Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Possible
- Size
- 1–2
It will be possible to human trigger a PERSISTENT SLAB 1 to 2 ft deep on all aspects above 2500' today, and picking out exact problem locations will be difficult. We have not seen any recent avalanches for 3 days. The most likely locations for triggering an avalanche are as follows:
- Locations with firm, faceted bed surfaces previous to March 12-16th new snow that ALSO previously released/slid during the late January avalanche cycle
- Terrain that was wind affected or cross loaded during Tuesday's 24 hr wind event, which gusted SE 31-50mph
- Locations with a shallower snowpack and more uniform bed surface comprised of old sun crusts OR slabs formed by the wind
We are coming out of a LONG duration LOW avalanche paradigm followed by small incremental storms with 10-12" of new snow March 12-16th followed by a 24hr wind event. The combination was just enough stress to overload weak layers, resulting in a natural avalanche cycle. What we know about persistent slabs is they can linger for weeks. So although numerous people have been skiing and riding this week without incident, specific locations exist where triggering an avalanche remains possible.
Use hand pits and pole probes to identify slabs. Look for hard snow sitting over weaker snow, smooth and rounded or lens shaped features. Shooting cracks or whumping are signs of instability and indicators of this avalanche problem, and may not occur prior to triggering an avalanche.
Remotely triggering an avalanche will be in the deck of cards today.
When the avalanche problems are complex and some uncertainty exists, it’s time to focus on safe travel protocol. In avalanche terrain, only expose one person to steep slopes at a time, choose safe zones out of the reach of avalanches for grouping up and expect avalanches to run further than you think, always have a spotter in a safe location, everyone carries a beacon, shovel, and probe, and are prepared and practiced for companion rescue.
Wet Loose: Wet loose avalanches are typically the first avalanches we see as the snowpack warms up. Wednesday and Thursday brought the warmest weather this week, which also resulted in more wet loose activity. Wet loose can be triggered by snow heating up especially near rocks and cliffs on steep slopes. While they are relatively small and manageable on their own, they can be dangerous if they pick up enough volume, or if they push you into terrain traps. They can also trigger larger slab avalanches as they travel downhill. They are typically preceded by rollerballs or pinwheels rolling down slopes as they heat up. This in an easily avoidable avalanche problem. Expect to find this avalanche problem on steep southerly aspects in the afternoon if temperatures rise and/or the sun comes out full force.
Cornices: Debris from cornice fall was observed this week on leeward aspects after Tuesday's strong wind. Cornices can also break off naturally, especially during warm weather when they start bending downhill. Simply avoid being underneath or near the tops of large cornices, as they break farther back than expected.
Dry Loose: Expect to trigger small dry loose sluffs in steep northerly terrain where low density cohesionless snow exists. Expect sluffs to be slow moving and manageable. While it is very unlikely that sluffs will be large enough to bury a person, they can be dangerous if you get carried through terrain traps like cliffs or into gullies that can accumulate debris.
Natural persistent slab avalanche on SSE Skyscraper. Photo was taken the afternoon of 3/20. The date and time of this avalanche is unknown at this time but likely occurred sometime on the afternoon of 3/19.
Photo: Andy Dennis