Flood Potential Outlook
Issued by NWS Blacksburg, VA

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Winter/Spring Flood Potential Outlook
National Weather Service Blacksburg VA
1200 PM EST Thu Jan 19 2017

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

...River flood potential slightly above normal due to forecast
rain...

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 slightly above
normal in the first week of the outlook period due almost
entirely to rainfall expected January 22-23. Flooding is likely to
be confined to portions of the Dan and possibly lower Roanoke
rivers. Flood potential drops back to near normal for the second
week of the outlook period.

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

Recent Precipitation:
January to date has seen precipitation from rain and snow ranging
generally from 1.5 to 2.5 inches across the HSA which is close to
normal for the period. December precipitation was also near
normal with a mean of 3.28 inches across the network of 58 NWS
COOP and ASOS sites versus the long- term (1981- 2010 period)
average of 3.32 inches or 99 percent of normal. There was a wide
range however, with amounts ranging from around 1.50 inches in the
far southeast to over 5 inches in parts of the far western HSA.

At longer durations the picture is somewhat mixed. November was
much drier than average with a mean of 1.30 inches across the
network, with the southeast areas the driest. Only a late month
rain event prevented the driest November on record at many
locations and stopped a consecutive days with no rainfall streak
that was also approaching record duration at some sites.

Looking back 90 days from mid-October through mid-January there
remains a very dry signal over southern portions of the HSA where
some counties along the VA-NC border are at less than 50 percent
of normal precipitation for that period. Meanwhile parts of the
northwest (mainly in WV) are above 110 percent of normal
precipitation for the period.

Drought:
Moderate drought (D1 drought category) continues to be shown on
the weekly U.S. Drought Monitor map across portions of southwest
Virginia and most of northwest North Carolina. Much of the
remainder of the HSA is in D0 or Abnormally Dry on the Drought
Monitor map. Nearly all of southeast West Virginia and parts of
far western Virginia are free of any drought designation.

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

Snow cover:
There is no snow anywhere in the HSA.

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

River ice:
There is no river ice as recent temperatures have been well above
normal. January is running 3 to 5 degrees above average so far.

Streamflow:
Streamflows are also somewhat variable and reflect the general
precipitation patterns. Near to above normal flows are observed in
the west and north while river basins in the south show below to
much below normal flows. Several USGS stream gages in the Dan and
upper New River basins in far southern Virginia and northern North
Carolina are below the 10th percentile of all flows for this date
at durations ranging from 7 to 28 days but are still close to
median flows 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)
show the driest soils are across parts of the southern Virginia
piedmont and foothills and extending into parts of northwest
North Carolina. Anomalies range from about 1 to 2 inches with the
worst area located right along the VA-NC border. Soil moisture is
currently close to normal in the western and northern sections of
the HSA.

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 12z Thursday
January 26th) show a wet period is likely. QPF of 0.25 to 0.50
inches is forecast though early Sunday (Jan 22) as a weak low
moves through the Ohio Valley. Heavier precipitation is forecast
for Sunday into Monday, January 22-23 as a strong area of low
pressure will move from the lower Mississippi Valley across the
southern Appalachians to the mid- Atlantic coast bringing periods
of moderate to heavy rainfall. QPF is currently forecast to be in
1 to 2.5 inch range depending on the final strength and track of
the low. Temperatures are expected to remain well above average
through the entire event meaning the precipitation will be all
rain.

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 February 1st show temperatures likely to remain well above
average early in period but trending closer to normal toward the
end of the period. Precipitation chances are forecast to be near
average early in the period but trending toward below normal late
in the 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 January 26th indicate some potential for river flooding
due to the forecast rainfall event of January 22-23. Forecast
precipitation from the ensemble models vary considerably but the
median values range from about 1.50 inches across the western
(New, Greenbrier, upper Roanoke) and northern (James) river
basins up to 2.50 inches in the southern and eastern basins
(Yadkin, Dan, lower Roanoke). The ensemble hydrologic models
currently show minor flooding likely (70 percent chance) only on
the lower Dan River at South Boston and a chance (30 percent) for
minor flooding further upstream on the Dan River toward Danville.
Any flooding would likely occur January 24-25 due to the lag in
runoff reaching the mainstem rivers. No flooding is indicated on
any of the other major rivers although rises to near Action Stage
are possible.

Ensemble river forecasts (MMEFS) are available at:
www.weather.gov/erh/mmefs

Summary:
The river flood potential is slightly above average through week
one of the outlook period, based on an incoming rain event and
near average for week two.

Next issuance:
The next flood potential outlook will be issued on or around
February 2nd, 2017.

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


$$

pc



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