As mentioned in passing in the Urban 10 Essentials thread ...
... an ARkStorm (for Atmospheric River 1,000 Storm) is a hypothetical but scientifically realistic "megastorm" scenario developed and published by the United States Geological Survey, Multi Hazards Demonstration Project (MHDP). It describes an extreme storm that might impact much of California causing up to $725 billion in losses (most caused by flooding), and affect a quarter of California's homes. The event would be similar to exceptionally intense California storms which occurred between December 1861 and January 1862. The storm would be a 1-in-1000-year event.[1] The name "ARkStorm" means "Atmospheric River (AR) 1,000 (k)."

ARís are real. Iíve seen a Pineapple Express hit the PNW following an early snow. All the snow melted and the rain came down on top of that; the rivers got really full. It made a mess and thatís in an area that routinely gets quite a bit of rain.

$.02 ó food for thought.