Atmospheric Rivers (AR) are a phenomenon reasonably new to science with much of the research having matured only in the past decade. These events result in considerable, sustained rainfall. A modelled AR event (based on the 1861/1862 storm) by the U.S. Geological Survey, Multi Hazards Demonstration Project (MHDP) for California projected damages in excess of US$700 billion (including $400 billion for property).
The animation below is from NOAA Climate and "shows shows a satellite view of the storm clouds, then a color-coded animation of moisture flowing out of the tropics, and finally, how the atmospheric river banked up against the storm's cold front and moved inland with it. Animation by Dan Pisut, NOAA Environmental Visualization Lab, based on GOES IR imagery from CLASS and total precipitable water data provided by the University of Wisconsin/SSEC MIMIC."