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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 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.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

South Coast.

An intense storm will impact the region over the day on Thursday. Storm slabs will build over the day and will likely become touchy once enough snow accumulates. The snow will also load a buried weak layer. Treat the danger as HIGH if you find 30+ cm of accumulation.

Confidence

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

Weather Forecast

WEDNESDAY NIGHT: Increasing cloud with early-morning snowfall, accumulation 5 cm, light increasing to strong southwest wind, treeline temperature -3 C, freezing level 800 m.

THURSDAY: Cloudy with snowfall, accumulation 15 to 25 cm and the most in the north of the region, strong southwest wind, treeline temperature -1 C, freezing level 1000 m.

FRIDAY: Early-morning snowfall then a mix of sun and cloud, accumulation 5 to 10 cm, light southwest wind, treeline temperature -4 C, freezing level 700 m.

SATURDAY: Cloudy with snowfall, accumulation 10 to 15 cm, light south wind, treeline temperature -5 C, freezing level 600 m.

Avalanche Summary

A few small (size 1) wet loose avalanches were observed in the North Shore mountains on Tuesday.

Avalanche activity is expected to spike on Thursday during the stormy weather.

Snowpack Summary

The next storm on Wednesday night into Thursday will drop around 20 to 30 cm of snow, with the most expected in the north of the region. 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.

In some of the region, around 50 to 80 cm of snow overlies a layer of surface hoar on north-facing aspects near and above treeline. The layer has been found to be reactive snowpack tests, such as in this MIN and this MIN. 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.

The remainder of the snowpack is well-settled. The snowpack depth varies from around 250 to 300 cm at the peaks of the North Shore mountains (1400 m), tapering rapidly with elevation. 

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.
  • Be aware of the potential for large avalanches due to the presence of buried surface hoar.
  • 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.