California Flood Threat From "Atmospheric River"

Meteorologists use the term "atmospheric river" to describe a long, narrow plume piping deep moisture from the tropics into the mid-latitudes.  One type of atmospheric river you may have heard of is the "Pineapple Express", a pronounced plume tapping moisture from the Hawaiian Islands to the U.S. West Coast.

Amazingly, according to NOAA's Earth System Research Laboratory (ESRL), a strong atmospheric river can transport as water vapor up to 15 times the average flow of liquid water at the mouth of the Mississippi River!  

Suffice to say, if an atmospheric river stalls over a particular area, significant flooding can be the result.  In fact, a study by Ralph et al. (2006) found atmospheric rivers responsible for every flood of northern Calfornia's Russian River in a 7-year period.  

That said, they're also important for western water supply considerations.  

According to NOAA/ESRL, 30-50% of the average annual precipitation in the West Coast states typically occurs in just a few atmospheric river events.

With that in mind, one such atmospheric river is now soaking parts of the West Coast and will continue to do so through the weekend

http://www.weather.com/news/weather-forecast/california-flood-threat-20121126