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Lower than Normal Flood Risk
Drought Concerns in the West
(as of March 12, 2004, updated March 19 for
Northeast)
For the most part, Winter has been kind from a flooding
perspective. The chance of flooding across much of the U.S. is less than normal
as spring approaches. The flood risk is above average only in northeastern Montana,
the upper peninsula of Michigan, northern Ohio, northern Maine, and parts of the
South. Another area of concern is the burn areas in southern California,
where flooding and mudslides could occur with intense rains. River flood
risk is gauged by analyzing
hydrometeorological conditions such as precipitation, soil moisture and ground
cover conditions, streamflow, snowpack, river ice, and reservoir storage.
Dry conditions and drought have plagued much of the
interior West for several years. In the West winter snows in many
areas are the source of year-round water supplies. The streamflow expected to result from
melting snow is running from average to less than half of what typically occurs
across the West. Much of this runoff will be captured in reservoirs to
provide water supplies throughout the dry summer and fall.
Flood Risk
Flooding is a concern on the Milk River and many of its tributaries in eastern
Montana. Flooding is also anticipated on the Poplar River in eastern Montana. Minor to
moderate flooding is expected on the Missouri River and other tributaries downstream of the
confluence with the Milk River in eastern Montana and northwestern North Dakota.
In eastern Montana, the snow
water content is generally in the 2 to 6 inch range with a few reports
of over 8 inches. In these areas, runoff and river flows are slow
because of the relatively flat terrain. A continuation of recent warm
temperatures, especially if accompanied by rain makes the flood risk above average.
There
is a heavy snowpack of 2 to 3 feet in the upper peninsula of Michigan, holding up to 10 inches
of
water. Frost depth of the soil ranges from 2 to 3 feet. These frozen
soils will allow snowmelt to runoff quickly if there is a rapid warm-up. This presents a flood risk. Recent
snowfall has left 10 to 15 inches of new snow across northern Ohio and
northwestern Pennsylvania, creating an above average risk of flooding.
Cold temperatures
over the winter in the Northeast resulted in
thick ice on rivers across most of northern New York and northern New England.
River ice 1 to 2 feet thick is found on mainstem rivers in northern Maine.
The potential for ice jam flooding
in northern Maine is above
average The threat of ice jam flooding
will persist as long as there continues to be a substantial ice cover and will peak later in the spring as
temperatures warm. Significant snowpack across
the Northeast is confined to northern New York and northern New England.
Depths reach
as much as 20 to 30 inches in higher elevations of the Adirondack Mountains in
northern New York, the Green Mountains in northern Vermont and the
headwaters of the Connecticut River in northern New Hampshire. This snowpack holds as much as
10 inches of
water and is another factor contributing to the above average flood risk.
Rivers from the Northeast southwestward to the Gulf Coast are generally running near to
above normal levels for this
time of year. Due to wet soils
and high river levels, portions of Louisiana and Mississippi
are deemed to have an
above average risk of flooding.
Warmer conditions and moderate snows over the winter in the upper Midwest
make this area less prone than normal to spring snowmelt flooding. The flood risk is below average from the Southwest to the Plains and Great
Lakes region, areas which have been experiencing drought conditions.
Across Nevada and the eastern Sierra Nevada, as well as
much of the central Rockies, precipitation totals have been near to above
average for the last 90 days, but the area is still suffering
from a multi-year drought, as indicated by the U.S.
Drought Monitor. The snowpack
was running near to somewhat above average for
early March, but recent very warm conditions caused rapid erosion of the snow
pack. These factors combine to indicate a below average
spring snowmelt flood risk. Across Arizona and parts of New Mexico, precipitation has ranged from
25 to 75% of normal for the last 90 days, contributing to the below average flood
risk for this Spring.
The flood risk across
Alaska is estimated to be average for this Spring as well. Both snow
cover and ice thickness are running between normal and below normal. March
temperatures have been seasonal, and the flood risk hinges on the timing of
warmer conditions. An early warm-up will reduce the flood risk, while cold conditions in April and early May followed
by a rapid warming could cause rapid snowmelt. Quickly rising streams could
cause flooding and ice jams that would exacerbate flooding.
It is important to remember that heavy rainfall
at any time can lead to river flooding, even when overall river flood
potential is considered below average.
Water Supplies
For the interior West -- from Idaho and Montana in the north, to Arizona and
New Mexico in the south -- March 1 snowmelt runoff forecasts ranged from about normal in the
north to less than 70% of normal in the south, with many basins in Arizona
expected to see less than half of normal runoff. However, recent very
warm temperatures are causing unusually early snowmelt in many areas and April 1
streamflow forecasts are likely to be revised downward. As of March 1st, reservoir
storage across the West shows the influence of the multi-year drought.
With reservoir levels in
Arizona and New Mexico running at less than a third of capacity, it will be
difficult to meet all water needs in these states this year. While states to
the north will be in better shape, water managers will still face
significant problems in allocating this precious resource.
Snowmelt runoff in California, Nevada, Oregon and Utah will be near or
above normal in most basins. This runoff, combined with current reservoir
levels means that water supplies in Washington, Oregon and especially
California should be at least adequate in most areas this year. In spite of
normal runoff in Nevada, there is continuing concern because reservoirs
are
less than a quarter full there.
Rainfall across the northeastern U.S. has been plentiful
during the past year. For the 90
days from November through January, precipitation totals
were as much as 150% of normal across much of this
region and 175% of normal in parts of the upper Ohio Valley.
Conditions in February and so far in March
have been drier and warmer. After two years of drought with significant
water supply deficits in the East, the substantial rains have replenished
reservoirs, which are now spilling the excess water. This includes the large
reservoirs in upstate New York which provide drinking water to New York
City. The New York City and New Jersey water supply systems are
currently 7 to 10 percent above normal storage capacity.
Joanna Dionne
Hydrologic Information Center
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