Flood Potential Outlook
Issued by NWS Blacksburg, VA

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Winter/Spring Flood Potential Outlook
National Weather Service Blacksburg VA
238 PM EST Thu Feb 20 2020

...Winter/Spring Flood Outlook...Number 3

...River flood potential near average to finish out the month of
February...

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. Snowmelt can contribute to the flooding as it did
dramatically in January 1996, when rain on top of rapid snowmelt
produced record flooding throughout parts of the Mid-Atlantic
region, including the Greenbrier River.

Flood Potential Outlook: For the Blacksburg HSA, the river flood
potential is near average for the next two weeks (through March
5th), based primarily on significantly above-average observed
stream flows, but average to below-average precipitation expected
over the next two weeks.

Current Flooding: No flooding is occurring within the HSA at this
time.

Recent Precipitation: February 5th-7th was extremely wet as 2 to
locally 4 inches of rain were observed across the James and
Greenbrier River basins, while widespread amounts of 3 to 5
inches, locally 6+ inches, were observed across the remainder of
the HSA. Numerous daily rainfall records were set during this
event and minor to moderate flooding was observed in nearly all
major river basins across the HSA.

Since then, precipitation has been close to normal, with 1 to 2
inches of mainly rain observed across the mountains, and 1.5 to
locally near 3 inches observed across the foothills and Piedmont.

January precipitation was slightly above average with 3.77 inches
across the COOP network versus the January mean of 3.32 inches or
118 percent of normal. December precipitation was also slightly
above average at 3.92 inches versus the long-term mean of 3.32
inches or 118 percent of normal. November was slightly drier
across the region with mean precipitation of 2.59 inches or 76
percent of the normal mean of 3.42 inches.

Drought: No drought conditions are currently observed or expected
to develop through March 5th within the HSA.

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

Snow cover: A weak storm passing to the south of the HSA on
February 20th is expected to bring up to 2 inches of snow, mainly
south of Highway 460. This snow will be hydrologically
insignificant and will melt by the coming weekend.

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 average streamflows at 7, 14 and
28-day durations are all indicating well above normal flows for
late February, given recent heavy rainfall and low
evapotranspiration rates.

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) as of February 20th indicate soil moisture ranging
from near normal across north-central Virginia to around a 3-inch
surplus across the Highlands of North Carolina

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, Smith Mountain Lake,
and the Western VA Water Authority reservoirs near Roanoke remain
at or near full capacity due to recent heavy rainfall and
continued high stream flows.

Future Weather Conditions: Quantitative precipitation forecasts
(QPF) from the Weather Prediction Center (WPC) over the next 7
days (through 7 AM Thursday February 27th) indicate rainfall
amounts ranging from 0.50 to 1.25 inches across the HSA, with the
highest amounts expected across the mountains and foothills of
northwest North Carolina. This precipitation will fall as a
combination of rain and snow.

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 period indicate near-
normal temperatures and slightly above-normal precipitation.
Temperatures and precipitation are expected to transition to
below-normal during the 8 to 14 day period.

Probabilistic/Ensemble river forecasts: The Meteorological Model
Ensemble River Forecasts (MMEFS) from the GEFS and NAEFS ensembles
valid through about February 26th indicate that all rivers are
expected to remain below flood stage.

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

Summary: For the Blacksburg HSA, the river flood potential is
near average for the next two weeks (through March 5th), based
primarily on current high stream flows, but average to below
average precipitation expected over the next two weeks.

$$


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