The Characteristics of Landfalling Atmospheric Rivers during Water Year 2017 and Northern California's Record-Breaking Precipitation

The Northern Sierra 8-station Index recorded 94.3 inches of precipitation during water year 2017 (through 31 July 2017), ~6 inches greater than the previous record set during water year 1983. Numerous other high elevation locations along the United States West Coast (USWC) experienced anomalously high and record water year precipitation accumulations. The record-breaking precipitation was driven by the frequent and persistent landfall of more than 60 atmospheric rivers, defined as long and narrow corridors of integrated water vapor and integrated water vapor transport. This study creates a subjective climatology using objective criteria in order to identify the frequency, distribution, orientation and intensity of the atmospheric rivers during water year 2017 and previous water years. This climatology seeks to gain an in-depth understanding of the meteorological conditions that contributed to the active AR landfall and anomalous precipitation accumulations. Composite analyses are constructed in order to compare the synoptic-scale flow regimes of water year 2017 and previous water years. Preliminary results suggest that a large contributor to the active water year 2017 was a southerly shifted storm track that directed a large number of ARs towards Northern California.