Purcells

Fair - Intensity of incoming weather is uncertain on Thursday

The Gulf of Alaska drives a relatively moist weather pattern for the second half of the week. Wednesday: Around 5 cm of snow above 1500m during the day Wednesday. Freezing level is relatively well behaved staying around 1500 m. Winds out of the SW, light at treeline, moderate in the alpine. Thursday: Trace of snow above 1500m. Winds SW, Lht at treeline, mod at ridge-top. Friday. Snowfall starting near lunchtime, 5 - 10 cm expected, winds SW, Lht @ treeline, Mod @ ridge-top.

Very little avalanche activity was observed on Monday aside from point releases out of steep solar slopes. I've left some of the narrative in from last week as it offers a humbling look into the deep slab avalanche problem: On Friday, a very large avalanche was triggered by a cornice in Gorman Lake (Dogtooth Range). It was wide, deep and covered sled tracks. Photos here: https://bit.ly/GMOYCv . There were four other close calls in the Purcell Mountains last week, all involving very large slabs releasing either from a distance or with a light load. On Thursday, a snowmobiler remotely triggered a size 3.5 slab on an east aspect at 2400m at Oldman Creek, east of Quartz Creek (video here https://bit.ly/GLHIbg). It failed 1-2 m deep, was 200-300 m wide and ran full path, taking out mature timber. A second slope also failed, either remotely or sympathetically.

10 - 20 cm of new snow fell Monday evening. This snow rests on a variety of old surfaces. Before the new snow, professional operators in the Purcells were reporting moist snow below 2000 m and wet snow below 1900m. The old snow surface, now down 5 - 20 cm consists of a suncrust to ridgetop on S, SW & SE facing slopes & a melt freeze crust to 2000 m on all other aspects. The recent warm temps have really helped to settle out the upper snowpack. Professionals throughout the region are reporting a very well settled & firm upper snowpack below the recent storm snow.The spring sun is increasing in strength as each day grows by a few minutes. It takes very little time for S, SE & SW aspects to feel the heat of the sun now. W & E facing slopes are just starting to be affected by the sun as we head into the tail end of March.The effect of the sun becomes very important at this time of the year. We're expecting a few cm's of snow everyday through Friday. As soon as the sun comes out, watch for roller-balling & pin-wheeling to start surprisingly quickly. Wet point release avalanches won't be far behind on slopes receiving direct sun.There's a lot of strong snow between recreationists and the February surface hoar layers and it's been a few days since there was a large deep avalanche in the region. That being said, it could still be triggered. I'm thinking of three different scenarios at the moment:1. The most likely scenario is a large cornice or icefall failure impacting the slope below and producing a very large avalanche. Managing this piece of the hazard puzzle is done by monitoring what's happening above you. Cornices or slopes receiving direct sun are suspect. If you feel like there's a question as to whether or not a piece of cornice is warming to its breaking point, then, there's no question, it's time to get off that slope.2. There was an outbreak of large human triggered avalanches last week, see the avalanche observations for the details of these incidents. This makes me think that it may still be possible for skiers or sledders to trigger very large avalanches failing 1.5 - 2m in depth. The most likely location for this type of failure is where the snowpack goes from thick to thin. This is most likely near ridge crests, the edges of slopes and especially around rock outcroppings.3. While I don't foresee it occurring in the next few days, a slope that does not experience an overnight refreeze is suspect for producing large avalanches. This is something to think about as we flip our calendars to the April page.