Friday January 15, 2021
Loose Wet, Wind Slab
Travel & Terrain Advice
Be cognizant of snow conditions and utilize small slopes to test and investigate the recent storm /wind slab snow as it develops over the weekend and its reactionary properties.
Careful and cautious route finding when transitioning from scoured areas into areas of wind loaded snow. Avoid traveling below and above cornice features as they are large and touchy.
Avoid open and steep slopes during periods of warming and rain; even small loose wet avalanches will have enough mass to push a mountain traveler into gullies and over cliffs.
Midweek there were several reports from the mountain snowmobile and backcountry ski community of human triggered avalanches size 1-2 on leeward/downwind terrain. The layer responsible for the triggering is a wind slab occurring primarily on convex rolls.
On Saturday a rain and snow event is in the forecast. And while the North Island is set to receive snowfall, the Below Treeline elevation band throughout Vancouver Island will receive precipitation in the form of rain. The Below Treeline elevation band will be subjected to additional loading and critical warming as a result of this weather event and we should see wet avalanches initiate on steep unsupported terrain as a result.
Location: Widespread on all aspects at the Below Treeline Elevation band
Possibility of triggering: During warming and rain events it is possible that humans will trigger this problem, natural avalanches are also possible.
Size: If triggered, expect these avalanche to be small but they will entrain more snow and they fall down the slope and could become large up to size 2 with enough mass to bury injure or kill a mountain traveler.
Found on predominantly North aspects, this avalanche problem overlies a variety of surfaces that include (below 1300 M) a crust and above this elevation (above 1300M) old storm snow. Expect that this problem will continue as a result of strong winds in the forecast (Friday and Saturday) and new precipitation amounts arriving overnight Friday into the early hours Saturday morning.
Location: North aspects and found in both the Alpine and at Tree line specific to terrain just below ridge top and in down wind areas.
Possibility: Triggering of this avalanche problem is likely from light loads such as skiers and snowmobilers. Natural avalanches are possible.
Size: If triggered expect these avalanches to be small, and on isolated terrain features large, size 2.
Over the past forecast period intermittent moderate to heavy snowfall and intense wind transport was observed. A storm system delivered significant quantities of snow up to 90mm of precipitation on the west side of the island with moderate to strong winds from the South West. Evidence of intense wind transport of snow to North aspects was observed in both the Alpine and Treeline.
Overall snowpack depths have been measured from 200cm to 400cm. A variety of crusts exist in the upper snowpack. Numerous snowpack tests indicate that these crusts are now beginning to bond to the storm snow.
The mid snowpack has an unreactive 20cm layer of facets that can be found down 120cm and deeper. This layer may be isolated to areas that are sheltered and at higher elevations above 1400 meters presents as a melt freeze crust/ poly grain layer.
The lower snowpack is dense and very well settled as many reports from island backcountry users and numerous snow profiles have indicated.
|Surface||Variable snow quality where melt freeze cycle has occurred (reports indicate a melt freeze crust exists mid and south island at Treeline elevation band where warming and subsequent cooling have occurred|
|Upper||Storm snow and wind slabs from past storms can be found over a crust below 1300 m and over old storm snow above 1500 m.|
|Mid||A 20cm layer of facets can be found down 120cm and deeper.|
|Lower||Well settled and dense.|
Past precipitation amounts topped out at 90 mm on the west side of the island with snowfall amounts nearing 45cm on the eastern side. The winds and air temps increased and have factored into a combination of wind slab and developing melt freeze snow surface at the Treeline and Below Treeline elevation bands.
Friday: 1-2mm Rain, Winds Moderate from the South increasing to Strong South mountain top winds in the afternoon, Freezing level will reach a high of 1200 meters.
Saturday: 5mm Rain and up to 10cm Snow North Island (with majority of precipitation during day and into evening, Winds Strong from the South at mountain top in afternoon, Freezing level steady at 1200 meters and overnight Saturday the freezing level will rise to 2100M into the early hours Sunday morning.
Sunday: The storm event from the previous day will continue into the early hours Sunday morning (Precipitation of up to 10 cm Snow and/or wet flurries at Below Treeline Elevation band), Winds Strong from the West, Freezing level will reach a high of 1250 meters during day.
*Weather modelling indicating less substantial wind activity (Moderate Wind speeds at south end of forecast zone) therefore the potential for Wind slab activity may be much less prevalent at the southern reach of the polygon.
Posted on Friday January 15, 2021 by Ryan Shelly