February 12, 2010 Avalanche Advisory

Good morning backcountry travelers! This is Steve Karkanen with backcountry avalanche information for Friday, February 12, 2010 from the West Central Montana Avalanche Center.

Thanks to Bob Skiles of Seeley Lake for the excellent snowmobile safety class he provided Avalanche Center and other Forest Service employees this past week. Bob’s dedication to snowmobile and winter backcountry travel safety is commendable as he volunteers his time to teach snowmobile safety classes to many people in the community especially youth in the local school district.

Current Avalanche Danger

On all slopes steeper than 35 degrees and on all aspects above 6000 feet, the avalanche danger is MODERATE. Natural avalanches are unlikely, human triggered avalanches are possible. On all other terrain above and below 6000 feet, the avalanche danger is LOW, natural and human triggered avalanches are unlikely.

Weather and Snowpack Analysis

Once again, SNOTEL sites recorded minimal amounts of precipitation, if any at all. Mountain temperatures have cooled this week mostly due to the sun not breaking through the stratus clouds hovering at mountaintop locations most of the week.

The cooler temperatures are allowing faceted crystals to persist in the upper 20cm of snow as well as a thin layer of buried surface hoar about 18 inches deep in the snowpack.

We received an excellent but unconfirmed report from skiers in the southern Mission Mountains who triggered an avalanche last Saturday on a 40-degree ENE aspect at 8400 feet. They dropped a cornice and performed a ski cut with no results. One skier dropped in and the slope released while being skied. They estimated the slope propagated about 300 feet across and ran 1200 vertical feet. They reported this as a size class 2 to 2.5 and the bed surface was the buried surface hoar 16”-18” deep.

We were unable to confirm this but it is significant in that this avalanche shows the variability in stability across the region.

The group had radios, communicated the fracture to the skier who immediately cut left into a safe zone and safely watched the slide go by.

This is an excellent example of using safe skiing protocols, having outstanding communication equipment and skills as well as recognizing the potential hazard even after slope tests showed stable conditions. If the entire party chose to ski the slope together, this could have been a tragic situation. We are seeing many examples of poor route finding and poor safe skiing technique and happily, this group has shown how important good communication and decision-making is during periods when conditions cannot be trusted.

Yes, we are rating the hazard at Moderate and Low these days, but remember what we have been saying all along this winter. We have buried facets at the ground and at a couple of weak layers in the upper snowpack. Yes, it takes a lot of force to produce failure at these layers but when they fail, they fail with energy and will be very difficult to escape if you are caught in a slab avalanche under these conditions.

In the Rattlesnake, a layer of facets that formed between two ice crusts on slopes that were exposed to the sun last weekend are the weak link and have the most potential to fail if we receive significant snowfall. Weather conditions have been similar throughout the advisory area so I would expect to find similar upper snowpack conditions on any southerly slope above 6000 feet. This weakness frequently begins to develop during the type of weather conditions we have been watching the past 3 weeks. We measured steep temperature gradients in the upper 20cm of the snowpack (between the 2 crusts) which allowed the faceting process to continue and cooler temperatures this week are allowing this condition to persist. This is limited to slopes that are most prone to warming during the clear warm days (south to southwest) the past 2-3 weeks. A YouTube video we shot yesterday clearly shows this feature.

We still find a buried surface hoar layer at mid-pack and depth hoar at the bottom 20cm but these layers are showing a lot of strength in all our observation site test pits, and it takes a lot of force to get these layers to fail.

Nevertheless, faceted snow cannot be trusted and the variability from one slope to another has to be factored into your decision-making.

Weather Forecast and Avalanche Outlook

Two weather systems will move through the Northern Rockies early Friday through midday Saturday. Each system will weaken dramatically as they move through a longwave upper ridge thereby limiting snowfall accumulation.

Without much new snow in the forecast, the avalanche danger will remain constant except for areas that see a lot of wind moving snow around. Cooler weather will allow the weaker layers to persist and high winds will quickly load leeward terrain even with the minimal amount of snow we received the past 4 days. Our overall snowpack is in a mostly safe condition but there are enough weak layers and so much variability that you have to assume dangerous conditions exist on some terrain features at the higher elevations. The avalanche reported to us from the Missions is all the evidence you need to know that it is possible to trigger an avalanche in some areas.

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If you get out and see avalanche activity or want to send us quick snow observations, send us a note at [email protected] or call us at 406-530-9766. 530-9SNO. Dudley will post the next observation on Presidents Day, Monday, February 15th.