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Avalanche Forecast

Archived

Apr 11th, 2023–Apr 12th, 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 unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Below Threshold.

Regions

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Storm slabs will likely be reactive to human triggers, especially in wind-affected terrain and/or where the recently formed slabs are sitting on weak facets.

Choose conservative terrain and watch for signs of instability.

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 new 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

Tuesday night

Partly cloudy / 20 km/h south ridgetop wind / Temperature at treeline around -12 C

Wednesday

Cloudy with isolated flurries; 0-5 cm / 20 km/h south ridgetop wind / Temperature at treeline around -9 C

Thursday

Mix of sun and cloud / 20 km/h southwest ridgetop wind / Temperature at treeline around -8 C

Friday

Mix of sun and cloud / 20 km/h east ridgetop wind / Temperature at treeline around -8 C

More details can be found in the Mountain Weather Forecast.

Terrain and Travel Advice

  • Continue to make conservative terrain choices while the storm snow settles and stabilizes.
  • 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.
  • 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.

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.