Watch out next week - slabs

A view from the top of Silver King yesterday shows that the mid-April layer will be a concern again with warming next week. Three large slabs were observed on N to NE aspects between about 5500' and 6500'. These appear to have come down in the brief warming last weekend. The great corn of late owes it's existence to unusually cool temperatures, especially hard freezes at night. The resultant hard crust is a little misleading as to what it means about deeper snowpack consolidation.

It looks like those cool temperatures will go away next week. Slabs of 4' or so seem possible with cornice release, deeper surface sluffs, or possibly just increased stress from the warming. The concern should be greatest on the suspect aspects and especially at higher elevations.

Thanks for the information Gary. 

Would you mind refreshing our minds with respect to what the events/ conditions were in mid-April and provide a little more explanation of what you believe has been going on in the snowpack to lead to the slab releases you observed?  AND/OR what signs one could look for in a pit to find this "mid-April" layer?

I think we sometimes let our guard down in the spring/ early summer and appreciate your expertise in this area.

Thanks,

J.P.

J.P>

Would you mind refreshing our minds with respect to what the events/ conditions were in mid-April and provide a little more explanation of what you believe has been going on in the snowpack to lead to the slab releases you observed? AND/OR what signs one could look for in a pit to find this "mid-April" layer?

Remember there was heavy snowfall in mid-April and a number of accidents in both the Casacades and Sw'ern BC. So, the bond must have been poor (I was in Red Rocks at that time). So, since this is the first time it has warmed up this much and especially for a prolonged period, I'd expect tthe bond to be weakened by the warming. The strength of the upper snowpack, then, is little more than the strength of the surface crust. As that weakens with warming - especially wind and clouds as Sunday nt through Monday nt - slabs are possible. South slopes appear stronger, which makes sense with more consolidation, but also get more warming which can get water down to the weakness. North slopes are far less consolidated - just look at the amount of recent snows still on rock faces as a good guage of that - but they are also cooler during clear periods. Still, north slopes are the slopes that have cornices hanging above them and many cornices are monster this year, and nor many have fallen yet. Those big cornices should trigger some large slabs.

Also, wet slabs may release on sunnier aspects when there is sufficient saturation of surface snows and penetration or percolation down to firmer layers. Also, glide cracks are opening rapidly. I could see that difference between Sat AM and Mon AM on the north side of Whitehorse, and also on the Colonial Creek headwall.

The large slabs on NE aspects near Crystal indicate the concern for slabs on NE aspects.

I was up around the Dutch Miller Gap area, central Alpine Lakes, this weekend at the beginning of the warmup and we saw a lot of scary looking slopes of the type Gary mentions. Specifically, many of the steep rock slabs on the peaks still had their full winter snowpack but were showing signs of creep (glide cracks, compression humps), and there were still a lot of big cornices. Just during the day Sunday snowpack up on west facing rock slabs of Bears Breast Mountain, above Williams Lake, showed signs of new glide cracks, and now it has been warmer for two additional days. We saw evidence of creep on all aspects. Looking at the telemetry for last night it appears that a lot of locations barely made it below 60 degrees! Please be very careful if you are going out this week.

Yeah, I bagged my ski day today after seeing that telemetry data! Plus we had some clouds last night.