Avalanche Advisory for 2015-12-20 07:02:39

Observations were made in several locations on Mt. Shasta yesterday in an effort to gauge any changes to, or the existence of the PWL (persistent weak layer) in the snowpack structure.  A ski tour up Broadway was a pleasure with mostly sunny skies with the occasional brief cloud bank.  New snow amounts from Friday made for excellent skiing/riding on stable snow.  A quick ECT (extended column test)  was performed at the top of the easterly aspect that drops into Old Ski Bowl resulting in an a score of ECTX (no failure).  This was our first look at the stability of slopes above treeline.  I dug a snow pit at the base of the westerly aspect of Green Butte Ridge that falls into Avalanche Gulch at approximately 8775 ft. to perform a full profile and conduct stability tests.  I found the crust layer that has been producing the PWL at lower elevations (7,000-8,000 ft.) on similar aspects, however, the faceting of snow crystals within this layer was not present.  Additionally, the depth of the crust layer combined with a "right side up" (softer snow layers lying on top of denser layers) structure in the density of the snow layers is resulting in a strengthening snow pack with no obvious weak layers.  This hypothesis was confirmed with multiple ECT's all with no results.   

Observations of the snow on the WSW aspect of Grey Butte where the PWL has been most present also revealed a strengthening snowpack with multiple extended column tests resulting with scores of ECTP 21-25.  What this says is that the PWL still exists, but is healing up with the weight of added snow on top and more bonding of the crystals within and around the crust layer.  Another thing to note is that when the weak layer failed it did not "pop" but it did "drop".  What this means is that there is no longer a lot of energy built up in the layer, hence the lack of a "pop" during failure.  It did "drop" though which says that there is still a slight weakness within the layer, and the crystals can collapse.  This PWL is stubborn to trigger, however, caution should be taken when choosing your line.  Multiple people tightly spaced, or a snowmobile cutting across the right trigger point in an isolated spot in steeper terrain could still result in causing an avalanche.

Avalanche danger will rise with additional snow fall from current and expected storms forecast for early today and then on Monday.  Monday's storm will bring the greatest chance for storm slab instabilities as the storm is forecast to be "upside down" meaning it will start off with cold air then warm up as the storm passes leaving colder, drier snow underneath wetter, heavier snow.   

Full Pit Profile @ base of Green Butte Ridge in Avalanche Gulch from 12-19-2015 by Jon Dove

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The Sand Flat cross country ski trails are snow covered and ready for your cross country skis or snow shoes. These are backcountry trails marked with blue diamonds on trees. Trails are not groomed.  Snow shoers, please blaze a parallel trail to cross country skiers staying out of the skin track. Thank you, and Enjoy!

The Pilgrim Creek Snowmobile trails will be groomed soon! Head to our "Education" tab on our website and find the snowmobile section for trail information and other sledder resources!

The Five Red Flags of Avalanche Danger any time of year include:  1) Recent/current avalanche activity  2) Whumpfing sounds or shooting cracks  3) Recent/current heavy snowfall  4) Strong winds transporting snow  5) Rapid warming or rain on snow.