ARkStorm stands for an atmospheric river (“AR”) that carries precipitation levels expected to occur once every 1,000 years (“k”). The concept was presented in a 2011 report by the U.S. Geological Survey (USGS) intended to elevate the visibility of the very real threats to human life, property and ecosystems posed by extreme storms on the West Coast.

California’s seasonal weather is influenced by El Niño and La Niña – temporary climatic conditions that, depending on their severity, make the weather wetter or drier than normal.

El Niño and La Niña episodes typically last 9 to 12 months, but some may last for years. While their frequency can be quite irregular, El Niño and La Niña events occur on average every two to seven years. Typically, El Niño occurs more frequently than La Niña, according to the National Oceanic and Atmospheric Administration (NOAA).