Debris Flow — Craig Coastal, Alaska

2020-10-26 · near Craig, Craig Coastal, Alaska

Event narrative

A very strong atmospheric river moved over western Prince of Wales and impacted areas around Craig, Klawock, Hollis, Black Bear Creek, Shaheen Creek and Staney Creek. The Klawock airport reported 3.60 of rain in 24 hours and was the 4th highest daily amount on record. Most of the rainfall fell in a short period of time and it was very exceptional from 600 AKST to 1300 AKST. The 1 hour amount of 0.74 was a 100 year return period. The 6 hour amount of 2.13 was a 25 year return period and the 12 hour amount of 3.42 was a 25 to 50 year return period. All the rainfall produced widespread debris flows that blocked roadways on forest services roads, main highways near Black Bear Creek, between Klawock and Hollis, and Port Saint Nicholas road near Craig. There was so much runoff along with debris in the water that the Klawock water treatment plant was shut down as the reservoir was overflowing and debris clogged the intake pipes. Residence were put on a water boiling notice for a time. The city of Craig declared a disaster declaration after this event.

Wider weather episode

A strong atmospheric river impacted Southeast Alaska on October 26th. This was associated with a southerly jet stream that brought moisture up to Southeast Alaska from the tropics that caused widespread damage across Prince of Wales Island. Due to the effective transport of available moisture, rainfall rates with the associated storm were unusually high. This led to multiple landslides across Prince of Wales Island. Rain amounts that triggered the debris ranged from 2-3.5 in 12 hours with a near 100 year return period for a 1 hour rain accumulation. There were about 65 debris flows identified from this one event.

View location on OpenStreetMap → (55.4004, -133.0280)

Source: NOAA Storm Events Database, event_id 925574. Narrative written by the NWS forecast office that issued the report.