Record High Temperatures Cause Major Runoff In Pacific Northwest

Posted by Christina Herrick|May 10, 2016

The entire Pacific Northwest experienced record high temperatures throughout April, causing much of the remaining snowpack to melt and runoff. More than 80% of all Snowpack Telemetry (SNOTEL) sites with at least 15 years of data set all new melt rate records for April. During two separate high-pressure weather systems in April, SNOTEL sites experienced minimum daily temperatures exceeding 20 degrees above normal. Due to the rapid snowmelt, runoff was above normal and Washington State’s rivers and streams were able to contain it without flooding.

“The latest NWS short-term forecasts are calling for warmer than normal with some chance of seasonal precipitation. Long-range forecasts for the early summer continue to be warmer than normal with equal chances of precipitation. Normal rainfall during this period is only about 13% of the annual total. The leading climate factor this summer will be temperature, said NRCS Water Supply Specialist, Scott Pattee.

The May 1 statewide SNOTEL readings were 87% of normal. The Tolt River Basin reported the lowest readings at 44% of the 30-year median among those with remaining snow. Potato Hill SNOTEL near Mount Adams had the highest percentage with 121%. Most basins reported considerable decreases from last month. Most areas reached peak snowpack by April 1 or before which is 2 to 3 weeks early. Westside medians from SNOTEL, and May 1 snow surveys, included the North Puget Sound river basins with 83% of normal, the Central and South Puget river basins with 60% and 78% respectively, and the Lower Columbia basins with 93% of normal. Snowpack along the east slopes of the Cascade Mountains included the Yakima area with 72% and the Wenatchee area with 74%. Snowpack in the Spokane River Basin was mostly melted out and the Walla Walla River Basin had 76% remaining.

The state received much below normal precipitation for the month of April; however, year-to-date statewide SNOTEL averages remain above normal at 124%. The Pend Oreille, which includes parts of Idaho and Montana, recorded the highest percentages in the state at 71%. Sheep Canyon SNOTEL near Mount St. Helens received the most rain last month with a total of 4.9 inches or 46% of normal.