A Lagrangian Analysis of the Inland Penetration of Atmospheric Rivers over Western North America

The interior western U.S. is sometimes impacted by high winds, heavy precipitation, and flooding associated with cool-season atmospheric rivers (ARs). However, relatively few ARs approaching the U.S. west coast penetrate into the interior, and forecasts of high-impact weather over this region would benefit from improved understanding of the factors contributing to the inland penetration of ARs. To differentiate between penetrating and non-penetrating ARs, low- (950-hPa) and mid-level (700-hPa) forward trajectories are initiated within cool-season ARs as they approach the west coast of North America. These trajectories are then classified as coastal decaying, inland penetrating, or interior penetrating based on whether they remain within an AR upon reaching selected transects over the western U.S. At initiation, interior-penetrating AR trajectories are associated with a more amplified trough-ridge pattern over the northeastern Pacific and western U.S., more southwesterly (vs. westerly) flow, and larger water vapor transport (qu), particularly west of the Sierra Nevada. Such interior-penetrating AR trajectories most frequently originate along the Oregon coast, but the greatest fraction of trajectories that eventually penetrate into the interior is found along the Baja Peninsula. Inland- and interior-penetrating AR trajectories are often able to maintain large qu by experiencing increases in wind speed (u) that offset decreases in specific humidity (q), particularly in the vicinity of high topographical barriers. Despite experiencing similar decreases in q, coastal-decaying AR trajectories do not experience increases in u, and qu decreases more rapidly. Therefore, synoptic conditions favoring larger initial qu and increases in u over the interior are two keys to differentiating between ARs that will or will not penetrate into the interior.