The chance of significant flooding along the St. Croix and Mississippi rivers rose to a near certainty in the latest National Weather Service flood outlook with fresh powder this week falling on an already deep snowpack in one of the state's snowiest winters on record.
The amount of flooding will depend on rain and temperature patterns in the coming weeks, wrote meteorologist Craig Schmidt, but big snow piles this late in March mean it's becoming more likely that the first warmup of spring will let loose a torrent.
"With no strong signal as yet for April, we will be at the mercy of individual weather systems to determine whether all of that snowpack ends up causing flood problems," said Schmidt, who added in the March 9 report that he plans to issue an updated spring flood outlook in two weeks because of this year's threat.
The flood outlook has set off alarms among Stillwater city staff, with Public Works Director Shawn Sanders reporting to the City Council this week that he plans to build a dike in the city's riverside green space known as Lowell Park. Sanders said he's begun ordering sandbags, jersey barriers, poly and fencing in preparation.
The Weather Service said that, for now, the St. Croix River at Stillwater has a 91 percent chance of moderate (88 feet) flooding and a 73 percent chance of major (89 feet) flooding. (The gauge in Stillwater marks the river's depth starting at 600 feet above sea level, so 89 feet would mean the river is at 689 feet above sea level.)
The worst-ever flood in Stillwater took place over Easter in 1965, when the river crested at 94.1 feet.
Families were evacuated from riverside communities like Afton, Bayport and Lakeland while teenagers and college students home on Easter Break did much of the work behind a 4-to-8 foot dike running along railroad tracks through Lowell Park. The "Teenager's Dike," as it was known, held back the river and saved downtown Stillwater.
Since then, the river has topped 90 feet just four times, in 1969, 1997 and twice in 2001.
In other areas, the Weather Service flood outlook included the following predictions:
- The Mississippi River at St. Paul has an 81 percent chance of moderate flooding and 61 percent chance of major flooding.
- The Mississippi River at Red Wing has a 83 percent chance of moderate flooding and a 67 percent chance of major flooding.
- The Mississippi River at St. Cloud has about a 50-50 chance of moderate flooding.
- The Minnesota River at Savage has a 69 percent chance of moderate flooding and a 36 percent chance of major flooding.
Snow forecast this weekend likely will place this season's snowfall in the Twin Cities among the 10 snowiest on record.
The snowpack across much of Minnesota remains at 12 to 20 inches deep, and portions of the Arrowhead and the western border area are seeing the deepest snow on record for this time of year.
Using a measurement called the snow water equivalent, the National Weather Service estimates that much of the Minnesota and Mississippi river basins have 3 to 5 inches of snow water equivalent on the ground. The Chippewa and St. Croix basins have even more, with 6 to 7inches. That's enough to put this year in the top 10 in the past 70 years.
River levels were normal to low at the start of winter, so there's room for some snowmelt in riverways before flooding would occur. Marshes, creeks, ponds and lakes also were at very low levels in late summer, according to the Weather Service, and these areas could intercept some of the snowmelt and alleviate the burden on creeks and rivers.