Avalanche Caribou Creek, N aspect

Location Name: 
.5 mile SE Caribou Lake, N aspect
Region: 
Selkirk Mountains
Date and time of avalanche (best estimate if unknown): 
Sun, 02/14/2021 - 14:00
Location Map: 
United States
48° 25' 33.042" N, 116° 39' 32.2416" W
US


Red Flags: 

Observation made by: Public
File attachments: 
https://dev.idahopanhandleavalanche.org/sites/default/files/ava-obs-files/birkeland_chabot_2006.pdf
Avalanche Observations
Avalanche Type: 
Slab
Slope: 
35degrees
Trigger type: 
Skier
Crown Height: 
1 ft
Aspect: 
North
Weak Layer: 
Other - explain below
Avalanche Width: 
300ft.
Terrain: 
Near Treeline
Elevation: 
5 900ft.
Bed Surface: 
Old Snow
Avalanche Length: 
500ft.
Number of people caught: 
0
Number of partial burials: 
0
Number of full burials: 
0
More detailed information about the avalanche: 

Information on events is second hand, snowpack observations performed in person two hours after event. Party digs a pit on slope of interest observing CT 23 on surface hoar, down 65 cm. Further upslope while on the way down the first skier going one at a time, in a group of 6, stops out of caution before approaching a convexity. From where first skier stops, a crack travels 15 ft downhill where propagation then occurs across slope of start zone at 35 degrees and down a slope sustained above 30 degree. No people were caught.

Weak layer is 1/23 surface hoar, variable SH size across bed surface. Bed surface on or just above 1/12 rain layer, bed surface boot penetation is 5 cm. Variable crown depth. From point of orign, lookers right half is a deeper and harder crown, 4F hardness with average crown depth 70 cm. Lookers left side, F hardness, 40 cm crown depth.  SS-ASu-R4-D2.

Pit dug by party on the way up was a safe representative location which ended up being 15 m from the point of origin, where first skier had stopped. From this pit, the avalache propagated above right and then downslope where left stauchwall was 2 m away from right side of pit wall, leaving pit area untouched. An ECT conducted two hours after event in the same pit resulted in ECT X. With no test results adjacent to a slope that was a human triggered slide I question myself in this appears to be poor craftsmanship. Yet with two additional hard hits after 30 the entire extended column, ECTP, slides out easily, Q1/SP, on 1/23 surface hoar. Once block is out, back and sides of ECT appear cleanly and evenly cut. No collapsing was observed around the area. It is possible the SH collapsed before or during avalanche event therfore changing test results at pit location. 

In this situation it would have been interesting and advantageous to find a safe spot to profile the crown. With the opposing test results and avalanche event next to one another, spatial variaibility could be another factor at play here. This possibly illustrates the complexity of the current snowpack. Persistent weak layer, here surface hoar, distribution and reactivity is variable in scale, the same slope as well as whole panhandle forecast area. Today we know a Considerable danger rating with a well known persistent weak layer. There is no one best test, formal and informal ways are both helpful to gain observations, its cumulative. For varying reasons, tests somtimes give false stable results, demonstrating importance in further safely seeking out known instabilities in gaining observations and making decisions; to "searth for signs of instability from as many data sources as possible (Birkeland and Chabot, 2006). If interested, attached file is a paper from Birkeland, Chabot 2006. The content under its title, "...Why digging more snowpits is a good idea" may prove to be valuable.

Weather Observations
Blowing Snow: 
No
Air temperature: 
Below Freezing
Wind Speed: 
Calm
Precipitation: 
Snow
Air temperature trend: 
Cooling
Wind Direction: 
Accumulation rate: 
Less than 1 in. per hour
More detailed information about the weather: