Avalanche Forecast
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.Â
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.Â
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.Â
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.Â
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.Â
Sunny weather continued Thursday and Friday with very light winds and slightly warmer temperatures.Â
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.
Avalanche Problems
Wind Slabs
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.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Possible
Expected Size: 1 - 1
Persistent Slabs
Release of a cohesive layer of soft to hard 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 Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Unlikely
Expected Size: 1 - 1