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Yellowstone River

Looking northwest across the Yellowstone River from the south side of the river. Water levels in area rivers are beginning to rise.

A week of very warm temperatures has melted more than half of winter snowpack in some areas of Montana, and if the trend continues snowpack could disappear by the end of the month — four weeks earlier than normal.

“Usually we don’t see melt rates this high, this early,” said Brian Domonkos, water supply specialist for Natural Resources Conservation Service in the Bozeman office.

He said high temperatures in the valleys translate to above-freezing temperatures at night at high elevations. In Billings this week, temperatures have been in the 90s.

It’s possible that as temperatures in the valleys cool this week, mountain nighttime temperatures in the 30s could slow things down, he said.

“But once it starts coming out that rapidly, it’s hard to slow down,” Domonkos said.

Much of the melt water will go directly into reservoirs to be stored for irrigation and recreational use later in the year. Most reservoirs reported average content at the start of the month.

Some basins have been hit harder than others. Domonkos said that 81 percent of the snow in the Musselshell Basin has melted away. Normally just 49 percent would be gone in the second week of May. For the Yellowstone Basin, 48 percent of snowpack has come down the river. Normally at this time of year, only 17 percent has melted.

“We’ve lost a lot snowpack over the last week,” echoed Todd Chambers, meteorologist at the National Weather Service in Billings. “It really started May 8.”

At the first of May, he said, the Yellowstone River at Billings was flowing at its winter level of about 2.5 feet. By May 11 it had climbed to 5 feet and was at nearly 8 feet on Wednesday. For this time of year, that’s the 90th percentile. A few days ago, it was below the 10th percentile. Flood stage at Billings is 13.5 feet.

Because snowpack wasn’t heavy during the winter and early spring, no flooding is forecast.

Chambers said the Yellowstone and other Montana rivers will continue to rise, but more slowly than in the past few days. Forecasts call for temperatures in the valleys in the 60s during the day and in the 40s and 50s at night. That means that at higher elevations, nighttime temperatures could be at or near freezing, he said.

In the Beartooth Mountains above Cooke City and in the Crazy Mountains, about half the snowpack below 8,000 feet was lost in the last seven days, Chambers said. Those mountains help fuel the upper Yellowstone River — the stretch between Yellowstone Park and Custer.

Snowpack in the Bighorn Mountains, which helps water the lower Yellowstone River, is half gone below 7,000 feet. A quarter of the pack above 8,000 feet is gone, too.

If the current melt rate continues, he said, the snow will vanish long before it normally melts out toward the end of June, he said.

What could be the biggest threat is a continuing lack of rain in what should be the wettest months of the year. There hasn’t been enough precipitation to support much more than a pathetically sporadic green-up this spring.

But Chambers said that cooling weather later in the week also brings a 40 percent chance of precipitation, most likely in the form of thunderstorms.

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