How do they forecast when floodwaters will crest?
CHANHASSEN, Minn. — By the weekend, several Minnesota rivers are expected to reach dangerously high levels. Some might even break records. Predicting if and when that could happen is complicated.
How do they forecast flooding? And what tools are used to measure rising rivers? Good Question.
Harriet Island Park in St. Paul better resembles Lake Harriet at the moment, as the Mississippi River swells past its banks more and more each day. The major flood threshold for that part of the river is 17 feet. As of Tuesday night, the National Weather Service was forecasting it would crest at 20.9 feet on Saturday.
Shawn Devinny, a senior meteorologist with the National Weather Service, shared what could make that number get even higher.
"Certainly how much rainfall we get between now and then could impact it," said Devinny.
When WCCO interviewed him on Monday, the Mississippi River was only expected to crest at 20.5 feet, proving that rain on Tuesday was causing it to rise.
Devinny shared how meteorologists forecast flooding.
"It includes observational data, analysis of how much rain has fallen, or in the winter how much snow is on the ground and how much of that is going to melt," said Devinny.
Meteorologists analyze upcoming precipitation, including how much will fall and where it will land. They also take into account whether the soil is dry or saturated, which right now is the latter given our extremely wet spring.
"Between all those things, we're kind of able to predict the flow of how much water is going to be flowing through the rivers at these places. And then, kind of convert that to a height that impacts the average person, like how high is the river going to get," Devinny said.
Tools help them measure exactly how high the water gets, like river gauges. Set alongside a bridge over the Crow River, the gauge works like a weather radar. It sends a beam down toward the water to calculate the water's height.
"There's also another type that is at the same location called a bubbler, which is a line underneath the ground inserted under the middle of the river channel. It uses pressure to tell how much water is above it," he said.
"It's extremely important (to have an accurate forecast). A lot of the cities and the counties are making decisions on roads to possibly close, places to block off," he said.
Forecasting also helps communities get a jump start on filling sandbags, building flood walls, and evacuating buildings.
Devinny says both floods and storm systems are difficult to predict.
"I've done both, and they're both incredibly difficult to predict," said Devinny. "I'm not gonna say one's easier than the other. They're very, very complicated and predicting the future is never easy, no matter what you're trying to do."
