What's powering West Coast mega storms? The 'pineapple express'

The massive winter storm that began to strike Northern California and Nevada over the weekend has caused flooding and mudslides, and prompted evacuations across the region, with a second storm set to hit Monday evening.

With California's ongoing drought entering its sixth year, the massive rainfall is unusual and welcome, by some. The weather pattern driving the rain is a meteorological phenomenon known as a "pineapple express," not to be confused with the Seth Rogen and Dave Franco comedy film of the same name. 

A pineapple express is a type of "atmospheric river" – a large weather pattern that stretches across the Pacific from Hawaii to California. While there are other types of atmospheric rivers that are recognized in other parts of the globe, pineapple express storms are one of the most commonly recognized by meteorologists, characterized by warm, moist air traveling across a long, narrow band to another part of the world. These events are usually associated with large amounts of rainfall in California, and their absence can have extreme negative effects.

"The 'pineapple express' has been pretty much absent the last five years," Mark Moede, lead forecaster for the National Weather Service in San Diego, told CNBC. "That's why the drought has been so persistent." 

This narrow band of moist air from the waters off of Hawaii has been dumping rain on Northern California and Nevada since Saturday, driving floods and power outages throughout the region. Up to 15 inches of rain are expected to be dumped on the foothills of the Sierra Nevada mountains before the storm moves out on Monday, with heavy snowfall expected at the mountaintops.

That snowfall is expected to help replenish the water supply from the Sierra Nevadas, which was found last week to include only 53 percent of the early-January average water content in its snow packs, according to CNBC.

But despite the much-needed precipitation, the pineapple express is also causing problems in the form of flooding and avalanches. Runoff in the already-saturated lower regions of the Sierra Nevada have prompted a number of voluntary evacuations as the area braces for a second storm Monday night.

"I don't think I've seen this much rain since I moved here six years ago," Bob Elsen of Sparks, Nevada, told CBS News. "It's why I moved out of Washington, to get away from this stuff."

As rain continues to fall across California and Nevada, many businesses have been temporarily closed, but emergency crews are out in full force in both states. So far, one fatality has been reported in connection with the storm in California.

The ongoing storms may be some of the largest in years for California, but the precipitation is not expected to end the drought in the state. California's central and southern region, the areas still most affected by the drought, continue to remain dry despite the pineapple express's rain delivery.

"At this point it's been Northern California receiving the bulk of the precipitation," Ted Thomas, spokesman for the California Department of Water Resources in Sacramento, told CNBC.

This article contains material from the Associated Press and Reuters.