Flood crest predictions were revised downward slightly Thursday but preparations continue for what could be Minnesota's third siege of near-record high rivers in 12 months.

Northbound Interstate 35W near the Minnesota River was reduced to one lane Thursday night to allow workers to start building a temporary dike on the shoulder to protect from floodwaters expected to crest next midweek. The work between Black Dog and Cliff Roads was scheduled to resume after Friday morning's rush hour and all lanes should reopen later in the day. Heavy rain and runoff could further narrow the road, officials said.

In St. Paul, next week's predicted Mississippi River crest is now both 2 1/2 feet lower and two days later than earlier forecasts from the National Weather Service. That would be about 6 inches above its 2010 crest and the eighth-highest ever, but 7.5 feet below the record set in 1965.

Still, St. Paul emergency operations manager Rick Larkin said he believes that up to a foot of new snow in central Minnesota, which will drain into the Mississippi River, will likely lead to a second, later peak at St. Paul.

"I expect it's going to be up for a longer period of time and extremely sensitive to precipitation and warmer temperatures," Larkin said. "I'm very pleased at seeing the lower crest. I'm concerned, though, that with the cold weather, people could drop their guard."

One of St. Paul's best-known weather-related landmarks, its main impound lot, was scheduled to be closed Friday in advance of the high water. About 550 already towed cars will be moved from the main lot at 850 Barge Channel Rd., close to the river near St. Paul Downtown Airport, to the city's "snow" lot at 1129 Cathlin Av. The operation should take about two days.

In Stillwater Thursday, officials decided to start on a "modified levee" that should be finished by this weekend.

"During the spring flooding season, the St. Croix River has historically been one of the last rivers in the state to crest," said John Gannaway, the Stillwater chief of police. "We anticipate a second crest, but as [for] what level and what time frame, we are not sure."

The Weather Service's North Central River Forecast Center is expected to release a comprehensive flood update Friday for Minnesota's rivers, including the Red. Flooding is expected to affect up to 50 of the state's 87 counties, officials said.

"We're going to be in for a long haul," said Wade Setter, deputy director of Homeland Security and Emergency Management for the state Department of Public Safety, which opened an emergency operations center Thursday. It will post flood information online at www.eoc.state.mn.us.

In addition to the I-35W dike, MnDOT workers also planned to build a dike along both sides of Hwy. 101 in Savage early Friday. The 250-foot dike between Zinran Avenue and Hwy. 13 will reduce the eastbound portion of the highway to one lane. The 2,000-foot dike on 35W between Cliff and Black Dog Roads will include concrete Jersey barriers and a line of large, expandable, sand-filled "Hesco barriers" developed by the military for protection and being used in the metro area apparently for the first time. They were used in Fargo-Moorhead in 2009 and 2010. The dike system also will involve submerged barrels and pumps to deal with runoff from rain.

The Associated Press contributed to this report. mcaul@startribune.com • 612-673-7646 paul.walsh@startribune.com • 612-673-4482