Avalanche Advisory for 2015-12-16 07:03:18

The obvious sensitivity of the persistent weak layer displayed by stability test results from December 13th (see videos) has resulted in a nervous tension among backcountry skiers/riders/snowshoers as it is an avalanche problem that is not often an issue with our maritime 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.  That being said, the fact that no natural or human triggered avalanches have occurred is puzzling.  It would seem that with the results of our many tests a couple of days ago, the forecast area would see some avalanche activity. That has not been the case. This lack of 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. So, while yes a small 30 by 90 cm block will fracture and propagate, a full slope will not propagate due to rocks, stumps and down trees that are creating an uneven bed surface where the old rain crust in question exists. 

Despite lack of any observations from yesterday, it should be noted that the persistent weak layer avalanche problem has not past, and may still be present on the aspects listed above in the 7,500-8,500 ft. elevations near treeline.  We do believe the persistent slab problem still remains an issue, especially with continued cold temperatures. While it remains unlikely to trigger an avalanche on this layer if a user could find the right combination of weak layer with slab on top, it remains possible. There is hope on the horizon that the faceting that is occurring to the snow crystals around the crust layer 40-50 cm deep will begin to heal with warming temperatures on Thursday.  Hopefully the facet growth will cease and the snow crystals will begin to round and bond with some melting influenced by warmer temps.  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 added weight of additional loading from wind slab development, a skier or rider, or snowmobile on this persistent weak layer could be enough to tip the stress vs strength scale where all ingredients for an avalanche come together perfectly.  Be observant, be cautious, and travel accordingly.

The geographic location of this persistent weak layer has been difficult to track. 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.  The persistent weak layer has been found on NW-W-SW-S-SE facing slopes in this elevation band.  In some areas the persistent weak layer is better developed than others. 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!

Recent light density snow has been readily available for wind transport. Strong Northerly winds ruled Mt. Shasta yesterday continuing to build wind slabs in size and spatial distribution. Strong winds can deposit wind slabs far down the slope below traditional avalanche start zones as well as creating wind slabs below 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. 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. 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.