Avalanche Forecast

Issued: Feb 16th, 2018 4:54PM

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

jmcbride, Avalanche Canada

Lizard-Flathead.

Periods of intense loading from snow and wind are expected to begin late Saturday afternoon into the evening. If the storm arrives earlier than forecasted, avalanche danger will increase sooner.

Summary

Confidence

Moderate - Timing, track, or intensity of incoming weather system is uncertain on Saturday

Weather Forecast

Snowfall is expected to begin Saturday afternoon, increasing to heavy snowfall during the evening, then easing overnight.SATURDAY: Snow. Accumulation 20-30 cm. Ridge wind light to moderate, east. Temperature -8 C. Freezing level valley bottom.SUNDAY: Mainly cloudy, light flurries. Accumulation trace. Ridge wind moderate to strong, east. Temperature -20. Freezing level valley bottom.MONDAY: Mix of sun and cloud. Ridge wind light, east. Temperature -18. Freezing level valley bottom.

Avalanche Summary

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. Explosive control work on Thursday also produced storm slab results up to size 1.5 on northerly slopes between 1700-1800 m.Explosive control work and ski cutting on Wednesday produced widespread storm slab results up to size 2, between 1600-2100 m.On Tuesday skiers were able to cut size 1.5 wind slabs on north east features at treeline. Wind slabs to size 2 were also reported, starting at the base of steep head walls. Last weekend we received reports of impressive sizes of cornice chunks breaking off a tree line ridge feature near Harvey Pass (Flathead area), running far into the forest and meadow below. See here for photos. Avalanche activity on deeper layers has tapered off for now, but it's best to avoid likely trigger points such as thick-to-thin snow pack areas and also close to the base of rocky headwalls.

Snowpack Summary

About 30-50 cm of recent storm snow and wind have created wind slabs at upper elevations. This new snow 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 at 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. Conservative terrain use is recommended while avoiding overhead hazard.

Problems

Storm Slabs

Recent storm snow and wind have formed storm slabs and loaded lee areas near ridgecrests and exposed features at treeline. This new snow overlies a variety of old surfaces including hard wind scour, old wind slab and crusts on solar aspects.

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

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely

Expected Size

1 - 2.5

Cornices

Cornices have triggered very large avalanches recently. Be especially wary of long runout distances in avalanche paths, and the possibility of mature timber being taken out by a surprisingly large avalanche.

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

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

2 - 3.5

Deep Persistent Slabs

Large avalanches have been running on deep weak layers. 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 exposure to overhead avalanche terrain, large avalanches may reach the end of run out zones.Be aware of the potential for large, deep avalanches.Avoid steep convexities or areas with a thin or variable snowpack.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

2 - 4

Valid until: Feb 17th, 2018 2:00PM