February 12, 2001 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 light dusting of snow, temperatures and wind moderated. By Sunday, a ridge of high pressure was building, although weather forecasters are predicting it^Òs end by Wednesday. Computer modeling of weather mid week conflicts, with some predicting moderate snowfall with brisk winds, while others predict only lightly unsettled weather and no significant snow. SNOWPACK: The cooler temperatures did not help upper snowpack strength, causing the storm snow to facet and loose some strength. The cool nights after the high pressure formed caused surface hoar development, with large crystals found at all elevations, especially in the alpine. Inland, south aspects developed another surface sun crust. The mid snowpack is still relatively strong except in shallow snowpack areas, and the weak bottom comprised of the November facets. These are particularly weak in areas of less than 100cm of snow. AVALANCHES: Surface slab activity to size 1.5 resulted from human triggering. FORECAST OF AVALANCHE DANGER UP TO THURSDAY EVENING (Feb 15th) ALPINE ^Ö Moderate (Rising rapidly if heavy snow develops) TREELINE - Moderate (Rising rapidly if heavy snow develops) BELOW TREELINE ^Ö Low (Rising rapidly if heavy snow develops) TRAVEL ADVISORY: The weather is hard to predict, with a ridge of high pressure decaying, a big storm building in the Gulf of Alaska, and cold air flowing into the region from the North East. With heavy loading from rapid snowfalls and/or wind, avalanche potential will build as loading increases. NORTH COLUMBIA REGION WEATHER: A mix of weather conditions occurred this weekend, with stiff winds in the alpine Saturday as skies cleared off, a very pleasant Sunday, then a light dusting of snow in the alpine greeting observers Monday morning. The weather forecast calls for the ridge of high pressure to break down Tuesday, with light to moderate snow developing in the Cariboo range and spreading southward. Temperatures should climb, and winds will pick up once again. There is a chance of heavy snow in the north parts of the region. SNOWPACK: A mixed bag of surface conditions exist. Most observers report the surface faceted and grew weak just before the weekend snow, which added only about 10cm on top of the facets. Surface hoar was buried under this 10cm dusting in some areas, and in other exposed locations, a thin windslab formed. Down about 100cm, an improving but persistent buried surface hoar layer gives some lingering instabilities. The lower snowpack continues to be the weak November facets, which are most troublesome in shallow snow areas. AVALANCHES: Some surface releases to size 1.5, generally with ski cutting and other human triggers. Throughout the region, the occasional size 3 to 4 continued to show up, either with cornice fall, explosive testing, or human triggering. These big releases are on the November facets and exhibit wide propagation. FORECAST OF AVALANCHE DANGER UP TO THURSDAY EVENING (Feb 15th) ALPINE ^Ö Considerable, rising if heavy snow arrives TREELINE - Considerable, rising if heavy snow arrives BELOW TREELINE - Moderate TRAVEL ADVISORY: These occasional large full depth releases represent avalanches which are potentially very deadly. Generally, big triggers like explosives or cornice fall are necessary, except for the occasional super weak zone which fails from the weight of a single skier or sledder. The best remedy for a situation like this may be to avoid big alpine features like large glaciers and steep bowls. These are the areas where finding the sweet spot that triggers the November facets could have VERY serious consequences. At least with smaller terrain features, triggering deep into the November facets may involve much less propagation and hopefully smaller, less dangerous avalanches if something does go wrong. SOUTH COLUMBIA REGION WEATHER: A ridge of high pressure began to affect the region on the weekend, bringing a general clearing trend. A light snowfall arrived for Monday morning. Weather forecasters predict the ridge to break down by mid week, but the amount of moisture which will accompany the breakdown is uncertain. The scenario with the most likelihood is moderating temperatures and light snowfalls by Thursday. SNOWPACK: About 15cm of new snow arrived late in the weekend, which has settled and gained strength since then. A buried surface hoar layer down about 70cm continues to fail in some areas, but it is also gaining strength overall. The lower snowpack is still weak facets formed in November, and this weakness is most pronounced in areas with less than 100 cm of overall snowpack. AVALANCHES: Some surface releases were observed, generally involving the storm snow and producing avalanches only size 1.5 or less. Of much more concern is some explosive testing conducted in the central Purcells, which produced large full depth releases propagating widely and running right to valley bottom. This indicates with large triggers like cornice fall, the November facet layer could be the failure plane. FORECAST OF AVALANCHE DANGER UP TO THURSDAY EVENING (Feb 15th) ALPINE ^Ö Moderate, areas of higher danger TREELINE - Moderate, areas of higher danger BELOW TREELINE ^Ö Low, areas of higher danger TRAVEL ADVISORY: More load in the form of rapid snowfall or high winds transporting snow into features will raise the danger. If heavy snowfall arrives, expect the danger ratings above to be much higher. The explosive test results indicate it may be prudent to avoid large features such as glaciers and open alpine bowls due to the potential for release on the November facets from something like cornice drop. ROCKIES REGION WEATHER: An arctic high pressure brought predominantly clear skies and cold overnight lows for the weekend. This is forecast to persist till Tuesday, when skies will become increasingly cloudy as the ridge of high pressure over BC breaks down. There is a chance of snow by Wednesday from a disturbance tracking in from the Pacific and likely moving right over the Columbia Icefields area. The amount of snow resulting is somewhat uncertain, but most expect the storm to track north and give moderate snowfalls in the alpine. SNOWPACK: Unfortunately, the cold weather caused further faceting in the snowpack, penetrating right into the lower layers. West of the Continental divide, where temperatures did not dip as low, this problem is less severe. A light dusting of snow sits at the surface and on Monday this has not been affected by wind. Surface hoar lies buried under the storm snow in the South Rockies. AVALANCHES: Ski cutting produces surface sluffing and smaller results in most areas. FORECAST OF AVALANCHE DANGER UP TO THURSDAY EVENING (Feb 15th) ALPINE - Considerable TREELINE - Considerable BELOW TREELINE - Moderate TRAVEL ADVISORY: If the wind picks up as the BC low tracks in, expect some pockets of higher danger to develop as wind transported snow fills in lee slopes and cross loads terrain features. If the storm tracks south and gives Fernie a dump, remember the buried surface hoar layer, which was still relatively un-reactive Monday, may become a failure plane. Prudent Rockies snowpack veterans are avoiding all big slopes in the backcountry on principal this winter, as the potential to trigger full depth releases into the November facet layer still remains significant.