'Atmospheric river' to bring heavy rain, flooding to US Pacific Northwest

WASHINGTON (NYTIMES) Scattered showers were falling across the Pacific Northwest on Monday (Feb 28) as meteorologists expected an "atmospheric river" to bring heavy rain and flooding across the region through the middle of the week.

The brunt of the storm system, which was described as a stream of water vapour, was expected in the mountains of northern Oregon and western Washington. Up to 18cm of rain could flood the rivers that flow off the mountain ranges, said Mr Dustin Guy, a meteorologist at the National Weather Service in Seattle.

"Amounts like that, combined with pretty high snow levels, can push our rivers up significantly," Mr Guy said.

By Monday afternoon, areas between Seattle and Portland, Oregon, had received about 7.6cm of rain, which caused some snowpack to melt, increasing the chances of flooding along some rivers in the region, said Mr William Churchill, a meteorologist with the Weather Prediction Centre.

"A lot of what's driving the flood concerns for this event is how rapidly the snowpack is melting while the rainfall is falling," Churchill said.

He added that although Seattle and Portland would be spared severe flooding, the cities could still receive significant rainfall this week.

The weather service in Portland on Monday succinctly summarised the weather on Twitter: "If you haven't been outside yet, well it's kinda wet."

An atmospheric river is a trail of water vapour that moves through the sky, much like the way a river moves over land, according to the National Oceanic and Atmospheric Administration.

Think of it as "a long ribbon of very moist air being aimed right at us," Mr Guy said.

On Monday afternoon, Seattle's rainfall total over the past 48 hours - 9.5cm - was nearly the same amount the city averages over the entire month of February, the weather service in Seattle said on Twitter.

Flood warnings were issued for a slate of Washington rivers, including parts of the Grays, Skokomish, Puyallup, Nisqually, Cowlitz and Naches, the weather service said.

A flood watch was in effect from late Monday through Wednesday afternoon for parts of northwest Washington and west-central Washington, including the cities of Everett, Olympia and Seattle, the weather service said.

Drivers should not try to drive around barricades or cross flooded areas, meteorologists said, adding that most flood deaths occur in vehicles.

Sections of the Washington Cascade Mountains from the Canadian border to the Columbia River were also under an avalanche warning through Monday evening.

The Northwest Avalanche Centre said Sunday that "very dangerous conditions" for avalanches could be created as heavy rain saturated the snow this week.

On Sunday, the weather service in Seattle said parts of Oregon and western Washington could expect heavy rain into Tuesday morning.

In Olympia, about 96km southwest of Seattle, about 3cm of rain had already fallen as of 10am Monday, breaking the previous daily rainfall record of 4.3cm set in 1972, the weather service in Seattle said on Twitter.

Gusty winds and "distant energetic ocean waves" were expected along the coast, according to the weather service in Portland.

It is "definitely not unheard-of" to have an atmospheric river in the West at this time of year, with the end of the wet season just weeks away, Guy said.

The atmospheric river this week will be at least the second to pummel the Western United States in the past several months.

In October, an atmospheric river converged with a bomb cyclone to unleash heavy rains in the San Francisco Bay Area. About 100,000 customers lost power as landslides and dangerous road conditions were reported in areas across Northern California.