Avalanche Advisory for 2015-12-17 07:00:27

The obvious sensitivity of the persistent weak layer displayed by stability test results from December 13th (see videos) has added an interesting twist to our current snowpack structure.  Stability tests were performed again yesterday in the same area as the Dec. 13th tests providing similar results.  An extended column test was performed, as was a propagation saw test (see test results and scores below).  The persistent weak layer has resulted in an air of caution amongst backcountry skiers/riders/snowmobilers 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 is a sensitivity that has been stubborn to trigger.  It would seem that with the results of our many tests performed couple of days ago, and then again yesterday, 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. 

With snowpack test results from yesterday confirming the weak layer is still sensitive, it should be noted that the persistent weak layer avalanche problem has not past.  Persistent weak slab formations 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 difficult 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 suspect crust layer will begin to heal with warming temperatures today.  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.

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. 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!

Strong northerly winds with the last storm cycle (Dec. 12th-13th) transported snow onto NE-E-SE-S-SW aspects. Due to lack of observations wind slab size and spatial distribution are not known.  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. 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.