Distinguishing Features of Atmospheric River Storms Linked to Debris Flow Initiation on Mt. Hood, Oregon, and Mt. Rainier, Washington

Strong eastern Pacific storms characterized by tropical-sourced moisture and heat are often referred to as Atmospheric Rivers (ARs) and are associated with the triggering of debris flows in the Cascade Mountain Range, USA primarily in the fall season. These storms typically feature freezing levels above 3000 m and heavy precipitation that can saturate slopes and rapidly melt shallow early season snowpack. In a study of periglacial debris flows on Mt. Hood, Oregon and Mt. Rainier, Washington, this combination of factors is proposed to initiate slope failure and subsequent debris flows. However, not all ARs trigger debris flows and other storms not associated with ARs may also lead to debris flows. The presence of these non-triggering storms has led to the question: what features distinguish the storms that trigger debris flows, and do these conditions differ between ARs and other storms? ACARS soundings are used to develop temporally detailed information about freezing levels and storm structure. Supplemental data from the SNOTEL network and NWS WSR-88D radar sites allow for better delineation of storm features and their impact on the ground. Antecedent snowpack, atmospheric temperature profiles, precipitation, and oragraphic enhancement are examined for storms associated with debris flows and those that failed to trigger events to determine what characteristics best differentiate the storms from one another. Specific features within the triggering storms, such as the presence of temperature inversions, are also examined for links to the elevation and geomorphic character of these periglacial debris flow initiation sites.