BISMARCK, N.D. — The mayor of North Dakota's largest city declared an emergency Monday and asked residents to help fill 1 million sandbags in preparation of major Red River flooding.

"This is a very serious flood forecast, and we'll meet it with a serious response," Fargo Mayor Tim Mahoney said during a news conference.

The latest National Weather Service outlook says "significant" snowmelt flooding is likely this spring in the Red River Valley of eastern North Dakota and northwestern Minnesota after last week's massive late-winter storm brought heavy rain and snow to the Upper Midwest. The chance the river will reach major flood stage in Fargo has increased from 50 percent to 90 percent.

Fargo and neighboring Moorhead, Minnesota, experienced a record flood 10 years ago. The two cities with a combined population of about 165,000 residents have implemented several measures since then such as home buyouts and levees, but Mahoney said there still are areas that could be vulnerable to major flooding.

"We cannot be complacent," he said.

If the river crested 4 feet above flood stage, about 2,500 properties could be impacted, according to Nathan Boerboom, a city engineer. The weather service outlook shows a 25 percent chance of that happening. There's about a 10 percent chance of the river reaching the same level it did in 2009, but Mahoney said the city will still prepare for that level.

The 2009 flood destroyed about 100 structures and caused millions of dollars in damage. Fargo, which sits lower than Moorhead, was saved only by a massive sandbagging effort by 100,000 volunteers involving more than 7 million bags.

The river is expected to crest in Fargo in mid-April. Sandbag-filling operations begin March 26 and will last about 10 days, according to Mahoney. School students will help, as they did in 2009, and Mahoney asked other residents to again volunteer, calling on "the spirit of Fargo."

The city also is working on flood plans to handle such things as emergency sheltering, feeding of displaced people, help for vulnerable populations and traffic control.

"Quick response teams will be ready," Mahoney said, adding that about 90 percent of the city's staff has experience from the 2009 flood.

The emergency declaration will help pave the way for federal aid to reimburse the city for flood-fighting expenses.