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 atmospheric rivers.

This weekend, Jeff Michalski, a meteorologist at the National Weather Service in Seattle, said that Washington's coast could be hit by significant 20-foot swells.

Pineapple and rum

People may know atmospheric rivers from their nicknames, such as Pineapple Express or Rum Runner Express. These names describe where the atmospheric river originates. A Pineapple Express, which came across the Pacific Ocean from Hawaii in late October, caused devastating flooding and multiple deaths in the Southwest's Four Corners regions.

Atmospheric rivers aren't just a West Coast phenomenon. Some larger nor'easters on the East Coast can pack a "nasty" atmospheric river, UCSD's Ralph said.

The term atmospheric river is one of a few technical science terms that have gained traction on social media and in the news over the past few years, along with El Niño and Kelvin waves.

But is there a relationship between El Niño and atmospheric rivers? Do the warm ocean temperatures associated with El Niño and the heat those waters release make the rivers stronger? That was Ralph's theory back in 2015, but his research showed the opposite.

"[Atmospheric rivers] don't dance to the tune of El Niño," Ralph said

Climate scientists say there also isn't enough data to connect the weather pattern to human-caused global warming, although a warming world does mean more water vapor in the atmosphere.

"We've got to keep our eyes" for atmospheric rivers, Ralph said. "Whether it's an El Niño year or a La Niña year or a No Niño year."