The atmospheric river, a narrow conveyor belt of extreme moisture streaming from the deep tropics into the northwest Lower 48, has maxed out the scales as a top-tier Level 5 event. The ranking is a product of the feature’s impressive moisture-loading and the long duration over which it is affecting the coast.
Marty Ralph, who directs the Center for Western Weather and Water Extremes in La Jolla, Calif., and who helped develop the scale for atmospheric rivers, said Category 5s, considered “exceptional,” occur on average one or twice per year along the West Coast.
Ralph said in an interview that the European and American computer models predict the event to reach Category 5. “There’s pretty high confidence,” he said.
Every hour early Tuesday, up to 760 million gallons of liquid-equivalent atmospheric moisture were sweeping into the Pacific Northwest, fueling the heavy downpours and mountain snowfall in the region. That’s enough to fill more than 27,000 Olympic-size swimming pools per day.
The moisture is arriving from places as far away as Hawaii, with some from the West Pacific, nearly 4,000 miles distant.
“It’s got all the ingredients to be a significant rainmaker,” Ralph said.
Flash-flood watches are up west of the Cascades in Oregon and Washington state, where a widespread two to four inches are likely in the lowlands and four to eight inches for the windward, or western, slopes of the higher elevations. The Willapa Hills in Washington and areas of north Oregon had picked up around four inches as of early Tuesday.
A few 10-inch amounts can’t be ruled out, particularly in the Coastal Range and Cascades.
Seattle had reported 1.69 inches by Tuesday morning, with nearly three inches in Olympia. Peak rainfall rates weren’t overly heavy — generally between a quarter-inch to a half-inch per hour — but the relentless nature of the rainfall will allow amounts to steadily climb.
“Rain gauges have reported 6-hour rainfall totals between 1.5 and 2 inches across the higher terrain,” wrote the National Weather Service in Portland.
Humptulips, Wash., about 80 miles southwest of Seattle near the coastline, had reported 4.86 inches of rain in 24 hours. At Hoodsport, on the Hood Canal connecting to the west side of Puget Sound, 4.36 inches was measured. Nearby Jefferson Creek had seen just over four inches.
The National Weather Service Weather Prediction Center included parts of northwest Oregon and southwest Washington in dual “moderate risk” bull’s eyes, a Level 3 out of 4 risk tier in their excessive-rainfall forecast.
Another localized hot spot may include parts of far-southern coastal Oregon and northwestern California, mainly between Gold Beach, Ore., and Crescent City, Calif. That’s where the core of the atmospheric river will be pointed Tuesday night into early Wednesday before the moisture flow curls offshore, pinches off and dissipates.
Whereas atmospheric rivers of lesser intensity can be very beneficial for the water supply in the West, Category 5s unload so much water they are considered primarily hazardous, given their likelihood to generate flooding and mudslides.
Ralph said the severity of the impact depends significantly on the conditions preceding the event, such as whether the ground is saturated, and the levels of lakes and rivers.
The high-end atmospheric river is the latest in a siege of storm systems that have buffeted the Pacific Northwest. More than a foot of rain has fallen in Seattle since the start of December, with Portland approaching double-digit totals.
That has left the ground saturated in most spots, unable to absorb much additional water. That bolsters the risk of excessive runoff, flooding and mudslides.
“With already saturated soils and rain continuing, slopes are unstable and will remain so for the next few days,” wrote the Weather Service in Seattle.
Melting snowpack below 8,000 feet is also contributing to additional runoff, flowing into rivers and exacerbating the ongoing flood threat. An initial dose of snow dropped up to 19 inches of snow on Snoqualmie Pass, east of Seattle on Interstate 90.
Mount Rainier, at an elevation of over 14,000 feet, is predicted to receive over 100 inches of snow through Wednesday.
According to the U.S. Geological Survey, about half of all rivers in western Washington were running above normal or much above normal.
Meanwhile, a mudslide had blocked the northbound lanes of Highway 101 near the North Bay in Washington, about 100 miles southwest of Seattle.
Ralph said the toll of this particular atmospheric river event will depend on exactly where the heaviest rain occurs and whether people and infrastructure are in the way. Pinpointing in advance the locations that will be hit hardest remains a challenge in forecasting.
In addition to heavy rain and flooding, strong winds are expected in coastal and western Oregon in the heart of the atmospheric river. Widespread gusts of 50 to 60 mph, with a few to 70 mph, were likely into the afternoon hours Tuesday. Then a more significant burst of strong winds could accompany a passing cold front early Wednesday.
Confidence is low surrounding that potential second burst of strong winds, but gusts up to 75 mph could occur along the central coast of Oregon, with gusts upward of 85 mph in the Cascades in southern Washington and northern Oregon.
Conditions will rapidly improve in the Pacific Northwest from north to south Wednesday, but another atmospheric river looks to target the region Friday with additional heavy rains and flooding.
The ongoing series of storms and atmospheric rivers is commensurate with what’s expected during a La Niña weather pattern, which usually favors stormy weather and plentiful rainfall in the Pacific Northwest. An active Pacific jet stream serves as a relentless guideway, shuttling storm after storm into the waterlogged region.
Jason Samenow contributed to this report.