Snow tubes, stream gauges and satellites: How do experts forecast floods?

Late winter is a busy time of year for flood forecasters. It's when they gather information to predict whether Minnesota will see flooding when the snow starts to melt.

The National Weather Service is predicting a higher-than-usual risk of flooding this spring, thanks to the winter's heavy snowfall.

How does a flood outlook come together? It takes lots of agencies, lots of researchers, lots of data — and time

What are the main factors that indicate flood risk?

The biggest driver of flooding is the snowpack — the amount of snow covering the ground that's been accumulating all winter. Or, more accurately: the amount of water that snow contains.

For instance, say there's 30 inches of snow on the ground, but it contains the equivalent of 5 inches of water when it melts. That is the critical number.

Another key factor, said James Fallon, a hydrologist with the U.S. Geological Survey, is whether the ground was already saturated from autumn rains before it froze: "How wet was it before winter started in the fall? How much water is in the soil and already taking up space when everything thaws out?"

The depth of rivers and streams now, before the snow melts, is also critical because it determines how much water they can take in before they start overflowing their banks.

Another important factor: How far down is the ground frozen? The deeper the frost, the longer it will take for the ground to thaw — and be able to absorb water from the melting snow.

"Frost keeps the water from going into the ground when all this melts," Fallon said. "So, that water will then run off if it can't go into the ground."

What data goes into the flooding analysis?

Several state and federal agencies gather information that's used to assess flooding risk. The National Weather Service collects all the data and analyzes it to come up with a spring flood outlook.

The Minnesota Department of Natural Resources and the U.S. Geological Survey use stream gauges and other equipment to monitor the depth and flow of streams and rivers around the state.

Monitoring technology has improved in recent years, making it possible to collect more accurate, up-to-the-hour data.

In Minnesota, the Geological Survey has about 115 telemetry stream gauges that use remote monitoring to provide real-time data, Fallon said. The National Weather Service uses about half of those as forecast points.

The gauges measure the river levels. Staff also go out and check the velocity of the river, which allows them to calculate the river's flow rate. That information helps forecasters determine how high the river will get.

And then there's snow. Researchers from the U.S. Army Corps of Engineers measure the snowpack by traveling around the state and collecting snow cores using long tubes. They weigh the snow to determine its water content.

In addition, the National Oceanic and Atmospheric Administration uses remote sensing to measure the snowpack from the air with airplanes or satellites.

And the DNR has volunteers around the state who measure snow and report those measurements to forecasters.

How are things looking so far this year?

So far, not great — although the weather in the next few weeks will play a major role in the flooding forecast once the thaw begins.

Much of Minnesota is covered with snow 18 to 30 inches deep — with as much as 30 to 40 inches in some areas, mostly in northern Minnesota.

"One thing that makes this winter stand out a bit is that we have such a general snow cover across the entire state," said Pete Boulay, a climatologist at the DNR. "Most years, you see a cluster of heavier snow in one part of the state."

That snow contains a lot of water. Most of the snowpack around the state has the equivalent of 3 to 4 inches of water in it, said Craig Schmidt, a senior service hydrologist with the National Weather Service in Chanhassen, Minn. In some places, like the Arrowhead in northeastern Minnesota, it's as much as 6 to 10 inches.

And because the ground was already saturated from autumn rains by the time winter began, it won't likely be able to absorb much more water.

"We had a fairly wet fall, so we had quite a bit of moisture in the soil," Schmidt said. "Even once it does thaw out, it will be pretty wet, and there won't be any room for much more water to soak in."

So it will run off — to rivers and streams, many of which are already at high levels, Fallon said.

"If the rivers are already high, then there's not a lot of area left for them to go before they start going up," he said.

But the biggest unknown factor is how quickly the snow will melt.

The best-case scenario would be a slow warmup, with the temperature dropping at night to pause the melting.

"Hopefully, we get a nice, gradual warmup and we don't have any rain. Then the water can kind of leak out," Fallon said. "But we have to prepare for the worst and hope for the best."

If the weather stays cold and there's a sudden, swift thaw, that could spell trouble, said Schmidt.

"Say our temperatures suddenly rise, and they stay high for three, four or five days with warm lows at night not dropping below freezing. That would be the way to most quickly melt the snowpack," Schmidt said. "That would be probably the most severe, in terms of flooding."

Are there certain parts of MN that are more at risk of flooding?

Experts are most concerned about southern Minnesota, including the entire Minnesota River and parts of the Mississippi River from St. Paul through Red Wing.

"At this point though, we have so much water on the ground that anything could be in play," Schmidt said. "But those are probably the main ones we are watching right now."

The likelihood that a river might spill over its banks can vary greatly. It depends on the flooding factors, but also on the characteristics of the river basin, including the steepness of its banks and how far the water can spread, Schmidt said.

Land use in the river basin also plays a role. Wetlands can hold excess water after the snow melts. But if they've been filled in and paved over for development, water can flow more quickly across the land and accelerate flooding.