Fargo Mayor Tim Mahoney pulled out a big three-ring binder after forecasters delivered the bad news on Thursday — winter storms coming this weekend and next week are raising the odds of major spring flooding there and in river towns throughout Minnesota.

Like Mahoney, who was calculating how many sandbags will be needed to protect homes and businesses, leaders of towns from the Red River Valley down to those along the banks of the Mississippi, Minnesota, Crow, St. Croix and Cannon rivers are bracing for spring flooding after the National Weather Service’s latest update.

Last week, forecasters warned of a higher-than-usual chance of spring flooding. On Thursday, they upped the odds for major flooding in many areas.

Over the past two weeks, steady snowfall has added another inch of water to the deep snowpack that blankets most of the state, according to Craig Schmidt, senior hydrologist with the National Weather Service in Chanhassen.

The heavy, wet snow expected to fall this weekend and a storm next week that may deliver rain and snow will add even more, he said.

Cold weather to date has kept much of the snowpack from melting, leaving more snow on the ground heading into the spring thaw.

“This year has been different because a lot of times it would snow early in the year and then we would lose some. Snow some more, lose some,” Schmidt said. “Every flake that’s fallen is pretty much still out there.”

To calculate how much, weather observers across the state trudge outside to collect core samples of snow in a tube, measuring the snowpack’s depth, then melting it to see how much water it will produce. The water from the snowpack amounts to 2½ to 4½ inches across a large swath of the state, with some areas reporting 5 to 6 inches, Schmidt said. “This ranks near historical high levels for early March,” he said in his report.

Underneath all that snow is frozen ground, with frost 2 to 4 feet deep. That will add to the trouble if spring temperatures soar quickly. A rapid snowmelt over frozen ground will mean a rush of water that could overload rivers and streams, flooding roads, fields and possibly neighborhoods.

Hope for a gentle melt

In Fargo, there’s now a 50 percent chance the Red River will reach about 35 feet, said Amanda Lee, hydrologist for the National Weather Service in Grand Forks. That compares to minor flooding that occurs when the river reaches 18 feet, she said.

With the new flood forecast in hand, Mahoney will meet with his department heads on Friday to discuss contingency plans in case a rapid meltdown threatens to raise the river.

“The next few weeks will be important to us,” he said. “We’re a little concerned about April showers.”

Fargo, like many other cities, has spent millions of dollars to mitigate damage caused by overflowing rivers. Many have built or bolstered levees, moved homes and businesses and raised some buildings to withstand rising waters.

But if the Red River rises to 35 feet, homes and other areas in Fargo will have to be protected, Mahoney said. The city will put up some flood gates, build clay dikes and fill 500,000 to a million sandbags, according to the guidelines in his three-ring binder that provide a course of action at various flood stages.

All of it has been tried, tested and refined over the years when floods inundated parts of the city.

“We have a town that understands floods, and we’re well prepared,” he said.

In 2009, the river reached 40.8 feet, he said. “The town looked like a combat zone. We had 7.5 million sandbags — the same height as the Empire State Building — in every neighborhood. … It looked like we tore up the whole town to get some protection.”

As of now, Mahoney said the city has things under control. Fargo can easily defend itself against a river that rises to 35 feet, he said.

“When you get over 35 feet, that’s when you start to look like 2009,” Mahoney added.

In Granite Falls, where the Minnesota River cuts through town, Mayor Dave Smiglewski said about $40 million spent to mitigate flooding should keep damage to a minimum if the waters rise to major flood stage. Right now there’s only a 50 percent chance of minor flooding, he said.

“The forecast is reassuring, but we’re going to have high water here,” Smiglewski said. “We’re a lot better prepared for it. We have sandbags and equipment to handle it. At this point, I know hope isn’t a very good strategy, but we’re hoping we don’t have to use that.”

For now, a forecast for lower-than-normal temperatures could slow the melt, said the Weather Service’s Schmidt. Temperatures over the next couple of weeks may rise into the 30s during the day and fall into the 20s at night, which is perfect for a gradual thaw. A higher sun angle also will melt some snow, he said.

That adds up to a gentle melt, he said.

“At least that’s what we’re seeing for now,” Schmidt said.