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PROFESSIONAL OBSERVATION FORM
 Start Date: 12.31.18 Name: Davis Operation: SAC
 Drainage/Route:  South of McDonald Peak Observed Terrain:  7,200-9,800, N-E-S Zone/Region:  Sawtooth Mountains
WEATHER OBSERVATIONS
 HN24: HST: 15-20 cm Sky: CLR Precip: Temps:
 Ridgeline Wind: Light from the N
 Blowing Snow: None Snow Available for Transport: Small amounts
 Comments: There were some clouds hanging onto middle elevations in the Boulder and Smoky Mountains that burned off by 1300. I observed blowing snow along the highest points in the Boulders. Other than than it was clear cold and mostly calm.
AVALANCHE OBSERVATIONS
 No. Location Date Size Type Trigger Aspect Elevation Comments
 1 Between McDonald and Cabin Peak Yesterday 2 SS N SE 9,450' Below/adjacent to a windblown ridge exposed to north wind. Looks to have started in fresh wind slab and stepped down into buried persistent weak layers. (12/11) seems likely based on a representative pit.
 1 Between McDonald and Cabin Peak Yesterday 1.5 SS N E 9,600' 200' below corniced ridge. Crown depth was much thinner. Did not step down.
 Comments: I observed at least 4 other good sized debris piles (D2 or slightly smaller). I assume most were small wind slab pockets but some could also be small dry loose.
SNOWPACK OBSERVATIONS
 HS: 60-135 cm 
 Upper Pack: New snow and some thin wind slab. 
 Middle Pack: Surface hoar layer down 40 cm atop a solid 1F mid-pack. 
 Lower Pack: 30 cm of 4F facets above 20 cm of F depth hoar at the ground.  
AVALANCHE PROBLEM ASSESSMENT
Avalanche Characteristics
Character/Avalanche Problem:
Persistent Slab

Layer/Depth:
40 cm

Likelihood of Triggering
Sensitivity:
Touchy

Spatial Distribution:
Specific
Terrain
  Primary Concern Comments: Found a SH layer that fits the depth of (12/11). This is the likely deeper failure interface pictured.
 
Avalanche Characteristics
Character/Avalanche Problem:
Persistent Slab

Layer/Depth:
100-135 cm

Likelihood of Triggering
Sensitivity:
Stubborn

Spatial Distribution:
Widespread
Terrain
  Problem 2 Comments: Depth hoar at the ground. Dug near the middle/upper elevation transition.
 
Avalanche Characteristics
Character/Avalanche Problem:
Wind Slab

Layer/Depth:


Likelihood of Triggering
Sensitivity:


Spatial Distribution:
Specific
Terrain
  Problem 3 Comments: Based on avalanche activity.
STABILITY TESTS
 Stability Test Results: Dug a pit in a slope representative to that with the slide. CT12 and ECTP12 SP on SH to 1 cm. CT18 SC and ECTP after a few extra "thumps" on Depth hoar at the ground.
SNOW STABILITY
 Snow Stability Rating for your observed area: Good Confidence: MODERATE Trend: Steady
BOTTOM LINE
 Comments: I was surprised to see a general lack of activity aside from a handful of small wind slabs. However, the avalanche pictured is a good reminder that weak layers in some areas of the Sawtooths are quite sensitive to relatively small loading events. If there is one pocket like this, there are bound to be others. I don't know that surface hoar played a role in that slide but it's certainly possible after finding an obvious 1 cm stripe of it in my pit at a similar aspect and elevation.
FILE ATTACHMENTS

181231_McDonald_06.jpg
(12/31/18) Wind slab that stepped down to deeper weak layers.

181231_McDonald_12.jpg
(12/31/18) Crown close up.

181231_McDonald_16.jpg
(12/31/18) Likely failure interface located down 40 cm in this location.

181231_McDonald_11.jpg
(12/31/18) Alpine wind affect.