Avalanche Forecast

Issued: Feb 17th, 2018 4:46PM

The alpine rating is high, the treeline rating is high, and the below treeline rating is considerable. Known problems include Storm Slabs, Cornices and Deep Persistent Slabs.

jmcbride, Avalanche Canada

Lizard-Flathead.

Intense snowfall beginning late Saturday afternoon and overnight, will increase avalanche danger on Sunday. Conservative terrain with no overhead hazard is a good choice.

Summary

Confidence

Moderate - Forecast snowfall amounts are uncertain

Weather Forecast

TONIGHT: Snow. Accumulation 10-20 cm. Ridge wind moderate to strong, northeast. Temperature -16. Freezing level valley bottom.SUNDAY: Cloudy, light flurries. Accumulation trace. Ridge wind moderate to strong, northeast. Temperature -20. Freezing level valley bottom.MONDAY: Mix of sun and cloud. Ridge wind light, northeast. Temperature -16. Freezing level valley bottom.TUESDAY: Mix of sun and cloud. Ridge wind light, southwest. Temperature -12. Freezing level valley bottom.

Avalanche Summary

On Saturday explosive control work produced cornice failures up to size 2, triggering storm slabs on the slopes below.Reports of natural and skier triggered storm slab and loose snow avalanches up to size 1.5 were reported during the storm on Friday, primarily in leeward areas in the alpine and at treeline.On Thursday, natural storm slab and loose storm snow avalanches up to size 1.5 were reported to have occurred overnight with active loading during the storm, while explosive control work produced storm slab results up to size 1.5 on northerly slopes between 1700-1800 m.Earlier in the week, explosive control work and ski cutting produced widespread storm slab results up to size 2. Avalanche activity will increase as a result of Saturday's storm. Expect widespread storm slabs and new cornice growth from intense snowfall and wind-loading. Rapid loading also has the potential to wake up buried weak layers. Conservative terrain with no overhead hazard is recommended.

Snowpack Summary

About 40-60 cm of recent storm snow and wind have created storm slabs at upper elevations. This overlies old wind slabs on leeward slopes at treeline and above, as well as sun crusts on solar aspects and a mix of surface hoar and facets in sheltered areas near treeline. Below 1700m, this new snow covers a rain crust that was buried on February 8th.For the past month there has been regular avalanche activity on multiple weak layers in the mid and lower snowpack. A widespread weak layer from mid December composed of facets, crusts, and surface hoar is 120-160 cm deep. Also, a rain crust with sugary facets buried in late November is near the base of the snowpack.The take home message is that several weak layers are still lurking in the snowpack and they deserve a lot of respect.

Problems

Storm Slabs

Heavy loading from snow and wind will form widespread storm slabs. This new snow overlies a variety of old surfaces including hard wind scour, old wind slab and crusts and will likely be touchy.

Avoid avalanche terrain during periods of heavy loading from new snow and wind.If triggered, storm slabs may step down to deeper layers resulting in large avalanches.Watch for whumpfing, hollow sounds, shooting cracks or recent avalanches.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Very Likely

Expected Size

1 - 2.5

Cornices

Cornices may be weak and have the potential to trigger large avalanches running long distances. Avoid lingering in runout zones below large corniced slopes, and stay well back from corniced ridge lines if traveling above them.

Avoid steep slopes below cornices.Give cornices a wide berth when traveling on or below ridges.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

2 - 3.5

Deep Persistent Slabs

The potential for large avalanches running on buried weak layers exists. Wind loading, fragile cornices, and smaller avalanches are all possible triggers, as well as thin or rocky snowpack areas where human triggering may also be possible.

Avoid steep convexities or areas with a thin or variable snowpack.Avoid exposure to overhead avalanche terrain, large avalanches may reach the end of run out zones.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

2 - 4

Valid until: Feb 18th, 2018 2:00PM