Regions
Vancouver Island.
Cornice Fall and wet loose avalanches observed in steep unsupported terrain (size 1) observed at Below Treeline elevations on Monday April 12th. Suspect similar events on terrain and aspect will take place at higher elevations with greater consequence as the freezing level rises above mountain top on Vancouver Island for the first time in many months.
Past Weather
Several snow storms and overnight cold temperatures have elongated winter on polar aspects on Vancouver Island. This melt freeze chapter will come to a close as "summer like" air temperatures will visit Vancouver Island for the next several days.
Weather Forecast
Near double digit air temperatures near mountain top as "day time high's" will inhibit snowpack cooling overnight and consistently weaken near surface snowpack layers and cornices. Wednesday: No new precipitation Winds light from the North, Freezing level rising during the daytime to 2250 meters. Temps at 1500 meters up to +8 degrees.Thursday: No new precipitation, Winds light shifting from South to North at mid-day, Freezing level rising to 2,400 meters. Temps at 1500 meters up to + 9 degrees.Wednesday: No new precipitation, Winds light shifting throughout the day from North to South, Freezing levels 2700 meters. Temps at 1500 meters 10 degrees.
Terrain Advice
Please respect the current closures at Mount Washington, no access is permitted at anytime on the ski areas slopes or terrain.During periods of intense warming and sunshine avoid solar aspect terrain.Avoid travelling both above and below cornices.
Snowpack Summary
New snow between 5 and 20 cm overlies a previously well developed melt/freeze crust. Warm (above zero) daytime temperatures and moderate settlement of this new snow has promoted bonding of this new layer of snow. Wind affected areas and continued cornice growth is evident and cornice fall is evident. The mid and lower snowpack are well settled and dense.Forecast temperature values are expected to rise rapidly and overnight cooling and snowpack recovery (refreeze) will dissapear as nighttime "low's" will not drop below zero degrees on Vancouver Island's mountain ranges. With these factors, the snowpack will begin to rapidly settle and is likely to become increasingly unstable as these elevated air temperatures have not been experienced on Vancouver Island for many months at the upper Treeline and Alpine elevation bands. Expect cornice collapse, glide slab failures, waterfall ice failures and perhaps other anomalous geo hazard events to occur as the mountains are going to see a rapid change from winter conditions to near summer like conditions in a 72 hour period.
Snowpack Details
- Surface: 5cm snow to 20cm snow (depending on elevation band) on Northerly terrain exists however will begin to melt with incoming air temps
- Upper: Large melt freeze crust (35cm thick) and above 1400M elevation (5cm thick)
- Mid: warm moisture laden snowpack at Below Treeline elevation and generally well settled and dense above 1400M
- Lower: Well settled and dense.
Confidence
High - Moderate - Avalanche danger ratings reflect the highest danger level expected over the course of the day. Weather forecast models are very well aligned and provide support to the forecasted avalanche problems.
Problems
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
Loose Wet
Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
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.