Western U.S. Superfloods in the Recent Instrumental Record
2021
Tarouilly, Emilie | Li, Dongyue | Lettenmaier, Dennis P.
We examine the characteristics of extremely severe and extensive floods across the Western U.S., which we term superfloods. We develop a system to score each day from 1950 to 2010, across the conterminous U.S. west of the Continental Divide, according to the severity of flooding and the extent of the area affected on that day. In order to augment the available stream gauge data, we incorporate model output runoff from the Variable Infiltration Capacity model aggregated to the HUC‐8 basin level. The superfloods we identify include well‐known events such as the Christmas floods of December 1964. Once the superfloods are identified, we use modeled snow water equivalent and snowmelt in combination with gridded observed precipitation and air temperature to identify superflood drivers. We find that the superfloods fall into three distinct categories: (a) winter events, most of which are associated with Atmospheric Rivers, many enhanced by rain‐on‐snow; (b) late spring floods associated with snowmelt, including some events with rainfall contributions; and (c) rainfall‐driven warm season floods associated with tropical storms (primarily in the Southwest). We find that rainfall is often the primary driver of superfloods, although snowmelt also contributes in rain‐snow transition zones. Regardless of superflood characteristics, extreme conditions in combination are a common feature, including combinations of rainfall and snowmelt as well as successive rainstorms. Our results provide a regional view of flood drivers and how they have combined to produce superfloods across the region over our 61‐year study period.
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