Turnagain Pass

Yesterday a party on Pastoral found the trigger point for a large (D3: could bury and destroy a car, damage a truck, destroy a wood frame house, or break a few trees) deep slab avalanche. The crown depth was estimated to be around 10 ft. deep. This avalanche was triggered from below as the two were crossing the slope. They reported, “It sounded like a distant explosion when it went, then we heard and felt the snowpack below us drop several inches.” When this was triggered it also sympathetically released a slope approximately half a mile away.  Last Friday the forecast said, “This deep persistent slab problem and will be guilty until proven innocent.  The 1st person on the slope or the 10th might trigger this type of avalanche. It is a total roll of the dice or Russian roulette set-up. Finding the shallow spot could have devastating results. This type of avalanche would be unsurvivable. There may be no obvious clues to indicate instability and digging to find the weak layer could be challenging.”  The verdict is out. This snowpack set-up is guilty and could be a scary issue for quite a while. The combination of faceted snow on or under a crust near the ground with a stiff slab above makes for an instability that is contiguous over terrain. A collapse of the weak layer in one spot can propagate/communicate a failure across the area. Translation: I step here. It avalanches over there. The way to manage this is to avoid avalanche terrain specifically in the Alpine. Shallow areas are more likely to be trigger points to cause the failure. This avalanche emphasizes paying attention to what is above you, what the terrain you are on is connected to and where other groups are. Thanks to the party involved for sharing details and all the other observations submitted.

• Triggering a dangerous deep slab avalanche is still possible above 3,000′. The avalanche yesterday was triggered around 3600′ and released at approximately 4200′.
• Shallow snowpack areas are most concerning (more trigger points and possibly more reactive facets).
• No red flags (recent avalanches, shooting cracks or whumpfs) are likely to be present to indicate an unstable snowpack. The party involved reported that there were tracks on the slope from the day before and no signs of instability (red flags) observed.
•  It might not be the first person on the slope that triggers the avalanche and remote triggering is possible.

Photo: Mike Ausman

 Crown photo: Mike Ausman

Photo of debris: Stephen Ellison

Yesterday observers reported stiff wind affected snow in the Alpine. There is still snow to move around and winds are forecasted to be easterly, gusting into the 40s today. Wind slabs will be possible on steep, leeward, loaded slopes. Look for drifting, cracking and pay attention to stiff snow under foot.  Even a small wind slab can be very dangerous in high consequence terrain.  In addition, give cornices a wide berth. They have been growing with each new snow and loading event. 

Wind loading and cornice along  -1/Warm-up Bowl.