February 7, 2002 Canadian Avalanche Association Bulletin NORTH COLUMBIA REGION WEATHER: A series of light disturbances tracked through this week, leaving behind up to 30cm of storm snow in total. Winds were generally light, but a 12 hour period of strong SW winds Tuesday caused some snow transport and wind slab development. The weather forecast calls for significant snow and very warm temperatures to begin Saturday and last through the weekend. Strong winds may develop Sunday. SNOWPACK: The upper snowpack has improved this week, but the continual light loading has slowed down improvement. Some wind slabs formed over less dense snow in the alpine, and these were occasionally reactive to ski cutting. By Wednesday, the January 2nd layer of buried surface hoar had become reactive to explosive testing. Observers report high variability in wind effect and storm snow amounts. AVALANCHES: Early in the week, explosive testing and ski cutting produced results in the storm snow. Following Tuesday's wind event, natural activity started to occur on lee slopes. By Wednesday, failures into the January 2nd layer down about 80cm were common. With rapid warming and more snow in the forecast, this will become more widespread. FORECAST OF AVALANCHE DANGER UP TO MONDAY EVENING (Feb 11) ALPINE - High TREELINE - High BELOW TREELINE - Considerable, deteriorating TRAVEL ADVISORY: High variability in conditions, regular natural and human triggered avalanche activity pre-storm, and rapid warming combined with heavy snow in the forecast will lead to a low confidence weekend. Conservative terrain will be a good place to be. Keep a wide margin for error due to the variability. SOUTH COLUMBIA REGION WEATHER: A ridge of high pressure over the area early in the week kept the weather favourable, but that changed Wednesday when the ridge retreated to allow some moisture inland. Up to 30cm of storm snow resulted. A brief break is in the weather forecast, until another more significant storm hits on the weekend. SNOWPACK: The upper snowpack had been improving until the new snow Wednesday, which developed some surface instability. Some wind slab development tended to make this worse in some areas. By Wednesday, the January 2 buried surface hoar layer, down 60 to 80cm at treeline, had become reactive. The November crust, down near the base of the snowpack, has begun to awaken due to the continual loading. AVALANCHES: By Wednesday, explosive testing and ski cutting reliably produced storm snow releases. Earlier in the week, a skier triggered a remote avalanche in the Purcells, which started on the January 2nd surface hoar layer and stepped to the November crust. Nearby in the Selkirks, a snow cat building an access trail triggered a slide that broke to the November crust the next day. FORECAST OF AVALANCHE DANGER UP TO MONDAY EVENING (Feb 11) ALPINE - High TREELINE - High BELOW TREELINE - Considerable, deteriorating TRAVEL ADVISORY: Keep your eye on the weather this weekend. The coming storm is predicted to be more intense in the North Columbias, but could easily track further south instead. Highly variable snowpack conditions leading up to the storm will make it more difficult to make safe decisions this weekend. Wise old snow dogs will be leaving a wide margin of error this weekend, and avoiding extreme terrain like the plague. ROCKIES REGION WEATHER: A ridge of high pressure over interior BC managed to keep the Rockies relatively dry until Thursday, when 10cm of snow fell in the Mountain Parks region, and up to 30cm in the Fernie area. The weather forecast calls for a series of light snowfalls to continue through the weekend, accompanied by mild temperatures and moderate winds. SNOWPACK: Warm temperatures lately have improved the snowpack somewhat by promoting settlement. Some wind slab development in the alpine represents a weakness at the interface below. The recent storm snow was failing at the interface to the old snow, particularly in the Fernie area. Down 50cm, a buried surface hoar layer with facets just above persists as a weakness. The November crust is reacting to large triggers. AVALANCHES: By Thursday, natural avalanches to size 2 were common, failing in the storm snow. A large cornice fall early in the week near Fernie was observed as the trigger for a large avalanche that failed to the November crust. In the Bow corridor near Banff, a similar cornice fall caused a size 3 that stepped to ground. FORECAST OF AVALANCHE DANGER UP TO MONDAY EVENING (Feb 11) ALPINE ^Ö Considerable, deteriorating if significant snow occurs TREELINE - Considerable, deteriorating if significant snow occurs BELOW TREELINE - Moderate TRAVEL ADVISORY: A lot depends on the weather this weekend. Some computer models are predicting significant snowfall as a possibility, while the extended public forecast predicts only occasional snow showers. With the frequent naturals observed, and two widely separated occurrences of large triggers awakening deep weaknesses, it is a time to avoid big terrain features. Anything that breaks deep will be highly likely to propagate widely to something quite large and deadly. Since cornice fall was the trigger for both deep releases, certainly keep an eye on that before entering a slope or grouping your party.