Flood in Glacier Bay, Alaska

The atmospheric river moved over the Glacier Bay area and Gustavus in the early morning hours of December 1st and persisted through the late evening hours of December 2nd. Heavy to moderate rainfall persisted over 36 hours with the highest amounts coming during the overnight hours going into December 2nd. In the first 12 hours of the event, 2.16 of precipitation fell which is a 5 to 10 year event, some of that precipitation did fall as snow at higher elevations. There was a few inches of snow at sea level but that quickly melted. More warm and moist air moved over the region into the evening hours of the 1st and persisted into the early morning of the 2nd. More heavy precipitation fell through this time frame and produces significant runoff as the ground was totally saturated as well as runoff from snowmelt. Precipitation totals were as high as 7.25 for the event from early December 1st through the early morning hours of December 3rd which is a 200 year event. The 24 hour precipitation amount reported at the Gustavus CRN site on December 1st was 3.76 which is a 10 to 25 year event. The 48 hour precipitation amount from Bartlett Cove in Glacier Bay National Park was 6.43 and Gustavus CRN reported 5.69 which is a 50-100 year event for both sites. The magnitude of the precipitation and snowmelt overwhelmed culverts and produced record water level on the Salmon River. Roads located near the Salmon River were flooded with up to 4 feet in places as wind fall in the river directed high water into the gravel pits and into the road systems of Gustavus. The hardest hit areas were Rink Creek Rd, Wilson Rd, Toad Rd along with areas near the Salmon River. The Salmon River began to rise rapidly through the morning hours of December 1st from the first batch of heavy rainfall. The water levels continued on a steep upward trend through the morning hours of December 2nd as it crested at 17.26 feet for a total rise of over 7 feet from the extreme amount of precipitation. This was a record stage for this site which was installed by the USGS in 2014. Flooding persisted for days after the heavy rain relaxed as it took awhile for the water levels to recede from the saturated sandy soils. The city declared a disaster declaration as homes were damaged from flooding along with road infrastructure damage.

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.