UDOT PLANNED AVALANCHE CLOSURES!!

Avalanche: MacDonalds Draw

Observer Name
M Dawson with investigation the following day by Bruce Tremper
Observation Date
Tuesday, December 11, 2012
Avalanche Date
Tuesday, December 11, 2012
Region
Salt Lake » Park City Ridgeline » MacDonalds Draw
Location Name or Route
Macdonalds
Elevation
9,700'
Aspect
Northeast
Slope Angle
40°
Trigger
Skier
Trigger: additional info
Sympathetic Release
Avalanche Type
Hard Slab
Avalanche Problem
Wind Drifted Snow
Weak Layer
Facets
Depth
2'
Width
100'
Vertical
400'
Snow Profile Comments

Snow profile and these comments done by UAC forecaster, Bruce Tremper the following day.

It was a classic wind slab on faceted snow. The faceted snow formed during 2 weeks of clear weather in November and is now buried about 2 feet deep. The strong winds from the past couple days deposited a hard, stout wind slab on the snow surface, which was 1 - 1.5 feet thick. It overloaded the buried faceted layer. Luckily, the party that triggered the avalanche recognized that the slope may be unstable so they chose a gentler spur ridge to the north. Their weight collapsed the slope and when that propagating collapse traveled about 80 feet and when it reached the 40-degree slope it triggered the avalanche. Luckily, no one was caught because it washed down into trees, which would not be good news.

The snow profile was different than some of the other avalanches we investigated along the Park City ridge line because there was no rain / rime crust on top of the faceted snow layer. Many of the other avalanches occurred because of faceted snow associated with that crust but in this case the crust was absent. As usual, if you have a weak layer and it's quickly overloaded with too much weight, you will get an avalanche. We should continue to avoid any slope with the combination of weak, faceted snow and a recent wind deposit on top of that layer. This combination exists mostly on the north through east facing slopes especially in upper elevation, wind affected slopes.

This slab-weak layer combination is quite variable from place to place and it's complex because of the subtleties of the rain / rime crust that was deposited on top of the faceted snow, among other reasons. My stability tests along the fracture line were quite variable. In some places, it would pop out easily on the faceted layer and in others it would not propagate at all. It's normal to have a lot of "spatial variability" when we are not getting a lot of snow. It's also common to get inconsistent results when you do the tests the day after the event. I'll bet that yesterday, the snowpit tests done on that slope where the avalanche occurred would have consistently produced spectacular results. But, of course, just approaching the slope would have triggered the avalanche as yesterday's avalanche clearly demonstrated. I'm glad this party had the knowledge to manage their terrain (they chose gentler slope angles). Otherwise, this investigation could have been much more serious.

Comments
Comments from the reporting party: Prior to the event, party of three experienced some collapsing during the ascent and evidence of natural sluffing. Due to wind loading on the ridge, Party decided to descend a low angle slope adjacent to, and to the North of, the culprit slope. The slide released sympathetically after the third skier descended. The start zone approached 40 degrees, and the slide ran over 400 feet vertical and well into the normal slope run-out zone.
Comments

Video of the snow profile by Bruce Tremper and Trent Meisenheimer.

Video
Coordinates