Case study: 2016 Natural glide and wet slab avalanche cycle, Going-to-the-Sun Road, Glacier National Park, Montana, USA

The Going-to-the-Sun Road (GTSR) is the premier tourist attraction in Glacier
National Park, Montana. The GTSR also traverses through and under 40 avalanche paths which
pose a hazard to National Park Service (NPS) road crews during the annual spring snow plowing
operation. Through a joint collaboration between the NPS and the U.S. Geological Survey
(USGS), a forecasting program primarily dealing with wet snow avalanche problems serves to
aid worker safety. The objective of this case study is to examine the meteorological metrics and
snowpack characteristics leading up to a noteworthy wet slab and glide avalanche cycle that occurred
16-22 April, 2016 during a period of unseasonably warm and sunny weather. Continuous
above freezing temperatures at upper elevations with daily maximum values reaching 10-15° C
persisted for four days. The nearby Flattop Mountain SNOTEL station reported a steady loss of
SWE of approximately 1.25 cm/day. River height and discharge on the Middle Fork of the Flathead
River (app. 20-35 km away from starting zones) increased from 1.26 m and 139.60 m3/s
(4930 cfs), respectively, on April 18 to 1.69 m and 267.59 m3/s (9450 cfs) on April 22. The ensuing
avalanche cycle began with three small glide avalanches on 17 April and culminated in three
large wet slab avalanches that released on wet, basal facets. These wet slabs were triggered by
glide avalanches releasing above and cascading over cliffs. Four of these avalanches crossed
plowed sections of the road, resulting in a three day delay in plowing operations. Finally, this
specific case was compared to previous statistical models for wet snow avalanches in this transportation
corridor. Out of 12 avalanche days, the model correctly predicted six of those days as
avalanche days. This case study allowed for a greater understanding of a wet slab and glide avalanche
cycle that occurred during a prolonged spring warming event and can serve as a reference
for future similar cycles.