Synoptic–Dynamic Characteristics of Subseasonal Precipitation Extremes over the United States

Precipitation extremes on subseasonal timescales can result in prolonged, high-impact flooding and can make significant contributions to water resources. Motivated by these impacts, this study investigates synoptic-scale dynamical processes characterizing these extremes. As a basis for this investigation, a climatology of subseasonal extreme precipitation events is constructed over the conterminous United States using the PRISM precipitation dataset for 1981–2022. Events are identified as areas of contiguous grid points at which the 21-day accumulated precipitation exceeds the climatological 98th percentile. The spatiotemporal characteristics of these events and their relevance for the hydroclimate across the United States are examined.

Synoptic-dynamic processes associated with events occurring in selected regions over the West Coast and central United States are diagnosed through reanalysis-based composite analyses and case studies. Results reveal a tendency for the events to occur in a connection with anomalous large-scale circulation regimes that persist and/or recur throughout the 21-day period. These regimes tend to be manifested by repeated propagation and breaking of synoptic-scale Rossby waves, promoting successive episodes of baroclinic development, forcing for ascent, moisture flux, and heavy precipitation over the same area. Events over the central United States tend to occur in connection with repeated anticyclonic Rossby wave breaking episodes downstream of a blocking ridge centered over the eastern North Pacific. Events over the West Coast are found to occur in regimes characterized by either a strong zonal jet stream or a high-amplitude blocking ridge over the North Pacific. These two regime types favor repeated cyclonic and anticyclonic wave breaking episodes, respectively, near the West Coast. Factors contributing to the formation, persistence, and recurrence of large-scale circulation regimes linked to subseasonal extreme precipitation events are briefly explored and discussed.