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Avalanche Forecast

Archived

Jan 12th, 2017–Jan 13th, 2017

Alpine
Natural avalanches unlikely, human triggered possible.
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.

  Recent wind slabs should be primarily on N-W-S aspects but be alert on all terrain aspects firmer wind transported snow.

Detailed Forecast

An upper ridge should drift across the Northwest on Friday. In the Olympics and Cascades this should cause some high clouds, light winds and gradually moderating temperatures west of the crest and at higher elevations.

Recent wind slabs should be primarily on N-W-S aspects but be alert on all terrain aspects for firmer wind transported snow.. The benign weather should bring a little more stabilizing to these layers on Friday. Remember that firmer wind transported snow is always your best sign of wind slab layers.

Small loose dry avalanches might be possible in steep wind sheltered terrain but will not be listed as an avalanche problem. 

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

Snowpack Discussion

Weather and Snowpack

A pair of warm fronts moved across the Northwest Saturday and Sunday causing see-sawing temperatures to near or above freezing for areas away from the Passes including Hurricane Ridge. 6 inches of new snow were reported by NPS staff Monday morning at Hurricane Ridge. 

Although the precipitation gage didn't record any tips, NPS web cameras showed steady light snowfall at Hurricane Ridge all day Tuesday with light upslope snow showers. Light N or NE winds were increasing in the afternoon.

Sunshine and light winds Wednesday and Thursday allowed for moderating 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 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 the fact the layer is deeper than 1 meter make human triggering of this layer 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 a 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.

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.