Atmospheric River Symptoms Repeat in the Northwest

This week’s pineapple express will not let up on heavy rain, snowfall, and brisk winds as we head into the end of the work week.

A conveyor belt of Aleutian low pressure systems is in place off the Alaskan coast, with these lows continuing to intensify as they approach the Pacific Northwest's coastline. Further south, tropical moisture is also being catapulted by a central Pacific low and a high pressure system off the coast of California, sending copious amounts of water vapor into the Northwest. In tandem, each of these features will work in tandem to create a soggy, snowy forecast over the next few days.

One round of precipitation will be found across the Northwest, northern Rockies and northern California for the rest of today and through Friday. Expect rain throughout western Washington, western Oregon and most of northern California. The lower elevations of the interior Northwest will likely see rain or a mix of rain and snow, though precipitation could change to all snow tonight and early Friday morning. Moderate to heavy snow will be piling up in the Cascades and northern Rockies.

In the near term, rain totals through Friday are variable, but 1 to 2 inches are expected in the Willamette Valley, southern Puget Lowlands, the Blue Mountains, and the Klamath Mountains of northern California, with lesser totals amongst the eastern slopes and Columbia Plateau. Ahead of the coastal ranges, 1 to 4 inches of rain are likely, with higher totals leaning for the central Oregon coast. Amongst the Cascades and coastal ranges, totals of 4 to 7 inches or higher of liquid-equivalent precipitation may be wrung.

In terms of snow, expect 4 to 8 inches of snow in the northern Rocky mid-elevations, central Rockies, most of the Blue Mountains, and the mid-elevations of the Olympics and Cascades. Eight inches up to 2 feet will pack onto the higher elevations of the Olympics and Cascades and along most ridges of the northern Rockies. However, at the peaks of Washington's Cascades, the Olympics, and north-central Rockies, snow accumulations will stack higher than two feet, with the peaks of the North Cascades breaking in at over 4 feet!

Currently, Flood Watches are posted for portions of western Washington and western Oregon. Winter Storm Warnings and Winter Weather Advisories are in effect across the Cascades and northern Rockies.

In addition to the rain and mountain snow, the winds will be howling across the Northwest, Great Basin and along the Rocky Mountain Front Range. Wind gusts will generally reach 40 to 60 mph, with locally higher wind gusts possible over the mountains and higher elevations.

High Wind Watches/Warnings and Wind Advisories are scattered from the Northwest to the High Plains.

Although this atmospheric river is weaker than its predecessor, its impacts will still be widespread. River and flash flooding is possible through today, with concerns for rockslides and mudslides from already saturated and unstable landscapes. Gusty winds could down trees and power lines, and mountain passes along the Cascades will not be navigable at times in low visibility and blowing snow. These conditions will pair at times with significant snow totals in the higher mountain passes to create blizzard conditions. 

As the weekend begins, the atmospheric river does not let up as it shifts southward. This will bring the greatest impacts to western Oregon and northern and central California.

Be sure to download the WeatherBug app to stay up to date on the latest on this changing weather. It’s never too early to have a supply kit packed in case of inclement weather. A simple kit including a weather radio, water, blankets, batteries, and non-perishable food items will go a long way in the event of a power outage.