Saturday March 23, 2019
Cornice Fall, Loose Wet, Deep Persistent Slab, Storm Slab
Travel & Terrain Advice
The snowpack has undergone extreme warming. Be vigilant of wet snow surfaces or isothermal (slushy or bottomless melting snow). Wet snow may be particularly prevalent on elevation bands exposed to rain events this weekend. Areas that receive rainfall will undergo additional loading and melting. As a result of this added loading, natural and human triggered wet loose avalanches are possible. Maintain cautious route selection that includes avoiding most southerly aspects or aspects with wet snow. Maintain conservative terrain travel by choosing routes that do not put your group in any kind of terrain trap that will increase the consequences from any size of avalanche (large or small). Do not travel above or underneath cornices as they still remain very weak due to warming and rain.
Mt Washington reported a size 1 wet loose Natural avalanche and North Shore also reported size 1 wet loose avalanches as a result of daytime warming this week. Snowmobile triggered size 1.5+ wet loose avalanche reported on steep solar cut block on Monday.
Cornice failure will remain a concern as rainfall and Treeline elevation band Freezing levels will exacerbate the ability of cornices to remain intact.
Below 1400M to 1700M of elevation, loose wet avalanches will remain a concern due to upper treeline freezing levels coupled with substantial rainfall below this elevation band.
Deep Persistent Slab
Down 30 - 50cm (depending on aspect) exists a surface hoar layer which is reactive to testing. Down 50 - 70cm (depending on aspect) exists a facet/crust layer which is reactive to testing. No avalanches have been reported on either of these layers however we suspect these persistent weak layers will continue to provide results and possibly become a increased hazard as the spring melt freeze cycle continues.
Alpine areas could see up to 12cm of snow overnight on Friday and early Saturday morning along with moderate winds. Leeward areas that are downwind from wind could be areas where a person could initiate a size 1 avalanche on the new storm snow and melt freeze crust interface.
The major warming event that took place this week on Vancouver Island has likely eliminated all cold powder snow. All aspects were exposed to these warm temps, Friday’s colder temps has created a 5cm thick melt freeze crust. High Alpine due North Faces may still hold some decent snowmobile or ski conditions as the snowpack in this area may have been able to remain a little colder and therefore possibly hold some powder snow conditions.
Fortunately, the incoming cooling for the next few days will help to slow the melting cycle taking place on Vancouver Island and set us into a more classing melt-freeze cycle. There remains 2 layers of concern within the top 70cm of the snowpack, so far these layers are only reactive to testing. Fortunately, no reports of avalanches have occurred on these deep persistent weak layers, as spring continues we will monitor these layers for changes in reactivity as the likelihood of initiation may increase as the warming trend persists.
|Surface||5cm melt freeze crust, below 1500M the snow surface is wet|
|Upper||Upper 30cm experiencing melt cycle at Treeline and below, 2 persistent weak layers down 50cm and 70cm|
|Mid||Well bonded mid-pack that does include several melt freeze crusts|
The week of March 18-21st saw major day time and night time air temperatures well above zero degrees. On Friday however, the freezing level dropped and a much needed refreezing of the snowpack surface had begun.
If the forecast is accurate, the melting and/or wet upper snowpack will now get a much needed sustained refreezing over the next several days. The elevation band at Treeline and above will remain below freezing this weekend. Incoming precipitation will fall in the form of rain on Friday and Saturday below 1400M of elevation however we will see snowfall above this elevation band.
Saturday - 12 mm of rain below 1400M and 14 cm of snow above 1400M, Moderate to Light Winds from the SW, Temps -3 to -0.5, Freezing Level 1400M
Sunday- 4 MM of Rain below 1500M and 4CM of Snow above 1500M, Light Winds from the SE, Temps -2 to 0, Freezing Level 1500M.
Monday - 4-6 MM of Rain below 1700M and 4-6 CM of snow above 1700M, Moderate Winds from SE, Temps +2 to 0, Freezing Level 1,700M
Posted on Saturday March 23, 2019 by Ryan Shelly