High-water risk rises even higher

Every passing snowstorm is raising the already-serious risks of springtime floods across Minnesota.

Chances of river levels reaching into the top five on record nearly doubled from 40 percent to more than 80 percent at Stillwater, where the Stillwater Bridge is almost certain to be closed. Chances also jumped to better than 80 percent at St. Paul and Jordan in updates released Thursday. That was thanks to up to 20 inches of snow from a Presidents' Day storm, the forecast for more next week, and the turn of the calendar toward thaw and thunderstorm season.

"We're now starting to get into a warmer time of the year, where the snow could melt quicker, and we have more rains," said Dan Luna, meteorologist in charge of the National Weather Service Twin Cities office. That means the risks of historic or record flooding, "can't go much higher," Luna said.

The Feb. 20-21 snowstorm steered south of the Red River Valley, so the risk of major flooding in the southern Red River Valley, including the Fargo-Moorhead area, has increased only slightly since the last update, Feb. 17. Downstream on the Red and along its tributaries farther north, the predicted flood risk has dropped at Grand Forks and Crookston.

Even so, the agency says there's about a 35 percent chance the river will exceed the record level posted at Fargo in 2009, when the river crested at just under 41 feet. There's an 80 percent chance of matching last year's crest of just under 37 feet, the sixth-highest flood on record.

Roughly 4 to 6 inches of water, accumulated over nearly four thaw-free months, remains stored in snow across much of the Minnesota landscape. Six to 8 inches rests in a 60-mile-long band along the Minnesota River, an area that bore the brunt of the Presidents' Day storm. Some areas near Marshall, Minn., are holding 8 to 10 inches of water.

The good news is that it's not quite like a 6-inch rain ready to fall, said Diane Cooper, a hydrologist with the North Central River Forecast Center in Chanhassen.

But a quick warmup could release a lot of that water in a hurry into streams and rivers. Rain on top of that could compound the problem, Cooper said.

The Red River Valley has less "snow-water equivalent" than other parts of the state, but that only means slightly lower risks of landmark flooding. That's because the Red River's channel is far smaller than the Minnesota's or the Mississippi's and the surrounding landscape is so flat, Cooper noted.

In southeastern Minnesota, ravaged by flash floods after an 8-inch downpour in September, about 3 inches of water is in the snow cover. But in Zumbro Falls, where the Zumbro River flooded every building downtown in September, the river has about a 3 percent chance of reaching that height again, with a 39 percent chance of "major" flooding, said Mike Welavaert, hydrologist for the National Weather Service La Crosse, Wis., office.

Last year's snowmelt was nearly ideal, Cooper added, since it developed southeast to northwest, allowing the downstream areas along the Minnesota and the Mississippi to carry floodwaters away early and make room for more. But the more normal course is for a west-to-east thaw, which allows snowmelt to converge into higher crests. Last March was also virtually snow-free.

Serious flooding in cities along rivers in Ohio this week might offer some kind of preview of a worst-case scenario to the Upper Midwest. Temperatures across the region leapt into the 50s and 60s last weekend, and thunderstorms dumped heavy rain on top of snow that was holding 2 to 3 inches of water, leading to serious flooding.

Minneapolis has a low flood risk because the river flows through a steep gorge and along park and industrial land. Much of the Mississippi from the Twin Cities to its headwaters, runs through areas rich with bogs and lakes, including reservoirs, which store floodwater.

The Associated Press contributed to this report. Bill McAuliffe • 612-673-7646