From NWAC - This is worth noting

Thanks for the good info in this thread. I have a protected slope in mind for this spring. What is the collective wisdom for those February layers being destroyed by warming? Is my usual waiting until after a couple warm close to 60 degree days in town or a thaw freeze cycle and calling it corn season fool hardy in this case? Is waiting through this current warmer weather not enough to destroy those layers?

Protected implies north facing. For these very deeply buried weak layers west of the crest (not White Pass) it should be hard to trigger as Mark Moore said in the avalanche statement I copied in this thread. North facing is likely OK for those old layers. Of course that doesn't mean you shouldn't evaluate for shallower, still triggerable weakness (also pay attention to the avy forecast).

Let me add, too, I would avoid steep convexities, but then, I almost always do.


I wouldn't have such a good feeling about south facing, however, when the surface isn't well-frozen and strong.  And east of the crest especially when it is warm and/or has snowed more than a few inches, all bet's are off.

Check this out (seen yesterday):

Here's one for the archives (this probably happened around 3/16 or 3/17): On a SE aspect I spotted what must have been about a 2' crown, now somewhat filled in. I then noted that an adjacent bowl, and then another,etc. also had such crowns. I was able to trace a single fracture line around six separate rib features for a total distance of 1.4 km! In 3 or 4 places the crown pulled onto the ridge crest at 15-20 degree angles from steeper terrain below. Debris in a visible run-out area looked to have been dry or at the most slightly moist (it was smooth) and has since been covered by about 18" of more recent snow.

Although this instability is most likely no longer relevant, it was by far the widest crown I've ever seen. The fact that the crown was uniform and only 2' or so thick and that it ran around numerous rib features suggested that it had little to do with wind transport. For such a shallow and likely not very cohesive slab to have propagated so amazingly indicates an extremely weak layer. Given the SE aspect, most likely there was a surface hoar layer sitting on top of a suncrust. That it pulled onto the ridgecrest also suggests surface hoar.

It shows that on occasion the Cascades get impressive weak layering.

On this day south facing slopes had a firm crust with just a few inches of loose wet snow.

Thanks, and yes it is North facing. I also do wonder if there are any rules of thumb or studies that show what temperatures are needed to effectively destroy buried layers. I understand being hesitant to publish something that would make people feel a slope was safe but it also seems that at some temps they either will have slid or consolidated.

I'm not aware of any studies that identify specific temperatures at which you could expect destruction of any particular weak layer. As I'm sure you're aware, there are many other factors to consider.

For the following list, forecast precision increases with depth in the list:

Think about the weather:

- Loading
- Warming
- Cooling

Now think about the snowpack:

- Layer depth
- Grain morphology
- Load on the weak layer
- Interface characteristics

Last, think about the terrain:

- Steepness
- Aspect
- Anchors
- Shape

Now, your ability to issue a precise forecast decreases as you strip away variables. So, using temperature only, it's possible to issue a high-confidence forecast that says "with moderate temperatures and additional loading, these weak layers will strengthen over the next 4 weeks". You won't be able to say where exactly, nor where exactly, but you can be relatively confident in this low-precision forecast.

If you want to know something about a specific slope... you need to go there. And if you can't go there, you need to look at an indicator slope nearby. If that isn't possible, then all you can do is speculate, albeit in an informed fashion.

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Nice post Gary. Your remarks about the shovel made me laugh.

There are a lot of factors to take into account. I am sometimes suprised at how quick even a little bit of sun can affect a slope on the other hand I have been on a sunny slope waiting for the corn to cook only to have a slight wind chill keep the slope too cold all day. It does seem that at some point every spring the slope does consolidate though so it seems like there has to be some relatively safe range of temperatures where we could assume the layers got melted together.

Rules of thumb are generally not a good thing if you are indeed going to bet your life on them.

Here's a way to think about it, too:

Given some particular weakness, bringing it back to life is kind of an exponential thing as you go through the season; such that initially a weakness will respond to a small amount of loading (snowfall or humans), and then as time goes on the weak layer itself gains strength and overlying layers deepen and strengthen, too, so that it then takes more load or more warming to make it fail than it would have previously. It needs to be a heavier dump of snow or a more dramatic and longer lasting period of warming than it would have before.

But the weak layer doesn't ever completely go away unless meltwater penetrates to the depth of the weakness and destroys it. In winter it is possible to refreeze this previously weak layer so that it becomes more like the rest of the snowpack, in spring this just doesn't happen except at very high elevations. So, even with meltwater percolation the weak layer will still be weaker than the surrounding snowpack. Consequently, in very warm or warm and wet periods it may still fail.

BTW, Thank you Cookie Monster.