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Thread: PNW snow pack
12-05-2005, 01:56 PM #1
PNW snow pack
post from Telemarktips - by Gary Brill
"We've already received significant snowfall this year (unlike last) with snow depths from 40" at 2700' to as much as 100" at 6000'. It however, has been a strange year already: A week of very heavy snow, nearly two weeks of clear, very warm weather, and now another 9 days of heavy snowfall at very low snow levels. We're now looking at ridging conditions the next week.
Yesterday's snowpack structure was quite interesting. A few inches of low density over 8" of denser storm snow, forming a soft slab. Beneath this was 2" of very loose graupel and beneath that 1/2" (1 cm) of very low density spatial dendrites - this is the weak layer. The bed surface is a smooth, slightly firmer layer of partially settled/new snow. This layer (4F-Fist) is still soft enough that, when skiing, one sinks above the knee. (The skiing is quite good).
Anyway, tests uniformly show that the top 12" come off as a slab in collapse or with very easy results. However, what enters into the equation is that the slab is not very cohesive (although it slides off as a unit in any tests). Accordingly it seemed that if a slab could be released it wouldn't propagate widely. Skiing confirmed this apparent behavior. Still, I wouldn't be positive of this interpretation because it is dependent on the hardness of the bed surface, and stiffness of the slab, both of which would vary with wind exposure. Skiing on opposite sides of glades confirmed this variation as did observation of wind effects on trees left and right of the glades. Accordingly, we kept our exposure to slope angles of 33 degrees or so, which felt safe, and test skiing confirmed this.
So, although there was a slab, a weak layer, and a bed surface, it was the lack of structure in the slab that really controlled the degree of slab hazard. The risk, however, had to incorporate two additional factors with regard to consequences - the speed at which any slab would be likely to slide which was defined by the bed surface/weak layer structure; and the fact that if a large enough slide could release, it would step down to about 40" - facets just above a crust from the 2 weeks of sunny, warm weather.
This provides a good example of how decisionmaking incorporates various factors that have unknowns and how balancing these factors is a significant part of evaluating probabilities and consequences: Risk = Probability X Consequences. In this case consequences were defined by size of slope (width and length) and the possible potential of deep slabs. The run-out was gradual and not channeled or gullied.
So, the solution to this problem with several unknowns (or hypotheses that weren't fully supported by hard evidence) was to assign a safety margin that was sufficiently large. A guess would be that avalanches might have released in the upper 30's degree angles. The slopes skied were 33-34 degrees max. They were skied singly, and initially on their edges with an astute attention to the "feel" of the snow while skiing.
12-06-2005, 01:45 PM #2Registered User
- Join Date
- Nov 2005
Those are interesting observations, and for where the approximate area where I think Gary was on 12/3 I can totally confirm them (even though I was on an E aspect), and I used the same decision making faced with the same information.
However, that information is pretty regional. We encountered different (more stable, no slab, less graupel layer) conditions one day later north of Stevens. Some of that but not all attributable to the extra settlement day.
Most of you probably know this already but you can get much more specific information @ http://www.avalanchenw.org/cgi-bin/search_reports.php (click the Search Reports button with no info entered to get all reports).