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Avalanche Forecast

Archived

Dec 2nd, 2017–Dec 3rd, 2017

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Olympics.

Take a conservative travel approach and avoid terrain traps until storm and wind slab have a day to stabilize.

Detailed Forecast

Light to moderate snow showers should persist Saturday night, then light showers should end on Sunday morning with light W to NW winds.

Storm slabs may continue to be reactive and may be a bit deeper on Sunday. Note of the two layers identified in the NPS ranger report from Saturday as they will now be buried a bit more deeply. Take a more conservative approach to enjoy the new snow safely.  Wind slab may be present on Sunday as well.  Approach ridges cautiously and watch for firmer wind-transported snow.

Early season terrain hazards still exist, such as poorly covered rocks, vegetation and creeks, particularly at lower elevations.

Snowpack Discussion

Warm, wet weather before Thanksgiving caused wet snow and glide avalanches as well as significant snowpack consolidation. There are no snowpack concerns below the Thanksgiving crust and in general new snow received post-Thanksgiving has reportedly bonded well. On Saturday at Hurricane Ridge, temperatures have hovered near the freezing mark likely leading to stabilization of the new snow that fell over the course of the week.

On Saturday a front and short-wave are moving across the Pacific Northwest and brought 7" of new snow in addition to the 5" of snow observed yesterday morning. Winds were generally out of the south in the single digits with maximum values in the low teen's. 

Observations

Friday morning: NPS rangers reported 5" of new snow along with active wind transport of the new snow. 

Saturday morning: NPS rangers report 7" of new snow. Small, 15 cm deep natural storm slab avalanches were being triggered by tree bombs along the road. Snow pit tests on a north slope at 5200' gave ECTP2 and ECTP4 @ 10 and 15 cm, respectively. Clean propagation in both case. On this test slope, the pencil hard Thanksgiving rain crust was down 55 cm.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.

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.