Flash Flood in Georgia, September 17, 2020

Area Forecast Discussion...UPDATED
National Weather Service Greenville-Spartanburg SC
443 AM EDT Thu Sep 17 2020

.SYNOPSIS...
Heavy rainfall and isolated thunderstorms associated with Tropical
Depression Sally will impact much of the area Thursday with showers
and storms tapering off by early Friday morning. Thereafter,
dry and cool high pressure will dominate the weather through the
middle of next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 400 AM EDT Thursday: A Flash Flood Watch goes into effect at 8
AM EDT this morning for portions of the area and will
continue through Friday morning.

Despite weak to moderate returns on radar imagery over the past few
hours, rainfall is finally making it to the surface with pockets of
heavy rainfall being reported after fighting to saturate a
previously dry atmospheric column associated with the sfc wedge in
place the past few days. Along with rainfall, widespread low stratus
and patchy fog prevails across the area this morning as the wedge
continues to retreat off to northeast and a front to the south
begins to lifts northward. Current temps are in the low to mid 60s.

As the remnants of what is currently tropical depression Sally moves
northeastward and overhead the Carolinas tonight (becoming extra-
tropical) and interacts with a front in the vicinity, expect
scattered to numerous showers across the area today and tonight with
even a few rumbles of thunder possible across the southeastern
portions as well, as this system brings plenty of moisture into the
region. Do think the better storm chances will be outside the area
to the south and east, but, could not entirely rule out a few
isolated tornadoes given the environment (sbCAPE nearing 1000 J/kg
in some areas and plenty of available shear). Winds could become
breezy at times as well, which with saturated soils could bring
trees down. Overall though, the main concern through the period is
the potential for heavy rainfall and flash flooding in urban areas
as well as along smaller streams and creeks. Rainfall totals with
this system will be slightly tricky given the higher QPF amounts
anticipated in the pockets of convective bands, as depicted by
latest CAMs. The latest WPC Excessive Rainfall Day 1 outlook places
the southeastern two-thirds of the FA in a Moderate and a Slight
elsewhere (across the NC mountains to the TN border). This lines up
well with latest guidance and it's placement of frontogenesis/axis
of better instability and higher rainfall amounts. Now through
through Friday morning, anticipate rainfall amounts of 3 to 5 inches
within the watch area with higher amounts up to 8 inches possible.
Outside of the watch area along the TN/NC border, 1 to 2 inches will
be possible. These amounts do not include what has already fallen so
far this morning. Max temps will fall about 10 degrees short of
normal.

With activity continuing into tonight, do expect a gradual
decreasing trend in intensity and coverage into Friday morning as
the system lifts off to the east-northeast. Min temps will remain
above normal.

&&

.SHORT TERM /FRIDAY THROUGH SATURDAY/...
As of 330 AM Wednesday: The weather following Sally will be
well-deserved as a continental polar airmass invades the northern
US Friday and 1030mb high pressure settles over the Great Lakes.
However, it will take most of Friday for residual moisture behind
Sally's post-tropical front to be pushed east by increasing dry
and cold air advection, so Friday looks to be mostly cloudy to
overcast with scattered light showers across the area rapidly
diminishing from west to east during the afternoon.

This sets us up for a significant change in the weather pattern
heading into the weekend as the large cP high pressure system
propagates slowly east into Quebec and overtakes much of the eastern
CONUS.  CAA will be well underway Friday night into Saturday as
a stout NE flow becomes established across the Carolinas.  The NE
flow will have an Atlantic fetch component which will keep skies
partly to mostly cloudy on Saturday though by Sunday sunshine
will dominate.  With NE flow persisting through Sunday, the cool
down will be dramatic, as highs fall to 9-11 degrees below normal
on Saturday and 12-15 degrees below normal on Sunday.  Lows Sunday
morning will reach the lower 50s outside the mountains.  This is
ideal weather for a trip into the mountains for some apple picking!

&&

.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
As of 330 AM Wednesday: The upper-level pattern stalls over
the western Atlantic for much of next week which allows the
fall-like high pressure pattern to persist through the week.
High pressure will gradually settle south and weaken over the
southern Appalachians by Wednesday as our connection to a colder
airmass is lost. Therefore, we have zero PoPs in the forecast
through the end of the period Thursday morning and temperatures will
moderate slowly through the period.  Expect highs on Monday to be
similar to Sunday, about 11-13 degrees below normal or closer to
our normal highs for early November!  Slight warming is expected
for Tuesday with highs about 6-10 degrees below normal, and by
Wednesday highs will be just a few degrees below normal which
is equivalent to the mid- and upper 70s east of the mountains.
Lows Monday and Tuesday morning will bottom out in the upper 40s
for much of the NC Piedmont and northern SC Upstate.  Enjoy!

&&

.AVIATION /09Z THURSDAY THROUGH MONDAY/...
At KCLT and elsewhere: Conditions continue to deteriorate across the
area from south to north as the atmospheric columns saturates and
precipitation begins to reach the ground. Increasing coverage of
rainfall combined with lowering cigs and patchy fog has allowed for
a mixture of MVFR/IFR/LIFR to prevail at the TAF sites this morning.
Expect this trend to continue and for flight restrictions to lower
to IFR/LIFR later this morning and continue through the day as Sally
moves closer and brings additional moisture to the area. There is an
increasing potential for TSRA at KAND/KGSP/KGMU/KCLT as the track
shifts slightly with latest model guidance. However, given lower
confidence attm, have only introduced TSRA at KCLT with this fcst.
Otherwise, expect moderate to heavy rain throughout the day and
tonight. Into Friday morning, conditions will begin to gradually
improve as the remnants of Sally move away from the area. Winds will
remain NE 5 to 10 kts with higher gusts possible through the morning
and afternoon hours.

Outlook: Conditions will gradually improve during the day on Friday
with VFR expected through the weekend and into early next week.

Confidence Table...

            08-14Z        14-20Z        20-02Z        02-06Z
KCLT       Med   67%     High  90%     High  90%     High  89%
KGSP       High  87%     High  87%     High  89%     Med   75%
KAVL       High  92%     High  89%     High  82%     High  81%
KHKY       Med   66%     High  98%     High  87%     High  92%
KGMU       Med   74%     High  89%     High  89%     Med   78%
KAND       Med   78%     High  87%     High  84%     Med   77%

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...Flash Flood Watch from 8 AM EDT this morning through Friday
     morning for GAZ010-017-018-026-028-029.
NC...Flash Flood Watch from 8 AM EDT this morning through Friday
     morning for NCZ035>037-056-057-062>065-068>072-082-502-
     504-506>510.
SC...Flash Flood Watch from 8 AM EDT this morning through Friday
     morning for SCZ001>014-019.

&&

SYNOPSIS...JMP
NEAR TERM...SGL
SHORT TERM...JMP
LONG TERM...JMP
AVIATION...SGL

Hazardous Weather Outlook
National Weather Service Greenville-Spartanburg SC
523 AM EDT Thu Sep 17 2020

GAZ010-017-018-026-028-029-NCZ035>037-056-057-062>065-068>072-082-
502-504-506>510-SCZ001>014-019-180930-
Rabun-Habersham-Stephens-Franklin-Hart-Elbert-Alexander-Iredell-
Davie-Catawba-Rowan-Macon-Southern Jackson-Transylvania-Henderson-
Cleveland-Lincoln-Gaston-Mecklenburg-Cabarrus-Union NC-
Greater Caldwell-Greater Burke-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-Union SC-Chester-Greenwood-
523 AM EDT Thu Sep 17 2020

...FLASH FLOOD WATCH IN EFFECT THROUGH FRIDAY MORNING...

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

.DAY ONE...Today and tonight.

Please listen to NOAA Weather Radio or go to weather.gov on the
Internet for more information about the following hazards.

   Flash Flood Watch.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday.

Hazardous weather is not expected at this time.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed. Please relay any information about
observed flooding to the NWS while following all local, state, and
CDC guidelines.

NCZ033-048>053-058-059-501-503-505-180930-
Avery-Madison-Yancey-Mitchell-Swain-Haywood-Buncombe-Graham-
Northern Jackson-Caldwell Mountains-Burke Mountains-
McDowell Mountains-
523 AM EDT Thu Sep 17 2020

This Hazardous Weather Outlook is for western North Carolina.

.DAY ONE...Today and tonight.

The remnants of tropical cyclone Sally are expected to pass close
enough to western North Carolina to bring periods of heavy rain
during the day on Thursday. While the heaviest rainfall and greatest
flooding threat is expected south and east of the Tennessee border
counties, bands of locally heavy rainfall will still impact the
interior mountains and isolated flooding of streams and urban areas
is possible.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday.

Hazardous weather is not expected at this time.

.SPOTTER INFORMATION STATEMENT...

None.

JMP