Avalanche Forecast
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.Â
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.Â
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.Â
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.Â
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.
Avalanche Problems
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: All aspects.
Elevations: Alpine, Treeline.
Likelihood: Likely
Expected Size: 1 - 1
Storm Slabs
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. 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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Elevations: Treeline, Below Treeline.
Likelihood: Likely
Expected Size: 1 - 1
Persistent Slabs
Release of a cohesive layer of soft to hard 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 Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Aspects: North, North East, East, South East, South West, West, North West.
Elevations: All elevations.
Likelihood: Possible
Expected Size: 1 - 1