Flood — Juneau Borough, Alaska

2020-12-01 to 2020-12-02 · near Douglas, Juneau Borough, Alaska

Event narrative

As the atmospheric river moved over the north central inner channels of Southeast Alaska it produced significant amount of precipitation from December 1-2. Heavy to moderate rainfall persisted over 36 hours with the highest amounts coming during the overnight hours going into December 2nd. By mid morning on December 1st, 2 to 3 inches of precipitation fell which is a 5 to 10 year event at the Juneau forecast office and the downtown Juneau LARC. Some of that precipitation did fall as snow, from 9 to 20 inches at elevations above 1000 feet. The snow level rose rapidly through the afternoon of December 1st to be above the mountain tops by the evening hours. This warming at elevation melted the fresh snow that fell with most gains lost by the morning of December 2nd. By the end of December 1st the Juneau airport reported a 24 hour precipitation amount of 4.93 which was a 100 year event and broke the all time daily precipitation record from 1946. Precipitation amounts for December 1st ranged from 3 to 5 inches with a number of daily precipitation records broken. Over the 2 days precipitation amounts were from 5 to 7. The Juneau airport 48hr total of 6.39 remained around the 100 year event. Other areas that did report heavy precipitation did not seem to get the intensity the airport reported. The Juneau forecast office 48hr total of 5.10 was near a 5 year event with the 24hr amount of 4.09 on December 1st was 5 to 10 year event. There was significant runoff from the intense precipitation and from the snowmelt in the mountains. Most of the small streams in the Juneau area began to rise rapidly through the morning hours of December 1st with Jordan Creek going over minor flood stage by the afternoon. As the heavy rain persisted into the evening and overnight hours going into December 2nd the soils continued to be super saturated to produce extreme runoff across the area. This produced record flooding along Jordan Creek in the early morning of December 2nd before it crested around 8am AKST at 11.20 feet for a total rise of 4.5 feet over 36 hours. Multiple homes around Jordan Creek and business in Jordan Square were flooded. Roads near the creek from Jordan Ave, Cascade St, Trout St and the main road of Glacier Highway all were significantly flooded with 1 to 3 feet of water. Due to the intense long duration heavy precipitation, additional snowmelt runoff, and high soil saturation there were multiple debris flows reported across the Juneau area near steep terrain which damaged homes, blocked a number of roads and overwhelmed culverts that damaged roads from erosion. Renninger Street, at the base of Heintzleman Ridge, accesses a city water reservoir along with a middle schools was washed out from erosion on both sides of the road. Residences along Mary Ellen Way on Vanderbilt Hill had multiple debris flows that damaged homes and deposited debris into yard that blocked drainages to divert water into homes. Multiple large debris flows occurred along the lower and upper Salmon Creek Rd and damaged the penstock that brings water from the reservoir to the ALE&P power plant and CBJ water treatment facility. This is the only water supply for DIPAC hatchery that supplies fish stock to all of Southeast Alaska. Without the water DIPAC 2021 stock will die impacting sport and commercial fisheries. A big debris flow was reported along Wire St which damages homes and blocked the Glacier Hwy. Another big debris flow was reported near the AWARE shelter along Glacier Hwy which deposited lots of mud around the shelter and surrounding homes. A home was damaged by a debris flow on Cohen Dr near Tee Harbor as the debris smashed into the side of the home removing a significant portion of the structural support. This was one of the most impactful flooding and debris flow events in recent memory and the data backups extreme impacts. The city of Juneau declared a disaster for this event.

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 → (58.2353, -134.3512)

Source: NOAA Storm Events Database, event_id 931871. Narrative written by the NWS forecast office that issued the report.