Detailed Observation

ObserverGareth Brown

General Observations

 12/24 Python Buttress up to 3200′.  At the trail head: 1700′, 2° F, calm.  At 3200′:  21° F, calm, light snow turning heavy later in the day, NE-NW aspect.

Found 30 cms of new snow sitting on two different layers.  The first was the 12/9 rain crust that had been exposed by the 12/19-21 outflow wind event on windward slopes.  This rain crust has been faceting due to the recent cold temperatures.  This will create a potential future problem below 4000′ when we receive additional snowfall.  The question will be: how much weight can it receive in its current state before it fails?  

The second layer we found the new snow resting upon was old wind slab on lee slopes formed by the 12/19-21 wind event.  We targeted the old windslab in two different pits on a lee aspect, NW.  We started on the periphery of the slab where the slab was thin, 2″.  Hand shear tests and Extended Column test produced very easy results, with propagation upon isolation, ECTPV down 30 cms.

We moved further into the lee of a cross loaded gully where the slab was thicker, 6-8″.  Extended Column tests produced ECTP10 and ECTP13 down 50 cms.  Tests failed on a 5 cm layer of 4 finger snow beneath 1 finger wind slab.  

These results indicate a persistent slab problem where a skier or snowmachiner could enter a slope without results where it is thicker at the top, and trigger an avalanche on the edge of the slab or towards the bottom, where wind slab becomes thinner.    

In our group today had a skier triggered avalanche on a cross loaded gully.  ASu-D1.5-R1-O, 30 cms deep, 20 meters wide, ran 200-300 feet.

Picture of skier triggered wind slab.


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