Subseasonal-to-Seasonal Precipitation Whiplash Events: Generation of a Database, Climatology, and Analysis of their Synoptic Drivers

Precipitation whiplash events, when one precipitation extreme immediately follows the opposite extreme, can result in cascading impacts across a region. For example, when a region rapidly transitions from drought to excessive rainfall (pluvial) conditions it can pose a considerable flood risk, as the antecedent drought conditions can result in the ground becoming impermeable. Conversely, pluvial conditions can lead to increased biomass across a region, which can then pose a considerable fire risk when the region rapidly transitions into drought due to the desiccation of the land surface. Such precipitation whiplash events have been previously studied; however, these studies have largely focussed on the annual scale - an extreme dry year followed by an extreme wet year - rather than on the sub-seasonal-to-seasonal (S2S) scale. Those studies that have examined precipitation whiplash events on the S2S scale have either examined them using calendar months rather than rolling periods or have only examined them across specific regions.

Therefore, this study develops a comprehensive analysis of precipitation whiplash events on the S2S scale across the contiguous United States between 1915 and 2020. Precipitation whiplash events have been identified by computing the standardized precipitation index (SPI) for 30-day rolling precipitation accumulation totals. A precipitation whiplash occurs at a grid point when the 30-day SPI moves from at or above 1.0 to at or below -1.0, or vice versa. Precipitation whiplash events were then objectively identified, considering the spatial continuity of grid points and implementing independence criteria, to create a database of individual precipitation whiplash events across the contiguous United States at the S2S scale. Using the developed database, events were clustered into regions using k-means clustering so that the climatology of these events and their synoptic drivers could be examined. Overall, we hope that the insights gained from this study will greatly enhance the understanding of precipitation whiplash events across the contiguous United States allowing for better predictability and preparedness into the future.