Avalanche Forecast

Archived

Feb 24th, 2018–Feb 25th, 2018

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches possible, human triggered probable.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Regions

Olympics.

You will be likely to trigger new and reactive slab avalanches on most slopes greater than about 35 degrees Sunday. Greater danger is expected in terrain receiving wind deposited snow. 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

A strong storm passage is expected by early Sunday. Very strong ridge crest winds and a period of heavy snowfall rates through early Sunday, along with a slight warming trend, will combine to cause a rapid increase in the avalanche danger Sunday.

You will be able to trigger new and sensitive wind or storm slab avalanches Sunday. You will be most likely to trigger these avalanches on slopes where winds have deposited snow such as below ridges. Use visual clues such as recent cornices, wind drifted or wind sculpted snow surfaces, to identify and avoid wind loaded slopes.

Any avalanche that begins in the most recent snow may step down to earlier storm snow or possibly to the persistent weak layer buried in mid February.

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 2-3 feet, shallow enough to be human triggered. Observations on the distribution and sensitivity of this layer can be difficult to gather. When uncertainty increases, safe terrain choices decreases. Avoid steep open slopes where a persistent slab avalanche may occur.

Be cautious on slopes above cliffs, creeks, and gullies where even small avalanches may have large consequences.

Snowpack Discussion

NPS rangers reported about 8 inches of fresh storm snow by Saturday morning. At that time, the new storm snow had little wind effect and remained of low cohesion, lacking the ability to propagate a slab avalanche. However, these observations were limited to below treeline.

As of Saturday afternoon, about 2-3 feet of settled snow sits on top of the weak sugary facets that formed on a strong crust. Areas of deeper snow exist on wind loaded terrain. 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.

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.

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.