Persistent Slabs in the Northwest

www.nwac.us/blog/2018/03/13/persisten-problems-pnw/

Please check it out.

I think it's important to point out that for several weeks NWACs avalanche forecast (at least for West Slopes North) predicted persistent slab on W-S-E aspects only. To quote from the earliest archived report on 3/6 "You may be able to trigger Persistent Slab avalanches in the upper snowpack on sun-exposed slopes (generally southerly aspects) greater than 35 degrees." This forecast only changed on 3/11. Which occurred after.....

.... the young man at Park Butte died on 3/10. Personally on March 3rd I saw the results of several very, very large natural avalanches down to the 2/8 crust on north facing aspects around the Mt Baker Ski Area that had occurred sometime during March 1st/2nd as 20" of new snow fell, which I did my best to report to NWAC via e-mail.

So yes I'm saying their forecast was basically wrong for several weeks. Personally I'm fine with that, I don't expect them to be infallible. They are a small group of people covering a very broad area. However their forecasts carry a certain offical-ness to them. Posted by ski areas, taken at face-value by many backcountry users. While useful I imagine users as a whole are over-reliant on NWAC vs their own observations and judgement in the field.

Something to consider. Thoughts?

I think it's important to point out that for several weeks NWACs avalanche forecast (at least for West Slopes North) predicted persistent slab on W-S-E aspects only. To quote from the earliest archived report on 3/6 "You may be able to trigger Persistent Slab avalanches in the upper snowpack on sun-exposed slopes (generally southerly aspects) greater than 35 degrees." This forecast only changed on 3/11. Which occurred after.....

.... the young man at Park Butte died on 3/10. Personally on March 3rd I saw the results of several very, very large natural avalanches down to the 2/8 crust on north facing aspects around the Mt Baker Ski Area that had occurred sometime during March 1st/2nd as 20" of new snow fell, which I did my best to report to NWAC via e-mail.

So yes I'm saying their forecast was basically wrong for several weeks. Personally I'm fine with that, I don't expect them to be infallible. They are a small group of people covering a very broad area. However their forecasts carry a certain offical-ness to them. Posted by ski areas, taken at face-value by many backcountry users. While useful I imagine users as a whole are over-reliant on NWAC vs their own observations and judgement in the field.

Something to consider. Thoughts?

The guy who was killed at Park Butte was a snowmobiler hucking 100' airs off a cornice onto a deep, hard wind slab. I doubt a skier could have triggered that slide under any circumstance. It's also worth noting that the snow level went up above 7k' pretty much everywhere this week, and the resulting settlement and crusts make triggering this layer even more unlikely, at least around the ski areas and other spots where people typically access touring on the westside. It would take a bigger trigger than a ski turn to get past these stable surface layers. Something to consider when you are skiing around WA Pass or anywhere that did not receive these warmups, or on the westside during on a hot day, but not when you are skiing 3" of dust on crust this weekend.

I think the focus on W-S-E was due to some persistent weak layers that formed on top of sun crusts. The avalanche which killed the two teenagers in the Alpental valley failed on such a layer.

However, the deep persistent slab problem (going back to that 2/8 raincrust with facets on top) has been consistently listed at all aspects and elevations, and I'd guess is most likely to fail on N aspects where the snow on top of that slab will be the most faceted. It looks like NWAC isn't saying for sure which layer the Park Butte avalanche failed on. From the picture I saw on SnoWest, it's a wind-loaded spot which could make it hard to tell.

It's getting hard for me to keep track of all the different layers which have occured since our high pressure period after 2/8. With the amount of snow that's fallen since then, that rain crust is not the only weak layer down pretty deep by now.

While conditions have changed since the NWAC blog came out (and will continue to), it's still worth a look to jog the memory or inspire further research.

Check out a discussion of an earlier persistent slab accident in Utah:
utahavalanchecenter.org/blog/38218

And this older discussion on consequence-probability and (loosely) PWL avalanches:
utahavalanchecenter.org/blog/30627

The Wasatch is used to persistent weak layers, and there are some common threads that tie into the NWAC blog post. I like this line, given recent accidents:

"The question is not whether these experts are well trained...the question is whether their world is predictable." - Daniel Kahneman/Amos Tversky

Stay safe, have fun, and ski well.

It would take a bigger trigger than a ski turn to get past these stable surface layers.

Generally agreed for deep persistent weaknesses, unless a skier gets unlucky and finds a thin spot in the snowpack where her weight can initiate a fracture.

Deep persistent problems are essentially foreign to us in the Washington Cascades; my margins of safety will remain large until these layers have a chance to round and lock up. That probably won't happen until the snowpack goes isothermal, whether by a pineapple or the changing of the seasons.

The nuance for us is anticipating when there will be enough surface melt to percolate all the way down to the facet-crust interface and lubricate it into a sliding layer. This will probably be one of those early Aprils of some historic slides.

March 10th forecast includes Deep Persistent Slabs as a problem. All aspects and elevations, Unlikely but Large to Very Large in size if triggered . . .

Below is from the forecast discussion

"Several older persistent weak layers exist within the snowpack. On E-S-W aspects a thin facet-crust combo (2/23) can be found. Snowpack test results show this layer healing but it has been reactive in some snowpack tests. An older deeper and more widespread persistent weak layer has been observed for several weeks. Weak sugary facets (2/13) sit just above a firm crust formed and buried in early February (2/. This crust is generally found about 3-4 feet below the snow surface."

www.nwac.us/avalanche-forecast/avalanche...de-west-north-baker/

visual indicators and spring almost acting like winter till the sun comes out: dont miss the pwl for the windslab...that will getcha if you head is only way down deep in the snow pack...some pretty good examples lately ..that part isnt rocket science..

Yeah, if my memory is correct (yes, I could just scan the archive to see for sure) NWAC has been mentioning that surface slabs or even loose snow slides could step down to the PWL/PWS even back before the wave of weekend deaths started up.

We still have multiple PWLs in the Baker area, I'm guessing other areas do too. Please make appropriate decisions out there.