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Avalanche Forecast

Archived

Dec 20th, 2018–Dec 21st, 2018

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.

The sun should shine on Friday, showcasing snow distribution and avalanche activity from Thursday's storm. Be wary of the bonding of all the recent snow, especially in lee terrain features near ridges, where snow deposits will be deep.

Confidence

Moderate - Due to the number of field observations.

Weather Forecast

THURSDAY NIGHT: Cloudy, freezing level dropping from 1000m to below valley bottom. FRIDAY: Partly cloudy, moderate northwest winds, alpine temperature -12°c, freezing level below valley bottom. SATURDAY: Cloudy with light snowfall, accumulation 5cm, moderate southwest winds, alpine temperature -7°c, freezing level below valley bottom. SUNDAY: Cloudy with snowfall, accumulation 5 to 10cm, moderate southwest winds, alpine temperature -5°c, freezing level 1000m.

Avalanche Summary

Natural storm slab avalanche cycles have occurred during each storm within the past week (see description and photos here). It is likely that a natural cycle occurred again on Thursday during the storm. Clearing skies on Friday should allow for views of previous avalanche activity.

Snowpack Summary

Around 30 to 40cm of snow fell in the region on Thursday with associated strong southerly winds. Deeper snow deposits are likely in the lees of terrain features. This sits on around 1 to 2m of storm snow that has fallen in the past two weeks. Below 1500m, Thursday's precipitation mostly fell as rain. In the north portion of the region, a weak layer of facets and surface hoar lies below all this storm snow, around 120cm deep. The weak layer appears most prominent around treeline, up to 2000m. While the layer is likely gaining strength, field observations show that slab avalanches remain possible on this layer where it exists. Near the base of the snowpack, a crust exists with weak and sugary facets beneath it. This could potentially still be of concern in high north-facing aspects where the snowpack rests on very smooth ground cover.

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.