Observation: Avalanche Professional

Advanced Information

Weather Summary

In the preceding weeks our terrain has evenly filled in many creek holes (not all of them), exposed rock, and medium-sized trees, creating a more uniform and smooth slope shape, especially in treed and rocky areas. Thus creating an above threshold snowpack- the minimum depth of snowpack needed to produce an avalanche.(Avalanche Canada) The Presidents Day storm cleared and cooled with unseasonably cold temps through the week of February 21-24. During this time, surface snow faceted over our midwinter snowpack. By Saturday, February 25th temperatures moderated to the low 20's and winds gradually increased through the morning, efficiently redistributing the loose cold, dry facets. Visibility was fair to good with any peeks of the sun ending fairly early. The winds were sustained at the upper end of moderate (17-25 mph) for nearly 24 hours with strong gusts. Cool temps continued into Saturday February 25th as evening precip started and continued throughout the day on Sunday February 26th. S1 continued with OVC skies and the occasional break for a couple BKN moments. Preliminary storm totals Saturday - Sunday are approximately 38cm. Storm snow was relatively light and estimated at 5- 7% SWE. (est 1.0" precip)

Avalanche Observation Comments:

Saturday morning winds gradually ramped up, creating a soft (4F-1F) shallow wind slab on facets. Leeward cross loaded steep terrain released naturally in 1 case, 10cm x 10m x 5m, remote triggered in another- by stomping with a ski was able to remote trigger a very small wind slab below a very small cornice 10cm x 3m x 3m, then able to intentionally trigger several leeward terrain features from small convex rolls to slopes from 35-40 degrees and up to 20cm deep. In another unique instance, intentionally triggered loose dry was able to gain a small amount of mass and trigger a windslab 15cm x 15m x 25m. On Sunday morning (HN12 15cm) reactivity continued in a similar sensitivity but different character. While the storm slabs were much softer (F) they were equally sensitive with an easy intentional skier trigger. In one case when a small shallow loose dry was able to gain enough mass (still quite small) and momentum to trigger a 15cm x 35m x 50m soft slab, the remaining flanks and crowns continued to be released by skier triggers. In another example, an intentional skier triggered slab, released in steep wind-loaded terrain running for several hundred feet. 25cm x 20m x 150m. The elevation dimension is representative of the crown, track, and deposit and not of the actual slab size.

Snowpack Observations

The snowpack obs with the best feedback were test slopes. By feeling with a pole for thinly covered crusts or deeper wind-deposited snow, one could easily trigger representative wind slab layers. However, in new strom snow drifts near ridge lines, trees, under small cornices with concave slopes only showed loose dry and no evidence of a cohesive slab release. By finding smooth convex rolls, the feedback was obvious. This is an important consideration when thinking of how to apply test slopes as part of observations. As great as they are, test slopes are also difficult to execute safely and effectively, with a representative slope being the key element for efficacy. Additionally, shooting cracks were present on smooth convex or planar slopes.

All other hand shears and column tests showed unconsolidated surface snow for storm slab obs.