Avalanche Forecast

Archived

Jan 6th, 2017–Jan 7th, 2017

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Regions

Olympics.

Lingering wind slabs are likely to persist, especially near and above treeline. Watch for wind deposited snow and avoid travel on or near likely trigger points such as areas with a shallower snowpack or above unsupported terrain features.

Detailed Forecast

Becoming cloudy Saturday with occasional very light snow, mainly during the afternoon. Only light amounts of new snow are expected through the day Saturday, if any. Winds should be mostly southerly and light to moderate.

Previous moderate E-NE winds through Wednesday should have loaded unusual aspects and built wind slabs on many open slopes, even to elevations well below treeline.

Watch for lingering wind slabs and avoid terrain where even a small slab avalanche could have unintended consequences. Remember to watch for firmer wind transported snow on all slope aspects or cross loaded slopes, especially in areas with varied terrain and modified wind directions.

Continue to identify the 12/17 buried PWL in snowpits and avoid areas where the overlying snowpack is shallower where affecting this layer would be more likely.

Snowpack Discussion

Weather and Snowpack

An Arctic air mass began moving into the Northwest Saturday night. This caused a change to cold north winds and upslope flow conditions along the north slopes of the Olympics, where Hurricane had about 15 inches of new snow for the 2 days ending Monday morning.

Fair and cold weather that started Monday continued through Wednesday with moderate NE winds at Hurricane Ridge through midday Wednesday.

Sunny weather continued Thursday and Friday with very light winds and slightly warmer temperatures.

Recent Observations

NWAC pro-observer, Matt Schonwald was in the field with NPS rangers in the Hurricane Ridge area Friday, 1/6. Matt traveled to the Steeple Rock area along the Obstruction Peak route. The persistent weak layer from 12/17 was very evident in the two distinct areas where snow tests were performed. The layer ranged in depth from 95 cm on SSW aspect to 135 cm on NW aspect. While reactive in deep snowpack tests and PST's, stiff overlying layers and being buried greater than 1 meter are indicating triggering this layer is unlikely. Of greater concern were the areas of recent wind slabs on a variety of aspects. There was no evidence of very recent avalanches, in that terrain, but older slides were seen, likely releasing during the strong wind events earlier in the week.

Problems

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.

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.