Avalanche Advisory for 2015-12-15 07:02:54

While we had some eye brow raising stability test results on December 13th (see videos), we have not observed or had any natural or human triggered avalanches reported since then. It would seem that with the results of our many tests, the forecast area would see some avalanche activity. That has not been the case. I've thought about this and what I've come up with is... perhaps because of our still shallow snowpack, the weak layer bed surface is not uniform and planar due to many ground protrusions still sticking up into the snowpack. 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 creating an uneven bed surface where the old rain crust exists. 

I want to make it very clear: by no means am I saying that this problem has past. Hence the name "persistent weak layer", I do believe the persistent slab problem still remains an issue, especially with continued cold temperatures. The classic notion of "we only get avalanche activity 24-48 hours after a storm" should not be abided by with this avalanche problem.

The details: the persistent weak layer in the form of near crust facets, first observed on December 11th, lies 40-50cm deep in the snowpack. Rounded facets likely remain loose and weak. 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. Continued wind loading or, a skier or rider on this persistent weak layer could tip the stress vs strength scale where all ingredients for an avalanche come together perfectly. Soft to hard slabs have been observed 1-2 feet thick. 

The geograhpic location of said weak layer is a tough one and has been difficult to track. For the time being, avoid wide open, smooth and planar looking slopes just below, near and slightly above treeline or in other words, between 7,500 to 8,500 feet.  The persistent weak layer has been found on NW-W-SW-S-SE facing slopes in this elevation band, some areas better developed than others. Unfortunately, we cannot nail this weak layer down perfectly. It is up to YOU to get your shovel out, dig a pit and identify the presence or absence of the weak layer. Further, conduct a few stability tests before considering skiing or riding on suspect slopes!

Recent light density snow has been readily available for wind transport. Wind slabs will continue to build in size and 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.