Flood — Juneau Borough, Alaska

2014-08-12 to 2014-08-13 · near Douglas, Juneau Borough, Alaska

Event narrative

This event was very difficult to identify due to the heavy rain masking the the much bigger event going on, the second release of the glacier dammed lake in Suicide Basin. It was about one month to the date when the first outburst flood from Suicide Basin took place. There were two days leading up to this event that had record rainfall values. The Juneau forecast office, near the headwaters but at lower elevation, had record rainfall on August 9th and 10th of one point thirty-nine and two point thirty-three respectively. The Juneau airport also reported record rainfall for those days but not as much as in the Mendenhall Valley. It is possible that the mountains and ice field that drains into the Mendenhall system had as much as double the rain amounts at the forecast office. All of the rainfall rose the Mendenhall Lake four feet to just below minor flood stage by July 11 and then it rose above minor flood stage after a burst of heavy rain in the amounts of one half to three quarters of an inch in the early morning of Tuesday July 12. The Mendenhall Lake continue to rise on a constant slope after the rain stopped. It can to light that there was a second release from Suicide basin on going. The Mendenhall Lake and River rose above moderate flood stage in the afternoon and crested at ten point eighty-eight and twelve point eight respectively. This is was the fifth highest record stage level on Mendenhall lake. After the crest both the Mendenhall River and Lake receded very quickly and were below minor flood stage by midnight on July 13.

This event flooded the Mendenhall Lake campground with about 2 feet of water at the entrance along with flooded out many campsites. There were fourteen occupied campsites out of sixty-nine site which were evacuated by the Tuesday afternoon. There was about 3 feet of water across west glacier spur road along Mendenhall Lake. A few hikers had to get rescued on the west glacier trail due to their van getting stuck because of the flooded road. There was significant flooding along the Mendenhall River near View Drive with one house getting twenty three inches of water in their garage. They also had water in their crawl space. This area was getting repair from damaged done from the previous outburst flooding event a month earlier.

Wider weather episode

A very warm front slowly moved over Southeast Alaska from August 9-12. This warm front produced areal precipitation amounts of three to four in half inches with possible double amounts in the mountains. Also there were very high freezing levels with the warm air over the area and this produced significant amounts of snow and ice melt. The runoff of the rainfall and snow/ice melt caused large rises on Mendenhall Lake and River on the 10th and 11th. Lake levels rose four feet from the runoff by the 11th and remained steady just below minor flood stage from the snow and ice melt with the warm temperatures. There was another burst of heavy rain of a half inch in three hours in the early morning hours of the 12th. This caused the Mendenhall Lake and River to rise above minor flood stage. The lake and river levels did not drop through the morning after the heavy rain stopped as there was something else at work. The hydrograph had indications of a second release of a glacier dam release from Suicide Basin due to the steady slope of rise. It was confirmed that indeed it was a second release and the Mendenhall Lake and River continued to rise through August 12 and crested in the afternoon at a level of ten point eighty-eight feet on the lake and twelve point eight feet on the river. Both of these crests were above moderate flood stage and produced significant flooding around the Mendenhall Lake area.

View location on OpenStreetMap → (58.4341, -134.5715)

Source: NOAA Storm Events Database, event_id 541758. Narrative written by the NWS forecast office that issued the report.