Image caption
As the cluster of storms and rainfall (green areas) known as the Madden-Julian Oscillation (MJO) travel around the tropics, they kick off planetary-scale waves in the mid-latitude atmosphere. Depending on the phase (location) of the MJO, the waves can alternately block or steer the flow of atmospheric rivers ("AR") around various high and low pressure systems and toward either the Gulf of Alaska or the U.S. West Coast . NOAA Climate.gov animation, by Cordelia Norris.