Record rainfall and melting snow packs will continue to cause rivers to overflow in large areas of the country, the National Weather Service said.
The greatest flooding danger includes much of the Mississippi River basin, the Ohio River basin, the lower Missouri River basin, Pennsylvania, New Jersey, most of New York, all of New England and portions of the West, including Colorado and Idaho.
"Overall moisture is unprecedented for this time of year over an area that extends over 1,000 miles," said Doug LeComte, a meteorologist at the government's Climate Prediction Center.
Joanna Dionne, a meteorologist at the weather service's Hydrologic Services section, added that "all the ingredients are there for flooding in this broad area and up into the northeast."
"American citizens should be on high alert to flood conditions in your communities. Arm yourselves with information about how to stay safe during a flood and do not attempt to drive on flooded roadways," said Vickie Nadolski, deputy director of the weather service.
The weather service posted flood and flashfrom Texas to Pennsylvania.
Heavy rains have dumped as much as a foot of rain in the Midwest this week, leaving behind more than a dozen deaths. Rivers were cresting above flood stage in Ohio and flooding also was reported in parts of Arkansas, southern Illinois, southern Indiana, Missouri and Kentucky.
The Meramec River was still cresting as of Thursday morning, threatening nearby towns where volunteers have started sandbagging, reports CBS News correspondent Hari Sreenivasan. In downtown St. Louis, the Mississippi River isn't expected to crest until the weekend.
On Thursday morning, high water closed the eastbound lanes of Interstate 70 - a major east-west highway - for about 4 miles in central Ohio's Licking County, the State Highway Patrol said. Cincinnati picked up 4.7 inches of rain and then traces of snow on Wednesday.
LeComte noted that a La Nina, an unusual cooling of the tropical Pacific Ocean has been under way and that often leads to wetter conditions in the U.S. Midwest.
However, he added, "what's happened in the last few months has not been a typical La Nina, the jet stream's been on steroids."
The forecast models differ on whether it will continue into summer, he said, "we'll have to wait and see."
The National Oceanic and Atmospheric Administration said more than 250 communities in a dozen states are experiencing flood conditions this week.
The spring flood forecast said:
While snowfall has been normal or above normal across most of the West this winter, dryness in many areas will prevent most flooding in this region. Runoff from snow pack is expected to significantly improve stream flows compared to last year for the West.
The spring forecast also looks at drought conditions and LeComte noted that while there has been some recovery dryness continues to affect large areas of the Southeast.
Lake Lanier in Georgia, for example, has come up four or five feet, he said, but is still 12 to 14 feet below where it should be at this time of year.
Dryness is also expected to continue in Florida, at least until summer thunderstorm season.
"Overall, the Southeast had near-average rainfall during the winter with some areas wetter than average. Nevertheless, lingering water supply concerns and water restrictions continue in parts of the region," the forecast said.
The forecast called for the drought to continue in parts of the southern Plains despite some recent heavy rain.
"Parts of Texas received less than 25 percent of normal rainfall in the winter, leading 165 counties to enact burn bans by mid-March. Seasonal forecasts for warmth and dryness suggest drought will expand northward and westward this spring," the forecast said.