Banff, Yoho and Kootenay National Parks Issued: Sunday, February 17, 2008 16:00 Valid Until: Monday, February 18, 2008 16:00 Bulletin Area: This forecast covers the east and west sides of the Continental Divide from the Wapta Icefields area in the north to the Sunshine area in the south. It also includes the Main Range area from Lake Louise to Bow Summit. Danger Rating: Sunday Monday Tuesday Alpine 3-Considerable 3-Considerable 3-Considerable Treeline 3-Considerable 3-Considerable 3-Considerable Below Treeline 2-Moderate 2-Moderate 2-Moderate Synopsis: Though natural avalanche activity has abated in the last 24 hours, the instabilities in the snowpack remain prime for human triggering in steeper, unsupported terrain. The key layers of concern continue to be: the recently developed surface slab at treeline and above, the January 27th facet interface, down 50 to 70cm and of course ubiquitous basal facets. Recent natural activity has involved each of these instabilities. Triggers have been either the failure of well developed cornices, or surface slabs pulling out of steep, unskiable terrain. Almost without exception, these triggers would subsequently step down, to the January 27th facet layer, then the basal facets, resulting in full depth slides running full path. Confirmation of just how touchy these weakness are was observed again today in field tests in the Bow Summit area, on a 30 degree, Northeast slope at 2300m. Rutshcblock scores of 3 and 4 were recorded below the new slab and at the January 27th interface, respectively. The surface slab failed with a sudden planar characteristic, while the January 27th interface failed with sudden collapse characteristics. Tomorrow, should the forecast clear skies be realized, temperatures in the valley bottom will climb to above zero, even in shady locations. On Southerly aspects, the solar affect will see such temperatures being felt well into the alpine by the afternoon. This rapid warming will result in natural avalanche activity on solar aspects, as cornices deform and break free and surface slabs creep at a rate that overcomes the shear strength with the interface below. Avalanches to size 3, as observed in the last couple days, will likely result. Expect this aspect to enter a High Danger Rating as daytime warming reaches its maximum. Continued caution must be exercised on steeper, unsupported slopes at treeline and above and route finding must consider overhead hazards, particularly on solar aspects in the afternoon. Avalanche Activity: No new avalanche activity was observed today. However, late in the afternoon yesterday, a number of new, size 2 to 3 avalanches occurred along Highway 93 North, on various aspects in the alpine. Slab avalanches were pulling out of steep terrain, or being triggered by cornice failure. Full depth, full path slides resulted. Outlook: It's a bit early to say good by to winter, but I'll be the first in line to say hello to spring as next week starts! Under the influence of a warm and dry high pressure system, we will see a daily pattern, of temperatures in the valley bottom and on solar aspects, climbing above zero, as the sun works it's magic. Natural avalanches should be expected on solar aspects, as instabilities in the upper snowpack are awakened by rapid warming and as hanging cornices deform and brake free, triggering slides below. As the week roles on we should also see a suncrust forming on southerly aspects. Keep this layer in mind when the next snowfall comes, as it will likely be our next performer. Travel Conditions: Ski penetration is dropping to 15 to 20cm, as the recent additions to the snowpack are settling out into a more cohesive base. Great turns can be found on most aspects at and below treeline, where the wind's affects have been limited. As you move into the alpine you'll quickly run into wind pressed and scoured conditions that vary significantly across terrain. Don't forget your sunscreen! MG