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Avalanche Forecast

Archived

Mar 4th, 2020–Mar 5th, 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 possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

South Coast Inland.

Snow and strong wind are forecast for Thursday, with the most snow in the north of the region. Slabs will rapidly grow and become touchy. Treat the danger as HIGH if you find more than 30 cm of snow or during periods of rapid loading.

Confidence

Moderate - Uncertainty is due to the timing, track, & intensity of the incoming weather system.

Weather Forecast

WEDNESDAY NIGHT: Cloudy with snowfall, accumulation 5 to 10 cm, moderate to strong southwest wind, alpine temperature -8 C, freezing level 600 m.

THURSDAY: Cloudy with snowfall, accumulation 10 to 20 cm in the north of the region and trace to 10 cm in the south, strong southwest wind, alpine temperature -6 C, freezing level 1300 m.

FRIDAY: Morning snowfall then mix of sun and cloud with isolated flurries, accumulation 5 to 10 cm, light southwest wind, alpine temperature -10 C, freezing level 1000 m.

SATURDAY: Morning snowfall then a mix of sun and cloud, accumulation 5 cm, light southwest wind, alpine temperature -10 C, freezing level 800 m.

Avalanche Summary

A few small wind slab avalanches were triggered naturally on Tuesday in alpine terrain. Otherwise, no new avalanches were observed.

Snowpack Summary

The next storm on Wednesday night into Thursday will drop around 15 to 25 cm of snow for the north of the region and around 10 cm to the south. The snow will likely consolidate into a storm slab as the day unfolds, with particularly touchy conditions during periods of rapid snow loading. The wind will be strong from the southwest during the storm, so expect wind slab development in lee terrain features, particularly near ridges.

A weak layer of surface hoar crystals and/or faceted grains may be found around 40 to 80 cm deep on northerly aspects at treeline and lower alpine elevations. The layer may have been destroyed by strong wind immediately before burial on February 22nd, but it may still exist in sheltered terrain features. Where it is found, it has been reactive in snowpack tests. This persistent weak layer warrants investigation and a conservative terrain use strategy. Check out the latest forecaster blog that offers a deeper dive into these conditions.

In the northern part of the region (i.e., Duffey/Hurley/Goldbridge), a deep instability exists at the base of the snowpack. It has not produced an avalanche since February 17th. Rapid warming or heavy loading by new snow/wind/rain events have the potential to re-awaken this layer.

Terrain and Travel

  • Storm slab size and sensitivity to triggering will likely increase through the day.
  • Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
  • Dial back your terrain choices if you are seeing more than 20 cm of new snow.
  • Storm slabs in motion may step down to deeper layers resulting in large avalanches.
  • Surface hoar distribution is highly variable. Avoid generalizing your observations.

Problems

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.