Characteristics Of Long-Duration Heavy Precipitation Events Along The West Coast Of The United States

Significance Statement We sought to understand atmospheric processes that result in heavy precipitation events along the U.S. West Coast that span long durations (>= 3 days) and thus pose significant flooding threats. We analyzed events over Northern California and identified four distinct flow patterns over the North Pacific that support their occurrence: two exhibiting a strong west-east jet stream and two exhibiting a wavy "blocked" jet stream. These patterns tend to persist for several days and promote the landfall of multiple extratropical cyclones in rapid succession along the West Coast, thereby maintaining the ingredients for precipitation over Northern California for extended periods (5-6 days). The physical understanding established by our results could have utility for forecasting heavy precipitation.Prolonged periods (e.g., several days or more) of heavy precipitation can result in sustained high-impact flooding. Herein, an investigation of long-duration heavy precipitation events (HPEs), defined as periods comprising >= 3 days with precipitation exceeding the climatological 95th percentile, is conducted for 1979-2019 for the U.S. West Coast, specifically Northern California. An objective flow-based categorization method is applied to identify principal large-scale flow patterns for the events. Four categories are identified and examined through composite analyses and case studies. Two of the categories are characterized by a strong zonal jet stream over the eastern North Pacific, while the other two are characterized by atmospheric blocking over the central North Pacific and the Bering Sea-Alaska region, respectively. The composites and case studies demonstrate that the flow patterns for the HPEs tend to remain in place for several days, maintaining strong baroclinicity and promoting occurrences of multiple cyclones in rapid succession near the West Coast. The successive cyclones result in persistent water vapor flux and forcing for ascent over Northern California, sustaining heavy precipitation. For the zonal jet patterns, cyclones affecting the West Coast tend to occur in the poleward jet exit region in association with cyclonic Rossby wave breaking. For the blocking patterns, cyclones tend to occur in association with anticyclonic Rossby wave breaking on the downstream flank of the block. For Bering Sea-Alaska blocking cases, cyclones can move into this region in conjunction with cyclonically breaking waves that extend into the eastern North Pacific from the upstream flank of the block.