Strong Storm Poised to Bring Heavy Rain to Oregon and Washington

An atmospheric river of moisture is poised to bring breezy winds and drenching rains to the Pacific Northwest late Friday into Sunday, with a second quick-hitting system sweeping the region Monday into Tuesday and delivering more rain and potentially stronger winds. Heavy rain could cause flooding and strong winds could knock out power and down trees.Forecasters are calling these typical winter storms, although this winter has been anything but typical for the region. Washington and Oregon are famously wet and gray in winter, and a bulk of their precipitation comes from these sorts of storms. But this year, both states saw unusual dry stretches in January.“We had weeks where it was sunny every day,” said Anna Ruhl, a meteorologist with the National Weather Service in Portland, Ore.Between Friday and Tuesday, Seattle is expected to record two and a half inches of rain, and Portland could pick up about two inches.Those amounts are in stark contrast to the rainfall totals from January. The Portland International Airport recorded a little less than three inches last month, down from the five it typically records in January.The discrepancy is more extreme in Seattle, which measured 1.92 inches of rain last month, compared with the 5.78 inches in a typical January.February was already on track to make up some of the deficit. A storm brought rain and snow to the region earlier this week, and the approaching atmospheric river is predicted to be strong, bringing a bigger dose of precipitation.Atmospheric rivers are ribbons of water vapor that stretch for thousands of miles and are carried through the sky by powerful winds. Upon hitting land, the atmospheric rivers can release heavy rain, especially as they move up and over mountains. This system will deliver multiple waves of moisture.As there often is with this type of storm, “there remains uncertainty as to the rainfall amounts” the region is expected to see, said Jacob DeFlitch, a meteorologist with the Weather Service office in Seattle. He warned that the rain would likely lead to “significant rises” on the region’s rivers, prompting a risk of flooding.Some of the highest precipitation totals are expected over the Olympic Peninsula in Washington, where over six inches of rain could fall at the coast and over 10 inches of precipitation in the mountains from Friday to Tuesday.Snow is expected to mainly fall at elevations above 7,000 to 8,000 feet, but more rain than snow is expected in the mountains with this system, which is warm and pulling in moisture from the subtropics, as is usual with atmospheric rivers. Rain is likely to fall on snow that’s already on the ground, and could trigger snow melt and water rushing down into streams and rivers.Rivers are forecast to swell, with several expected to get close to flood stage. The Skokomish River, which flows from the Cascade Mountains to Puget Sound in Washington, is poised to spill its banks over the weekend. The flood forecast for rivers is constantly changing, and people should turn to their local Weather Service offices for the latest information.To the north, the storm will impact British Columbia, with Vancouver Island expected to record seven to 10 inches of rain and Vancouver a little less, said Chad Hecht, a meteorologist with the Center for Western Weather and Water Extremes at the University of California, San Diego. To the south, the system will brush far Northern California, with locations across Del Norte County poised to get up to five inches of rain.Mr. Hecht said that anyone living between British Columbia and far Northern California should prepare for days of rain.“You could have a multiday event, maybe 72 hours-plus for some locations,” Mr. Hecht said. “It’s going to be a wet couple of days.”The big, wet weekend storm will be followed by a second “more compact” system that’s not an atmospheric river, Mr. DeFlitch said.“It may be strengthening as it approaches land, and that could result in strong winds,” he said. “There’s still uncertainty in the forecast.”