Mt Washignton reported several wind slabs avalanches (up to size 1.5) occurring as a result of explosives control work on the mountain.
Past Weather
Large amounts of snow and rain have fallen in the past 24hrs and the forecast indicates additional precipitation in the form of rain and snow for Saturday and Saturday evening. Alpine and Treeline elevation bands will prevail in terms of gaining the most amount of new snow without rainfall, however the freezing level will rise considerably (from 850M to 1800M in the span of several hours in the evening of Saturday Dec 19. In addition to moderate precipitation (rain/snow) Strong to Extreme winds on Saturday will exacerbate leeward loading on downwind slopes creating new wind slabs.
Weather Forecast
Saturday: 15cm Snow and 5mm Rain, Winds Strong to Extreme from the SW, Freezing levels 850m throughout day rising to 1850M in early evening, Temps @1500m -2 (rising to +4 at 7p).Sunday: 6cm Snow, Winds Strong from the South shift to Moderate SW winds in afternoon, Freezing levels to 1,050m, Temps @1500m -2. Monday: Trace amounts of snow possible, Winds Moderate from the NW, Freezing levels 500M, Temps @1500m -5.
Terrain Advice
Careful route finding and snow-pack evaluation are paramount during periods of sustained moderate to strong snowfall this weekend as well as a fluctuating air temperature (Critical Warming) and strong/extreme winds.Avoid traveling both above and below cornice features.Careful and cautious route finding when entering into or over convex rolls or steep features.Human Triggered avalanches are "likely" to be initiated by skiers/snowmobilers/snowshoers on Saturday, Sunday and possibly Monday if forecasted precipitation and winds arrive as scheduled. Finding lower angled terrain (generally less than 30 degrees) should be considered an appropriate choice given the increasing avalanche risk over the course of the next 3 days.Mt Washington Alpine Resort: Please no ski touring on Mt Washington as explosives control work and heavy equipment are on the mountain in several locations
Snowpack Summary
Limited observations at this time however snowpack depth decreases substantially lower than the 1200M elevation band. Recent snowpack summaries show a depth of 120cm of Snow at an elevation of 1400M in the Mount Washington area. Several melt freeze crust have developed on all aspects at the Treeline and Alpine elevation bands on Vancouver Island. The upper snowpack melt freeze layer is not yet completely frozen (in certain areas) and reports show that this layer is still isothermal (0 degrees) and slushy (in certain areas). In zones where freezing temperatures have remained consistent, the upper snowpack melt freeze crust is frozen completely, a facet layer is developing and is showing results on testing as a result of recent storm snow from this past week initiating on this melt freze crust interface. However, no reports of skier/snowmobile triggered avalanches at this time.
Snowpack Details
- Surface: Storm Snow and Wind Slab instabilities will exist at Treeline and Alpine Environments this weekend
- Upper: Well bonded and dense moisture laden snowpack that contains a melt freeze crust that is showing consolidation with new snow above
- Mid: Well settled
- Lower: Well Settled
Confidence
High - Weather models in agreement, Moderate field data available, insufficient Weather station data.
Problems
Wet Slabs
Wet Slab avalanches are the release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slabs can be very unpredictable and destructive.
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.