Persistent Slabs
- Alta quota
- Limite del bosco
- Sotto il limite del bosco
- Probabilità
- Likely
- Dimensione
- 2–3.5
The calendar says April, but we're facing mid-winter-style Persistent Slab problems (watch this video). We have two distinct problems to consider:
- "Fresh" Slabs: Yesterday, SAC forecasters remotely triggered multiple avalanches in neighboring zones (Banner Summit video), and an observer heard 2 natural avalanches release near Copper Mtn. You could trigger 1-2 foot thick slides on many middle and upper elevation slopes. The new snow that fell over the past several days has formed a soft slab above small facets and crust + facet combinations. Watch for obvious signs of instability like snowpack cracking or collapsing.
- Thicker, Older Slabs: While less likely, the possibility of triggering a massive 3-6 foot thick avalanche remains on the table. These hard slabs also rest on weak facets and crust + facet combinations. The most recent triggered avalanche involving this snow occurred Friday in the Frenchman Ck drainage near Smiley Creek when riders remotely triggered a slide that broke 4-5 feet deep and 800+ feet wide (photo at right/above). The riders were at least 500 feet away on the ridgeline above when they triggered the slide. Blocks of snow the size of small cars remained intact as they traveled downslope, breaking green trees along the way. Triggering an avalanche involving this deeper weak layer is less likely than the thinner "fresh slabs", but the consequences would be devastating.
The multiple persistent slab problems are creating variable, spooky avalanche conditions. You may or may not see obvious signs of instability on unstable slopes. You may be able to trigger slides from long distances away. Slides could break much wider than you expect, wrapping around large terrain features. Tracks on a slope are not an indicator of stability Smaller avalanches can be effective triggers for these deeper weak layers as they travel downslope.
ADDITIONAL DISCUSSION: You may encounter fresh wind slabs in exposed middle and upper elevation terrain. Slabs will be thicker and more widespread in alpine terrain. Any wind slabs that you encounter may be perched above weak facets and/or crusts, making avalanches easier to trigger and wider than you'd expect. Relatively small wind slabs moving downhill could trigger the larger, more dangerous Persistent Slab avalanches discussed above. Look for wind slabs near ridgelines and on the sides of mid-slope terrain features like gullies and ribs. Surface textures like dunes, ripples, or pillows are clues that the wind has been at work. Cracks shooting out from your skis or machine are clear indicators that you've found an unstable slab.
Read the Forecast Discussion below for details on yesterday's bullseye information and observations.
(3/31/2023)
Crown of a very large avalanche that was remotely triggered by riders from at least 500 feet away on the ridgeline above. It broke 4-5 feet deep and 800+feet wide. This slope faces W at 9,300' in the Frencman's drainage near Smiley Creek. You can clearly see the two sets of persistent weak layers in our snowpack in this photo.