Flash Flooding and Debris Flow in North Carolina, September 14 to 18, 2018

Area Forecast Discussion...UPDATED
National Weather Service Blacksburg VA
1135 AM EDT Fri Sep 14 2018

.SYNOPSIS...
Hurricane Florence will track inland over southeast NC today,
then shift into South Carolina Saturday into early Sunday,
before moving northward across the southern Appalachians into
the eastern Ohio Valley early next week. Period of heavy rain
and gusty winds will accompany this storm and bring the threat
of significant flooding.

&&

.NEAR TERM /THROUGH SATURDAY/...
Update as of 1115 AM EDT Friday...

Based on latest projections for extended river flooding along
portions of the New, James, and Dan rivers, the going flash
flood watch has been updated to add counties along and southeast
of the Blue Ridge as well as the I-81 corridor for this weekend.
Could also see flash flooding in these areas espcly in the later
Saturday through Monday timeframe so keeping the same timing for
now. May need to add more counties later on pending later model
guidance and QPF.

Previous update as of 1000 AM EDT Friday...

Morning soundings show low level moisture spinning westward
beneath a drier subsidence inversion aloft this morning well to
the northwest of Florence. However low level winds quite strong
aloft espcly off the GSO raob where seeing 50 kts just above the
boundary layer. Some of this already mixing down across the
piedmont where upped wind gusts to around 35 mph over southern
sections per output from the latest HRRR/NamNest. Otherwise will
see breezy conditions increasing elsewhere espcly near the
southern Blue Ridge but overall below 30 mph. Appears any rain
threat to be light and along the eastern slopes through early
afternoon before bands to the south unfold north and mix with
some weak instability out east where bumped pops to more
widespread likelys later on. Shower threat appears less west of
the Blue Ridge given lingering subsidence and more northeast
flow so overall chance coverage there. High temps look to remain
in the 70s but still some breaks possible mainly south and west
of the Blue Ridge.

No changes to the going flash flood watch for now but given
forecast river flooding along the Dan and Roanoke Rivers may
need to extend as early as midday to cover these areas.

Previous discussion as of 340 AM EDT Friday...

Hurricane Florence continues to weaken in terms of winds but
will still bring an impact to our forecast area in terms of
rainfall, especially Saturday and beyond. Despite the weakening
some gusts to 35 to 40 mph are not out of the question this
afternoon into Saturday, mainly in the piedmont of VA/NC to the
foothills of NC and some of the higher terrain. The 00z model
package continues to show some differences in the track of
Florence with the ECM most consistent and close to the NHC
forecast.

With this solution, today will see increasing chances of shower
in the piedmont with less across the west. Some high-res models
are firing up convection over the WV mountains into far SW Va
this afternoon, which is not of the question. Overall went with
likely pops east of Lynchburg to Reidsville line, to slight to
low chance in the mountains. Thinking subsidence may limit
coverage along/west of I-81, except west of Bluefield to
Beckley. Isolated/localized flash flooding possible in the east
today into tonight, but again seems the better threat will be
Saturday into Monday as the flow turns more east then southeast
to south.

Saturday, will see Florence moving slowly from near Florence,
SC to Columbia, SC by evening. Models are favoring a nose of
higher rainfall totals surging toward the NC
mountains/foothills, with some outer banding of heavier showers
also possible across the NC piedmont near the VA border.

With the forecast rainfall we will be posting a flash flood
watch starting Saturday morning through Monday evening for our
NC counties from Watauga/Ashe in the mountains to
Rockingham/Caswell in the piedmont, then extending north along
the Blue Ridge to Floyd/Patrick/Grayson and Carroll Counties in
VA. Needless to say this has been highlighted for the past
several days, and additional watches are likely further north
with later forecasts.

Temperatures not changing too much with record warm lows
possible.

Confidence is high for this forecast period.

&&

.SHORT TERM /SATURDAY NIGHT THROUGH MONDAY NIGHT/...
As of 500 AM Friday...

Likely, the most significant impacts of a weakening Florence
will be felt across our area from late Saturday night into late
Sunday as the storm picks up speed on its way from central South
Carolina to the central Appalachians, before making a northward
turn toward far western Virginia. As the storm approaches the
western Carolinas, winds will gradually veer from northeasterly
Saturday night to an east-southeasterly direction by Sunday
afternoon, becoming more perpendicular to the ridgelines and
therefore increasing the upslope enhancement of rainfall
amounts. As such, believe Sunday and Sunday night is when we can
expect our heaviest rainfall, especially along the Blue Ridge
where locally 8-12 inches is possible. A flash flood watch has
already been posted for the Blue Ridge south of Roanoke and all
the northern North Carolina counties, which will likely be
expanded northward within the next 24 hours. The strongest low
level shear and helicity will also be realized on Sunday into
Sunday night, indicating an increased threat of a few, brief
spin-up tornadoes within the stronger rain bands. Winds during
this time will average in the 15 to 20 mph range sustained, with
some gusts approaching 35 mph for much of the area, though
ridge tops such as the Blue Ridge Parkway may experience gusts
up to 45 mph. As such, the issuance of a wind advisory within
the next 24 hours appears appropriate.

Between the combination of heavy rain and gusty winds,
preparations should be made for a wide range of hazards. Heavy
rain will quickly saturate our already moist soil conditions,
resulting in a strong potential for down trees and rock slides
which may begin to occur early in the storm, potentially cutting
off roadways. As rainfall continues, streams and rivers may
rise rapidly, resulting in a strong threat for flooding. Power
outages and losses of telecommunications are likely, which may
last for extended periods of time, perhaps several days. Folks
are strongly advised to anticipate these hazards and prepare
accordingly.

On Monday, the remnants of Florence will drift northward toward
the upper Ohio River Valley, resulting in decreasing rainfall
and windspeeds for our region. As such, rainfall will begin to
transition to scattered showers in otherwise decreasing cloud
cover, though locally heavy rain will still be possible in some
of the lingering rain bands.

On Monday night, the remnants will begin to turn northeast on a
track toward New England. Depending on how quickly the remnants
exit the Ohio River valley, Tuesday may become a day of drying
as winds shift northwesterly for the mid-Atlantic, though a few
showers and thunderstorms may develop with afternoon heating.

&&

.LONG TERM /TUESDAY THROUGH THURSDAY/...
As of 530 AM EDT Friday...

Little to discuss for the Tuesday night to Thursday time period
as high pressure builds in from the northwest, resulting in
relatively dry weather conditions under partly/mostly clear
skies. Temperatures will remain summer-like, with overnight lows
holding in the 60s and afternoon highs warming into the upper
70s to low 80s.

&&

.AVIATION /15Z FRIDAY THROUGH TUESDAY/...
As of 720 AM EDT Friday...

MVFR to IFR ceilings to stick around through the morning but
increase to MVFR to VFR by 18z. Showers will mainly be situated
east of the taf sites, but some will nudge into LYH/DAN at times
through the day.

Northeast winds will be in the 10-15kt range with gusts
20-30kts, the higher over DAN closer to the center of Florence.

Going into tonight the cigs will drop again below MVFR with
increasing threat of rain as far west as BCB. Visibilities will
also drop to MVFR to IFR at times. Winds stay up for the most
part through the period, but nothing over 30kts in the gusts are
expected.

Medium confidence in the forecast this period.

Extended Discussion...

Rainfall will spread north and west through the weekend with
poor flying conditions. Winds will swing to the southeast by
Sunday then to the southwest to west early next week. Will see
Florence shift north of us by Tue-Wed next week with conditions
improving.

Confidence gradually transitions to below average through
Sunday due to existing uncertainties associated with the
movement and placement of Florence.

&&

.HYDROLOGY...
As of 1130 AM Friday...

Latest model output continues to suggest heavy rainfall across
at least the southern half of the region with much of this
occurring Saturday night into Monday. Rainfall rates will likely
be higher across northwest North Carolina as deeper banding
occurs just to the northeast of Florence by Sunday. This may
spell excessive rainfall of 5 to 10 inches there with locally
heavier totals pending just how the bands pan out. Elsewhere,
appears that a wedge of high pressure aided by strong northeast
flow may tend to lessen rainfall rates heading north into parts
of Virginia until this cool pool exits by early Monday. This
would support a bit less rainfall heading north until things
become more banded and convective in nature on Monday. However
rainfall exceeding 5 inches appears likely espcly along and east
of the Blue Ridge in Virginia.

This has resulted in projections of moderate to major flooding
along the New, Roanoke and Dan Rivers by early next week. Thus
have extended the going flash flood watch to include most
counties that are within these basins.

Otherwise the effects of the wet antecedent conditions are
unclear because if it rains hard enough and long enough it may
not matter much but soil moisture and stream flows are well
above the seasonal norms. Output from the National Water Model
shows positive streamflow anomalies on nearly all our streams
and rivers for this time of year and similar numbers are shown
on the USGS Waterwatch website. The Roanoke River at Roanoke
forecast point for example, is running at the 98th percentile of
all flows for this date and over 500 percent of the mean flow
for the date. The New River at Allisonia, above Claytor Lake is
at the 95th percentile and over 200 percent of mean flow.
Because these flows are near their annual minima in early to
mid- September it may be a less of a factor but cannot be
discounted. With the existing QPF consensus, moderate to major
flooding within several river basins is a very real possibility.
The other concern is the risk for landslides which become start
to become a concern in steep terrain with 5 inches or more of
rain and a big concern above 10 inches. This has been included
within the current Flash flood Watches that are in effect,

&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...Flash Flood Watch from Saturday morning through Monday evening
     for VAZ009-012>017-022-032-033-043-044-058.
NC...Flash Flood Watch from Saturday morning through Monday evening
     for NCZ001>006-018>020.
WV...None.

&&

SYNOPSIS...MBS/WP
NEAR TERM...JH/MBS/WP
SHORT TERM...NF
LONG TERM...NF
AVIATION...KK/WP
HYDROLOGY...JH/PC

Hazardous Weather Outlook...UPDATED
National Weather Service Blacksburg VA
1131 AM EDT Fri Sep 14 2018

VAZ007-010-011-018>020-023-024-034-035-045>047-059-WVZ042>044-507-
508-151545-
Tazewell-Bland-Giles-Craig-Alleghany VA-Bath-Botetourt-Rockbridge-
Bedford-Amherst-Campbell-Appomattox-Buckingham-Charlotte-Mercer-
Summers-Monroe-Eastern Greenbrier-Western Greenbrier-
1131 AM EDT Fri Sep 14 2018

This Hazardous Weather Outlook is for central Virginia, south
central Virginia, southwest Virginia, west central Virginia and
southeast West Virginia.

.DAY ONE...This afternoon and tonight.

There is a low probability for widespread hazardous weather.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday.

Excessive rainfall from the remnants of Florence will impact the
area this weekend into Monday. Some areas could see flash flooding,
as well as river flooding. Although winds are not expected to be that
strong, a few gusts over 40 mph and saturated soils could lead to
downed trees, with isolated power outages. There could also be some
isolated weak tornadoes Sunday.

.SPOTTER INFORMATION STATEMENT...

Spotters are encouraged to monitor for flooding, downed trees and
power lines, along with possible tornadoes Sunday. Please email your
rainfall and severe weather reports to rnk.skywarn@noaa.gov or pass
them along on our US National Weather Service Blacksburg VA Facebook
page.

NCZ001>006-018>020-VAZ009-012>017-022-032-033-043-044-058-151545-
Ashe-Alleghany NC-Surry-Stokes-Rockingham-Caswell-Watauga-Wilkes-
Yadkin-Smyth-Wythe-Pulaski-Montgomery-Grayson-Carroll-Floyd-Roanoke-
Patrick-Franklin-Henry-Pittsylvania-Halifax-
1131 AM EDT Fri Sep 14 2018

...FLASH FLOOD WATCH IN EFFECT FROM SATURDAY MORNING THROUGH MONDAY
EVENING...

This Hazardous Weather Outlook is for north central North Carolina,
northwest North Carolina, south central Virginia, southwest
Virginia and west central Virginia.

.DAY ONE...This afternoon and tonight.

There is a low probability for widespread hazardous weather.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday.

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

   Flash Flood Watch.

In addition, although winds are not expected to be that strong, a
few gusts over 40 mph and saturated soils could lead to downed
trees, with isolated power outages. There could also be some
isolated weak tornadoes Sunday.

.SPOTTER INFORMATION STATEMENT...

Spotters are encouraged to monitor for flooding, downed trees and
power lines, along with possible tornadoes Sunday. Please email your
rainfall and severe weather reports to rnk.skywarn@noaa.gov or pass
them along on our US National Weather Service Blacksburg VA Facebook
page.

Special Weather Statement
National Weather Service Blacksburg VA
1054 AM EDT Fri Sep 14 2018

NCZ003>006-019-020-VAZ032-043>045-058-059-142100-
Surry-Stokes-Rockingham-Caswell-Wilkes-Yadkin-Patrick-Henry-
Pittsylvania-Campbell-Halifax-Charlotte-
Including the cities of Dobson, Danbury, Eden, Yanceyville,
Wilkesboro, Yadkinville, Stuart, Martinsville, Danville,
Lynchburg, South Boston, and Keysville
1054 AM EDT Fri Sep 14 2018

...Gusty Northeast Winds Into Late This Afternoon...

As the circulation of Florence moves inland across southeast
North Carolina through this afternoon, northeast winds will
strengthen across the foothills and piedmont sections with gusts
to 40 mph possible. These speeds may be enough to cause tree limbs
to fall resulting in sporadic power outages. Showers are also
possible this afternoon which could briefly bring down stronger
winds in isolated areas.