Avalanche Forecast
Regions: Olympics.
Avoid travel on or below recent large cornices found along ridges, mainly N-NE facing. Give a wide berth and avoid travel on slopes below. Also, watch for wind slab formation on isolated terrain features.
Detailed Forecast
A transitory ridge of high pressure should move across the area Saturday to allow for cool weather, light winds and some possible sun breaks. This weather should maintain the excellent surface snow conditions around the Hurricane area.Â
Watch for steep slopes were loose dry slides are possible, these would be most dangerous above terrain traps such as steep V shaped terrain which was seen Friday by observers.
Avoid terrain where wind may have built isolated wind slab layers, mainly higher elevations near ridges, most likely on NW-E facing terrain.
Watch for terrain traps which were noted below steep slopes. Give large cornices a wide margin and watch for areas of recent wind slab deposits. Â
Snowpack Discussion
A heavy rain event earlier in December caused wet snow to penetrate to the ground in the Hurricane Ridge area. This formed a crust around December 9th with good bonding to storm snow that fell last week as temperatures cooled. Several strong storms over the past week have now built about 1-2.5 feet of new snow over the crust layer.
NWAC pro observer, Matt Schonwald and an NPS ranger travelled extensively Friday around Hurricane Ridge testing the recent storm snow. At around 5500 feet the snow depth is about 4-5 feet with the December 9 crust buried about 1.5-2 feet from the recent storm snow. The snowpack in the Hurricane Ridge area was mostly unconsolidated low density surface snow with a gradually increasing density down to the well bonded crust, a very good profile. No slab character was noted in the upper snowpack during testing. Only isolated wind slabs were noted with no releases. Extensive cornices were noted on N-NE ridges. Skiing was very good.
In the higher elevations, above tree line, strong winds recently have scoured much of the snow from a wide variety of exposed slopes, leaving much exposed rock and vegetation.Â
Avalanche Problems
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind lips of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
Cornices can never be trusted and avoiding them is necessary for safe backcountry travel. Stay well back from ridgeline areas with cornices. They often overhang the ridge edge can be triggered remotely. Avoid areas underneath cornices. Even small Cornice Fall can trigger a larger avalanche and large Cornice Fall can easily crush a human. Periods of significant temperature warm-up are times to be particularly aware.
A corniced ridgeline. A large cornice has formed at the top of the ridge. A smaller cornice has formed to the left of the trees from crossloading.
Cornices are easy to identify and are confined to lee and cross-loaded ridges, sub-ridges, and sharp convexities. They are easiest to trigger during periods of rapid growth (new snow and wind), rapid warming, and during rain-on-snow events. Cornices often catch people by surprise when they break farther back onto flatter areas than expected.
Aspects: North, North East.
Elevations: Alpine, Treeline.
Likelihood: Possible
Expected Size: 1 - 1
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: North, North East, East, North West.
Elevations: Alpine, Treeline.
Likelihood: Possible
Expected Size: 1 - 1