January 4 Avalanche Advisory

Weekend Avalanche Advisory
Posted January 4th at 0600

This is Steve Karkanen at the West Central Montana Avalanche Center with the weekend avalanche advisory for January 4th -6th, 2008. The avalanche danger rating expires at midnight January 4th. This report is based on field observations and data collected on Thursday and describes conditions seen at that time. This advisory is the sole responsibility of the Forest Service and does not apply to operating ski areas.

Weather and Snowpack Analysis

The New Year ushered in a bit of a break from the seemingly non-stop snow and wind all areas within the advisory area have seen the past several days. We received reports of large natural avalanches in the Bitterroot near Lolo Peak, and in the Southern Swan and Southern Mission Mountains that released on or before December 31st during the last of the storm cycle that blasted through the area prior to New Years Day. Most locations received close to 20” of new snow since last Friday with high W-SW winds associated with the storm passage.

Since New Years Day, temperatures and wind speeds moderated and allowed the snowpack to settle out and gain strength. While most observation sites are showing much improved stability, the weak layers we have been describing for several weeks have persisted and are now deeply buried*. This is somewhat disconcerting as it takes a lot of force or weight to trigger failure at this level but when it fails it fails with a vengeance, as if it were spring-loaded. Although it takes a lot of force or weight to initiate failure, this type of slab can be very destructive. With this in mind, the best way to mitigate the hazard is to expose only 1 person or 1 sled on any steep open slope.

*(see a good representative photo and snow pit profile of this in our photo gallery at http://www.flickr.com/photos/missoulaavalanche)

All observers are reporting very similar conditions from Lost Trail Pass to Lookout Pass in the Bitterroot, the Rattlesnake and Southern Swan and Mission Mountains. Of course the depths vary greatly but the layer of weak faceted snow is a widespread condition. Some locations also have buried surface hoar at varying depths but the predominant concern is with the faceted snow that formed early in December after the first avalanche cycle and warm up that occurred. This layer can be found between 50 and 80cm from the ground at most high elevation sites. It can be found on all aspects and seems to be more sensitive on shallower snowpack.

Today’s Avalanche Danger

Above 5000′ in all advisory area locations, the avalanche danger is MODERATE. Natural avalanches are unlikely. Human triggered avalanches are possible. Be aware that there may be pockets where the avalanche danger is CONSIDERABLE especially on recently wind loaded terrain steeper than 35 degrees. Natural avalanches are possible and human triggered avalanches probable. The most likely place to find these pockets of unstable snow is on East to North aspects.

Below 5000′ the avalanche danger is LOW. Natural avalanches are very unlikely and human triggered avalanches are unlikely. Low avalanche danger does not mean there is no avalanche danger. Snow is as variable as the mountains we like to play on and you can almost always find a pocket of snow that will avalanche given enough weight or shock applied to it.

Weather Forecast and Avalanche Outlook

An upper level trough will move into our area Friday bringing snow accumulation and gusty winds to the higher terrain. This storm starts out warm then gradually cools. This is just what we want for the New Year, more snow and no rain.

Expect snowpack stability conditions to gradually strengthen until significant precipitation returns to the area.

As mentioned earlier, stability conditions have greatly improved over the entire area. The main concern will be how well new snow will bond to the current snowpack and just how much weight or shock the weak layer can take. We have seen 2 active avalanche cycles, the latest over the holidays, all associated with facets that formed on the old snow surface after the warm weather during the first week in December. This most recent period of avalanche activity was notable in that the size and depth of the reported avalanches is of a nature that we have not seen here for several years. We’ve enjoyed very stable conditions in Western Montana for the past 6 years and many of us have grown accustomed to this. Don’t let your guard down. It may take a lot to get a failure at this deeper instability but the consequences would be devastating. If you must ski or ride on avalanche terrain remember; 1 at a time, watch your partner from a safe area, always wear and carry rescue equipment (transceiver, probe and shovel) and have a plan if something bad happens. Your companions need to rescue you, nobody else can.

The next avalanche advisory will be posted on January 11th, 2008.