Debris Flow — Craig Coastal, Alaska

2023-11-20 · near Klawock, Craig Coastal, Alaska

Event narrative

Reports began coming in of landslides near the Klawock airport, close to Black Bear Creek, and near Craig in the mid morning hours of November 20th. As the day progressed, more landslides were reported across Prince of Wales Island, including along the Hollis Highway, near Hydaburg, and along Port Saint Nicholas Road. In Coffman Cove, roads were reported to be washed out, and a landslide blocked the road leading to Luck Lake. Some homes were impacted, but for the most part the landslides blocked roadways and impacted travel.

In addition to the heavy rain, the maximum sustained wind measured at the Kasaan FAA webcam was 51 mph at 350pm, a peak wind gust of 59 mph measured at the Thorne River RAWS at 554pm, and peak gusts of 55 mph were measured at Craig and Klawock between 4-6pm. At 5pm there was a report of power poles snapped in half in Hydaburg followed by a report of roofs blown off and flying debris in the same community at 631pm. There were other reports of trees down across the island and the local ferry service to Ketchikan was cancelled.

Wider weather episode

A hurricane-force 964 mb low lifted out of the North Pacific and into the Gulf of Alaska through the early morning hours of November 20, 2023. This low-pressure system continued on a north-northwest track, with the warm front moving over the southern and central Panhandle through the day and the front pushing north through the evening hours. There was a cold air mass over the northern half of the region with a stalled Arctic front over the central Panhandle. This cold air over the northern Panhandle and the Yukon produced strong high pressure over the Yukon, creating a tight pressure gradient over Southeast Alaska. This weather system also had significant subtropical moisture associated with it and characteristics of an atmospheric river with most of the moisture transport going over the southern half of the panhandle.

As the associated weather front moved over Southeast Alaska through November 20th, the cold air in place over the northern half of the panhandle and a strong high pressure in Canada produced a very strong pressure gradient. This set up an overrunning snowfall event with an excess of 12 inches and with the combination of strong to high winds produced blizzard conditions and the development of a mountain wave event near downtown Juneau. Most of the winter impacts occurred over the northern half of Southeast Alaska. Over the southern half of the panhandle there was shallow cold air in place which did produce some snow at the start of the event in the evening hours of November 19th into the early morning of the 20th. The snow changed over to rain as the warm air associated with the atmospheric river moved over the region along with high winds. Heavy rain moved over Prince of Wales Island (POW) and the southern half of Southeast Alaska in the morning hours of November 20th and by the end of the day rain amounts ranged from over 6 inches on the western side of POW to 3 inches on the eastern side of the island and over the southern inner channels. This produced minor flooding near Thorne River as the river went over its banks and flooded the main road that accesses the town of Thorne Bay from the afternoon hours of the 20th and receded below its banks in the late morning of the 21st. There were reports of roads damaged by culverts washing out due to the heavy precipitation near Coffman Cove. This affected several homes and cut off access by car to the rest of the island.

The rainfall rates that moved over the southern half of the panhandle were very high and ranged from 0.35 to 0.75 inches per hour. This intense precipitation persisted, resulting in impressive 3-hour, 6-hour, and 24-hour rainfall totals and precipitation frequencies. The precipitation frequencies ranged from 10 to 50 year rainfall event. The combination of these very high rain rates and high winds produced widespread shallow landslides across POW and a deadly one near Wrangell.

View location on OpenStreetMap → (56.1094, -133.7160)

Source: NOAA Storm Events Database, event_id 1148551. Narrative written by the NWS forecast office that issued the report.