Avalanche Advisory for 2015-12-18 07:02:01

The obvious sensitivity of the persistent weak layer displayed by stability test results from December 13th (see videos) and then again on Dec. 16th has added an interesting twist to our current snowpack structure.  We are continuing to monitor and investigate the distribution of this persistent weak slap avalanche problem.  The persistent weak layer has resulted in an air of caution amongst backcountry skiers/riders/snowmobilers. Rightly so because it is an avalanche problem that is most often not an issue with our maritime (wetter) snowpack.  This caution is warranted as the sensitivity to added weight and disturbance to the snowpack in areas where the right combination of factors exist could result in an avalanche.  Reports of slide activity caused by heavy OSV equipment a couple of days ago on the the SSW aspect of Grey Butte indicate that the weak layer can fail with the right trigger.  It is a sensitivity that may not be as stubborn to trigger as previously thought.  The lack of prior evidence of this "sensitive" persistent weak layer is perhaps the result of our still shallow snowpack.  The weak layer bed surface is not uniform and planar due to many ground protrusions still poking up into the snowpack disrupting uniform distribution. This fact will limit the size of slabs that may release if triggered to being small.  It does not take much snow, however, to push you into trees or over other obstacles.  Size does not matter.  In the right circumstances a small slide can still bury you!

We do believe the persistent slab problem still remains an issue, especially with continued cold temperatures. Persistent weak slab formations may still be present on the aspects listed above in the 7,500-8,500 ft. elevations near treeline.  Some variability and uncertainty surrounds persistent weak layers and persistent slab avalanches.  While it remains difficult for a person on foot to trigger an avalanche on this layer, if a user happens upon the right combination of weak layer with slab on top, and the right trigger point, it remains possible.  Hopefully the facet growth will cease and the snow crystals will begin to round and bond with some melting influenced by warmer temps.

For today, continue to avoid wide open, smooth and planar looking slopes just below, near and slightly above treeline in the 7,500 to 8,500 ft. elevation areas.  Large triggers like multiple people tightly spaced on a slope or a snowmobile would have better chances of possibly triggering a persistent slab.  The presence or absence of the avalanche problem will vary widely from one location to another due to depth of snow, aspect and elevation, and ground anchors/protrusions.  The persistent weak layer has been found on NW-W-SW-S-SE facing slopes in this elevation band. Unfortunately, we cannot nail this weak layer down perfectly. It is your responsibility to get your shovel out, dig a pit and identify the presence or absence of the weak layer. Conduct a few stability tests before considering skiing or riding on suspect slopes! Be observant, be cautious, and travel accordingly.

Strong northerly winds with the last storm cycle (Dec. 12th-13th) transported snow onto NE-E-SE-S-SW aspects. Observations made yesterday at treeline along the edge of Sun Bowl indicated that over the past couple of days wind slabs have had to opportunity to bond with underlying snow.  This may not, however, be the rule as one ascends higher on the mountain where colder, windier conditions may have prevented bonding. Winds were strong enough that the usual areas along ridge tops where wind slabs typically deposit have been wind scoured.  Wind slabs have been deposited further down the slope below traditional avalanche start zones as well as creating shallow wind slabs near treeline. Avoid these areas by traveling in areas less than 30 degrees in slope angle without steeper terrain above. Even on scoured, windward aspects small wind slabs can develop which was observed while traversing along Broadway towards Sun Bowl. We've seen the full spectrum of wind loading on all aspects. Local orographics allow non-typical aspects to wind load on Mt Shasta. Over the course of the past week, we've had wind from all directions though the highest wind speeds have been blowing from the north/northwest.  Once again, uncertainty exists for wind slab sensitivity to triggering due to lack of upper elevation observations. Watch out for near and above treeline terrain near/below ridgetops, on the leeward side of rocks and tree outcrops, round/hollow sounding snow pillows, shooting cracks, narrow couloirs and extreme terrain.