Mississippi flood risk remains unusually high
The likelihood of a record or near-record spring flood on the Mississippi River remains unusually high, according to personnel at the National Weather Service office in La Crosse, Wis.
“It is pretty likely” that the Mississippi will equal its second highest level (recorded in 2001) at many river towns, hydrologist Mike Welvaert said Thursday.
There is a 30 percent to 40 percent chance that it will equal the all-time 1965 level, he said.
“While 40 percent wouldn’t sound all that bad if you were talking about the chance of rain tomorrow, statistically speaking that is really high when you are talking about the chance of a record flood,” Welvaert said.
In a normal year, he said, that chance would be in the 2 percent to 3 percent range.
“The longer we hang on to our snow cover (in Minnesota and Wisconsin), the greater the risk of a rapid warmup accompanied by seasonal rains” – a scenario that could feed three swollen main tributaries (the Minnesota, St. Croix and Chippewa) into the Mississippi all at once, Welvaert said.
NWS meteorologist Jeff Boyne said prevailing La Nina conditions will tend to delay the thaw of a snowpack that contains the equivalent of 4 to 6 inches of water.
“It doesn’t look like we will see a huge warmup right away,” he said
Boyne said he thinks the crest would likely occur in the April 15 to 20 range.
Because of above-average precipitation last fall and this winter, the Mississippi has remained at least a couple feet higher than its normal winter level, he said.
The severity of the 2011 spring flood, Boyne said, will depend upon the pace of the thaw and the amount of spring precipitation.
“If they come together we could approach the flood of 2001 if not the flood of 1965,” he said.
As of this evening, the river stood at 8.55 feet at Lansing with an 18-foot flood stage; 9.46 feet at McGregor with a 16-foot flood stage; and 8.45 feet at Guttenberg with a 15-foot flood stage.The 1965 record heights at those locations, respectively, are 22.5 feet, 25.38 feet and 23.65 feet.