Northwest Coastal

A storm has overloaded weak layers in the snowpack. Large avalanches are very likely.

High

On Sunday expect lingering flurries with some clearing throughout the day. On Monday and Tuesday the region should see a mix of sun and cloud as a dry ridge of high pressure develops. Ridgetop winds should remain mainly light for the forecast period. Alpine temperatures will hover around -13 on Sunday, -17 on Monday and about -20 on Tuesday.

Recent explosives avalanche control in the Shames area produced several size 1.5 slab avalanches on a variety of aspects between 1250m and 1330m. A surface hoar layer buried in mid-December was the suspected culprit in these events. Moving forward, new snow and wind is expected to spark a round of destructive natural storm slab activity on all aspects and elevations. Due to underlying persistent weak layers, human triggering will remain a very real concern for several days after the storm has ended.

By Sunday morning I expect up to 45cm of new snow. Strong to extreme southwest winds are forecast to redistribute much of this snow into deeper, more destructive deposits in higher elevation lee terrain. The new snow overlies a very touchy layer of sugary faceted snow and well-developed surface hoar which is reported to exist on all aspects and elevations. Due to the persistent nature of these underlying crystals, I expect this newly formed storm slab to remain reactive for the foreseeable future. Adding to the mix of weaknesses in the upper snowpack are a few other surface hoar layers that developed throughout December which you may now find buried in the top 100cm. The weight of the new snow may cause these layers to "wake up" resulting in larger, more destructive avalanches.