Monday January 29, 2018
Loose Wet, Deep Persistent Slab, Storm Slab
Travel & Terrain Advice
As temperature begin to drop below zero on Monday we can expect a rain crust to form on all elevations and aspects. New snow in moderate amounts will then deposit on this crust. The bond to this crust and the amount of new snow will be the thing to watch over the next few days. Expect this bond to be poor and slow to gain strength. Avoid areas of deep deposit like wind loaded N-NE aspects in the alpine, treeline and treeline like features below treeline. Large open slopes, wind filled gullies, convex rolls and steep unsupported slopes will be prone to triggering naturally and certainly with human activity. Use ski poles, probes or other devices to test new snow depths as you travel and check the bond to the crust, even with simple hand shears.
Friday - Numerous loose dry sloughs up to 1.5, all aspects on steep slopes in the treeline and below running fast (no alpine obs).
Saturday - Sensitive storm slabs observed in the upper snowpack. Many active windslabs on NE-NW asp at treeline size 1-2 around Mt Washington and Cain, triggered with explosives, ski cutting and skier accidentals. One natural triggered size 3 in the closed North bowl zone at Mt Washington. The majority of these were running down to a depth between 20-50 cm on the interface between the huge snow event last Saturday-Sunday and the new storm snow that fell in the days after. No observations for Strathcona Park reported.
Sunday -Moderate new snow overnight then moderate to heavy rain produced numerous loose wet up to size 1 and a few wet slab up to size 2 with ski cutting and explosive work on Mt Washington. No other reports for this period but would expect this to be a widespread avalanche cycle for the entire forecast area.
Rain will continue to destabilize the upper snowpack until temps and freezing levels drop Monday. This additional weight and temperature will result in loose wet avalanches on all aspects and elevation bands. We can expect loose wet slides up to size 1.5 that will possibly trigger naturally and are very likely to trigger with human activity. Avoid exposure to terrain traps and steep slopes during this warm period.
Deep Persistent Slab
Failures in tests over the past few days and avalanche activity hint to the existence of deep persistent slab issues. These weaknesses will continue to be loaded with additional snow over the next few days. Cool temps and dropping freezing levels will reduce concerns but the possibility still exists. These slabs could exist on all elevations and aspects and are unlikely to trigger up to size 2.5 with human loading. Large triggers like cornice failures, explosives or other avalanches creating a step down effect could possibly trigger these deep layers up to size 2.5.
Moderate new snow fall is expected late Monday and forward into this forecast period. This new snow may have a poor bond to the rain crust that is forecast to develop after the rain Sunday into Monday. This new snow may result in storm slabs up to size 2 and will exist on all aspects in the alpine, treeline and treeline like features in the below treeline zones. Pay special attention to open N-NE aspects where wind loading may increase this effect. These slabs (if they form) will possibly trigger naturally and likely trigger with human loads.
Moderate amounts of new snow have fallen since the huge snow event two weekends ago. Numerous shears/failures have been active in this new snow, with the most interesting between the new and old big snow event down 30-50 cm. Failures have released with moderate to easy results (natural, and human triggered) and are smooth and planar (ie. scary, evidence the size 3 natural in Mt Washington’s North bowl). On Sunday the cold temps and amazing powder skiing we had came to an abrupt end with high freezing levels, warm temps and moderate to heavy rain. The upper snow pack became moist and wet Sunday and travel became difficult. Rapid settlement of the upper snow pack resulted due to the rain and lead to widespread instability.
|Surface||Rain crust on all aspects and elevations due to Sunday's rain event.|
|Upper||Moist down below the crust.|
|Mid||Numerous weak layers between last weeks new snow interfaces. Test results producing easy-moderate smooth failures|
|Lower||Well settled including the large snow events accumulation.|
Moderate snow fall and very cold temps for the island (-4 to -8) Thursday night though till Sunday morning for the entire forecast region. Sunday mid morning moderate to heavy rain fall all the way to the top of the islands summits as the freezing level and temperatures took a 24 hour spike. Winds were Light to moderate from the SE-SW.
Temps will drop and we will see light to moderate new snow accumulation. Winds will be strong to moderate mainly form the S-SW and freezing levels will remain low.
Monday - More rain 7-24 mm switching to snow 3-16 cm, positive temps around 2 to 3 will drop by the end of the day down to -2 to -4, with strong to moderate winds from the SW. Freezing levels 2000 m dropping to 1000-800 m.
Tuesday - Moderate new snow 10-25 cm, with cold temps -3 to -6, and strong to moderate SW winds. Freezing levels 900 m to 500 m.
Wednesday - Light to moderate snowfall 4-20 cm, cold temps -4 to -6, and light to moderate winds from the SW-SE. Freezing levels 300 m to 800 m.
Posted on Monday January 29, 2018 by Bill Phipps