Flood Potential Outlook
Issued by NWS Blacksburg, VA

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Winter/Spring Flood Potential Outlook
National Weather Service Blacksburg VA
447 PM EDT Thu Mar 16 2017

...Winter/Spring Flood Outlook...Number 6

...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 next week (through March 23rd). No heavy rain is forecast
over the next seven days. Contributions from snow melt west of
the Blue Ridge will be insignificant due to low snow pack and only
gradual melting.

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

Recent Precipitation:
During March 2017, a strong frontal boundary and its associated
rainfall produced anywhere from 0.25 to 0.50 inches across the
Piedmont to 1 to 1.50 inches over the mountains the first two days
of the month. However, precipitation since then has been sparse,
with multiple cold fronts passing across the region triggering
only scattered precipitation that seldom exceeded a quarter inch
for a single location. 0.50 to 1.00 inches fell early on March 14
across the Piedmont of Virginia and North Carolina, while
generally less than 0.50 inches was observed further west.

February 2017 was extremely dry with average precipitation across
the NWS network of COOP and ASOS stations of 0.92 inches or only
30 percent of the 30-year (1981-2010) normal 3.03 inches. It was
the driest February in 20 years of HSA COOP records, just slightly
drier than 2002 which had 0.94 inches. Several sites had their
1st or 2nd driest February on record with data back more than 100
years including Roanoke which had 0.54 inches, second only to 1938
with 0.51 inches.

January precipitation was generally near or above normal across
the HSA ranging from 3 to 5 inches with some slightly higher and
lower amounts. It was somewhat wetter in the western mountains
with the driest area in the northern Blue Ridge of Virginia. The
average was 4.23 inches of precipitation versus the long-term
normal of 3.32 inches or 127 percent of normal.

Looking back 90 days from mid December through mid March, a dry
signal persists for northern North Carolina and most of Virginia
in the HSA, where many stations are at 50 to 75 percent of normal
precipitation. Portions of far western Virginia and southeast West
Virginia are closer to normal, ranging anywhere from 90 to locally
125 percent of normal precipitation for the period.

Drought:
Dry conditions since February have resulted in a gradual increase
in drought conditions, with D1 conditions (Moderate Drought) now
covering much of the Piedmont and the foothills of Virginia, as
well as all of northern North Carolina. D0 conditions (Abnormally
Dry) cover most of the remaining Virginia counties in the
Blacksburg HSA. Drought conditions are not currently observer in
southeast West Virginia.

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

Snow cover:
Snow cover as of March 16th is confined to southeast
West Virginia, far western Virginia, and the highlands of
North Carolina, with an average depth of one to three inches,
though elevations above 3000 feet potentially have up to 6 inches
on the ground. Much of this snow cover is expected to melt
gradually through the next week as daytime high temperatures warm
generally into the 40s and 50s while 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:
There is no river ice after the extremely warm winter of
2016-2017.

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)
have changed little for the first half of March as compared to the
end of February. A swath of below normal values, ranging from 1
to 3 inches, remains in place from northwest North Carolina
northeastward through the Blue Ridge foothills and Piedmont of
both Virginia and North Carolina. Southeast West Virginia remains
close to normal for mid March.

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, March 23th) show amounts ranging from up to 0.50 inches
for locations east of the Blue Ridge, while locations along and
west of the Blue Ridge can expect up to 1.25 inches with locally
higher amounts across western Greenbrier County. Storm systems
riding across the Ohio Valley will be the main source of our
precipitation, however moisture is limited due to lack of a strong
fetch from the Gulf of Mexico.

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 March 29th show a likelihood of temperatures near to
slightly above normal through the entire period. There is the
potential for precipitation to be above normal during the 6 to 10
day period, due to a strong storm system with the potential for
widespread, possibly locally heavy precipitation to affect the mid
Atlantic around March 25th. Precipitation is expected to be near
to slightly above normal for the 8 to 14 day 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 March 22nd 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
March 30th, 2017.

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

$$

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