North Dakota, Minnesota, South Dakota and Iowa were listed as the greatest flood threat, and the National Oceanic and Atmospheric Administration said parts of the South and East are also more susceptible than usual to high water.
A winter influenced by the El Nino phenomenon left large areas soggier than usual, NOAA said.
This year's snowpack tops last year's in many areas with as much as 10 inches of water resulting in some regions.
"We are looking at potentially historic flooding," NOAA Administrator Jane Lubchenco said at a briefing.
"It's a terrible case of deja vu, but this time the flooding will likely be more widespread. As the spring thaw melts the snowpack, saturated and frozen ground in the Midwest will exacerbate the flooding of the flat terrain and feed rising rivers and streams," Lubchenco said.
In particular, she said, flooding is expected along the Raccoon, Des Moines, James and Red rivers of the North.
Fargo set a record last year and "we'll likely see similar but more widespread flooding" this year, said National Weather Service Director Jack Hayes.
The current snowpack is among the top 10 since World War II with much of it remaining on the ground because of the cold weather, Hayes said, so rivers in the Midwest are likely to remain above flood stage for weeks or more.
Des Moines Mayor Frank Cownie said his city, still recovering from previous flooding, has closed off some low-lying roads and is bringing in extra pumps "just in case."
Gary Carter, director of hydrology for the Weather Service, said it is extremely unusual to see this amount of wetness two years in a row.
"In the South and East, where an El Nino-driven winter was very wet and white, spring flooding is more of a possibility than a certainty and will largely be dependent upon the severity and duration of additional precipitation and how fast existing snow cover melts," Hayes added.
El Nino, a periodic warming of the tropical Pacific Ocean can affect weather worldwide. It is expected to continue at least through spring.
In addition to the region of highest flood hazard, there is an above-normal chance of flooding across the south from West Texas to the Atlantic Coast and north to the southern two-thirds of New England and much of the Midwest.
Regions listed as expecting below normal flooding are northern Michigan including the Upper peninsula and adjacent areas of Wisconsin; also the Pacific Northwest, most of California, Nevada, Idaho, Montana, Wyoming and northern Utah and Colorado.
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