‘Long-duration’ storm to blast Pacific Northwest with flooding, snow newsbhunt

Where it’s cold enough in the high elevations of the Cascades and Olympic mountains, multiple feet of snow are possible.

The heavy precipitation is predicted to trigger flooding and landslides and disrupt travel. As storms continue to pound the region this weekend and early next week, temperatures are forecast to rise, which will raise snow levels. The combination of heavy rain and melting snow could cause river flooding.

A ‘long-duration’ winter storm

The National Weather Service is calling the onslaught of storms a “long-duration” event that will last several days.

The Pacific Northwest is forecast to catch the brunt of the storminess, but some heavy precipitation will also spread into the northern Rockies.

Multiple zones of low pressure crashing ashore in western Canada will fling long, narrow jets of heavy precipitation known as atmospheric rivers into the Pacific Northwest.

The atmospheric rivers draw moisture from deep into the tropics toward higher latitudes and can produce copious precipitation in a short time.

The Center for Western Weather and Water Extremes at the University of California at San Diego projects that the atmospheric river intensity could peak at Category 4 out of 5, which is considered “mostly hazardous” because of the potential for flooding, with the most intense conditions forecast for southwest Washington and northwest Oregon. Much of the Pacific Northwest is forecast to experience at least Category 3 conditions, which the center states brings a “balance of beneficial and hazardous” effects.

Seattle will live up to its wet reputation for days to come. Its forecast calls for rain until at least Wednesday. It will be raw, too, with highs in the 40s through Friday moderating into the 50s early next week.

About 1 to 2 inches of rain is forecast through the weekend, with totals ballooning to 3 to 6 inches by midweek.

Rainfall will be even heavier around Portland. Around 3 inches of rain could fall by Sunday and 6 inches through Wednesday. The coastal ranges near Portland expect extremely high amounts, with as much as 20 inches for the mountain peaks.

In the high elevations of the Cascades and Olympics, 2 to 4 feet of snow are forecast through Sunday morning. “Travel will be difficult and hazardous due to both heavy and blowing snow,” the Weather Service writes.

The forecast in the mountains becomes more complicated with time as temperatures increase and snow levels rise. The Weather Service forecasts snow to switch to rain over all mountain passes through the Cascades over the weekend. “Several inches of rain will bring the threat of river flooding late this weekend into next week,” the Weather Service writes.

Periods of moderate to heavy snow also spread eastward through the northern and central Rockies, dropping up to a few feet in the high spots of Idaho, Wyoming and Utah. As precipitation cuts off somewhat abruptly across Northern California, just a few inches are expected for the Sierra Nevada and to the south.

As rain totals increase and snow switches to rain at higher elevations, the Weather Service office in Seattle is also warning of a growing flood risk Sunday into next week.

It says snow levels starting between 1,500 and 2,000 feet Thursday will probably to rise to 6,000 or 7,000 feet early next week, as atmospheric rivers draw in milder air from the southwest.

“River flooding will be possible on more susceptible rivers such as the Skokomish River in Mason County, but may also include other rivers in the Olympic and Cascade basins,” the Weather Service writes.

There is also some risk of urban flooding given runoff within population centers during the heaviest rain, the Weather Service said.