Central Sierra Nevada

Triggering an avalanche that fails deep in the snowpack has become unlikely on a regional scale. Isolated lingering instability remains a concern. If you are choosing to step out into avalanche terrain, see the forecast discussion for ways to minimize the likelihood of triggering a very large, unexpected avalanche.

Normal caution

The faceted snow near the bottom of the snowpack that made up the Dec 1 persistent weak layer (PWL) predictably contributed to an impressive avalanche cycle from Dec 1 to Dec 12. Since the end of the last storm, we have made it four consecutive days with no additional reports of avalanches that failed deep in the snowpack on the Dec 1 PWL which is now buried some 4 to 6+ feet deep in the snowpack . We have also received no reports of snowpack collapses or whumpfing triggered by backcountry travelers following the last deep persistent slab avalanches that occurred in the Dec 11 - Dec 12 timeframe. Snowpit data has shown a general but not perfect trend of decreasing reactivity and increasing strength.

Isolated instability is always a concern at the tail end of a persistent slab or deep persistent slab avalanche cycle. Choosing to travel in avalanche terrain on NW-N-NE aspects, especially near and above 8,000' where the Dec 1 layer was weakest, is not without some amount of lingering risk of being caught in a very large, high consequence avalanche. Here are some things you can do to minimize the likelihood of triggering that unlikely, but very large avalanche:

1) Avoid complex terrain with lots of gullies, exposed rocks, and cliffs. These features create relative shallow spots in the snowpack where triggering deep instability is easier.

2) Don't jump off rocks, cliffs, or cornices. The additional force when landing on the snowpack increases your triggering force on the slope and any weak layers.

3) While exposing only one person at a time to avalanche terrain, use safe zones for the rest of the group that have an extra margin for error. Anticipate that in the unlikely event that a large avalanche occurs, it will be wider than expected, travel further downslope than expected, and take out some trees. Make sure your safe zone is truly out of harm's way. Many highly experienced individuals have been caught in an avalanche while stopped in an intended "safe zone" that was not safe enough when the avalanche triggered was larger than expected.

A few other items:

• E winds over the last 24 hours have scoured snow from N-NE-E aspects near treeline and above treeline.
• Some drifting of this scoured snow has occurred with very little new wind slab development.
• Any wind slab avalanches that occur today are unlikely to exceed size D1.
• Thin sun crusts have formed on the snow surface in many areas of direct sun exposure, mainly on SE-S-SW aspects.