Avalanche Forecast

Archived

Jan 2nd, 2017–Jan 3rd, 2017

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches unlikely, human triggered possible.

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.

Watch for wind transported snow and wind slab on a wide variety of non-traditional aspects and features Tuesday. Wind slab layers will require careful terrain and snowpack evaluation and caution. The 12/17 persistent slab has become less likely to trigger. Avoid travel in open terrain and especially with large groups as these slides may require a larger load to trigger.

Detailed Forecast

A weakening upper trough should linger over the Northwest on Tuesday. Meanwhile a cold Arctic air mass will slowly push further south over the BC-Montana area causing cold offshore surface flow over the Northwest. Alpine N-NE winds should ease a bit should ease at Hurricane with some clearing.

The recent shift to N winds at Hurricane should make new wind slab most likely NW to SE aspects on Tuesday. But due to recent shifting winds and since Matt noted wind slab on other aspects on Saturday watch for wind transported snow and wind slab on other aspects as well.

The 12/17 PWL persistent slab problem is becoming more deeply buried and less sensitive in snowpack tests and has been removed from the avalanche problem set.

Snowpack Discussion

Weather and Snowpack

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

Recent Observations

There were two triggered avalanches Friday in the Hurricane area, including a solo traveler who was partially buried after triggering a soft slab avalanche and was fortunately able to self rescue.

NWAC pro-observer, Matt Schonwald was back in the field in the Hurricane Ridge area Saturday, 12/31. Matt found evidence of widespread wind transported snow near ridges and cross loaded features, cornices on multiple ridges and evidence of naturally triggered storm slab avalanches, likely during recent storms late last week. In multiple test pits the 12/17 PWL was found still intact buried consistently 60-80 cm below the surface. However, strong, well settled snow above and good bonding is indicating that triggering this layer is becoming unlikely.

The NPS ranger on Monday reported loose dry avalanches along the road on Sunday. No further loose dry avalanches were seen along the road on Monday. Low visibility limited observations at the ridge on Monday.

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.