Avalanche Forecast

Archived

Dec 18th, 2015–Dec 19th, 2015

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

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.

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.

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.

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.

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 drifts 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.

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.