The early heat wave across most of the Northwest is forecast to start winding down Wednesday. It might have felt nice while it lasted, but the unusual warmth --record-setting, in some cases-- compounded the rapid melting of the Northwest's precious mountain snowpack.
When winter officially ended last month, snow measurements showed near normal to above normal snowpack across the Northwest. In four short weeks though, the snowpack in Oregon, Washington and Idaho has significantly eroded.
For instance, Washington State Climatologist Nick Bond noted the snow gauge at Snoqualmie Pass east of Seattle stood at 79 inches on April 1. By Tuesday morning, the snow depth had diminished to 48 inches.
The reason why is no mystery. Bond said the snowpack was primed to melt by above average temperatures since late winter.
"One thing I have been struck by especially is how warm the temperatures have been at night,” Bond said. “That is actually one of the reasons why the snow is melting so fast. Snow is obviously very reflective. So it doesn't actually absorb a lot of heat from the sun. Solar radiation is mostly reflected. But if you have these warm nights, those really contribute to the melt."
A map of temperature anomalies shows daily average temperatures in April so far running significantly above normal throughout Idaho, Washington and Oregon. A blog post by Rex Block of the Spokane office of the National Weather Service estimated temperatures averaged about six degrees Fahrenheit above normal in the Northwest.
Block wrote that a number of weather stations are reporting the warmest start to April on record, including Quincy, Lind, Colville and Mazama, Washington, as well as Priest River, Idaho.
Bond said the water supply outlook remains "adequate" thanks to full reservoirs, but he's concerned about low stream flows and dried-out landscapes come late summer.
The National Weather Service office in Spokane said minor river flooding is possible in central Washington and north Idaho beginning Thursday as rivers swell from rapid snow melt in the mountains.
Reservoir operators with the U.S. Bureau of Reclamation are seeing high inflows to water storage reservoirs earlier than average around the Northwest.
"We’re trying to capture that, keeping in mind to make sure to leave room for flood control," said hydrologist Mary Mellema in Boise during an interview with public radio on Tuesday. "We're trying to adapt and manage."
"It makes it trickier to operate the reservoirs," Mellema added. She said her agency continues to anticipate having adequate water supply to provide full deliveries to irrigators through the summer.
"We're not talking about any curtailments yet," she said.