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West Slopes South - South of I-90 to Columbia River

Issued: 12:00 AM PST Thursday, December 6, 2018
by Matt Primomo

Bottom Line

Winter is about to begin in earnest. An abrupt change in our weather is headed our way and with it, a rise in avalanche danger. You will likely be able to trigger slab avalanches formed from both new snow and wind. In some locations these may be very touchy due to weak, old surface snow. Expect a stormy pattern to begin this weekend and continue into the week.

 

Travel Advice

  • Plan and prepare for changing conditions, and avoid steep slopes if you see warning signs such as shooting cracks, whumphs, and fresh avalanches.

  • Think “slab” once 6” inches of new, or wind deposited snow builds on top of the old snow surface.

  • Practice avalanche rescue. The beginning of winter is a great time to replace the batteries in your avalanche beacons and refresh your avalanche rescue skills.

 

Snowpack and Avalanche Discussion

The mountains have been cold and stagnant this last week. Only a few small wet loose and dry loose avalanches have been reported. The big story is that widespread surface hoar has formed in many locations.

Photo: Surface hoar glistening in the sun on 12/5/2018 at 5,000ft in the Table Mountain area of the Northwest Cascades.  Andrew Kiefer photo.

In addition, the low density upper layer of the snowpack has become weak. There is a thick and supportable crust beneath this layer, found from 6-12” down from the surface in most areas. Where this is the case, there are no significant layers of concern. However, new snow and wind will change that beginning this weekend. The timing of the increased danger will occur differently depending on geography; First in the Olympics, then the Volcanoes, the cascade crest, then the East Cascades. Conditions may become very touchy as the weekend progresses. Refresh your knowledge of storm slabs and wind slabs HERE and HERE. 

Some areas did not receive rain during the Thanksgiving storm, such as the North East Cascades. In these areas, no firm crust exists. Instead, expect a layer of weak sugar snow (known as basal facets) near the ground on shaded aspects near and above treeline. In addition to new snow and wind slab concerns, we have concerns for deeper slabs that will be harder to predict on this layer. See photo below.

Photo: Red lines indicate crowns from an avalanche that likely occurred on 11/26/2018. Matt Primomo photo.