Avalanche Forecast

Archived

Mar 31st, 2021–Apr 1st, 2021

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 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, human triggered possible.

Regions

Northwest Coastal.

New storm snow accompanied by strong southwest wind will likely build fresh and reactive storm slabs at upper elevations.

Confidence

Moderate - Forecast snowfall amounts are uncertain.

Weather Forecast

Unsettled conditions will exist on Thursday as the cold front weakens and a new low develops near Haida Gwaii.

Overnight Wednesday: Snow amounts 10-20 cm with strong ridgetop wind from the southwest. Alpine temperatures near -7 and freezing levels valley bottom.

Thursday: Snow 10-20 cm. Ridgetop wind generally light with gusty periods from the southwest. Alpine temperatures near -4 and freezing levels 600 m.

Friday: Snow amounts 5-15 cm. Ridgetop wind strong from the South and freezing levels 600 m.

Saturday: A mix of sun and cloud. Ridgetop wind light from the southwest and freezing levels 700 m.

Avalanche Summary

By Wednesday afternoon a glide slab avalanche size 1.5 was reported from a 35-degree rocky slope at 500 m. Avalanche activity is expected to increase on Thursday with new snow and strong southwest wind.

Snowpack Summary

Storm snow amounts will continue to accumulate overnight Wednesday building reactive storm slabs by Thursday.

The recent snow adds to the 30-60 cm that fell at upper elevations last weekend. Much of the recent precipitation fell as rain at lower elevations and the snow surface below treeline is a melt-freeze crust in many areas.

At higher elevations, the new snow has added significant load to a couple of deeply buried weak layers. The first persistent weak layer was buried in mid-March and is around 100-180 cm deep. It consists of surface hoar on sheltered northerly aspects around treeline and down between 150 to 250 cm is another layer of surface hoar and facets, buried in mid-February. At this time, these layers linger in the snowpack but haven't recently been a reactive avalanche problem.

The mid and lower snowpack is well settled and strong in most areas.

Terrain and Travel

Watch for fresh storm slabs building throughout the day.
Look for signs of instability: whumphing, hollow sounds, shooting cracks, and recent avalanches.
Start with conservative lines and watch for clues of instability.
Pay attention to cornices and give them a wide berth when traveling on or below ridges.

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.

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.