DEG 10KT 3594 8143

RM