If the storm arrives ahead of schedule on Wednesday, danger will quickly rise to Thursdays forecast values. This bulletin is based on a very small amount of data, significant variations in conditions exist.
Confidence
Fair - Due to limited field observations
Weather Forecast
A massive low pressure system in the gulf of alaska is driving a moist & active weather pattern that will persist through the weekend. The majority of the system gets eaten up on the coast, but the Purcells will still see a good shot of snow. I expect the first pulse to produce 5 cm Thursday. Thursday night should produce another 5 cm or so. Look for light snowfall Friday to produce another 5 - 10 cm during the day and another 5 cm overnight. Saturday's pulse could deliver 10 - 20 cm's to the region with the Bugs receiving the most snow. I expect 20 - 50 cm of total storm snow to be in place by Sunday morning. Freezing levels should stay around 1500m all weekend. Strong to Extreme alpine winds out of the W/SW ramp up Wednesday night and should persist through the weekend.
Avalanche Summary
A few observations from last weekend have trickled in. They mostly involve the November 17th surface hoar layer. Riders triggered a few different avalanches on this layer with most of them coming in at size 1.5. There is an unconfirmed report of at least one partial burial in the Quartz Creek area. Thankfully, it sounds like everyone is okay. Crown depth was around 40 cm.
Snowpack Summary
An incredible amount of variability exists in the region. In the north the alpine snowpack is near 2 m in depth, in steep contrast alpine depths in the south are just over a foot. Current surface conditions range from zipper crust to surface hoar. These layers may be players as storm totals begin to stack up late Thursday evening. A deeper surface hoar layer we're calling the November 17th SH can be found down around 60 cm in depth. This was a player in at least one close call last weekend. There's a stout rain crust near the ground that we're calling the early November crust. There are some facets below it too, but it's unreactive at this time. There is still a sharp transition in snow depth from treeline to below treeline making backcountry travel challenging under the 1300 m in elevation. Remember, it's still November.
Problems
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.