Debris Flow — Taiya Inlet, Alaska

2020-12-02 · near Skagway, Taiya Inlet, Alaska

Event narrative

As the atmospheric river moved over the northern inner channels of Southeast Alaska it produced significant amount of precipitation from December 1-2. The snow level was near sea level at the on December 1 with accumulations of 5 to 7 inches with as much as 40 inches at high elevations over 2 days. The snow level rose above the mountain tops through the early morning hours of December 2 and started to melt the fresh snow to increase the amount of runoff. Through December 2 there was about 15 inches of snowpack loss at high elevation and almost all the snow melted off at sea level which led to significant amount of runoff into local rivers and overwhelmed culverts that damaged roads from erosion. In Dyea very heavy precipitation of 4 inches fell in 10 hours through the early morning hours of December 2 with a total of 7.5 in 2 days fell. In Skagway, December 1 broke the all time daily precipitation record with 3.67 and it also broke the 2 day all time precipitation record with 6.09. The COOP in Skagway reported 9.92 in 3 days which was close to a 1000 year event. The Taiya River near Dyea rose 4.5 feet in 12 hours and cresting in the afternoon of December 2nd just over bankfull from all the precipitation and snowmelt runoff. That is very high river levels for this time of year along with how fast the river rose. The soils were very saturated from the heavy precipitation and snowmelt to help destabilize the steep slopes at the same time there was some windy conditions to increase the risk of debris flows. Multiple debris flows blocked the Dyea road cutting them off from Skagway in the early morning hours of December 2 when the highest rain rates were occuring.

Wider weather episode

A large plume of concentrated moisture or Atmospheric River (AR) with an associated low-pressure system moved across the Pacific Ocean and transported tropical moisture from the southwest which impacted Southeast Alaska (SEAK) from December 1st through December 2nd. This event brought widespread significant, and sometimes historic record setting rainfall to locations across Southeast Alaska. There were multiple waves along the main front to produce a quasi-stationary front and allowed heavy precipitation to persist over the panhandle for a long duration. Rainfall amounts were as high as over 15 at Little Port Walter with most other locations reporting 5 to 12 over the 2 days. Juneau, Skagway and Haines airports broke their all time daily precipitation record on December 1st along with the all time 2 day total when both days are combined. Rainfall intensity recurrence intervals ranged from a 50 to 200 year event when looking at 6,12,24,48 hour time periods. In some locations the precipitation started out as snow, minor accumulations but then changed quickly over to moderate/heavy rain. Significant snowfall did occur mainly at high elevations and along the Haines and Klondike highways before transition to rain as temperatures rose above freezing all the way up to 6000 feet with the impacting warm front. Snow accumulations were very impressive from December 1st into the 2nd with 18 at 800 feet near the Haines US customs border station and a staggering 59 at 2560 feet on Mount Ripinski above Haines. There was persistent precipitation that occured the previous weeks to keep the soils on the wet side. The heavy precipitation and warm temperatures melted snow that had already accumulated on the ground at sea level then as the snow level climbed above the mountain top this increased the amount of runoff significantly from the mountains. All the excess runoff resulted in significant and in places record flooding of roads and homes, erosion that damaged roads and infrastructure, and widespread mass wasting events such as landslides and debris flows. Most of these impacts were reported to have started in the early morning hours of December 2nd and persist into December 3rd or longer. The northern half of Southeast Alaska saw most if not all of these impacts with a few specific rivers, Salmon River in Gustavus and Jordan Creek in Juneau, seeing record river stage values. Widespread damaging debris flows impacted Juneau, Skagway and Haines that took out roads, destroyed homes and caused 2 fatalities in Haines. To go along with the heavy precipitation there was also very strong damaging winds as the waves moved along the front over the panhandle. These strong winds damaged power lines from falling trees.In total there were about 7 communities (Haines, Juneau, Skagway, Gustavus, Hoonah, Tenakee Springs, and Hyder) that declared a disaster from this event.

View location on OpenStreetMap → (59.4588, -135.3584)

Source: NOAA Storm Events Database, event_id 931827. Narrative written by the NWS forecast office that issued the report.