Flood Risk Above Average in the Northeast,
Ohio Valley, Midwest, middle and lower Mississippi Valley, Idaho, Mountains of
western Wyoming, western Colorado and New Mexico
(March 14, 2008)
Copious
precipitation, areas of substantial water held in the snow pack, wet
soils, and above normal to much above normal streamflows
have produced a risk of minor to moderate flooding across a swath from
eastern Kansas and Nebraska through much of the Mississippi Valley and its
tributaries, Indiana through to much of New York, New England, and the
northern mid Atlantic. Heavy winter snow combined with recent rain indicates
parts of Wisconsin and Illinois should see minor to moderate flooding,
with as much as a 20-30% chance of major flooding on some rivers in
southern Wisconsin and northern Illinois. An above average risk of flooding exists as well for portions of middle Rocky Mountains,
Colorado and the headwaters of the Snake River basin in Wyoming. There is substantial snowpack in
parts of the West, with water
content of the snow reaching over 150% of normal in some
areas of Utah and Colorado. Wisconsin received record amounts of snowfall
this year, and Vermont has up to a foot of water held in the snow. Parts of
northern New York and New England have 2 to 4 feet of snow on the ground. The
potential for ice jams persists in parts of northern New England. River ice in New York
is showing signs of weakening, while the ice is firmer farther north. River
ice is still very thick in Maine, with thickness up to 2.5 feet in northern
Maine. A potent storm system dropped up to 20 inches of snow across Ohio
March 7th and 8th and up to 3 inches of rain across
southern New England, contributing to the elevated risk. Recent
temperatures, particularly in the daytime, across the Midwest and Ohio
Valley have been above normal, leading to the beginning of the spring
snowmelt season. Snow melt plus recent
rainfall is causing rising river levels and some minor to moderate
flooding. Flooding is
occurring from northeastern Texas through parts of Arkansas into the Midwest
and Ohio Valley.
The flood potential from snowmelt and ice
jams this spring breakup season throughout Alaska is currently rated as
average. The March 1 ice thickness data are available for a limited number
of observing sites in Alaska. Measurements indicate that ice thickness range
from well below normal in southwest and southcentral Alaska to above normal
in the eastern interior. Accumulated freezing degree days are now between 78
and 112 percent of normal with the highest percentage in southeast Alaska.
Generally ice thicknesses are greater in areas of the state with less
snowpack. An analysis of the March 1 snowpack by the Natural Resources
Conservation Service (NRCS) indicates normal to above normal snowpack
throughout much of the western half of the state, the Kenai Peninsula, and
southeast Alaska. Some basins in southern southeast Alaska have greater than
150% of normal snowpack. The eastern half of the state including the
Susitna, Copper, Tanana, the Yukon above Tanana, upper Koyukuk and north
slope drainages all show below to well below normal snowpack. Regardless of
climate averages there is enough snow in most areas to produce significant
snowmelt runoff peaks if subjected to a rapid warming pattern.
Drought
conditions have contributed to the below average flood risk across the
Southeast and northern Plains. The continuing drought across the West has
mitigated the flood potential from the above average snow pack as much of
the runoff will either be absorbed by the
dry soils or be collected in area reservoirs. Flood risk is gauged by analyzing hydrometeorological conditions such as
precipitation, groundwater conditions, streamflow, snow conditions, river
ice, and reservoir storage.
Rainfall across the U.S. has varied from less
than 50% of normal during the past 90
days in parts of the northern and southern Plains to over 150% of normal
over much of the West and from the central Plains to New England. An active
storm pattern across the northeastern and western U.S. brought repeated
storms across these regions through December
and February, soaking the
soil and priming the areas for later flooding. Copious rainfall over this
same period mitigated the effects of long standing drought in parts of the
Southeast. Recent
rainfall across the drought stricken Southeast has continued to ease some of the
dryness, but has not elevated the flood risk.
Temperatures varied this winter, with unusual
warmth across much of the eastern U.S. and northern Plains in December
which continued into January
across the northern states. This warmth contributed to much of the snowpack
in the lower Midwest melting through the winter, leaving little on the ground
as of March 7.
Colder than normal temperatures spread across the Plains and Midwest in February,
allowed an enhanced snowpack to return to the Midwest. Frost depths across the north
central U.S. range from 1 to 3 feet in the most northern areas. The
frost depths are as deep as they are because there has been less than normal
snowfall over much of
Minnesota
and
North Dakota
this winter. Chances of flooding
are low in these areas because of the lack of snow.
However, if they were to receive late season snows and rain during
snowmelt then the frozen ground will enhance runoff rates and lead to
flooding. The recent March storms across the eastern U.S. added
substantial snow across the Midwest, with up to 20 inches in Columbus, Ohio.
Where the precipitation fell as rain, it helped saturate the soil, increase
streamflow, and add water to the water content of the snow pack.
Plentiful rainfall has resulted in very wet
soils from the central Plains to New England as well as portions of the
West. This wetness is reflected in
the Palmer Drought Index, as well as soil
moisture conditions used by the National Centers for Environmental
Prediction in their forecast models. These conditions have produced nearly
saturated soils, and any additional rain will run off quickly and result in
flooding. Below average precipitation across the northern and southern
Plains as well as the Southwest combined with long term dryness and drought led to drying and very dry soils, with reduced flood risk in these
areas.
Rivers in a
swath from eastern Kansas, across the Midwest into the Ohio River Basin to much of
southern New England and parts of the southern Appalachians are running above normal. Much above normal precipitation
across the Northeast during February resulted in much above normal stream
flows, with many sites across southern New England and southeastern New York
setting new record daily flows for March 13. Rivers across much of the rest
of the nation are running at normal to below normal levels for this time of
year. River ice is widespread across the Northeast, with some softening
noted in southern areas. River ice is still quite thick and widespread in
Maine and the northern tier of states. Most rivers across interior Maine are nearly 100 percent
ice covered with thickness ranged from 0.5 to 1.5 feet. Rivers across the
northern tier of states contain ice, but few ice jams have been noted. The
Rock River in Illinois had ice jams along much of the river, exacerbating
flooding and pushed the river to record levels in the Moline Area. Ice jams
have also been common in southern Iowa, northern and central Illinois, and
southern Michigan during the past month. Ice jams are likely as frozen rivers and streams thaw
through the spring.
As a result of the plentiful precipitation
this winter across the Northeast, water
supply reservoirs across the region are generally at or above target levels
for early March. The New York City water supply was at 99.5 percent as of
March 14, which is 12 percent above normal. Other reservoirs are at or above
normal pool levels. The Scituate reservoir which provides the drinking water
for northern Rhode Island has reached 97 percent of capacity.
Across the Southeast, drought affected
reservoirs have shown recent improvement. In Virginia, the
Carvins Cove reservoir, a major source for Roanoke drinking water, is now
15.7
feet below the spillway. By way of comparison the water level fell to 34.1
feet below the spillway at the height of the 2002 drought. In contrast, pool levels of
most major reservoirs in north and central Georgia are running near target
levels for this time of year except for Lake Lanier and Carters Lake. It is
during the spring that many operators plan to fill their reservoirs to
summer pool levels. Normal rainfall could take care of that situation.
However, above normal rainfall would be needed for Lake Lanier and Carters
Lake to fully recharge. In western South Carolina, the levels of Lakes
Hartwell and Russell are near the record low levels.
The water supply outlook across the West has improved
this year compared to last. The majority of
mountain snowpack is average or above
as is the total precipitation
since October 1, providing the potential for maintaining water supply levels
or seeing some improvement. The water
contained in the snow is also ranging general above average as well. Reservoir
storage as of March 1 shows the effect of the persistent drought, with
most levels below average. So the potential of somewhat above average streamflow
will only
provide limited improvements. Of the four major irrigation reservoirs
in Montana, Lima Reservoir had 125 percent average storage, Clark Canyon was
holding 74 percent of average water, Gibson Reservoir had 47 percent of
average stored water, and Fresno Reservoir had 55 percent of average stored
water. The major hydroelectric reservoirs in Montana (Canyon Ferry and Fort
Peck) had end of February storage in the average to below average range; 93
and 58 percent of average water, respectively. End of February reservoir
storage in Wyoming was 68 percent of average in Boysen Reservoir and 111
percent of average in Buffalo Bill Reservoir. Month-end storage was 101
percent of average in Bighorn Reservoir on the Wyoming/Montana border. Most of the annual streamflow in the western
U.S. originates as snowfall that accumulates in the mountains during the
winter and early spring. This snow accumulation can continue into April. If March and early April have below normal
precipitation, the snowpack could fall back to sub normal levels.
In Summary; recent heavy rain over saturated
soils has caused ongoing minor flooding in parts of the Midwest and southeastern
New England. Long term above normal precipitation combined with areas of
deep snowpack have resulted in a broad swath of above average flood risk
from the central Plains and Arkansas into Wisconsin and across the Midwest
and Ohio Valley to New England and the northern mid Atlantic. In southern
Wisconsin there is a better than 50% chance of major flooding which
contributes significantly to the better than 50% chance of moderate flooding
along the Mississippi River. Above average
snowpack for this time of year across much of the West should help mitigate
some of the drought conditions, resulting in an average to below average
flood risk. The exceptions are some portions of middle Rocky Mountains,
southern Colorado, the headwaters of the Snake River basin in Wyoming where
an above average risk of snowmelt flooding exists. Drought conditions, dry
soils and subnormal precipitation led to a below average risk of flooding
across the northern Plains, western Texas and the Southeast.
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 and ice jams can cause rapid
water level rises on rivers with significant ice accumulation.
For current flood information:
http://www.noaawatch.gov/floods.php)
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