A top-tier atmospheric river soaked the Pacific Northwest during the first half of the workweek, bringing extreme rainfall approaching 10 inches, severe wind gusts and a litany of mudslides. The “exceptional” atmospheric river, rated a Category 5 on a 1 through 5 ranking, is the latest in storms and conveyor belts of moisture to buffet Oregon and Washington state.

Because of the high winds, more than 600,000 customers were without power in the Pacific Northwest on Wednesday morning (about 540,000 in Washington, and 74,000 in Oregon), according to PowerOutage.us.

At its peak Tuesday, the atmospheric river was trucking ashore 665 million gallons of moisture every hour, about a quarter of the flow rate of the Mississippi River. Flood watches blanketed western Oregon and Washington during the height of the episode, which drew in moisture from the tropical West Pacific up to 4,000 miles away.

Seattle has seen 7.45 inches of rainfall since Jan. 1, marking the wettest start to a year on record.

In Oregon, scattered landslides were reported, including a “significant” landslide affecting West Burnside Street in Portland. Oregon Highway 126 was also shut down for a time Tuesday night.

East of Portland, hillsides in Multnomah County, Ore., and Skamania County in southwest Washington were described as “unstable” and “extremely dangerous.” A number of landslides were reportedly threatening homes along Interstate 84, prompting urgent evacuation orders by emergency management officials.

One person was unaccounted for as of Wednesday morning, believed to be in a vehicle that was swept away in a debris flow. Firefighters were unable to locate any potential victim with thermal imaging cameras, and the area remained “too dangerous” for a ground search, according to the Multnomah County Sheriff’s Office.

“With the landslide within the Columbia River Gorge — the highway is still closed — I’m assuming [officials] are in the process of clearing Interstate 84,” said David Bishop, a meteorologist at the National Weather Service in Portland. “There has been a clearing east of Dodson, Oregon, to allow those people within the city of Dodson to evacuate and get to a shelter in Hood River, Oregon.”

“The situation has been deemed too dangerous to send in rescue crews,” tweeted the National Weather Service in Portland. Emergency management officials had even provided residents with direction on which routes to evacuate to avoid additional unstable terrain.

Up to 10 inches of rain fell thanks to the atmospheric fire hose, which blasted areas west of the Cascades for 36 to 48 hours. A jackpot of 9.63 inches was measured in the Willapa Hills of Washington, with just under eight inches coming down at Abernathy Mountain.

Twin maxima were found along the Coast Range and the Cascades, where the height of the mountains helped to focus rainfall. Also, 8.52 inches fell in the northern Oregon Cascades at Log Creek, with 8.59 inches at South Fork in the mountains near the coast.

Meanwhile, 8.96 inches were recorded near Quinault in Grays Harbor County, Wash., northwest of Olympia.

Amounts were highly variable depending on elevation, since atmospheric rivers are most efficient in moisture transport several thousand feet above the ground. That’s why the heaviest rainfall is usually found on the western, or windward, side of mountain ranges.

Warm air accompanying the influx of moisture also translated to melting snow below 8,000 feet, contributing to additional runoff and flood concerns. Mount Rainier, on the other hand, more than 13,000 feet tall, was expecting 100 inches of snow, give or take, at its summit— more than 8 feet.

Snoqualmie Pass, about 50 miles east of Seattle, received 23 inches of snow between Monday and Tuesday. It has accumulated 236 inches this winter, the most in 10 years, according to Scott Sistek, a meteorologist for KOMO News in Seattle.

At Seattle-Tacoma International Airport, more than 3.25 inches of rain fell between Monday and Tuesday, totaling 14.1 inches since Dec. 1. January is running about three and a half times normal for rainfall, a story widespread across the Pacific Northwest. Soil moisture content is maxed out, and the ground is saturated.

“The current atmospheric river event brought in some heavy rain after previous heavy rain events, as well as a low-pressure system that moved through the area,” said Mary Butwin, a meteorologist at the National Weather Service in Seattle. “We’ve had the normal amount of monthly January rainfall right at the beginning of the month.”

More than half of rivers in western Washington are reporting levels above or much above normal. In Oregon, multiple rivers were experiencing moderate or major flooding.

In addition to extremely heavy rainfall, strong winds caused damage while further destabilizing hillsides. The strongest winds occurred in the foothills and mountains of central Oregon, where gusts behind a cold front Tuesday night topped 80 mph.

Cape Perpetua on Oregon’s central coast clocked a gust to 95 mph, with winds of 80 mph in the Willamette Valley. A station two miles south of the summit of Mount Hood reported a 97 mph gust.

“That’s at Palmer Lift, about 7,900 feet,” said Bishop.

In Washington, heavy downpours helped mix stronger winds to the surface, prompting an overnight severe thunderstorm warning for areas east of the Haro Strait north of Seattle. A gust to 64 mph was reported in Oak Harbor, Wash., as the line moved through.

“We issued [the warning] due to the high wind gusts associated with heavy rain showers, though there was no actual thunder itself,” explained Butwin.

Areas near the Strait of Juan de Fuca gusted between 50 and 60 mph, with a gust of 50 mph at Seattle’s airport.

Conditions were dramatically improving from north to south Wednesday as dry weather built in. Another potential atmospheric river event is expected late in the week.