Central Sierra Nevada

Keep your guard up and follow best practices for travel in avalanche terrain. Isolated instability remains a lingering concern. Consider avoiding shaded gullies, cliffs, couloirs, and slopes with lots of exposed rock where there is a greater chance of encountering isolated instability.

Normal caution

Isolated instability is always a concern at the end of a persistent slab or deep persistent slab avalanche cycle. If the recent avalanche cycle was not associated with a persistent weak layer, I would be operating differently and perhaps moving into terrain more aggressively. Since it was not, I'm making cautious, incremental steps into avalanche terrain on NW-N-NE aspects, especially near and above 8,000' where the Dec 1 PWL was weakest. I'm choosing terrain very deliberately, seeking out simple terrain with very few obvious trigger points.  I'm taking what precautions I can while traveling in avalanche terrain to minimize the likelihood of triggering that unlikely isolated, but potentially very large avalanche.

Here are some things you might consider doing if choosing to travel in avalanche terrain, especially on NW-N-NE aspects:

1) Avoiding complex terrain - gullies, slopes with lots of exposed rock, cliffs, and couloirs. These features create relatively shallow spots in the snowpack where triggering deep instability is easier.

2) Avoid jumping off rocks, cliffs, or cornices. The additional force when landing on the snowpack increases the 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 incorporates 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. 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:

• Periods of moderate speed E winds over the last couple of days have scoured snow from N-NE-E aspects near treeline and above treeline with very little new wind slab development in lee areas.
• Thin sun crusts have formed on the snow surface in many areas of direct sun exposure, mainly on E-SE-S-SW aspects.
• The faceted snow near the bottom of the snowpack that made up the Dec 1 persistent weak layer (PWL) and the Dec 8 buried surface hoar layer contributed to a predictable but impressive avalanche cycle from Dec 1 to Dec 12. Since Dec 12 we have received no additional reports of new avalanches, snowpack collapses, or whumpfing triggered by backcountry travelers. The Dec 1 PWL is now buried some 4 to 6+ feet deep in the snowpack. Snowpit data has shown a general but not perfect trend of decreasing reactivity and increasing strength.