Flood Potential Outlook
Issued by NWS Blacksburg, VA

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Winter/Spring Flood Potential Outlook
National Weather Service Blacksburg VA
148 PM EST Thu Feb 15 2018

...Winter/Spring Flood Outlook...Number 4

...River flood potential slightly below normal despite improved
precipitation...

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
below normal for the next two weeks (through March 1st). Moderate
widespread rainfall over the past week has resulted in a
significant improvement of the dry conditions that have been in
place for much of the winter. While rounds of rainfall are
expected to pass across the HSA in the coming week, none of these
rounds are expected to be heavy enough to result in widespread
river flooding. All major stem rivers in the HSA are below 1/2
bankfull, with many in the 1/4 to 1/3 bankfull range, indicating
there is storage capacity available to handle another widespread
moderate rainfall event.


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


Recent Precipitation:
Precipitation has improved significantly from late January through
the early half of February for nearly all basins in the HSA. Since
the beginning of the month, nearly all reporting stations across
the mountains and the foothills have experienced between 3 and 5
inches of precipitation, the vast majority falling as rain.
Locally higher amounts along the crest of the Blue Ridge of 5 to 6
inches have been observed. Storm tracks directing precipitation
further to the north have resulted in lesser amounts along the
lower reaches of the Staunton and Dan River basins ranging from
1.5 to locally 3 inches during this time.

Monthly totals for January ranged from 1 to 1.50 inches in parts of
the James and New River basins up to over 3 to 4 inches in parts of
the Tennessee and upper Yadkin River basins. Single station monthly
totals ranged from 1.11 inches at Covington Filter Plant in
Covington City, VA to 4.77 inches at Boone 1 SE, in Watauga County,
NC. For 61 available stations, the mean precipitation for January
was 2.42 inches or 73 percent of the 30-year normal of 3.32 inches.

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 only slightly wetter with an observed mean
monthly rainfall of 1.19", which is 35 percent of the average
3.42" per the 1981-2010 normals.

At many stations it was among the top 5 driest November through
January periods on record. This included Roanoke with only 3.20
inches, the second driest such period since records began in 1912.
Blacksburg had 3.51 inches, also for the second driest since
1952.


Drought:
Multiple rounds of widespread precipitation since the end of
January have resulted in a significant improvement in dryness
across the HSA. As of February 13th, D0 conditions (Abnormally
Dry) have been removed from generally from the crest of the Blue
Ridge westward, but remain in place across the foothills of the
Blue Ridge. The coverage of D1 conditions (Moderate Drought) has
been reduced, but remains in place across the lower reaches of the
Staunton and Dan River basins, which have not received as much
precipitation as areas further north and west.

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


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

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


River ice:
There is no river ice anywhere in the HSA at this time.


Streamflow: USGS real-time streamflows have increased since the last
posting of this outlook. Seven-day streamflows for nearly all
basins have risen to the 90th percentile, in many cases 200% to
300% what has been observed historically at this time of year. The
exceptions remain the lower Dan and Staunton River basins, which
are running near normal.

Twenty-eight day streamflows have also improved across the entire
HSA. Streamflows across all basins are near normal, with flows
running anywhere from 75% to 125% of what is typically observed 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 improved soil moisture conditions due to recent
widespread precipitation. Greatest deficits remain observed across
the Piedmont of northern North Carolina into central Virginia, an
area which is experiencing 1" to 2.5" below-normal conditions.
Deficits decrease to near normal soil moisture conditions further
west across the foothills and mountains.

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
guide curves where applicable.


Future Weather Conditions:
Quantitative precipitation forecasts (QPF) from the Weather
Prediction Center (WPC) over the next 7 days (through 7 AM
Thursday, February 22nd) show amounts of about 0.75 to 1.25 inches
for nearly all of the HSA, with the possibility of locally higher
amounts approaching 2.00 across the Bluestone and Greenbrier River
basins. Overall, this is slightly above normal precipitation but
would not present a significant flood threat at these amounts.

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 28th show a strong likelihood of above-normal
temperatures across the entire HSA. Precipitation is expected to be
slightly above normal early, and near to slightly above normal late
in this 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 February 21st indicate low 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 slightly below normal through
the outlook period, based on current conditions and forecasts.


Next issuance:
The next flood potential outlook will be issued on or around
March 1st, 2018.

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

$$

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