Flood Risk High and Flooding Imminent for the Red River of
the North Basin; Above Average Risk for Upper Midwest and from the lower Great Lakes to Illinois and part of New
England
(March 13, 2009)
General Assessment
Copious precipitation
during the fall,
wet soils before freeze-up, and areas of substantial water in the snow
pack have produced an imminent risk of major flooding along the Red River of the
North and a risk of minor to moderate flooding in much of eastern North
Dakota and northern Minnesota. Major overland flooding is expected in
eastern North Dakota and northwest Minnesota beginning the week of Sunday,
March 22. This will subsequently lead to major, and possibly record,
flooding along the Red River of the North and its tributaries. During the previous 6 months numerous
locations have received above normal precipitation. Heavy rainfall (in February
and early March) falling on
already saturated soils caused recent flooding and very high flows on rivers across
southern Michigan, northern sections of Ohio, Indiana and Illinois.
These conditions make future flooding more likely in this area. Areas
of the Nation suffering from drought with accompanying low
soil moisture and sub normal to non existent snowpack have a below average
risk of flooding this Spring. The
risk of flooding is gauged by assessing the cumulative impact of
hydrometeorological factors such as precipitation, groundwater conditions,
streamflow, snow conditions, river ice, and reservoir storage. Areas of the West, with sub normal
snowpack and dry soils, are expected to have below normal streamflows,
negatively affecting the water supply across the region.
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.
Special note: After the end date of
this assessment, temperatures across the upper Midwest rose significantly, resulting
in numerous flood advisories for melting snow and potential ice jams.
Specific Areas of Above Average Flood
Risk
Upper Midwest
Based upon forecasts produced by National Weather
Service field offices using the Advanced Hydrologic Prediction Service (AHPS),
significant spring
flooding is expected across the upper Midwest, including the
Red
River Basin
. A sharp cold snap immediately
following late fall rains, which produced 200 to 400% of normal
precipitation for October and November, created saturated soils and a deeply
frozen surface before winter snows fell. The water content
of the snow pack ranges from 3 to 6 inches across much of the region. These
conditions will enhance runoff when the snow pack melts, increasing the risk
of flooding.
Major flooding
is expected along the entire Red River of the North, with major to near
record flooding expected at Fargo, ND and major flooding in the Grand Forks
area. With flood severity in the basin contingent upon
the timing and rate of snow melt, tributaries such as the
Sheyenne River and Wild Rice River in North Dakota are expected to have
major to near record flooding. Major flooding also remains likely along
the James River in northern South Dakota.The temperature
of the snow pack is still very cold, which indicates that it has not yet ripened, delaying the
onset of snowmelt. As Spring progresses, the chances of a
rapid warm up increase, thereby increasing the likelihood of a rapid and
continuous melt and the risk and severity of flooding. Colder than normal
temperatures this winter along with high river flows at the time of
freeze-up have generated a thick ice cover which heightens the risk of ice
jam flooding on the Red River and its tributaries. In addition,
widespread overland flooding is expected due to frozen drainage networks.
Extended-range AHPS-based forecasts indicate that
Devils Lake has a nearly 100% chance of establishing a new record high water
level, exceeding the record of 1,449.2 feet set in May, 2006.
Moreover, there is a 65% chance the water level at
Devils
Lake
will reach 1,451.2 feet. The permanent dike that protects the city of
Devils
Lake
provides protection up to a lake level of 1,454 feet. At
Fargo
,
ND
, there is a 100% chance the Red River will exceed the major flood level of
30 feet, which is 12 feet above flood stage. There is also close to a 25% chance of exceeding 40.1 feet, the
record flood level set at
Fargo
on April 7, 1997. At
Grand Forks
, the
Red River
has a 100% chance of exceeding the major flood level of 46 feet or 18 feet
above flood stage. The top of the
Grand Forks
dike system, built after the 1997
Red River
flooding, is at a level of 60 feet. The Red River at
Grand Forks
has a 10% chance of exceeding 54.35 feet, the record flood level set on
April 22, 1997. These outlooks
are subject to change based on the timing of the Spring melt combined with
future precipitation and any river ice-jam flooding.
Lower Great Lakes to Illinois and part of
New England
Wet
soils combined with up to 150% of normal winter
precipitation, sub normal winter
temperatures and frozen grounds have produced an area with an above
average risk of Spring flooding from the southern Great Lakes into
Illinois. Streamflows are
running well above historical averages and a number of locations are already
near or
above flood stage, setting the stage for additional flooding.
A significant snowpack still exists across
much of northern New York and the rest of New England. An above average area
of flood risk exists across a portions of east central New York into southern
Vermont and New Hampshire.
Alaskan Potential
The flood potential from snowmelt and ice jams this spring breakup
season throughout Alaska is currently rated as average. The March ice thickness data are available for a limited number of observing
sites in Alaska. Measurements indicate that ice thickness ranges from below normal on the the Yukon at Nulato and the Tanana at Fairbanks
to above normal at the Yukon River at Eagle. Accumulated freezing degree days are generally between 100 and 120 percent of normal over most of
mainland Alaska. On the North Slope they are around 84 percent of normal while southeast Alaska has values ranging from 146 to 164 percent of
normal. An analysis of the March 1 snowpack by the Natural Resources Conservation Service (NRCS) indicates an above normal snowpack throughout
much of mainland Alaska. Exceptions include below normal snowpack in Southwest Alaska, the Matanuska/Susitna basin and the North
Slope drainages while the Kenai Peninsula is below to well below normal. Above normal snowpack is reported in much of southeast Alaska. There is
enough snow in most areas to produce significant snowmelt runoff peaks and potential flooding if subjected to a rapid warming pattern.
Temperature and Frost Depth
Temperatures this winter have been variable
across the United States. The north central states saw temperatures 5 to 6 degrees
below normal in December and January,
with the deep cold extending eastward across the northeastern quarter of the
Nation in January. Above average temperatures across the West, High Plains
and into the central and southern Plains reached as much as 4 degrees above normal
for the winter. February
warmth across the Northeast reduced the amount of snow cover in southern
New England and western and southern New York. It also contributed to
melting much of the snowpack across the southern Great Lakes into the
Midwest, generating the ongoing minor to moderate river flooding and creating an above
average risk for Spring flooding. The warm temperatures
from parts of Montana through the central and southern Plains have reduced the
extent of snow cover and the amount of water held in the snow, contributing
to the lower than average risk of flooding. Frost depths across the north
central U.S. range from 2 to more than 3 feet deep in eastern North Dakota as a result
of saturated soils preceding the freezeup in November and December. Depths
run from 1 to 2 feet in parts of the southern Great Lakes states to 2 to 4 feet in
northern sections. Frost depths in southeastern Michigan are less than a
foot due to early season snow insulating the ground.
Precipitation and Snow Conditions
Precipitation across the U.S. has varied from 50%
or less of normal during the past
90 days across the central and southern Plains and Southeast to in excess of 150% of normal in parts of the northern and
central Rockies and the upper Missouri Valley. Substantial Fall
precipitation across the northern and central Plains into the Great Lakes
resulted in very wet soils which froze going into the winter. Several warm, wet storm
systems in mid February through early March brought heavy rainfall across Illinois, Indiana,
northern Ohio, northwest Pennsylvania and western New York. These systems,
with accompanying snow melt and ice jamming, produced widespread flooding across
the region. A late season storm in
early March deposited from 4 to 6
inches of precipitation across the Midwest and southern Great
Lakes. Winter storms during March also deposited substantial amounts of precipitation
across parts of northern California. A preliminary
graphic of the 5 month precipitation
(October 2008 - February 2009) across the West shows above normal precipitation throughout the higher
elevations of southeast
California, southern
Nevada
and over the western High Plains including much of
Montana. Precipitation is well below normal across much of central and
northern
California, eastern
Oregon, southwestern
Idaho, southeast
Arizona, and southern
New Mexico.
Snowpack as of March 1st across the West is
above the long term average over much of the Rockies and below normal across
much of the west coast states. Although some basins in the
Sierra Nevada Mountains saw impressive gains to the snowpack from a wet storm
system that lasted from March 1st through March 4th, snowpacks remain below
average. Across the northern tier of states, snowpack
is substantial with 4 to 6 inches of water
held in the snowpack. In the Northeast, substantial snowpack
is confined to a portion of eastern New York, New Hampshire and Vermont where the risk of flooding is above normal.
Soil Moisture and Drought Conditions
Plentiful rainfall has resulted in very wet
soils from the northern and central Plains, across the middle Mississippi
Valley and across the southern Great Lakes as well as parts of northern New
England. The wetness is reflected in the Palmer
Drought Severity Index as well as soil
moisture conditions. This wetness can enhance runoff, especially in
areas with ample snowcover. Dry soil conditions across the southern Plains
to the Southeast as well as areas west of the Rockies combined with above
normal temperatures and substantially below normal precipitation to create a
reduced flood risk. Drought
conditions have contributed to the below average flood risk across the southern
tier of states and much of the West.
River Conditions
Rivers in a
swath from New England across the southern Great Lakes into Illinois and
eastern Iowa and Missouri are running above normal as of March 13th. Warm temperatures
combined with rain from the Midwest to the mid Atlantic and southern New England caused snow melt in mid February which generated above average streamflows
across this area. River ice in much of southern New England and the
Midwest
was eliminated due to these recent warm temperatures and rainfall.
However, river ice is expected to be a problem in the
Upper Midwest
as ice is unusually thick this Spring due to the colder than normal Winter.
This will lead to increased chances of ice jam flooding on the Red River of
the North and its tributaries. River ice also persists across northern
New York
and northern
New England, creating the potential for ice jam flooding in the region.
Water Supply
The water supply
outlook as of March 1st across the West is significantly lessened
from that of the same time last year. Mountain
snowpack is variable, with significant surpluses in scattered basins
across the West and over the interior and panhandle of Alaska. Large
deficits dominate much of the Upper Columbia River basin and the southern
Arizona-New Mexico border. Precipitation during fall
and winter was generally below normal. The map of water
contained in the snow shows large areas of sub normal content.
Reservoir
storage as of March 1st shows the effect of persistent drought, with
most levels below average. The anticipated sub-average streamflows will not
improve the drought situation. 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.
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 mid March. Groundwater monitoring wells across New England and New
York state show levels near to above normal. Lakes and reservoirs are near
or above target levels for mid March. As of March 3rd, the New York
City water supply system was at 85.8 percent of capacity, which is 1.6
percent above normal. Water supply is not anticipated to be an issue going
into spring for this area. Across the Mid Atlantic, water supply conditions
are generally close to average. In the Southeast, reservoir conditions are
near normal except for portions of northeast Georgia and western South
Carolina. Lake Lanier, Lake Hartwell and Lake Thurmond are running very low
even though they have seen some beneficial rainfall lately.
Summary
Early season
heavy rain saturated soils which froze deeply before snow fell across the
northern Plains, combined with substantial snowpack, has created an area of
above average flood risk. Major overland flooding is expected in eastern
North Dakota and northwest Minnesota beginning the week of Sunday, March 22.
This will subsequently lead to major, and possibly record, flooding along
the Red River of the North and its tributaries. There is a 100% chance that the Red River at
Fargo ND
will exceed the major flood level of 30 feet, and close to a 33% chance of
exceeding 39.57 feet, the record flood level set on April 17, 1997. At
Grand Forks, the
Red River
has a 100% chance of exceeding the major flood level of 46 feet. The top of
the permanent Grand Forks
dike system, built after the 1997
Red River
flooding, is at a level of 60 feet. The
Red River
has a 10% chance of exceeding 54.35 feet, the record flood level set on
April 22, 1997. Deeply frozen
rivers in the region have created an above average risk of ice jam flooding.
Major flooding also remains likely along the James River in northern South
Dakota. Heavy rains and saturated soils has caused flooding on rivers from
Illinois
to the lower
Great Lakes
region. Additional heavy spring rainfall is likely to cause renewed
flooding. Elsewhere, substantial snowpack across an area of eastern
New York
and southern
New Hampshire
and
Vermont
has created a greater than average risk of flooding this Spring.
Ongoing drought conditions and below average snowpack led to a below average
risk of flooding across portions of the West, as well as portions of the
Southwest into southern
New Mexico
and
Texas
. In addition, generally dry conditions have yielded a lowered risk of
flooding across parts of the Southeast and upper Great Lakes.
For current flood information:
http://www.weather.gov/ahps
http://www.noaawatch.gov/floods.php
Joanna Dionne
Hydrologic Information Center
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