An Assessment of Atmospheric Rivers as Flood Producers in Arizona

Atmospheric rivers (ARs) are long, narrow plumes of concentrated water vapor that are a critical factor in the transport of moisture from oceans to continents in the mid-latitudes. Much of the existing research on the impact of ARs on the United States focuses on the Pacific Coastal states and their importance as contributors to precipitation, their impact on water resources, and their role as flood producers. The objective of this study is to determine the importance of Pacific Ocean ARs penetrating further inland and affecting flooding in the state of Arizona. The following questions were addressed: (1) Are certain regions in Arizona more susceptible to AR-related flooding? (2) Do ARs produce flooding events of greater magnitude in Arizona than floods produced by other mechanisms? (3) Are there identifiable variables or conditions that influence the frequency, magnitude, and location of AR-related flooding in the state? Based on a study of selected watersheds throughout Arizona, results showed that the most active region of AR-related flooding in Arizona is associated with the abrupt increase of elevation along the Mogollon Rim of the state's Central Highlands Transition Zone physiographic region. The percentage of AR-related flooding events in this region can reach over 50% for some watersheds, such as the Verde and the Salt. The influence of ARs on flooding is weaker to the north, in the Colorado Plateau region, and to the south, where summer convective storm activity in southeastern Arizona's Basin and Range physiographic region is a more common flood producer, and where the most extreme floods are associated with late-summer tropical cyclones. When ARs did affect northern or southern Arizona, they did not have the same degree of influence on flood magnitude and frequency as in the Mogollon Rim/Central Highlands watersheds, which implies that watersheds in the Mogollon Rim/Central Highlands have characteristics that are favorable for AR-related flooding. Lastly, in addition to the importance of the Central Highlands' orography on AR flooding, another finding of this study points to the importance of the trajectory of the inland-penetrating AR as a factor. The corridor along which the AR enters the region can strongly affect which ARs will produce floods and which ARs will not, with a south/southwesterly trajectory across Baja California producing the largest percentage of AR floods in Arizona in this study