Avalanche Forecast

Archived

Apr 12th, 2023–Apr 13th, 2023

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Below Threshold.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Below Threshold.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches unlikely, human triggered possible.

Below Treeline

Below Threshold.

Regions

.

Wind slabs may remain reactive to human triggers on lee aspects at treeline and above.

Watch for recently formed and reactive wind slabs as you transition into wind affected terrain.

Confidence

Moderate

Avalanche Summary

We are still waiting on observations from the weekend's storm. We are expecting a natural storm slab cycle to have run during the peak of the storm and we wouldn't be surprised to see evidence of large, persistent avalanches as the weather clears.

In the White Pass area on Saturday, a rider triggered a cornice fall from a distance, which produced a size 3 persistent slab avalanche on a northeast aspect at 1750 m. The avalanche was 200 cm deep and was suspected of having failed on a layer of facets above a crust buried in January.

Observations are limited at this time of year, please consider sharing any information or photos you have on the Mountain Information Network to help guide our forecasts.

Snowpack Summary

30-40 cm of recent snow has likely seen some redistribution by southerly winds, forming deep deposits in leeward terrain features. The new snow sits over previously wind-affected snow on north and east-facing slopes and a crust on windward south-facing slopes.

A weak layer of surface hoar/crust/facets buried in early January is now buried well over 1 m deep. This layer has produced recent avalanche activity in the White Pass region and remains a concern in terrain where the snowpack is thin. The lower snowpack consists of basal facets, particularly in shallow areas.

Weather Summary

Wednesday night

Cloudy with isolated flurries; 0-5 cm / 20 km/h south ridgetop wind / Temperature at treeline around -8 C

Thursday

Mix of sun and cloud / 10 km/h south ridgetop wind / Temperature at treeline around -7 C

Friday

Mix of sun and cloud / 20 km/h east ridgetop wind / Temperature at treeline around -7 C

Saturday

Cloudy with flurries; 3-10 cm / 30 km/h southeast ridgetop wind / Temperature at treeline around -7 C

More details can be found in the Mountain Weather Forecast.

Terrain and Travel Advice

Watch for newly formed and reactive wind slabs as you transition into wind affected terrain.
Avoid steep, rocky, and wind effected areas where triggering slabs is more likely.
Use caution on large alpine slopes, especially around thin areas that may propagate to deeper instabilities.
Cornice failure may trigger large avalanches.

Problems

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.