Flood Potential Outlook
Issued by NWS Blacksburg, VA

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Winter/Spring Flood Potential Outlook
National Weather Service Blacksburg VA
1211 PM EST Thu Jan 18 2018

...Winter/Spring Flood Outlook...Number 2

...River flood potential remains well 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 next two weeks (through February 1st). No heavy rain is
forecast over the next seven days and area rivers have ample
storage capacity to handle this runoff. Contributions from
snowmelt will be insignificant as the current snow holds very
little water and will be melting quickly.

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

Recent Precipitation:
The first half of January has seen a slightly wetter pattern than
in the incredibly dry preceding two month period, but not in all
areas. Monthly amounts through today (Jan. 18th) range from a
about 0.50 to 0.75 inches in parts of the New River and Yadkin
River valleys up to over 3 inches in Watauga County, NC. For 52
available stations, the mean precipitation to date is 1.52" which
is just slightly below normal for that limited period.

Both November and December 2017 were exceptionally dry for the
entire HSA. For December, the mean monthly rainfall was 0.96" or 29
percent of the average amount 3.32" per the 1981-2010 normals. 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. Numerous COOP
stations were in the top 3 driest November-December periods on
record. Roanoke Airport saw only 1.06" for the two-month period,
the driest since records began in 1912.

Fortunately, October had above normal precipitation, with a mean
monthly rainfall of 5.52", or 176% of the normal 3.13".

Drought:
Due to the persistent dry conditions the start of November, the
foothills and Piedmont region of Virginia and North Carolina 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:
Snow cover as of January 18th ranges from less than 1 inch in many
mountain areas up to 10 inches in parts of the piedmont, a
reversal from the typical pattern. All the snow cover is expected
to melt quickly over the next few days as daytime high
temperatures warm into the 40s and 50s for the next week.

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

River ice:
Rivers that froze up in early January melted in a brief but sharp
warmup last week and the renewed cold has not been sufficient to
reestablish significant 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. Many are lower than the 10th percentile of all
flows observed on this date especially at longer durations. The
Dan River at Francisco (USGS Station 02068500) in Stokes County,
NC which drains 129 square miles of the upper Dan River basin, is
near a record low flow for the date.

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 including John H. Kerr, W. Kerr
Scott, Philpott Lake, Lake Moomaw and Smith Mountain Lake are near
or below guide curves where applicable. Flood capacity, where
available remains at the maximum given the low current elevations.

Future Weather Conditions:
Quantitative precipitation forecasts (QPF) from the Weather
Prediction Center (WPC) over the next 7 days (through 7 AM
Thursday, January 25th) show amounts generally under 0.25 inches
for nearly all of the HSA. All of which is expected to come from
a single storm system and cold front crossing the area on Tuesday,
January 23rd.

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 31st show a likelihood of both temperatures and
precipitation to be near to slightly above normal through the
entire period.

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 25th 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
February 1st, 2018.

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


$$

PC



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