March 5, 2001 This Bulletin Sponsored by Survival on Snow NOTICE OF UNUSUAL AVALANCHE CONDITIONS: Valid until further notice. This season avalanche professionals between the Inner Coast Mountains and the Rocky Mountains have observed snowpacks that are unusually weak. The combination of below normal snow depth and low temperatures has produced layers of facetted grains and surface hoar with a low strength. These types of weak layers can persist for a long time, and the avalanche danger will increase with every load of new snow onto this weak base. The present snowpack is less stable than in most other years and remarkably different from the snowpack at the same time last winter. Back country users will have to pay much attention to snowpack structure and strength during this winter. SOUTH COAST REGION WEATHER: After a nice addition to the alpine snowpack, skies cleared and bright sun blanketed the winter playground by Monday morning. Winds died down to a light breeze in the alpine. The weather forecast calls for a ridge of high pressure to continue to build, keeping this pattern going for a few days. SNOWPACK: The new snow was reactive when it first fell. Many areas report a surface hoar layer buried just under the storm snow. The warm weather is promoting settlement and bonding. South facing aspects are loosing strength in the latter part of the day due to solar radiation. The midpack remains relatively strong, although low elevation moraine features are reported as weak throughout. The base is the weak November facet layer. AVALANCHES: The weekend started out with naturals and human triggered events to size 2.5, running loose in storm snow. This diminished to occasional naturals to size 1.5 by Monday, running as soft slabs and loose. FORECAST OF AVALANCHE DANGER UP TO THURSDAY EVENING (March 1st) ALPINE ^Ö Moderate, deteriorating in the afternoons TREELINE ^Ö Moderate, deteriorating in the afternoons BELOW TREELINE ^Ö Moderate, deteriorating in the afternoons TRAVEL ADVISORY: Watch for the effects of solar heating on southern aspects, weakening the snow and increasing the likelihood of natural and human triggered avalanche activity. Crevasse bridges will be weakened again by this period of sunshine. Watch shallow snowpack areas for triggering into the November facets. Check for cornices above you as you travel and particularly if you stop for a break. NORTH COLUMBIA REGION WEATHER: A Gulf of Alaska low pressure roared through the BC interior, leaving behind up to 25cm of storm snow by late Saturday, when skies began to clear again. Sunday was filled with bright sun, before cloud and light snow returned Monday. The forecast calls for a ridge of high pressure to form Monday night, bringing clear skies by Tuesday, although overnight lows will remain quite cool. SNOWPACK: Various sun crusts exist in the upper snowpack on south aspects. The storm snow fell on a widespread and very significant surface hoar layer, which is now down approximately 20-30cm. This layer was very reactive and will remain so for a while. Midpack, several buried surface hoar layers are still reactive after several weeks, and continue to let go with human triggering on steep terrain features. The lower snowpack is the November facet layer. AVALANCHES: Frequent avalanche activity to size 3.5 occurred this weekend, although most human triggered events were 2.5 or less. Reports of several close calls, including one full burial in an experienced party, were reported. The midpack surface hoar layers were reported failing with human triggers as well. . FORECAST OF AVALANCHE DANGER UP TO THURSDAY EVENING (March 1st) ALPINE ^Ö High, improving TREELINE ^Ö High, improving BELOW TREELINE ^Ö High, improving TRAVEL ADVISORY: Expect solar heating to cause deteriorating stability on southern aspects as the day progresses. As the storm snow settles, the potential for propagation will increase. Cooler overnight temperatures should help snowpack temperature recovery each night. Crevasse bridges will be weakened by the sun. SOUTH COLUMBIA REGION WEATHER: After a Gulf of Alaska low tracked through on the weekend, leaving behind some moisture, skies cleared briefly. Almost immediately, a low pressure system which came onshore in Oregon began pumping moisture northward into the region, causing heavy cloud and a mix of snow and rain. The weather forecast calls for a general clearing trend to begin by Tuesday. SNOWPACK: A widespread surface hoar layer was buried under the two pulses of snow that had occurred by Monday, and this layer remains reactive. Several surface hoar layers are buried in the mid snowpack and still showing up as shears in snowpack tests, as is a layer of facets down about 110cm. With the recent warm temperatures and new snow load, the weak November facet layer near the base of the snowpack is beginning to be reported as a failure layer again, particularly in areas with less snow toward the eastern edge of the region. AVALANCHES: Frequent releases were reported early in the weekend, failing on the surface hoar just under the storm snow. By Monday, occasional naturals to size 2.5 were still being reported, some failing to ground in shallow snowpack areas. FORECAST OF AVALANCHE DANGER UP TO THURSDAY EVENING (March 1st) ALPINE ^Ö High, improving TREELINE ^Ö High, improving BELOW TREELINE ^Ö High, improving TRAVEL ADVISORY: Watch for deteriorating stability on south aspects as the sun bakes down. The reports of failures to ground indicate that shallow snowpack areas are nearing the threshold were we may see more widespread activity on the deep November facets that have been down at the base of the snowpack all winter. ROCKIES REGION WEATHER: After starting the weekend out with some cloud, wind, and snow, the weather started on a trend toward clear skies on Monday. The weather forecast calls for a ridge of high pressure to continue to build, ushering in blue skies and sunshine by Tuesday afternoon. SNOWPACK: About 10-20cm of snow was added to the alpine, which was promptly moved around by the wind in exposed areas. The midpack continues the trend toward gradually becoming weaker, while the base of the snowpack consists of the weaker facets and depth hoar formed in the spring. AVALANCHES: Naturals and human triggered events were reported this weekend, including one slide in a wind loaded gully feature which took the life of an Alberta snowmobiler near Sparwood. Several incidents of slides releasing to ground were reported. One close call occurred when the bowl above Cascade falls let loose, with the climbers below narrowly escaping the tragedy which avalanches have caused this time of the year on this popular climb in previous winters. FORECAST OF AVALANCHE DANGER UP TO THURSDAY EVENING (March 1st) ALPINE - Considerable TREELINE - Considerable BELOW TREELINE - Moderate TRAVEL ADVISORY: The number of avalanches and human triggered incidents which involve avalanches failing to ground confirms the need to be particularly aware of the unusual conditions we face this winter due to a shallower than average snowpack. The strength of the midpack layers is slowly fading as spring approaches. Slopes that were consistently safe in previous years need re-evaluating before you go onto or below them this year, due to the unusual conditions that exist this season.