Debris Flow in Alpine County, California

Heavy rain in the vicinity of the Tamarack Fire Burn Scar caused a slide over State Route 4 that was reported by CHP to be 3-4 feet deep and closed the highway in both directions. No rainfall reports in the exact location, but estimates of 0.5-1 were reported to the south and 2-3 to the north, which occurred in a few hours. This is in addition to the heavy rainfall the day before.

A deep layer of monsoonal moisture pushed northwest and north into California, Sierra, and western Nevada in a southerly upper flow that marked the western margins of an area of strong high-pressure ridging into the Great Basin region. Thickening mid-level moisture coupled with steep afternoon lapse rates and warmer surface heating initiated deep columns of slow-moving moisture-laden showers and thunderstorms with torrential rains over the 2021 Tamarack burn scar. This resulted in widespread flash flooding and debris flows in Alpine County CA that created a number of impacts. Rainfall rates of 2.22 inches per hour were observed during the peak of the storm in Markleeville, CA which corresponds to at least a 500 year recurrence interval and possibly 1000 year, per NOAA 14 atlas.