A new winter forecast, heavily influenced by La Nina conditions, rates the chances of a cold and wet winter in the Northwest higher than a month ago.
Washington, in particular, can expect below-average temperatures and above-normal precipitation for December, January and February, the National Oceanic and Atmospheric Administration’s Climate Prediction Center said. A month ago, the center rated the state’s odds of a cold and wet winter at 50-50.
Washington State Climatologist Nick Bond said that La Nina almost always leads to a healthy amount of snowpack to supply water for summer irrigation.
“There’s always a chance for a rude surprise, but right now, in terms of water supplies for next year, it looks good,” Bond said.
The long-range forecast, scheduled to be updated Dec. 21, reinforced the role La Nina, a lowering of Pacific Ocean temperatures, will have on the U.S. winter. The new outlook generally increased chances for wetter weather in the northern U.S. and drier weather in the southern U.S.
Climatologists expect the La Nina to remain “weak,” a measurement of how much lower than normal sea temperatures are. However, the prediction center noted than a large reservoir of cold water across the Pacific could foreshadow a stronger La Nina.
Bond said he doubts a strong La Nina will develop, though a moderate La Nina could evolve. Even a weak La Nina influences the weather, he said. La Nina conditions were weak last winter, but a large snowpack supplied irrigation systems through a hot and dry summer.
Bond said La Nina conditions have their greatest effect in the Northwest after the first of the year.
“As the season goes on, its influence grows,” Bond said. “I would say the evidence that it’s going to be colder than normal in December is on the skimpy side, but there’s no evidence it will be warmer than normal.
“For later in the winter, there’s more of an argument that it’ll be on the chilly side.”
Snowpacks in basins throughout Washington are well above normal for mid-November. Bond said the early start in accumulating snow doesn’t necessarily telegraph what the rest of winter will be like.
“The connections are pretty weak,” he said. “Things can change.”
Besides Washington, Western Oregon and the Idaho panhandle are forecast to be colder than normal for the three-month period. There is no strong signal for Eastern Oregon, Northern California and most of Idaho, according to the prediction center.
Southern Idaho falls in the portion of the country expected to have a winter warmed by La Nina.
Washington, Idaho and the northern half of Oregon are expected to be wet. The southern half of Oregon and Northern California are forecast to have average precipitation.