Multiple atmospheric rivers are expected to roll across the northwestern US over the next few days, bringing heavy rain, feet of snow and strong winds.
In total, more than half a dozen western states are under some type of winter weather alert Sunday, as these atmospheric rivers spread from Washington and Oregon eastward into Colorado and Wyoming.
An atmospheric river (AR) is a plume of moisture that helps carry saturated air from the tropics to higher latitudes, delivering unrelenting rain or snow.
“The Winter Storm Severity Index (WSSI) shows major Impacts are possible in the highest elevations of the Pacific Northwest and the Rockies, likely causing hazardous to even impossible travel conditions in these impacted areas,” the Weather Prediction Center cautioned.
At least 5 to 10 inches of rainfall is forecast along the coastal areas of Oregon, Washington and northwestern California. For higher elevations, as much as 1 to 3 feet of snow is expected to accumulate through the weekend.
An avalanche warning has been issued for areas of Oregon and Washington including Stevens and Snoqualmie Passes, the West Slopes of the southern Washington Cascades, and Mt Hood.
“Very dangerous avalanche conditions will develop over the next 24 hours,” the Northwest Avalanche Center in Seattle said. “Travel in avalanche terrain in not recommended.”
A sequence of back-to-back atmospheric rivers, called an AR family, began Saturday and will continue across this region through Wednesday without much of a break period in between systems. This lack of recovery time will lead to an increased risk for flooding.
“In scenarios where we see multiple atmospheric rivers move onshore in the form of a family, hydrologic impacts tend to be exacerbated due to the lack of time for rivers and soils to recede back to baseline,” said Chad Hecht, research and operations meteorologist at the Center for Western Weather and Water Extremes at UC San Diego.
A level 4 out of 5 AR event is forecast to hit roughly the entire coastline of Oregon.
Not all atmospheric river events are bad. In fact, AR event levels 1 and 2 are considered mostly beneficial rains and are much needed across the western US to build water supply levels. But AR event levels 4 and 5 are more hazardous than they are beneficial as the risk for flooding and travel dangers outweighs the benefits.
“Given enhanced rainfall during the weekend, the soils will be more vulnerable and thus the potential for greater run-off and flooding,” the Weather Prediction Center said. “Snow levels are initially expected to be rather high, and therefore western facing slopes of the Cascades may be subject to an additional risk of flooding due to snow melt.”
A slight risk for flooding, which is a level 2 out of 4, has been issued for Sunday, Monday and Tuesday across western Oregon and Washington in anticipation of the excessive rainfall amounts.
Gusty winds up to 50 mph also are likely and could bring down trees and power lines.
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