Flood — Ketchikan Gateway Borough, Alaska

2025-09-15 · near Ketchikan, Ketchikan Gateway Borough, Alaska

Event narrative

Through the late morning hours of September 15th reports from the local emergency manager reported significant runoff from the steep hillside that surrounds the Ketchikan area from the very heavy rain. This overland flow continued and increased into the afternoon, which flooded numerous roads that had to be closed due to the depth of water and the risk to life and property to the drivers. Some of this urban flooding did occur in poor drainage areas, during the tourist season, and affected traffic and the safety of the visitors. In the afternoon, the emergency manager reported brown water in the runoff, causing elevated concern for landslides, although no landslides were recorded from this event. The peak wind measured at the airport during this event was 44 mph at noon AKDT.

The Ketchikan airport recorded a new daily rainfall record for September 15th of 7.66 inches. This broke the previous daily record of 2.65 inches set in 1981.

From the record rainfall, local hydro-electric dams rose significantly, ranging from 3 to 4 feet. Along Ketchikan Creek, which is fed from overflow of Ketchikan Lakes Dam, the water level rose almost 3 feet in 6 hours, cresting just below 62 feet. Even though it remained below minor flood stage of 63.5 feet, there was a lot of water moving down the creek and right through downtown Ketchikan which caused some concerns.

Wider weather episode

A strong low pressure system moved out of the North Pacific Ocean on September 14th and into the Gulf of Alaska through September 15th. The weather front associated with this strong low tapped into sub-tropical moisture as it lifted out of the North Pacific and transported this moisture in a very narrow but long plume which is described as an atmospheric river.

This atmospheric river moved over the Ketchikan area of Southeast Alaska through Monday and produced very heavy, localized, rain for approximately 12 hours. Area precipitation stations reported a range of rainfall amounts of 2 to 8 inches throughout the duration of the event (36hrs), including a new daily rainfall record for the airport and the 4th wettest day on record ever. When looking at rainfall intensity return periods, this 24 hour amount was a 5 year return period, but most of that fell in a 12 hour period with 6.60 inches from 0700AKST to 1900AKST (a 25 year return period). Looking at a finer scale, the 6 hour amount at the Ketchikan airport of 3.94 inches was a 10 year return period.

This very heavy and record breaking rainfall produced significant runoff across the Ketchikan area. This significant runoff produced mainly urban flooding of roads causing road closures and did raise area hydroelectric dams significantly.

View location on OpenStreetMap → (55.3758, -131.7554)

Source: NOAA Storm Events Database, event_id 1293441. Narrative written by the NWS forecast office that issued the report.