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Avalanche Forecast

Archived

Feb 20th, 2021–Feb 21st, 2021

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural and human triggered avalanches likely.
Alpine
Natural and human triggered avalanches likely.
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 unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

South Coast.

Avoid avalanche terrain. Heavy snowfall, rain, and wind will create very dangerous conditions on Sunday.

Confidence

Moderate - Uncertainty is due to rapidly fluctuating freezing levels. We are confident about the likelihood of avalanche activity, what is less certain are their possible size.

Weather Forecast

A warm front will bring some intense weather the next few days.

SATURDAY NIGHT: Heavy snowfall with 25-35 cm by the morning, strong southwest wind, freezing level climbing to 1100 m by the morning with treeline temperatures around -1 C.

SUNDAY: Another 20-25 mm of precipitation throughout the day with freezing level climbing from 1100 to 1500 m so wet snow will transition to rain, strong southwest wind, treeline temperatures around +2 C in the afternoon.

MONDAY: The storm continues with another 30-50 mm of precipitation and freezing levels dropping from 1500 to 800 m resulting in rain transitioning back to snow, strong west wind, treeline temperatures around -2 C.

TUESDAY: Mix of sun and cloud, moderate northwest wind, treeline temperatures around -3 C.

Avalanche Summary

No notable avalanche activity has been reported since the last decent storm on Monday. With the sheer amount of precipitation expected on Sunday and Monday we can certainly expect avalanche activity, however there is some uncertainty about the transitions between snow and rain which will impact the exact character of the avalanches (e.g. dry storm slabs vs. wet loose). The greatest danger will be on slopes that accumulate lots of fresh snow before it turns to rain. High alpine terrain in the region may not receive any rain, resulting in enormous amounts of snow and extra dangerous conditions.

Snowpack Summary

The snowpack will undergo rapid changes on Sunday and Monday with transitions between heavy snow to heavy rain and then back to snow. A total of 80 mm of precipitation is possible over this period. Forecasts on Saturday suggest the North Shore mountains could get 25-35 cm of snow before it starts to rain on Sunday morning. Once the rain starts low elevations will get wet saturated snow, while higher elevations will continue to get heavy snowfall with strong wind promoting natural avalanches.

Leading into this storm the upper snowpack has an interesting mix of thin crust layers and some soft faceted snow between 30 to 70 cm deep. While avalanches this weekend are most likely going to fail in fresh storm snow layers, there is a remote possibility for these deeper layers to also become reactive (see the latest video from North Shore Rescue for a picture of the snowpack before the storm).

Terrain and Travel

  • Avoid all avalanche terrain during periods of heavy loading from new snow, wind, or rain.
  • Only the most simple non-avalanche terrain free of overhead hazard is appropriate at this time.

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.

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.