California is facing its 12th atmospheric river this winter after a historic, yearlong drought.

California's reign of drought is over, but its water problems are not. Renewed heavy rain powering in on countless atmospheric rivers from the west are bringing devastating floods.

Storms have also been producing lightning and hail. Flooded roads and torrential downpours have created difficult commutes stretching from the San Francisco Bay Area to Los Angeles for many residents.

Atmospheric rivers are narrow, flowing columns of water vapor in the sky. When they reach land, they can produce tremendous and sometimes dangerous, amounts of rainfall.

In the Pacific Ocean, atmospheric rivers originate over the warm tropical waters near the equator. There, water evaporates into water vapor and is swept up by large Pacific low-pressure systems.

As the water vapor is transported, it's concentrated and often driven towards the west coast of the US. Once the moisture reaches land, elevated topography along the coast acts to 'squeeze' moisture out of the air producing heavy rain. At higher elevations, this falls as snow.

Heavy rain and melt-water can lead to very serious and sometimes life-threatening impacts. Flash flooding and debris flows enhanced by topography are among the most frequent hazards.

The phenomenon is usually responsible for 50% of California's yearly precipitation, so under normal circumstances has benefits of replenishing reservoirs.