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Avalanche Forecast

Archived

Jan 8th, 2017–Jan 9th, 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.

New wind slab is most likely on Monday on northwest to southeast slopes mainly in the near and above treeline. Avoid slopes above terrain traps in the below treeline where recently formed weak layers may have been buried by new snow.

Detailed Forecast

Several inches of new snow or more should be seen at Hurricane by Monday morning. Temperatures will be cooler by Monday morning.

Southwest winds aloft should decrease on Monday. Light snow showers should decrease on Monday with further cooling.

If you travel into the back country near Hurricane on Monday watch for new wind slab mainly on northwest to southeast slopes mainly in the near and above treeline. Previous wind slab may linger on other slope aspects also mainly in the near and above treeline. Watch for firmer wind transported snow.

New storm slab is possible Monday on sheltered slopes where there was more than several inches of rapidly accumulating snowfall.

New wind and storm slab layers should be more stable by the end of the day on Monday.

Remember that recently formed surface hoar and surface faceted snow has been seen lately in the below treeline. It will be a good plan to avoid slopes above terrain traps in the below treeline where these weak layers may have been buried by new snow.

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

A cold dry Arctic air mass was over the Northwest last week. The main weather event last week was a day or so of east winds on Wednesday that went on a powder wrecking rampage, scouring windward slopes and re-distributing it to lee slopes or other areas.

Reports indicate the winds eliminated most of the faceted surface snow and surface hoar that formed in the near and above treeline during the cold weather. But these weak surface crystals are still reported below treeline.

A warm front is moving south to north over the Northwest on Sunday eroding and replacing the Arctic air mass and causing warmer temperatures. Southeast winds aloft on Sunday will shift to southwest by late Sunday and Monday.

 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 Steeple Rock on the Obstruction Peak Road. The persistent weak layer from 12/17 was evident in the two distinct areas where snow tests were performed. The layer ranged in depth from 110 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.

Matt also relayed to us photos of a skier triggered hard wind slab avalanche on a 35 degree, W-NW slope at Hurricane on Saturday. Apparently the skiers that triggered the avalanche were not caught.

Skier triggered hard wind slab avalanche on a 35 degree, W-NW slope at Hurricane on Saturday. Photo by Gary Holmquist.

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.

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.

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.