Climatological Characteristics of Atmospheric Rivers and Their Inland Penetration over the Western United States

Narrow corridors of moisture transport known as atmospheric rivers (ARs) contribute to extreme precipitation and flooding along the west coast of the U.S., but knowledge of their influence over the interior is limited. Here, we use ERA-Interim reanalysis data, CPC precipitation analyses, and SNOTEL observations to describe the characteristics of cool-season (November–April) ARs over the western U.S. We find that AR frequency features a maximum along the Oregon/Washington coast, a strong transition zone over and west of the Cascade–Sierra Ranges, and a broad minimum over the deep interior, southwest, and Great Basin east of the high Sierra Nevada (high Sierra), located south of Lake Tahoe. East of the Cascade–Sierra Ranges, the AR frequency is greatest over the interior northwest.

The fractions of cool-season precipitation and top-decile precipitation events attributable to ARs are greatest over and west of the Cascade–Sierra Ranges. Further east, these fractions are greatest over the northwest and southwest interior, with distinctly different large-scale patterns and AR orientations enabling moisture penetration into each of these regions. In contrast, AR-related precipitation over the Great Basin east of the high Sierra is rare. These results indicate that water vapor depletion over major topographic barriers is a key contributor to AR lysis, with ARs playing a more prominent role in the inland precipitation climatology where lower or less continuous upstream topography facilitates the inland penetration of moisture.