Avalanche Forecast
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
The upper ridge should linger over the northeast Pacific Ocean through the weekend with some moisture moving through the ridge. In the Olympics and Cascades this should cause periods of mid and high clouds, light winds and further moderating temperatures especially 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 Saturday. Remember that firmer wind transported snow is always your best sign of wind slab layers.
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.
Small loose dry avalanches might be possible in steep wind sheltered terrain but will not be listed as an avalanche problem.Â
Snowpack Discussion
Weather and Snowpack
A pair of warm fronts moved across the Northwest last weekend 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.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.
An upper ridge over the northeast Pacific Ocean has caused fair weather Thursday and Friday over the Olympics and Cascades with mostly light winds and moderating temperatures especially west of the crest and at higher elevations. 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.
NWAC pro-observer Matt Schonwald was out on Klahane Ridge on Friday and on a south slope at 5170 feet found about 90 cm of snow with no reactive layers over the 12/17 PWL. The PWL gave a PST60/100 End result so we still need to watch this layer.
Also here is advance notice that faceted surface snow and surface hoar seen in some areas lately due to the cold weather will need to be watched when snow and potential rain begins to arrive next 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: 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