Avalanche Forecast

Archived

Feb 23rd, 2018–Feb 24th, 2018

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 possible, human triggered probable.

Below Treeline

Natural avalanches possible, human triggered probable.

Regions

Olympics.

You will be able to trigger new and reactive wind slabs on a variety of aspects Saturday. Use visual clues such as snow drifts, fresh cornices, and uneven snow surfaces to identify and avoid steep wind loaded slopes. Persistent slabs are still possible in the Olympics. Stay off of large steep open slopes where you may trigger this difficult to manage avalanche problem.

Detailed Forecast

New and reactive wind slabs will form Friday night and Saturday on lee slopes near and above treeline. You will be most likely to trigger these avalanches on slopes where winds have deposited snow such as below fresh cornices, on snow dirfts, or near uneven snow surfaces. Use these visual clues to identify and avoid wind loaded slopes while you travel.

The persistent slab problem has not gone away. Snowpack observations on a variety of aspects Thursday confirmed that a persistent weak layer is still present and reactive. Weak sugar facets can be found just above a firm crust layer formed in early February on all but south aspects. This layer is down about 2 feet. This is perfect depth for human triggered avalanches. Observations on the distribution and sensitivity of this layer can be difficult to gather. When uncertainty goes up, terrain use should go down. Avoid steep open slopes where a persistent slab avalanche may occur.

Shallow storm slabs may develop with additional precipitation Saturday. These are expected to remain small and be most hazardous to travelers in terrain where consequences are higher. Be cautious on slopes above cliffs, creeks, and gullies where small avalanches may have large consequences.

Snowpack Discussion

As of Friday afternoon, about 2 feet of settled snow sits on top of the weak sugary facets. Areas of deeper snow exist on wind loaded aspects. Snowpack tests continue to suggest that these facets can fail and produce avalanches. This layer has not been reported on South aspects in the Hurricane Ridge area.

Loose surface snow existed in many locations prior to Friday afternoon’s storm.

There are no other significant layers of concern in the mid and lower snowpack.

Observations

On Thursday Matt Schonwald and NPS rangers observed 20-30 inches of generally right-side-up surface snow over weaker snow (facets). Snowpack tests indicated the weak facet layer could still fail and produce avalanches. Northeast winds during the day Thursday redistributed snow onto SW-W-NW aspects.

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.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of 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 Slab.