This Bulletin Valid Until: Sunday March 19, 2017 6 pm.
DANGER RATINGS (Make sure you understand the
danger level meanings)
Outlook |
Friday |
Saturday |
Sunday |
Alpine | CONSIDERABLE | CONSIDERABLE | CONSIDERABLE |
Treeline | CONSIDERABLE | CONSIDERABLE | CONSIDERABLE |
Below Tree line | MODERATE | MODERATE | MODERATE |
Confidence: Moderate: uncertainty in determining above 1800 meters in elevation the current upper snow pack conditions, specifically seeking the existence of the new melt freeze crust and its strength and bonding properties with new snow. The other uncertainty that resides above 1800 meters in elevation is the bonding properties, depth and distribution of the deep persistent crust. Seeking observations and information from public observers. If you have the training, experience and skill level to access alpine avalanche terrain , please forward your observations to forecaster@islandavalanchebulliten.com
Main Concerns: (Avalanche problems)
Wind Slab- New snow between 10 to 25 cm is expected during this forecast period combined with moderate south east and south west wind will produce wind slabs. Wind slabs will be present in lee terrain and located in features such as ridge top areas and steep convex rolls. With the exception of elevations above 1800 meters wind slabs will be seated on a recently formed hard crust and will be very touchy to human triggers . Avalanches from wind slabs could be large (size 2 )in size and will become very touchy to all triggers when temperatures begin to warm.
Persistent Slab- This persistent weakness within the snow pack is bonding moderately well at treeline and poorly in the alpine.The weakness is located down between 80 and 100 cm. On lee aspects (North East and North West) the persistent weakness is suspected to be down 200cm. Currently the new melt freeze crust is bridging the upper snow pack, providing this deeper crust with a protective layer from light to moderate loads. Further heavy loading and very warm temperatures may wake this deep instability and could produce very large avalanches in isolated terrain
Travel/Terrain Advice: Very dangerous avalanche conditions, travel in avalanche terrain is not recommended. If users intended to travel in avalanche terrain make conservative decisions, and utilize simple well support low angle terrain. Be especially aware of over head hazards (cornices) and avoid lee areas with deep pockets of fresh snow.
Past Weather: Rain, warm air and moist surface conditions were followed with a sharp drop in freezing levels and temperatures promoting the development of a melt freeze crust. This very hard surface crust has formed on all aspects and elevations below 1800 meters and boots or skis will not penetrate it. Up to 15 cm of snowfall then fell on this new crust in the last 48 hours combined with moderate Se wind and cooling temperatures.
Avalanche Summary:
Tuesday March 14, warm and wet conditions produced numerous natural wet loose avalanches on North steep aspects at treeline and below treeline. These avalanches were large in size (size 2) and a couple of them gathered enough mass to put load on the persistent weak layer , causing a step down and producing slab avalanches between 40 and 60 cm deep.
Wednesday March 15, a few very large natural avalanches (size 3) were observed and reported.These avalanches were located at 1800 meters on north to north east aspects and had deep crowns, estimated at 1.5 to 2 meters. The avalanches failed on the mid February crust during the warming period, and initiated at the top of steep convex rolls, with unsupported terrain below.
Thursday March 16, Skiing cutting late in the day at treeline on a northwest aspect, produced small (size 1) wind slabs just below ridge top, being very touchy and running on the melt freeze crust, These avalanches did not have enough mass to run far
Snowpack Description: New low density snow of up to 15cm is bonding poorly to a very hard surface crust that formed on all aspects and elevations below 1800 meters. The hard crust has bridged the snow pack and boots or skis in most areas would not penetrate it. Moderate south east and south west wind has transported this new snow to lee aspects. The persistent weakness can be found down 80 to 100cm and up to 200cm in lee areas in the alpine. Below is a well settled snow pack
Surface- new low density snow of up to 15cm overlie a well developed melt freeze crust approx 15cm in depth
Upper- 80 to 100cm overlie the persistent melt freeze crust
Mid- Well settled.
Lower-Well settled.
Weather Forecast:
A westerly flow will continue to bring moderate south east and south west wind with moderate snowfall. Temperatures are expected to remain cool with freezing levels staying below 1000 meters for the duration of the forecast period
Friday - up to 10 cm new snow Winds SE to 30 km/hr Freezing level 700 to 900 m.
Saturday- up to 10 cm new snow Winds SW to 25 km/hr Freezing level 700 to 900 m.
Sunday- up to 5 cm new snow Winds SW to 15 km/hr Freezing level 0 to 900 m.
Prepared by Jesse Percival