Atmospheric river slamming Pacific Northwest; floods, 'very large' avalanches possible

• A flood watch is in effect for more than five million people in Washington and Oregon.
• An avalanche warning is also in effect for the mountains of Washington.
• "A major positive of this event will be the substantial rain that will fall over far eastern Washington."

An "extreme" atmospheric river was barreling into the Pacific Northwest on Monday, and with it will come the threat of flooding and avalanches, forecasters say. 

"It will be an active stretch of days across the Northwest as we leave February in the rearview mirror and start the month of March," AccuWeather meteorologist Alex DaSilva said. "A parade of storms will keep the weather active across the Northwest through Wednesday."

Meteorologists said rainfall amounts will range from 2 to 4 inches early this week from Portland, Oregon, to Seattle, and locally higher amounts are possible on the windward sides of the mountains.

WHAT IS AN 'ATMOSPHERIC RIVER?':These rivers of water vapor can extend thousands of miles.

A flood watch is in effect for more than 5 million people in Washington and Oregon, the National Weather Service said. That includes the Seattle and Tacoma metro areas. "Flooding of rivers, creeks, streams and other low-lying and flood-prone locations" was possible, the Weather Service said.

An avalanche warning is also in effect for the mountains of Washington, and "very dangerous avalanche conditions" are expected, according to the Northwest Avalanche Center in Seattle.

"Avalanches may become very large and destructive," the center said, adding that "avalanches will be large enough to bury or kill you, and this will not be a day to try to outsmart the hazard."

CLIMATE CHANGE REPORT:UN panel's grim assessment: 'Parts of the planet will become uninhabitable'

The foul weather is courtesy of an "atmospheric river," a ribbon of water vapor that can extend thousands of miles from the tropics to the western U.S. At 250 to 375 miles wide, the river of moisture provides the fuel for massive rain and snowstorms that can cause flooding along the West Coast. 

"Some folks rate atmospheric rivers like hurricanes and tornadoes, and they rank this event as an AR4 (atmospheric river, category 4) out of a possible 5," said Seattle-based meteorologist Cliff Mass on his blog Sunday.

The news wasn't all bad, however: "A major positive of this event will be the substantial rain that will fall over far eastern Washington, helping to restore soil moisture for dryland farmers there," Mass said. "The strength and direction of this atmospheric river are well-structured to bring moisture into eastern Washington. Very good news."

Indeed, the heavy rain is coming off of a relatively dry start to February, according to CNN. As of Sunday morning, the Seattle-Tacoma airport had only received just over 1 inch of rain this month, which is more than a 2.5-inch deficit for the month of February, CNN said.