Flood Potential Outlook
Issued by NWS Blacksburg, VA

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Winter/Spring Flood Potential Outlook
National Weather Service Blacksburg VA
1003 PM EST Thu Jan 4 2018

...Winter/Spring Flood Outlook...Number 1

...River flood potential remains below normal...

Each winter and early spring, the National Weather Service office
in Blacksburg issues a series of routine flood potential
outlooks. These outlooks estimate the potential for river flooding
(not flash flooding) across the Blacksburg office`s Hydrologic
Service Area (HSA). The HSA includes 40 counties covering parts of
southwest Virginia, far northwest North Carolina and far
southeast West Virginia. Major river basins in the HSA include all
or parts of the New, Greenbrier, Tennessee, James, Roanoke, Dan,
and Yadkin. This outlook is based on the current assessment of
hydrometeorological factors which contribute to river flooding.
These factors include, but are not limited to, recent
precipitation, soil moisture, snow cover and snow water
equivalent, stream flows, river ice and expected future weather
conditions.

In this part of the southern Appalachian and Mid-Atlantic region,
heavy rainfall is the primary factor that leads to river
flooding. Heavy rainfall can rapidly cause river flooding at any
time of the year, even when overall river flood potential is
considered low.

Flood Potential Outlook:
For the Blacksburg HSA, the river flood potential is below normal
for the two weeks (through January 18th). While no heavy rain is
forecast over the next seven days, nearly all rainfall that does
occur is expected to translate directly into runoff due to frozen
ground conditions across the entire HSA. Area rivers have the
storage capacity to handle this runoff. Contributions from snow
melt west of the Blue Ridge will be insignificant due to very
little snow pack and slow melting.

Current Flooding:
There is no flooding occurring or forecast at this time.

Recent Precipitation:
Both November and December 2017 have been exceptionally dry for
the entire HSA. For December, the mean monthly rainfall was 0.96",
29 percent the average amount 3.32" per the 1981-2010 normals.
The only precipitation of note for the entire month was a snow
event on December 7-8, which brought liquid equivalents ranging
from a trace in the far west to over 0.50" in parts of the eastern
Piedmont. A few light rainfall events comprised the remainder of
the scarce precipitation.

November 2017 was the driest November across the HSA since 2012,
with an observed mean monthly rainfall of 1.19", which is 35 percent
of the average 3.42" per the 1981-2010 normals. A handful of
rainfall events throughout the month generally all produced average
amounts less than 0.50".

October marked the last time that above normal precipitation was
observed, with a mean monthly rainfall was 5.52", or 176% of the
normal 3.13". Two major rainfall events made up the bulk of the
precipitation: the passage of Tropical Depression Nate on October
8-9, which resulted in localized flash flooding across the
mountains, and a strong upper level storm system tapping deep
tropical moisture during October 23rd, which brought 4 to 6 inches
of rain to the Blue Ridge region south of Roanoke VA.

Drought:
Due to the unusually dry conditions since the end of October, the
foothills and Piedmont region of Virginia are experiencing D1
conditions (Moderate Drought). D0 conditions (Abnormally Dry)
cover generally all of the mountain counties of the HSA, as well
as the foothills of northwest North Carolina.

Please visit the www.droughtmonitor.unl.edu/ for access to the
drought maps and additional information.

Snow cover:
Significant snow cover as of January 4th is confined mainly to
Greenbrier County of southeast West Virginia, where western parts
of the county have 1 to 4 inches on the ground. Elevations above
3000 feet in far western Virginia and the highlands of North
Carolina potentially have 1 to isolated 2 inches on the ground.
Much of this snow cover is expected to melt gradually through the
week of January 7-13 as daytime high temperatures warm generally
into the 30s and 40s, and overnight temperatures most nights fall
into the 20s and 30s.

Please visit www.nohrsc.noaa.gov for detailed information on snow
cover and snow water equivalent.

River ice:
Due to exceptionally cold temperatures during the last week of
December into the beginning of January, most rivers across the HSA
are experiencing thin layers of ice.

Streamflow:
USGS real-time streamflows, at all ranges from from 7 to 28-days,
remain very low relative to what has been observed historically
at this time of year.

For more detailed information on streamflow conditions see the
USGS WaterWatch website: https://waterwatch.usgs.gov

Soil Moisture:
Soil moisture analyses from the Climate Prediction Center (CPC)
indicates below-normal soil moisture for the entire HSA. Greatest
deficits are observed east of the Blue Ridge, where the Piedmont
of North Carolina and Virginia are experiencing 2.5" to 4" below
normal. Deficits generally decrease from east to west across the
mountains, ranging from roughly 2.5" below normal near the Blue
Ridge to around 1" below normal for southeast West Virginia and
far western Virginia.

For additional soil moisture information see:
http://www.cpc.ncep.noaa.gov

Reservoirs:
Major water supply reservoirs are near guide curve with normal
flood capacity at the flood control reservoirs.

Future Weather Conditions:
Quantitative precipitation forecasts (QPF) from the Weather
Prediction Center (WPC) over the next 7 days (through 00z Friday,
January 12th) show amounts generally ranging from 0.25 to 0.50
inches for nearly all of the HSA, all of which is expected to
come from a single storm system riding across the Ohio Valley on
Monday, January 8th.

WPC QPF is updated frequently and is available at:
www.wpc.ncep.noaa.gov/#page=qpf

The longer term outlooks for the 6 to 10 and 8 to 14 day periods
through January 18th show a likelihood of both temperatures and
precipitation to be near to slightly above normal through the
entire period. Precipitation during the 6 to 10 day period is
expected to come from a storm around the 12th-13th of the month.

For additional long range forecast information see:
http://www.cpc.ncep.noaa.gov

Probabilistic/Ensemble river forecasts:
The Meteorological Model Ensemble River Forecasts (MMEFS) valid
through about January 11th indicate no potential for river
flooding through the period. Ensemble river forecasts (MMEFS) are
available at: www.weather.gov/erh/mmefs

Summary:
The river flood potential is below average through the outlook
period, based on current conditions and forecasts.

Next issuance:
The next flood potential outlook will be issued on or around
January 18th, 2018.

For additional hydrologic or weather information, visit our
website at weather.gov/blacksburg.

$$



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