February 24, 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: A ridge of high pressure which formed early in the week was well enough anchored over the province to repel any low pressure systems that advanced, sending them north or south of the region. Skies were sunny until Thursday, and temperatures climbed above zero even in the alpine. The weather forecast calls for increasing cloudiness leading into the weekend, a chance of a light snowfall, then a rapid drop in temperature as arctic air invades from the north. Strong gusty winds may develop as the arctic front approaches. SNOWPACK: Surface layers settled rapidly, and apart from some afternoon surface instabilities related to solar heating, overall the snowpack improved in strength. Surface hoar is found in protected areas. South aspects will form a sun crust as the temperature drops. A wind slab may develop over the weekend. Some areas still find shears on a buried surface hoar layer down about 40-60cm. At the base is the weak November facet layer, which is a concern in shallow snowpack areas. AVALANCHES: Surface sluffs and slab releases occurred on solar aspects in the afternoons, some up to size 2. Nothing deep or large was reported. Surface windslab failures may be a problem by the weekend. FORECAST OF AVALANCHE DANGER UP TO MONDAY EVENING (Feb 26th) ALPINE - Moderate TREELINE - Moderate BELOW TREELINE - Low TRAVEL ADVISORY: Watch those cornices! Observers report crevasses are poorly bridged this winter and warm weather like we just experienced increases the likelihood of breaking through a bridge and falling into a crevasse. NORTH COLUMBIA REGION WEATHER: Friday afternoon and evening, an unexpectedly intense snowstorm swept through the region. Snowfall amounts appear to be highly variable in the alpine, ranging from 7cm of snow to more than 25cm. Snowfall rates of 3cm an hour were recorded in Revelstoke for some time Friday night. By Saturday, arctic air had invaded as predicted, clearing skies and bringing temperatures down. This is predicted to continue for the weekend. Wind will pick up Saturday. SNOWPACK: Although storm snow amounts vary, some areas received significant, rapid loading. This storm snow fell on a widespread surface hoar layer, which was sitting on a weak faceted layer on protected slopes, and a sun crust on solar aspects. With the sunshine Saturday, the storm snow is settling into a soft slab, made worse by the increasing winds. Midpack there are still three more buried surface hoar layers within the top 100 to 125cm. The November facet layer sits at the bottom. AVALANCHES: Naturals to size 2.5 by Saturday morning, running on buried surface hoar. More action can be expected, with a strong possibility of failures to or stepping down into lower instabilities. FORECAST OF AVALANCHE DANGER UP TO MONDAY EVENING (Feb 26th) ALPINE ^Ö Considerable, areas of High TREELINE ^Ö Considerable, areas of High BELOW TREELINE ^Ö Moderate, areas of Considerable TRAVEL ADVISORY: The general trend in snowfall appears to be that southern areas got the bulk of the new snow, although up to 15cm variation in amounts is being reported even between two adjacent drainages. SOUTH COLUMBIA REGION WEATHER: The snowstorm that hit the North Columbias Friday night was still well underway Saturday morning in the South Columbias. Wide variability in snowfall amounts were being reported, but observers expect up to 20 or more centimeters in some areas by the end of the storm. Some low elevation areas started the storm with moderate rain. Winds were picking up Saturday. The forecast is for skies to clear by Sunday. SNOWPACK: The new snow is falling onto an extensive surface hoar layer, which on solar aspects was sitting on a thick sun crust. Some buried surface hoar layers midpack are still reported as moderate shears in snowpack tests, and the weak November facet layer sits at the bottom of the snowpack. AVALANCHES: Saturday morning, no natural activity was being observed yet, although cloud and snow made alpine observations difficult. Avalanche professionals are expecting activity to begin before the storm ends, with the storm snow failing on the surface hoar layer just below. FORECAST OF AVALANCHE DANGER UP TO MONDAY EVENING (Feb 26th) ALPINE ^Ö Considerable, areas of High TREELINE - Considerable, areas of High BELOW TREELINE - Moderate TRAVEL ADVISORY: The sun is forecast to begin shining once again by Sunday, and when this happens expect solar aspects to react with natural avalanche activity. The surface hoar was much larger and more uniform on North aspects before it was buried, so the storm snow will be particularly touch there. Avalanches could easily step down once triggered. Shallow areas will be areas where human triggering to the November facets will be more likely due to the new snow load. ROCKIES REGION WEATHER: The storm that hit BC Friday night had arrived in the Rockies by Saturday, with steady snow accumulating 10 to 25cm by Saturday noon, and still more falling. Winds were light, but the forecast calls for an increase as the arctic air advancing southward sweeps through, likely on Sunday. Temperatures will cool and skies will clear once the front passes. SNOWPACK: With the weak Rockies snowpack this year, 25cm of new snow and some wind is a significant loading event. The storm snow was light and fluffy Saturday, and will need to settle or be wind affected before it begins to react as a soft slab. Midpack strength had deteriorated in the last few weeks, and the base is the ugly weak November facets and huge depth hoar crystals. AVALANCHES: Some natural activity was showing up in the South Rockies by Saturday noon, but most other areas still had not seen much activity. Human triggering potential is increasing rapidly. FORECAST OF AVALANCHE DANGER UP TO MONDAY EVENING (Feb 26th) ALPINE ^Ö Considerable, areas of High TREELINE - Considerable, areas of High BELOW TREELINE - Moderate TRAVEL ADVISORY: Snowfall amounts were variable, and most snow was low density. Even light wind will cause wind transport in the alpine, further loading lee slopes and cross wind terrain features. Human triggered soft slab avalanches will be a serious potential once the new snow settles a bit or gets drifted into a soft windslab.