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Avalanche Forecast

Archived

Jan 4th, 2017–Jan 5th, 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.

Moderate to locally strong E-NE winds Tuesday night and Wednesday should have loaded unusual aspects and built wind slab in all elevation bands. Allow fresh wind slabs time to stabilize and avoid terrain where even a small slab avalanche could have unintended consequences. Remember 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. 

Detailed Forecast

High clouds in advance of a weak weather disturbance should make for a mostly cloudy day over the Olympics. Temperatures will remain cold but winds will be much lighter on Thursday.

Moderate E-NE winds Tuesday night and Wednesday should have loaded unusual aspects and built wind slab in all elevation bands. 

Allow fresh wind slabs time to stabilize 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. 

Remember 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

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 mid-day Wednesday. 

Recent Observations

There were two triggered avalanches Friday 12/30 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.