Explainer: Spring flood forecast
"A large swath of the North Central United States is at risk of moderate to major flooding this spring," the National Weather Service said in its latest forecast on Feb. 24. Below are the scenarios by region.
North Central U.S.: above average
Heavy late summer and autumn precipitation (twice the normal amount since October in parts of North Dakota and Minnesota) have left soils saturated and streams running high before the winter freeze-up. NWS models show this year’s snowpack contains a water content ranked among the highest of the last 60 years.
The combination put a large portion of the North Central United States at risk of moderate to major flooding this spring, extending from northeastern Montana through western Wisconsin and along the Mississippi River south to St. Louis.
Information provided by NOAA on February 17, 2011, indicated Fargo, N.D., has a near 95 percent chance of exceeding major flood stage of 30 feet. At a stage of 30 feet, portions of downtown Fargo begin flooding and temporary dike construction is necessary. Approximately a 20 percent chance exists of reaching or exceeding the 40.8 foot record set in 2009. Grand Forks, N.D., has about a 95 percent chance of exceeding major flood stage of 46 feet. There is approximately a 40 percent chance of Devils Lake, N.D., exceeding 1,455 feet, which could partially inundate portions of the town of Minnewauken, including critical infrastructure and roads across the lake, emergency service routes and possibly a small section of the Amtrak train line.
There is potential for moderate to major flooding on the Milk River and its tributaries in northeastern Montana. The Milk River near Glasgow Montana has about a 90 percent chance of exceeding the major flood stage of 27 feet. Some minor ice jam flooding is already occurring in Montana; additional flooding resulting from ice jams is likely throughout the late winter and early spring.
The James River at Huron, S.D., has about a 90 percent chance of exceeding major flood stage of 15 feet and a 30 percent chance of exceeding the record 21.2 foot level set in 1997. The Big Sioux River at Brookings, S.D., has a greater than 95 percent chance of exceeding major flood stage of 12 feet and about a 30 percent chance of exceeding the 14.77-foot record set in 1969.
The Mississippi River is likely to see major flooding from its headwaters near St. Paul, Minnesota, downstream to St. Louis. St. Paul, MN., has about a 95 percent chance of exceeding major flood stage of 17 feet, where secondary flood walls are deployed to protect the St. Paul Airport. Further downstream, the risk of major flooding on the Mississippi (Iowa, Illinois and Missouri borders) will persist into the spring. Much of that region’s snowpack typically accumulates later in the winter. The quantity of spring rains and late-season snow will determine the magnitude of flooding in the Middle Mississippi Valley.
Northeast: above average
There is a small area of above average flood risk in portions of the Northeast, primarily across Southern New England and the Catskills Mountains in N.Y. state. As a result of October and November rain storms, these regions had above normal soil moisture levels prior to the winter freeze, followed by above average snowfall, and river icing in many locations.
If snowpack and river icing conditions were to persist beyond mid-March, this area could have an elevated risk of spring flooding during the melt period, especially if heavy rains fall during the melt.
Southern plains: below average
Fall and winter precipitation over Texas and New Mexico was significantly below average, ranging from 20 to 75 percent of normal from October 2010 to mid-February 2011. Portions of the Pecos River and the Rio Grande basins received as little as 10 percent of normal rainfall. Soil Moisture Analysis by the Climate Prediction Center show drier than normal soils from the surface to as deep as 2 meters.
This deficit will minimize the amount of water that can be converted to river flows during any rainstorm. Current stream flow conditions as measured by the US Geological Survey range from near average too much below average for stations across this region.
Mid-Atlantic, Southeast: below average
Fall and winter precipitation over the Mid Atlantic and Southeast ranged from 50 to 75 percent of average for this period. Isolated portions of South and North Carolina only received between 25 and 50 percent of normal precipitation. Therefore, soil moisture is well below normal across most of the Southeastern US and the Mid-Atlantic.
Deficits in the precipitation and soil moisture water contents translate into below average stream flow conditions for much of the region and a below average flood risk for the spring.
West: no forecast yet
Late February is too early to determine spring flooding potential across the Western U.S. Much of the snowfall which determines spring runoff in the mountain west accumulates during the remainder of the winter and spring.
Snowpack remains above and much above average in many regions. However, extreme high temperature can lead to elevated melt rates at any time during spring. There is still ample time left in the accumulation period for the spring flood potential to change.
Reuters contributed to this report.