Risk of spring flooding if trends continue

By Ann McCreary

While the statewide snowpack remains healthy, warmer-than-normal temperatures and lots of rain in recent weeks are quickly melting lower and mid-elevation snow and could pose the risk of spring flooding if the warm, wet trend continues.

“Overall snowpack [statewide] is at 101 percent of normal. We’re feeling pretty good about that considering we’re coming into mid-March,” said Scott Pattee, water supply specialist with the National Resources Conservation Service.

However, “rain has loaded the snowpack” creating water density that is 10 percent higher than normal, “putting us a month earlier in our melt cycle,” Pattee said Monday (March 7) in an overview of Washington’s water supplies.

“We are already seeing high water and runoff in some places” such as the southern Cascades and lower Yakima basin, he said.

The March 1 forecast for April-September streamflows predicts the Methow River, measured at Pateros, to flow at 122 percent of normal, Pattee said.

In April 2015 the streamflow forecast for the Methow River was at 73 percent of normal, and in 2014 it was 51 percent of normal. At this point, this year looks more promising than the past two summers.

“I don’t expect it to go below normal this year unless we have a major melt-out,” Pattee said.

Unfortunately, that doesn’t appear to be entirely out of the question, given the comparatively warm weather throughout the state, he said.

“The temperatures are killing us. They are 5 to 15 degrees warmer than normal in the mountains. That hastens the snowmelt in the mountains,” Pattee said.

Harts Pass leads state

There is still plenty of snow up there to melt, Pattee said. Harts Pass continues to lead the state in snow water equivalent readings (the amount of water held in the snow) at 145 percent of normal as of March 7.

That compares to 98 percent of normal on the same date last year, and 113 percent of normal in 2014.

“Harts Pass is the highest elevation [snowpack measurement] site in the state and it tends to hold pretty good snow even when others don’t,” Pattee said.

Snowpack readings elsewhere include 86 percent of normal at Mount Spokane, 85 percent of normal at Stevens Pass, and 113 percent of normal at Hurricane Ridge on the Olympic Peninsula.

Because that snowpack has absorbed so much rain, “the biggest concern is localized flooding when the snowpack is this ripe this early. If we have a sudden warm-up we could really see a lot of snow coming off at one time,” Pattee said.

“If we have 133 percent of snowpack and have a big Chinook [wind] come and take off the snow, you could have a problem.”

An early melt not only poses the risk of rare spring flooding, but “could potentially shorten the irrigation season,” Pattee said.

Heavier and earlier runoff could also disrupt the return of spring Chinook. “If we have really high flows at the time of the run, the fish could have a difficult time reaching spawning grounds,” Pattee said.

“We want to keep that water in the mountains as long as we can. It helps that there is more snow than normal.”

The National Weather Service forecast calls for cool and wet weather for the first half of March, followed by a warmer and possibly drier second half of the month. Long-term forecasts for spring continue to be warmer and drier than normal.