California megastorms could severely flood parts of state

California is currently experiencing the first of what many experts believe will be prolonged storms that threaten to severely flood parts of the state.

From Scientific American:

The relentless storms are being driven by a feature in the atmosphere you have probably never heard of: an atmospheric river ...

The real scare, however, is that truly massive atmospheric rivers that cause catastrophic flooding seem to hit the state about once every 200 years, according to evidence recently pieced together (and described in the article noted above). The last megaflood was in 1861; rains arrived for 43 days, obliterating Sacramento and bankrupting the state. The disaster is largely forgotten, but the same region is now home to more than six million people. Simulations of a 23-day storm there indicate that more than $400 billion of damage and losses would occur, far surpassing the $60 billion estimates for Hurricane Sandy's effects. New research also shows that climate change may make these storms more likely to occur.

What is causing these storms, and how might these floods impact the rest of the county?