- Associated Press - Thursday, March 17, 2011

WASHINGTON (AP) - Spring flooding has already begun and the worst is yet to come, with the greatest danger in the Northeast and Midwest, government forecasters said Thursday.

The highest flood risks are concentrated in the Upper Midwest _ particularly parts of the Dakotas, Minnesota and Montana _ and in the region around New York City including parts of New Jersey, Pennsylvania and New York State, the forecasters said.

“The stage is set for widespread, major flooding,” National Weather Service Director Jack Hayes said. “The worst is still ahead.”

Hayes told a briefing that the accumulated snowpack in the north central states is among the highest in 60 years.

Metropolitan areas that face a greater than 95 percent chance of major flooding include Fargo and Grand Forks, N.D.; St. Paul, Minn.; Davenport, Iowa; Rock Island, Ill.; and Sioux Falls and Huron, S.D., the forecasters said.

Warm temperatures could cause much of the snowpack to melt across South Dakota and southern Minnesota this week, setting off moderate to major flooding in eastern South Dakota next week, the forecasters said.

In addition, they said minor flooding could begin this week on the Mississippi River and its tributaries in southeastern Minnesota and southwestern Wisconsin, leading to moderate to major flooding by early April. The Mississippi River is already cresting at Cairo, Ill.

And a series of storm systems is expected to cross the region during the next two weeks bringing additional snow or rain on top of the remaining snowpack. That could result in the beginning of minor flooding in the southern headwater portion of the Red River of the North, eventually leading to major flooding sometime from the last week of March through early April.

According to the new forecast:

_There is a 95 percent chance that Fargo, N.D. will top the major flood stage of 30 feet, where portions of downtown begin flooding and temporary dike construction is necessary. There is approximately a 20 percent chance 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 about a 40 percent chance of Devils Lake, N.D. topping 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.

_ Moderate to major flooding is possible on the Milk River and its tributaries in northeastern Montana. Near Glasgow, Mont., the river has about a 90 percent chance of exceeding the major flood stage of 27 feet. There is already some minor ice jam flooding occurring in Montana.

_ 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.

_ There is likely to be major flooding on the Mississippi River from St. Paul, Minn. to St. Louis. St. Paul, Minn. 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.

_ Downstream, the risk of major flooding on the Mississippi in Iowa, Illinois and Missouri will persist into the spring.

_ There is a flood threat in southern New England and the Catskill Mountains as a result of October and November rain storms which left these areas with above normal soil moisture levels prior to the winter freeze. That was followed by above average snowfall, and river icing in many locations. If snowpack and river icing conditions 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.

_ It is too early to determine spring flood potential for the western states, although snowpack in the region is above average.

Overall factors contributing to the high flood dangers include heavy late summer and fall precipitation last year which left the ground saturated and streams running high before the winter freeze.

Following that there was a heavy winter snowfall resulting in deep snowpack. During the winter, temperatures were cold, keeping the heavy snow frozen so when spring warming arrives it melts and runs off into streams and rivers. The rate of spring melt and the potential for ice jams in rivers will determine the amount and location of flooding.


Online: https://www.weather.gov/oh/hic/nho/

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