Steamboat & Flat Tops

Saw an old avalanche, unknown trigger, on an east-aspect near treeline.

Clear skies that turned broken by afternoon. Above freezing temperatures on Willow Creek Pass and near-freezing at treeline. Light south winds with moderate gusts. No new snow.

Below treeline, the height of snow ranged from about 40cm near Willow Creek Pass to 85cm at 10,500 feet. Steeper south facing slopes were mostly bare of snow, and it seemed slopes below 10,000 feet held equal or less snow than they did during my last visit here 6 weeks ago. These slopes were supportable to ski travel but I sank to the ground when stepping out of my skis. I found a very poor structure in a snowpit on a low-angle north facing slope below treeline, where a 35cm soft slab rested atop very weak facets to the ground. With a little more snow to beef up the slab, I would be concerned. I did not find faceted snow on the surface of this snowpit or while traveling in other below treeline areas. Near treeline, the slopes I probed where I’d be concerned of avalanches held a deep snowpack, 115-170cm. These areas saw recent and previous wind-drifting. Surfaces held a thin wind crust. Under the crust was about 10cm of fist-hard snow and graupel that fell a few days ago, followed by a 60-90cm 1-finger and pencil hard slab. I did not see propagation in extended column tests on these slopes, and with these thick, hard slabs I would not expect to. Where the slab is thinner, perhaps in rockier areas or less wind-loaded areas, an avalanche would be possible as the snow that makes us the bottom 50cm of the snowpack is quite weak.