Snowmelt-triggered debris flows in seasonal snowpacks

Snowmelt-triggered debris flows commonly occur in mountains. On 14 June 2019, a flow occurred on steep, east-facing slope composed of unconsolidated glacial and periglacial sediments Yosemite National Park. Originating as shallow landslide, ~1,300m 3 ripe snow was instantaneously entrained into the carrying boulders, trees, soil downslope. The forested area at toe strained out leaving muddy slurry to issue across Highway 120 during dewatering. We document this mass movement assesses its initiation using local snowpack meteorological data well regional atmospheric reanalysis examine synoptic conditions. A multiday warming trend ripening prior event 500 hPa ridge amplified over western North America leading record warm 700 temperatures. Anomalous temperatures cloud cover prevented refreezing accelerated with meltwater contributing saturation. Similar conditions catastrophic 1983 Slide Mountain flow, also hypothesized be snowmelt initiated. With projected increases heat waves, our findings can support natural hazard early warning systems snow-dominated environments.