Persistent slab avalanche on Nyack Mountain

Location Name: 
Forecaster Observation - Nyack Mountain, Flathead Range
Observation date: 
Monday, January 15, 2018 - 14:30
Is this an Avalanche Observation: 
Yes
Observation made by: Forecaster

Tabs

Quick Observation

Traveled to Nyack Mountain to investigate the large Persistent Slab avalanche that occured late 1/12 or early 1/13. Details can be found under the avalanche tab.

Snowpack, Avalanche, Weather Images: 
Travel Details
Region: 
Flathead Range - Middle Fork Corridor
Route Description: 

Traveled the Rescue Creek trail to the location where the trail leaves the forest and enters the basin. Followed the lookers left avalanche path on the south side of the creek to the crown located on Nyack Mountain.

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
NE
S
SW
NW
Persistent Weak Layers: 
Buried
New Snow in the past 24 hours: 
0.00in.
Total Snow Depth: 
110"
More comments about the snowpack and weather: 
Recent warming has left a 2-3" breakable sun crust on sunny aspects at low elevations. Mid and upper elevations possess a surface slab that occasionally would crack under my skis but no shooting cracks, collapses or other signs of instability were noted. Surface hoar was forming at mid and upper elevations.
Avalanche Details
Avalanche Details: 
Date and Time of Avalanche: 
Saturday, January 13, 2018 - 01:00
Avalanche Type: 
Hard Slab
Failure Plane/Weak Layer: 
Old snow
More information or comments about the avalanche: 

This avalanche occurred either Friday night (1/12) or early Saturday morning (1/13) following a storm that deposited 18" of heavy dense snow at 6000' in nearby Wahoo Creek.

  • The avalanche started at 7500' on Nyack Mountain and was triggered by the heavy dense snow and/or the associated storm slabs directly above the crown.
  • Failure layer was weak faceted snow resting on a crust deep in the snowpack. Numerous rock outcroppings with associated weak snow are scattered across the face of Nyack Mt and the fracture line.
  • The crown varied from 2 feet to over 7 feet.
  • Of note was the ability of this slide to pull out a deep 8-10' slab off a low angle ridgeline.
  • Debris from this initial slide then triggered a Persistent Slab avalanche 1000 vertical feet below and off to the side of the path.
  • Most debris stopped at several terrain traps in the path but a relatively small amount traveled the entire 3500' to the valley floor.
  • Numerous storm slab avalanches were observed on either side of the path at upper elevations. 
  • Oddly, limited avalanche activity was noticed in the other slide paths that I could see in Rescue Creek.  
Hide Trigger
Trigger: 
natural
Hide Terrain
Start Zone Slope Angle: 
40
Aspect: 
North
Starting Elevation: 
7500'
Hide Size
Destructive Size: 
D4 - Could destroy a rail car, large truck, buildings, or swaths of forest
Relative Size: 
R4 Large
Crown Height: 
5 ft
Avalanche Length (Vertical Run): 
3500ft.
Avalanche Width (Average width): 
400ft.
Avalanche Location: 
Date and Time of Avalanche: 
Friday, January 12, 2018 - 01:00