Not Yo Mama's Storm Slabs

Location Name: 
Forecaster Observation - Lake McDonald Area
Observation date: 
Wednesday, January 1, 2020 - 16:30
Is this an Avalanche Observation: 
Yes
Observation made by: Forecaster

Location

Tabs

Quick Observation

We went to Central Glacier Park looking for feedback from the new storm slab and old persistent slab problems. We found both, and a scary new week layer buried by the New Year's storm.

  • Soft storm slabs 4-12 inches thick were easy to trigger on steep test slopes at middle and upper elevations. These failed within the top-heavy, up-side-down new snow and were most reactive where previous wind had thickened the slabs. We got cracking around our skis and occasional shooting cracks in the new storm snow. Slabs failed within the the snow between about 5,500' and 6,000' elevation.
  • Barring any deep whumpfing on old persistent weak layers, we dug a test pit at 6,800' where the snowpack was just over 3' deep. We got consistent results on depth hoar near the bottom of the snowpack.
  • The most exciting results of the day were between 6,000' and 6,800' on west, south, and east aspects where we found surface hoar preserved above near surface facets and under the very thin 12/31 rime crust. At 6,600' we experienced collapsing on the surface hoar and went to a nearby test slope. Stomping from a flat area near the break-over produced a D1 avalanche that propagated across the entire slope and remotely triggered part of the slope next door. The storm snow at this location had also been previously drifted.
  • We did not see any recent natural avalanches, but visibility was poor.
  • We stuck to low-angled, well supported, planar slopes and kept out of trouble.
Snowpack, Avalanche, Weather Images: 
Travel Details
Region: 
Glacier National Park - McDonald Lake Area
Route Description: 

To 6800'

Activity: 
Skiing
Hiking
Snowpack Details
Snowpack and Weather Details: 
Hide Terrain
Elevation of observation: 
3500-5000 ft
5000-6500 ft
Above 6500 ft
Aspect(s) of observation: 
E
SE
S
SW
W
NW
Red Flags: 
Shooting cracks
Collapsing / whumpfing noises
Blowing snow
Persistent Weak Layers: 
Surface Hoar
Facets or Faceted Crust
Depth Hoar
Buried
New Snow in the past 24 hours: 
8.00in.
Total Snow Depth: 
1m
More comments about the snowpack and weather: 
Light rain below 4000' all day. Snowpack below 5000' consists of a few inches of slush. Light snowfall with periods of S2 until 3pm. Light W winds and occasional light blowing snow. Obvious previous drifting from SW winds. 12/31 rime crust found on all aspects and elevations in our travels. It's very thin, very breakable, but probably helps the soft new snow propagate a fracture wider than your average storm slab. Small SH was preserved under the rime crust between 6000' and our turn-around at 6800'. We found it in almost every hand pit on W-S-E aspects. Below the SH are NSFs. Bed surface for our biggest test slope result was the thin, decomposing, 12/24 crust. 12/24 crust is thick and hard up to ~5500' and gets thin, but is still present up to at least 6800' on W-S-SE (probably sun crust here).
Blowing Snow: 
Light
Wind Speed: 
Light (Twigs in motion)
Wind Direction: 
West
Air temperature: 
Below Freezing
Snow line: 
4500'
Sky Cover: 
Obscured by fog, etc (X)
Highest Precipitation Rate: 
Moderate Snowfall (S2)
Avalanche Details
Avalanche Details: 
Date and Time of Avalanche: 
Wednesday, January 1, 2020 - 13:00
Number of avalanches: 
2
Avalanche Type: 
Soft Slab
Failure Plane/Weak Layer: 
Old snow
More information or comments about the avalanche: 
Hide Trigger
Trigger: 
skier
Trigger Modifier: 
Intentionally Triggered
Hide Terrain
Start Zone Slope Angle: 
40
Aspect: 
East
Starting Elevation: 
above-treeline
Hide Size
Destructive Size: 
D1 Relatively harmless to people.
Relative Size: 
R4 Large
Avalanche Length (Vertical Run): 
100ft.
Avalanche Width (Average width): 
50ft.
Avalanche Location: