Avalanche Forecast

Archived

Mar 15th, 2020–Mar 16th, 2020

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 unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Regions

Northwest Coastal.

Sustained above freezing temperatures with intense sun will maintain elevated hazard. Be alert to signs that snow is warming and weakening, and be prepared to dial back terrain choices. Avalanches could potentially run full path.

Confidence

Moderate - Uncertainty is due to difficult to forecast freezing levels. Uncertainty is due to how buried persistent weak layers will react with the forecast incoming weather.

Weather Forecast

Sunday night: Clear, light northeast winds, alpine temperature near 0 C, freezing level staying near 2300 m.

Monday: Clear, light northwest winds, alpine high temperature 2 C, freezing level near 2500 m.

Tuesday: Clear, light northeast winds, alpine high temperature 3 C, freezing level near 2200 m.

Wednesday: Partly cloudy, light west winds, alpine night temperature 3 C, freezing level near 2000 m.

Avalanche Summary

Avalanches are expected to become more reactive with sustained warming.

During the arctic outflow winds, operators reported small to large (size 1-3) wind slab avalanches releasing naturally in lee terrain features at treeline and alpine elevations.

Reports of avalanches releasing on persistent weak layers have continued through the week. Operators reported several large (size 2-3) avalanches breaking on both the March 9th and March 1st surface hoar layers from human, explosive, and cornice triggers. Several of these avalanches were remotely-triggered. Check out this MIN from the Shames area for a helpful example.

Two very large (size 2.5-3) natural glide slab avalanches were observed along the highway west of Terrace on Friday. Maintain conservative margins to avoid areas with glide cracks.

Snowpack Summary

On Sunday, freezing levels jumped to 2300 m with intense solar radiation, and alpine temperatures rose from -20 C to 0 C in the span of 24 hours. Sustained above freezing temperatures are forecast overnight and into another sunny day on Monday. These conditions are expected to destabilize the surface snow and cornices as well as add significant strain to buried weak layers.

Over the past few days, strong arctic outflow winds formed wind slabs in a reverse-loading pattern. Last week's storm delivered 30-60 cm of new snow to the region with strong southwest winds creating ample snow available for wind transport. The recent snow is sitting on a weak layer of surface hoar buried 30-60 cm deep in many areas, especially in sheltered areas at treeline and below treeline. See this MIN from the Shames area that shows this layer when it was on the surface. Observers have reported widespread whumpfing and propagation in snowpack tests on this March 9th surface hoar, confirming its propensity for human triggers.

There are several additional layers of surface hoar that are now buried 70-120 cm and 110-160 cm deep that may also be the most prominent around treeline. On south through west facing slopes, this surface hoar may be sitting on a crust, which may increase the potential for triggering avalanches on these layers.

A weak layer of facets that formed in January may be found about 150 to 200 cm deep, and an early season melt-freeze crust lingers at the base of the snowpack. These layers have produced a few very large natural and explosive triggered avalanches over the past two weeks. Sunday's warm-up may have the potential to re-awaken these deeper layers.

Terrain and Travel

Extra caution for areas experiencing rapidly warming temperatures for the first time.
Avoid steep slopes when air temperatures are warm, or solar radiation is strong.
Minimize overhead exposure; avalanches triggered by warming or cornice fall may be large and destructive.
The more the snowpack warms-up and weakens, the more conservative you`ll want to be with your terrain selection.
Be aware of the potential for large avalanches due to the presence of buried surface hoar.

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.

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.