Avalanche Forecast

Archived

Dec 17th, 2015–Dec 18th, 2015

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Regions

Olympics.

Dangerous avalanche conditions are expected early Friday, mainly near and above treeline. Best to limit objectives by avoiding steep and wind loaded slopes, watching for sensitive storm layers. Recent and newly wind loaded slopes in the near and above treeline zone should be found on a variety of aspects Friday.

Detailed Forecast

A frontal system overnight Thursday should give way to showers and cooling Friday.

Avalanche problems Friday should be associated with new storm and wind slab layers. New and recent snow will be deposited on lee NW through E aspects. Most avalanches should stay within the new storm snow, but a few may step down to older storm layers or crusts in isolated locations.

In areas below treeline, the warming and rain may have caused natural avalanches on some steep slopes. In addition cooling should begin to form a new crust layer below the elevation that experienced rain or wet heavy snow, likely about 5500 feet. This should limit any avalanches to mainly shallow new snow.

However, near and above treeline, increasing snowfall and winds are expected overnight Thursday and early Friday. This should cause sensitive storm layers. Therefore, best to avoid steeper slopes and those showing signs of recent wind loading.

Terrain anchors are still causing significant anchoring at the lowest elevations. Use caution near creeks which are still open in some areas.

Snowpack Discussion

An atmospheric river around Dec 9th pushed heavy rain above treeline in the Olympics. This caused consolidation and stabilizing of the older part of the Olympics snowpack. An active and cool weather pattern over the last week has produced about 2 to 3 plus feet of new snowfall near Hurricane Ridge that now sits over the 12/9 rain crust.

Last Sunday an NPS Ranger at Hurricane Ridge reported a storm slab release below the Sunrise Ridge likely on an east facing slope near Hurricane. The slide was likely triggered, but this was not confirmed. The avalanche crown was estimated at 2-3 feet and released about 150 wide and ran about 100 feet vertical - big enough to get into serious trouble.

Early this week, cool weather with light new snowfall occurred. A strong frontal system is producing moderate to heavy rain or snowfall at rising freezing levels Thursday. The heavy wet snow or rain should cause a significant increase in danger as the weak underlying snow becomes heavily loaded.

Problems

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.

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.