Rivers in the sky are raining down on the Pacific Northwest. What are they?

It's the wettest time of the year for one of the wettest parts of the country: the Pacific Northwest.This week is no exception as heavy rain, high tides and a rising sea level hit the West Coast.And more storms are on their way.Two atmospheric rivers, bringing heavy rain, will pound parts of Oregon and Washington state back-to-back on Friday and Saturday.Two other atmospheric rivers already dropped multiple inches of rain on both states earlier this week."What's really problematic is when you get compounding impacts from multiple atmospheric rivers," said Josh Weiss, a meteorologist with the National Oceanic and Atmospheric Administration's Weather Prediction Center.

Those impacts include flash flooding, urban flooding and even landslides, he said.This year's atmospheric river season, which runs from October to March, is off to a fast start, said Weiss.What do we mean by a river in the sky?An atmospheric river is a narrow band of water vapor pushed by the wind.Loaded up with moisture, it dumps rain or snow.

These rivers can be up to 2,000 miles long and 600 miles wide, said F.Martin Ralph, the director of the Center for Western Weather and Water Extremes at UC San Diego's Scripps Institution of Oceanography."It's literally a river in the sky," said Ralph, one of the leading researchers on atmospheric rivers.

These rivers are a natural climate pattern.In fact, about 50 to 60% of rain in California comes from atmospheric rivers, Weiss said.Residents in the Pacific Northwest are used to heavy downpours.

But the atmospheric rivers are hitting as El Niño's warm ocean waters contribute to rising sea levels and fuel storm systems.And right now, these natural weather patterns are coinciding with annual high tides, known as King tides.

That leaves coastal and inland waterways along the West Coast at risk for flooding.Ralph said that according to 40 years of data from the Federal Emergency Management Agency, 90% or more of all flood damage in the Western U.S.is from a...