January 31, 2003 This Bulletin Sponsored by Columbia Brewery SOUTH COAST REGION WEATHER: The South Coast mountains received from 20 to 30 mm of precipitation leading up to Friday morning. In the alpine this fell as 15 to 30 cm of snow above 1500 m, with rain occurring at lower elevations. A cooling trend and moderate westerly flow will cause continuing snow showers through early Sunday, with freezing levels gradually falling back below 1000 m. SNOWPACK: Wind slab of varying thickness is found overlying the melt freeze crust from last weekend, most prominently on North and East aspects. Most observers report that this recent storm snow is reasonably well bonded to the crust below. The main concern this weekend will be potential weaknesses within the storm snow itself. These will gain strength over time, but will continue to be subject to human triggers. Cornices have grown large over the past week and are reported to be prime to start falling as they grow this weekend. AVALANCHES: A few thin human triggered slab avalanches have been reported on North aspects. With light to moderate snowfall and moderate to strong winds in the forecast through early Sunday, this activity is expected to continue. The potential exists for falling cornices to trigger larger avalanches. FORECAST OF AVALANCHE DANGER UP TO MONDAY EVENING (FEBRUARY 3) ALPINE ^Ö Moderate TREELINE - Moderate BELOW TREELINE - Moderate TRAVEL ADVISORY: As the westerly winds continue to load those slopes at treeline and above, watch for areas of unstable wind slab, particularly on North and East aspects. Corniced ridges should be avoided as recent precipitation combined with strong winds and warm temperatures has promoted rapid cornice growth. NORTH COLUMBIA REGION WEATHER: The past week has seen light to moderate snowfall at relatively warm temperatures accompanied by moderate south westerly winds. Freezing levels were around 1000 m on Friday and were forecast to lower to valley bottom by the end of the weekend. As the flow turns more north westerly this weekend, weather forecasters predict continuing light to moderate precipitation through Sunday, accompanied by moderate NW winds. SNOWPACK: Last weekend^Òs warm precipitation event produced a substantial crust up to about 2100 m when temperatures dropped. Wind slabs from 20 to 40 cm thick have formed above this layer from treeline up into the alpine, mostly on North and East aspects. Subtle weak layers within this recent storm snow are a concern heading into this weekend, as wind and continuing precipitation will continue to add to the load. A series of buried surface hoar weaknesses in the upper two metres will also come under increasing tension as loading increases. AVALANCHES: Significant avalanche activity earlier in the week has tapered off, although isolated human triggered thin slabs were still being reported on Thursday. With continuing wind and snowfall this weekend, it will still be possible to trigger avalanches in wind loaded areas near treeline and above. FORECAST OF AVALANCHE DANGER UP TO MONDAY EVENING (FEBRUARY 3) ALPINE - Considerable TREELINE - Considerable BELOW TREELINE ^Ö Moderate TRAVEL ADVISORY: Watch for areas of significant wind loading at all elevations and take note of any signs of instability such as cracking and whumpfing. Cornices will have weakened with the warm weather. If they fall they can easily trigger avalanches on the slopes below, so keep an eye peeled to slopes above whenever visibility allows. SOUTH COLUMBIA REGION WEATHER: The past week has seen storm snow amounts varying from 20 to 50 cm in the region. Freezing levels have moved between 1000 and 1500 m during the week. Winds were strong out of the south. As of Friday, winds are shifting to the West and are forecast to be north westerly by late Saturday. Freezing levels will begin to fall Saturday. Light to moderate precipitation is forecast to continue through late Saturday. SNOWPACK: Wind loaded storm snow overlies a melt freeze crust from last weekend. In some areas at treeline and below, the crust is weak enough that deeply buried weak layers are still a concern. Potential shears in the slab over the crust will continue to be problematic this weekend, particularly on North and East facing slopes. Wind loading will continue on East and eventually Southeast aspects through the period. Deeper instabilities continue to suffer from increased loading, and still represent potential failure planes. AVALANCHES: Numerous natural and human triggered avalanches have been observed throughout the region up to Friday morning, mostly involving wind slabs overlying the crust. Isolated avalanches have been triggered on the deeper weak layers in the Purcells. Human triggered avalanches will remain likely through the weekend. FORECAST OF AVALANCHE DANGER UP TO MONDAY EVENING (FEBRUARY 3) ALPINE - Considerable TREELINE - Considerable BELOW TREELINE - Moderate TRAVEL ADVISORY: Snowfall is expected to continue through late Saturday, adding to the load on a variety of snowpack weaknesses. As the wind shifts to north westerly later in the weekend, watch for the pattern of wind loading to shift onto East and Southeast aspects. The forecast cooling trend is gradual and won^Òt significantly strengthen the snowpack in the short term. KOOTENAY BOUNDARY REGION WEATHER: The melt freeze crust from last weekend has been buried by 10 to 30 cm of dense snow this week. By mid-day Friday rain was again falling up to nearly 2000 m, forming a new surface crust. Freezing levels should fall back to below 800 m later this weekend and light to moderate snowfall is forecast to continue through late Saturday. SNOWPACK: Near surface instabilities above last weekend^Òs crust will be a concern this weekend. The deeper weak layers may be less affected by human forces now that a surface crust has formed, but near Nakusp, the crust was still not carrying skiers weight so this effect is still somewhat limited. New surface slabs are likely to form through the weekend with forecast precipitation and winds shifting to North westerly. AVALANCHES: Leading up to the weekend, activity has been confined to the storm snow overlying last weekend^Òs crust. Forecast snowfall and moderate winds will continue to load these near-surface weak layers over the weekend and human triggering of relatively shallow slabs will remain a possibility. FORECAST OF AVALANCHE DANGER UP TO MONDAY EVENING (FEBRUARY 3) ALPINE - Considerable TREELINE - Moderate BELOW TREELINE - Low TRAVEL ADVISORY: Watch for the development of new wind slabs at and above treeline this weekend. The surface crust that is forming on Friday may become a significant weak interface in the snowpack as time goes by. Slab thickness may be greater in alpine areas that were not affected by Friday^Òs rain. SOUTH ROCKIES REGION WEATHER: Light precipitation during the past week has buried a crust with 5 to 25 cm of storm snow. Some areas that received almost no snow report a surface hoar layer formed during the week. On Friday, freezing levels climbed to 1700 m and light rain fell at lower elevations, with snow above. Light to moderate snow is forecast into Saturday, with gradual cooling and winds shifting into the West and Northwest. SNOWPACK: A moist snow surface well into the alpine on Friday will form a weak crust as temperatures cool on Saturday. Weaknesses within the storm snow are a concern for this weekend. Last weekend^Òs melt freeze crust is helping to protect the deeper weak layers. If there^Òs sufficient snowfall over the weekend expect both surface instabilities and deep weakness failures to become increasingly likely. AVALANCHES: Avalanche activity leading up to the weekend has been confined to the storm snow overlying last weekend^Òs crust. As precipitation continues and temperatures slowly cool down this weekend, activity involving wind slabs up to 40 cm thick is expected to continue. Natural avalanche activity may decrease with cooling but human triggering will still be very possible. FORECAST OF AVALANCHE DANGER UP TO MONDAY EVENING (FEBRUARY 3) ALPINE - Moderate TREELINE - Moderate BELOW TREELINE - Moderate TRAVEL ADVISORY: Snowpack conditions are widely variable throughout the region. Take note of any signs of instability such as cracking or settling of wind slabs. Some areas may have a layer of surface hoar underlying the recent storm snow. Large triggers such as cornice falls or a compact group of people in one spot may still be a trigger for larger than expected avalanches.