Avalanche Forecast

Archived

Apr 1st, 2021–Apr 2nd, 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 unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely, human triggered possible.

Alpine

Natural avalanches unlikely, human triggered possible.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Natural avalanches unlikely.

Regions

Northwest Coastal.

Additional snow and wind will continue to form reactive storm slabs on Friday. Use small slopes with low consequence to test the bond of the new snow. Be especially cautious in wind affected terrain.

Confidence

Moderate - Forecast snowfall amounts are uncertain.

Weather Forecast

THURSDAY NIGHT: Snow; 5-10 cm. / Strong, southwest ridgetop wind / alpine low temperature -7 / Freezing level valley bottom.

FRIDAY: Snow/rain; 10-20 mm, and another 5-15 cm. overnight / Strong, southwest ridgetop wind / alpine high temperature -1 / Freezing level 800 m.

SATURDAY: Flurries in AM and clearing in PM; 0-3 mm. / Light, southwest ridgetop wind / alpine high temperature -1 / Freezing level 800 m.

SUNDAY: Mix of sun and cloud / Moderate, northwest ridgetop wind / alpine high temperature -1 / Freezing level 800 m.

Avalanche Summary

On Wednesday, naturally triggered glide slab avalanches were reported up to size 3. Glide cracks releasing as full depth glide slab avalanches are extremely difficult to predict. Best practice is to avoid slopes with glide cracks.

Snowpack Summary

Storm snow amounts will continue to accumulate overnight Thursday and throughout the day on Friday, forming reactive storm slabs.

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.