A PDF version of this report is available here.
On May 27, 2019 multiple natural avalanches stranded 13 cyclists on Glacier National Park's (GNP) Going to the Sun Road. This incident summary was prepared by U.S. Geological Survey (USGS) avalanche forecasters with the Going-to-the-Sun Road (GTSR) Avalanche Program.
Gradually warming temperatures from May 20-24 contributed to melting of the snowpack above the GTSR as it advanced from spring to more summer-like conditions (Figure 1). These increasing temperatures promoted instability in the surface and near surface layers of the snowpack resulting in eight natural wet-loose avalanches that reached the GTSR during this period (none larger than size D2). Temperatures during this interval peaked on May 24 and coincided with a substantial increase in afternoon natural wet-loose avalanching, especially in the Triple Arches and Slopes slide paths where the forecasters and GNP Road Crew spent approximately two hours clearing fresh debris (up to size D2) to exit the work area.
A small storm on May 25-26 impacted the GTSR alpine section. Temperatures decreased briefly and up to 6 inches of new snow (0.6 inches of SWE at Flattop) accumulated above 6000 feet. On Monday, May 27, skies cleared and temperatures increased immediately following the storm. This weather change quickly destabilized the new storm snow available for avalanching in the upper reaches of avalanche paths along the GTSR.
An avalanche (or multiple avalanches) deposited a substantial pile of debris along the GTSR in Triple Arches again sometime between May 24 and May 27. Numerous cyclists were reported along the upper reaches of the GTSR. Some of these cyclists chose to climb over the previously deposited avalanche debris in the Chutes Below Triple Arches, Triple Arches, the Chute Above Triple Arches and The Slopes to continue riding towards Logan Pass.
At approximately 3:00 pm on Monday, natural wet-loose avalanches began to release in Triple Arches avalanche path, causing 13 cyclists to remain on the road on the uphill side of the avalanche debris. The natural avalanches (size D1 to D2) continued intermittently until approximately 7:00 pm. At this time, the resultant avalanche debris pile (which includes the existing debris from earlier the week before and weekend) was approximately 80 feet wide and 20 feet deep on the uphill side.
The upper portion of the Triple Arches avalanche path is separated into two distinct starting zones (figure 2), and the path itself is characterized by steep snow slopes interspersed with cliff bands up to several hundred feet in height. The exact point release of the numerous wet loose avalanches is unknown, but, given the time of day, the wet avalanche activity likely started on the south aspect (in the sun in Figure 2) and continued to the southwest aspect as the day progressed. We suspect that the numerous avalanches that occurred that day would be classified as such: WL-N-R1-D1 to D2-I.
Overall avalanche path characteristics:
Starting Zone Elevation: 2723 m
Runout Zone Elevation: 1431 m
Vertical Fall: 1292 m
Length of Path: 2261 m
Aspect of Starting Zone: SW and S
Angles of –Starting zone: 48 deg
Track: 30 deg
Runout Zone: 12 deg
Entire Slide Path: 40 deg
It should be noted that the GTSR serves as a terrain trap for avalanche debris, and even small avalanches can potentially produce deep piles of debris on the road itself.
GNP officials previously closed the GTSR at The Loop midday on May 27 after a separate rock slide blocked the road and prevented emergency vehicle travel, although many cyclists were still above the closure at the time of closing. Glacier National Park volunteer bike patrol units contacted Park Dispatch and Dispatch initiated a response to rescue the stranded cyclists. GNP rangers, two GNP road crew members, and two USGS and GNP avalanche forecasters responded to ensure the safety and well-being of the stranded cyclists. Avalanche forecasters determined the snow above the path had stabilized and traversed across the debris to watch for any further avalanche activity as a road crew member removed the debris with a front-end loader. Forecasters also informed the cyclists of the response plan and confirmed that there were no injuries. The Road Crew was able to cut a path through the debris after approximately 45 minutes and provide a passage for the stranded cyclists at approximately 9:00 pm (Figure 3). All of the stranded cyclists were uninjured and safely returned down the road that evening.
Natural avalanching (up to size D2) in Triple Arches, the Chute Above Triple Arches, and the Slopes continued over the next few days as temperatures continued to rise to the highest temperatures of the year at that point in time.
Figure 1: Daily temperature (°F, top panel), accumulated precipitation (inches, middle panel), and snow water equivalent accumulated (inches, bottom panel) for Flattop Snotel site (6300 ft., approximately 10 km from avalanche site) for the period May 20-28.
Figure 2: Overview of Triple Arches avalanche path (shaded in gray).
Figure 3: A front end loader clears the avalanche debris from Triple Arches avalanche path on Monday, May 28.
Figure 4: Courtesy Image taken by a road cyclist during after avalanche activity on May 27, 2019.
For additional questions regarding the incident, contact GNP via their press release.
Flathead Avalanche Center Comments
We hope that by examining incidents like this one we can help the community better understand the events. We offer comments in order to help people avoid future avalanche accidents.
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