12-15-06, Tony Grove Lake, Upper Bunch Grass

                Toby Weed

When I got up to Tony Grove Lake on Friday, 12-15-06, I noticed four large natural avalanches from the day before.  A couple had run way out onto the ice.

Under the steep cliffs on the west side of the lake I noticed several crowns 1-2 feet deep and ~200' wide.  The avalanches occurred on the very steep east facing slopes above the lakeshore and below the cliffs above.

The slab avalanches released on a sugary buried weak layer, but yesterday's rain saturated the snow and it froze solidly overnight.  Today, I found the crust supportable.  In fact I could barely stand up on a hill, and I slid back down to the lake twice because I couldn't even dig my skis into the icy crust.

It rained up to around 8500' on Thursday 12-14-06

By Friday morning 3.4 inches of water had fallen at the Tony Grove Snotel site in the last 72 hours.  Heavy snow built a stout slab on weak snow.  Here's a shot of some sparkly frost or surface hoar crystals taken in the area last week on 12-7-06.

Upper Bunch Grass

Some bold backcountry travelers triggered the upper avalanche by knocking a cornice onto the slope below.  The cornice broke unexpectedly, and one member of the party fell onto the slope.  Luckily, the slab below had already released by the time he hit the slope, and he landed on the bed surface unharmed.  The lower slide is a natural, which probable occurred with several others in the area overnight on 12-14-06.

The crown was 1-2 feet deep and about 90' wide.  A finger-hard wind slab released on sugary snow formed near the surface during last week's cold weather. I also noticed frost of surface hoar crystals in the culprit weak layer.

Substantial hard slab chunks made up the weighty pile of deposition.  In addition to the intentionally triggered avalanche, the party reported extensive audible collapsing and surface cracking in the area.