Cold could prevent major flooding in Red Wing
This week, the National Weather Service is predicting the Mississippi River will crest Friday morning at around 13.5 feet. That's half of a foot shy of minor flood stage.
Two weeks ago, those predictions had the river reaching well over the major flood stage of 16 feet. The reason behind the drastic change in predictions?
"Primarily it's because it was so cold. We had snow out there, and none of that water was allowed to melt and run off," said Mike Welvaert, hydrologist for the NHS La Crosse Weather Forecast Office.
The cold weather prevented the snow that's already on the ground from melting, keeping it locked on land and out of the river, Welvaert said. This allows time for the snowmelt from a few weeks ago to travel downstream, before any new snow can melt and make the river flow beyond its banks.
The making of a prediction
River level predictions are based on a lot of factors, Welvaert said. Things like soil moisture, frost in the ground, snow on the ground, future temperatures and future precipitation all play a part.
A computer simulation - based on mathematical equations and physics - then takes all of those factors into account and gives a river crest prediction.
"This is just a lot of math and physics jammed together and being done with the help of the computer," Welvaert said.
Still it takes a trained hydrologist to read and interpret the computer's output.
"Without humans, it wouldn't be very accurate," Welvaert said.
Welvaert said hydrologists have been using computers to help predict river crests since the middle part of the century. Before that, they applied the same equations and physics that computers do today. It just took longer, he said.
While the recent cold snap brought lower river forecasts, it also brought the possibility of a double crest. This means the river will reach a highpoint - in this case, probably around 13.5 feet - before going back down as those flood waters flow south. When temperatures rise again in the next few days, new snowmelt will cause the river to rise and crest again.
In 2001, the Mississippi also saw a double crest. Then, the river peaked first on April 17 at 18.1 feet. Over a week later, on April 28, it crested again at 18.2 feet. Both crests are among some of Red Wing's highest - ranking in at numbers three and four.
The current forecast indicates that this year's double crest won't come anywhere near those numbers. Still, the National Weather Service shows that there is a 90 percent chance that the second crest will reach 15 feet the week of April 13.
In 2001, spring's snowmelt was augmented by 6 inches of rain in April, causing those two record crests - an occurrence that hasn't been completely ruled out for this year.