Rapid snowmelt causes stream forecasts to drop

YAKIMA, Wash. — Rapid snowmelt in Washington’s mountains and lowland flooding in May has switched streamflow forecasts for the rest of the summer from a surplus to a deficit.

A month ago, above-normal streamflows were forecast for May through September. Now below-normal flows are forecast for June through September by the National Weather Service River Forecast Center in Portland.

The upper Yakima Basin is expected to be at 62 to 72 percent of normal flow for June through September and the lower Yakima at 59 to 71 percent.

“That’s not good news, but everyone should be fine because of full Yakima reservoirs and soil moisture is good,” said Scott Pattee, water supply specialist of the Washington Snow Survey Office of the USDA Natural Resources Conservation Service in Mount Vernon.

While the NWS Climate Prediction Center estimates above normal temperatures and below normal precipitation — hot and dry — for Washington for June, July and August, there should be enough water for irrigators and everyone, barring extreme heat and dryness, Pattee said.

Basins with no reservoir storage, such as the Wenatchee and Entiat, should be OK unless the summer becomes excessively hot and dry, he said.

The five mountain reservoirs serving 464,000 acres in and around the Yakima Basin were at 99 percent of capacity and 111 percent of average on June 5, according to the U.S. Bureau of Reclamation.

Chris Lynch, a U.S. Bureau of Reclamation hydrologist, said he anticipates full water supply for junior and senior water right holders in the Yakima Basin.

May’s rapid snowmelt has pushed the “storage control date” forward to between June 11 and 20, he said. That’s when outflows from the five reservoirs, totaling 1,065,400 acre-feet of water, are greater than inflows to the point that drawdowns start. It was June 29 last year.

Yakima River flow at Parker, below Union Gap, was at 11,000 cubic feet per second through May 21 but dropped to 900 by June 3, he said.

“That’s a steady decline showing we’ve lost most of the snow. Flow is now stabilizing,” he said.

Pattee said statewide snowpack is now 97 percent of normal. But he said the number is almost meaningless since there’s virtually no snow left below 5,000 feet elevation.

Peak flooding is over and the Okanogan and Columbia rivers will begin dropping in the next couple of weeks, he said.

Other June through September streamflow forecasts: Columbia River at Grand Coulee Dam, 93 percent of normal; Columbia at The Dalles, 89 percent of normal; Snake River below Lower Granite Dam, 83 percent of normal; upper Columbia (Kettle, Colville, Similkameen, Okanogan and Methow rivers) range from 68 on the Kettle to 104 percent of normal on the Coville; central Columbia (Chelan, Entiat and Wenatchee rivers) 80 percent of normal.