Avalanche Forecast

Issued: Mar 12th, 2019 11:00AM

The alpine rating is considerable, the treeline rating is moderate, and the below treeline rating is low. Known problems include Wind Slabs and Persistent Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Wind slabs may be easy to trigger and dangerous at upper elevations. Some uncertainty remains with how the new snow is bonding to older, weak surfaces. Be alert of quickly changing conditions as the sun and temperatures begin to change the snow surface. Plan simple tours that allow for avoiding steep, upper elevation slopes. 

Summary

Discussion

Snow and Avalanche Discussion:

A wide range of snow fell across the area on Tuesday. The big story, however, is what did the winds do to the upper elevations? Plenty of low density, cold snow was available for transport prior to Tuesday. The winds increased during the day, and likely built reactive wind slabs at the higher and more exposed areas. Small wind slabs were still reactive on Sunday, where a skier triggered a D1 wind slab on a northeast aspect of Wedge Mountain. That occurred before this system blew through.

The recent storm buried a mix of sun crusts on southerly aspects and weak, faceted snow on northerlies. Dangerous conditions may be found where thick wind slabs overlie weak, faceted snow. The sun should make an appearance on Wednesday, and with it, a rapid change in snow surface conditions. Loose wet avalanches may run on steep, south facing slopes in areas that did pick up over a couple inches of new snow. 

Snowpack Discussion

March 10, 2019

February started stormy and cold with very active avalanche conditions on persistent weak layers. The month finished with continued cold temperatures but drier weather allowed avalanche activity to taper off. Persistent weak layers that were buried early in the month (Feb 8th) are now unreactive, though you can still find the grains.

Aside from the series of storms in early to mid-February, most regions have measured relatively light snow accumulation in the past 3 weeks. Snow has stayed soft especially on shaded slopes and faceting and surface hoar have been plentiful. While there has been plenty of sunshine since mid-February, very cold temperatures have kept melt-freeze crust to a minimum on sunny slopes.

A natural loose wet avalanche (D1.5) on a southeast aspect of Lichtenberg Mtn at 5,400ft. 3/10/2019. Photo: Josh Hirshberg

Recent Avalanches

Our attention shifted to the upper snowpack. Recent avalanche problems have largely focussed on wind transported snow and weak snow surfaces on both dry, shaded slopes and sun-exposed aspects. In some locations, a weak layer of surface hoar and facets was buried on March 6th. We’ll keep an eye on this interface as we move into the future.

A skier-triggered storm slab avalanche (D1.5) on Shuksan Arm, north aspect, 5200ft. 03/09/19 Adam U Photo. 

Moving Forward

As we move further into March, there are two points to consider:

  • The strength of the March sun: As the sun creeps higher into the sky and the days grow longer, the sun can have a greater impact on the snow surface. When the sun comes out, expect things to change quickly. You may see avalanches conditions change with natural loose avalanches originating from sunny slopes, surface snow becoming thick and heavy, and slabs taking on a moist to wet snow character.

  • Weak surfaces: facets and surface hoar have blanketed snow surfaces in many zones. Any significant snowfall will bury well-developed and widespread persistent weak layers. Recently, most storms have not delivered enough precipitation to cause a problem.

Problems

Wind Slabs

An icon showing Wind Slabs

The recent storm snow landed on a variety of surfaces from surface facets and surface hoar on shaded aspects to melt freeze crusts on southerly aspects. The slightly warmer temperatures associated with the storm, along with strong winds built slabs on upper elevation terrain. This heavy wind loading likely transported snow-both new and old, onto leeward slopes. The snow may fracture quite wide on shaded aspects where the recently buried layer is weak and faceted. Any avalanches on the old surface may step down a foot or so to layers from early March. Slides may run quite a distance, encompassing much of a terrain feature at upper elevations. Check for thick, cohesive layer of snow sitting over a weaker snow layer. If you see obvious signs of instability such as shooting cracks and recent slab avalanches on small steep slopes, it is time to stay off of, and out from underneath steep open slopes.

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

Persistent Slabs

An icon showing Persistent Slabs

It has become less and less likely to trigger a deeper slide, however, the poor structure with facets underneath a crust from early February remain concerning in some areas. At lower elevations, the snowpack is shallow and weak on shaded aspects. In the foothills, facets can be found near the ground beneath February's storm snow. These deeper slabs are most concerning on isolated, shaded slopes where stronger snow may rest over facets. This structure is more concerning than at upper elevations, where aside from the recent storm snow, the snowpack is generally deep and strong. Persistent slabs are unlikely at upper elevations and in the western portion of the zone.

If you find signs of instability such as cracking and collapsing, avoid steep slopes nearby. Stick to well-supported slopes and avoid places where firm slabs look to exist near shallow rocky zones. Travel one at a time from well defined safe zones when moving through avalanche terrain.

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, West, North West.

Elevations: All elevations.

Likelihood

Unlikely

Expected Size

1 - 1

Valid until: Mar 13th, 2019 11:00AM