Atmospheric River Symptoms Repeat in the Northwest

This week’s pineapple express will not let up on heavy rain, snowfall, and brisk winds heading into Friday.

A conveyor belt of Aleutian low pressure systems is in place off of the Alaskan coast, with these lows continuing to intensify into the Pacific Northwest's coastline. Further south, tropical moisture is being catapulted by a central Pacific low and a high pressure system off of the coast of California, sending copious amounts of water vapor into the Northwest. In tandem, each of these features are working to mold quite a workweek for the region.

After a particularly impactful burst of rain, snow, and wind before Wednesday afternoon, A lull in rainfall is ongoing this afternoon and evening. Impacts will still be prevalent across the ridges before another large burst of moisture begins to squeeze out later in the morning.

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 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.

Snow additions through Friday will generally rise before dropping again Friday morning. Still, deep blankets will build onto the peaks with enhanced snow ratios. Four to 8 inches are expected in the northern Rocky mid-elevations, central Rockies, most of the Blue Mountains, and the mid-elevations of the Olympics and Cascades. Eight inches to two 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 four feet!

Currently, Flood Watches are posted for portions of western Washington and northwestern Oregon. Winter Storm Warnings, Winter Storm Watches, and Winter Weather Advisories are in effect for portions of northwestern Montana, Idaho, the Cascades in Washington and Oregon as well as a portion of the Blue Mountains.

Additionally, High Wind Warnings and Wind Advisories are in effect for most of the coastal ranges as well as the Front Range of the Rockies and the eastern Cascade's foothills in south-central and northwest California.

Although this atmospheric river is weaker than its predecessor, its impacts will still be widespread. River and flash flooding is possible through the 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 rapid wind gusts of 40 to 65 mph or greater. 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, expecting to shift southward and bring the greatest impacts in 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.