• atmospheric river;
  • precipitation;
  • western United States


Atmospheric rivers (ARs) have increasingly been recognized for their contribution to high-impact weather and climate variability. A recent investigation based on observations located primarily in lowland valleys and basins of the western United States suggests that 10–50% of the total cool season (November to April) precipitation between water years 1998 and 2008 occurred on the day of and day following AR landfall (hereafter the AR fraction), as identified using Special Sensor Microwave Imager data. However, these results are based only on ARs crossing the North American west coast between 32.5°N and 52.5°N, which excludes those crossing the west coast of the Baja Peninsula. Here, we identify ARs in the ERA-Interim reanalysis and examine the AR fraction at high-elevation observational sites and in the NOAA/CPC Unified Daily Precipitation Analysis. At high-elevation snowpack telemetry sites, we find good agreement with the AR fraction obtained previously for valley and basin locations. We also show that including ARs crossing the west coast of the Baja Peninsula (as far south as 24°N) substantially increases the AR fraction over the southwestern United States. Copyright © 2012 Royal Meteorological Society