Rain - or even snow - this week will change runoff rates and patterns, complicating crest predictions.
A wintry early spring storm expected Tuesday through Wednesday could raise, delay or prolong crests on rivers across Minnesota -- or perhaps all three.
"Expect things to change," said hydrologist Diane Cooper at the North Central River Forecast Center. "This is a really complex situation."
The forecast for a rain/snow mix across southern Minnesota, including possibly more than an inch of precipitation, "has really thrown a wrench into the works," Cooper said. Winter storm watches were interspersed with flood advisories and fog alerts across the northern half of Minnesota Sunday.
Flood forecasts posted Sunday showed significant rises on rivers across southern Minnesota through the week. But those forecasts did not include the coming precipitation, Cooper said.
As of Sunday, the St. Croix River was expected to rise to within inches of the level that would close the Stillwater lift bridge by the coming weekend, and the rising Mississippi was expected to prompt the closing of Warner Road in St. Paul. Similarly, some flooding was predicted for Delano Friday, while the Minnesota River at Henderson was expected to reach its seventh-highest crest late in the week.
But those forecasts will be undermined by the coming system; expect updates Monday.
The snowmelt across southern Minnesota had been gradual enough to suggest that flooding might not be as severe as feared, but conditions this week are worrisome, Cooper said. Less snow on the ground could hasten runoff from new rain. Snow would be better, since it holds water back.
Rivers across northern Minnesota remain frozen.
At the Twin Cities, the official snow depth Sunday -- despite the large piles of shoveled and plowed snow along streets and sidewalks -- was 0 for the first time since Nov. 22. St. Cloud had 2 inches; Fargo, N.D., had 7.
Twin Cities daily high temperatures this week are expected to drop from 47 Monday to near freezing through the rest of the week. That could slow some runoff, but a slowdown also can cause concern.
"The longer you wait, the greater the risk of getting a big rain event that really makes things go fast," noted Pete Speicher, forecaster at the National Weather Service office in Grand Forks, N.D.
Bill McAuliffe • 612-673-7646