Thoughts on different weak layers and avalanche problems
November 26, 2018
We’re continuing our series on WYSAW take away points. Another theme that emerged in WYSAW was an effort to understand weak layers and specific avalanche problems. Here are highlights focused on persistent slab problems and avalanche forecasting:
Trends of Persistent Slabs Following Loading Events – Jason Konigsberg
Jason is a forecaster for the Colorado Avalanche Information Center (CAIC) and as such faces the challenge of when to lower the danger to LOW from time to time. Many forecasters agree that Low Avalanche Danger is probably the hardest level to go to and is particularly difficult in the land of persistent and deep slabs. Jason’s curiosity drove him to put some numbers to the number of days out from storms, so he looked at avalanche activity based on storm loads of 10cm in one day and sustained strong winds. Seeing that avalanches continued more than 7 days after these loading events, he looked at the subset of avalanches that occurred more than 7 days following loading. He found that even small loading events with as little as 1-2” of snow would be potential triggers for these persistent slabs. During periods of high pressure with no loading whatsoever there were very few avalanches more than seven days out. Now forecasters have some data to work with when they are looking at going to Low.
More details can be found in The Avalanche Review36 #3, February 2018, page 21. OR
Teton DDL – A Science Perspective – Patrick Wright
Patrick explored why the December Drought Layer may have been so persistent, as well as a good overview of the different periods of Activity and Re-Activity. The interesting thing that I gleaned was that the overall warmth of the snowpack may have contributed to the prolonged faceting of the early December snow over the Thanksgiving Crust. It was not a melt-layer recrystallization process as the crust was thoroughly frozen, but in many areas the snowpack had been isothermal on Thanksgiving so that there was a lot of residual heat that remained in the lower snowpack (below the crust). He hypothesized that heat may have prolonged heat transfer through the crust while diurnal faceting was occurring in the upper snowpack. That mechanism may have turbo charged the faceting process so that the facet layer was quite thick in many areas and the facets were well developed.
Teton DDL – A Practitioner Perspective – Don Carpenter and Don Sharaf
Don and I explored the DDL layer further by looking at its evolution (formation, burial, activity, dormancy, reactivation, extinction/removal) and then how we managed groups in the field given its persistence and frequent reactivation. Don C illustrated the challenges of opening terrain when there is still uncertainty for a low likelihood/high consequence avalanche. The best quote came from Brian Gorsage (WYDOT) who said “You don’t have to be right, you just can’t be wrong”
Take home points were:
Forecasting for an Untouchable Snowpack – Kevin Hammonds
Kevin is now a professor at MSU and runs the cold room for the university. In the past few years he has given us much to ponder regarding faceting around crusts, but in this talk he looked back at an accident (2010) that occurred while he was a climbing ranger on Mt. Rainier anda meterology student at the University of Utah. The thrust of his talk was that many areas are too remote, or too high to be easily forecasted for (either by conventional weather data or for avalanche hazard). James went through the weather data available at the time of the accident to see if he could have made a fair estimate of the amount of loading in the start zone of the avalanche. In 2010 with some expert analysis, he was able to create a fair estimate of precip amount (SWE) and wind speed and direction for 12,000’ (600 mbar). The meteorology products have become even better since then and with today’s commonly available products and familiarity with your forecast zone some estimates could be made for areas that lack instrumentation.