Making Decisions with Wind Slabs

Location Name: 
Forecaster Observation: Lake McDonald Area
Observation date: 
Monday, January 4, 2021 - 17:45
Is this an Avalanche Observation: 
No
Observation made by: Forecaster

Tabs

Quick Observation

We toured up the west ridge of Mt Edwards to investigate recently developed wind and storm slabs.

  • On the way up the ridge our main concern was the wind slab problem. The wind during the last storm had come from variable directions leaving wind slabs on different aspects of the ridge. We noticed cracks up to a 6 feet long in some drifts. We dug a pit in one of these slabs on a mid elevation, northwest aspect and got a propagating result in the wind slab. Up higher there were cornices building and we were able to knock off some large blocks, however it did not entrain any snow on the slope below them.
  • The day started out with calm winds and a high cloud ceiling. The winds picked up as a storm front moved in around noon, winds began gusting up to moderate with some ligh blowing snow at ridge tops.
  • We found the freezing rain crust from New Years at mid to upper elevations. The crust was more prounounced the higher we went. We felt some localized collapses that might have been this crust breaking or it could have just been buried logs in the snowpack.
  • At low elevations, below 4200', there was a rain crust from yesterday morning.
  • On open south facing aspects at low elevations we noted a sun crust from yesterday afternoon.
  • We did not notice any recent avalanche activity in the surrounding area during the morning while the visibility was good.
  • On the way down we did see rollerballs just starting to form on a south facing aspect at mid and lower elevations.

In order to manage the wind slab problem we decided to ski wind sheltered slopes where we felt ok skiing steeper terrain.

 

Snowpack, Avalanche, Weather Images: 
Snowpack, Avalanche, Weather Videos: 

1/4/21 - Making Decisions With Wind Slabs

Travel Details
Region: 
Glacier National Park - McDonald Lake Area
Route Description: 

To 7200'

Activity: 
Skiing
Snowpack Details
Snowpack and Weather Details: 
Hide Terrain
Elevation of observation: 
3500-5000 ft
5000-6500 ft
Above 6500 ft
Aspect(s) of observation: 
N
S
SW
W
NW
Red Flags: 
Shooting cracks
Collapsing / whumpfing noises
Blowing snow
Rollerballs / pinwheels
Persistent Weak Layers: 
Facets or Faceted Crust
Buried
Total Snow Depth: 
1m
More comments about the snowpack and weather: 
Snow surfaces consisted of 4F wind deposits on lee aspects where ski pen was ankle to knee deep, to a breakable sun crust on open solar aspects, to a few inches of light snow on top of old wind surfaces and rime crusts in fetches near and above treeline (ski pen 1"). The 1/1 crust was exposed by wind on some upper elevation ridgelines and was about 1cm thick there, elsewhere it was buried about 10cm down and was eggshell thin. We dug at 5500' on a NW aspect in a wind slab where we got some cracking: ECTP18 down 31cm in F hard DFs 5cm below the buried 1/1 cr; the WS here was F+. We dug on a S aspect at 6100' where HS=105 to look for the PS structure: ECTNs with easy and hard force down 20cm under the 1/1 cr, down 45cm at a 4F-1F interface, and down 70cm above the 12/9 cr; no propagation.
Blowing Snow: 
Moderate
Wind Speed: 
Moderate (Small trees sway)
Air temperature: 
Below Freezing
Snow line: 
3000
Highest Precipitation Rate: 
Light Snowfall (S1)