Avalanche Forecast
Regions: Olympics.
Freshly formed wind slabs should persist primarily on leeward westerly facing slopes, especially open slopes near and above treeline. Also, watch for cross-loaded slopes and lingering wind slab on a variety of aspects. Avoid travel on steep terrain with firmer wind transported snow and avoid freshly loaded slopes in all elevation bands.
Detailed Forecast
High pressure should cause a clear, cold night Wednesday and a sunny day Thursday with light winds and cold temperatures. There has been low density snow available for recent transport and with the expected cold temperatures, wind slabs will be slow to settle and stabilize.Â
The fresh wind slabs should be primarily on lee westerly aspects. However, also watch for cross-loaded slopes and lingering wind slab on a variety of aspects. Watch for firmer wind transported snow and avoid freshly loaded slopes in all elevation bands. Â
Small loose dry avalanches are possible in steep wind sheltered terrain, but will not be listed as an avalanche problem.Â
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.Â
Snowpack Discussion
Weather and Snowpack
A cold and dry Arctic air mass was over the Northwest last week. The main weather event during this cold snap was a day of strong east winds last Wednesday that created widespread variable snow surfaces by scouring windward slopes and re-distributing the snow to a variety of aspects.
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 were still reported below treeline prior to snowfall that fell over the weekend.
A pair of warm fronts moving through the PNW Sunday and again Sunday night left temperatures see-sawing 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.Â
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.
A partly cloudy day with plenty of sunshine and light winds Wednesday allowed for moderating 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 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.
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: Alpine, Treeline.
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