Debris Flow — Sitka Coastal, Alaska

2020-11-01 to 2020-11-02 · near Sitka, Sitka Coastal, Alaska

Event narrative

It had been very wet in the area with 3 to 5 inches of rain over the 3 previous days. Heavy rain from an atmospheric river moved into the Sitka area on November 1 and persisted into the morning of November 2 with an additional 3 to 5 inches of rainfall. Rain rates were greater than 0.5 inches per hour. The heavy rain along with the already saturated soils increased the risk of debris flow near steep terrain. In the early evening of November 1 a debris flow blocked Sand Dollar Road. One lane was cleared for travel in the evening but another debris flow came down during the overnight hours going into November 2 and the road was blocked again. One home at the top of Sand Dollar Road was not damaged but was now on a earth cliff that needed an engineering assessment for safety reasons. Overall there were around 18 debris flows from October 26-November 2. There was also extensive flooding around the Indian River and caused damage to the dam and fresh water diversion for the Sitka Sounds Science Center's hatchery. There was damage to the dam and the road to the water intake was washed. At the hatchery the motors for the drum screen that filters the fresh water were overrun, burned out and destroyed the motor and pumps.

Wider weather episode

Deep low pressure tracked northward along the outer coast causing impacts across Southeast Alaska beginning Halloween night through November 2nd. High Winds were felt over the outer coast and heavy rain caused rivers to rise out of their banks across the southern half of Southeast Alaska with minor impacts. The ground was already saturated by moderate to heavy rain leading up to November 1 and 2 with amounts ranging from 5 to 9 inches. Multiple mudslides occurred near steep terrain after rain amount of 3.5 inches over 36 hrs along with wet antecedent soil conditions. Tight pressure gradient along the Coast Mountains resulted in strong northerly outflow winds and cold air to push southward through the northern inner channels resulting in heavy snow.

View location on OpenStreetMap → (57.0785, -135.3751)

Source: NOAA Storm Events Database, event_id 927766. Narrative written by the NWS forecast office that issued the report.