Resetting the snowpack

[mogul5480]

Recently in Montana, avalanches have been sliding off of a weak, faceted layer near the ground. Observations of uphill and sympathetic propagation are widespread.

My question is, when you get this kind of widespread avalanche activity that rips off the entire snowpack down to the bottom-most weak layer, does this effectively "reset" the snowpack by clearing out the weak layer, or does the bed surface of the slide path generally form a new weak layer that causes persistent problems throughout the season?

Also - the weather is supposed to remain cold and clear in the aftermath of the storm. To what extent does the recent avalanche activity influence the ability for the snow to form new instances of facets or surface hoar?

This is mostly just for my education. Thanks very much.

[CookieMonster]

Widespread avalanching produces bed surfaces with variable characteristics. The chaotic interaction between terrain and weather makes it difficult to predict what will happen in general, and predicting what will happen specifically usually requires detailed information. The size of the area is another limiting factor, as high precision forecasts ( what will happen specifically ) are usually only available for small areas.

The exposed snow surface breeds instability, so the amount of time for which the bed surfaces remain exposed to the weather is another variable. To give you an idea of the complexity:

If a layer of facets remains on the bed surface, and the layer is then exposed to multiple melt/freeze cycles ... well, that's a completely different scenario than if a bed surface composed of faceted crystals is immediately reloaded during a storm. Melt freeze activity is often limited by aspect, so there is always the possibility for faceted crystals on cold aspect bed surfaces to continue to develop, while faceted crystals on warm aspect bed surfaces undergo entirely different changes.

With respect to the relationship between avalanche formation and facet development, I'm not sure any specific relationship exists. However avalanche formation thins the snowpack, and a thinner snowpack is definitely more facet prone than a thicker snowpack.

There really isn't a rule of thumb, and as always, you should use multiple observations to determine the likelihood of avalanche formation at a specific place and time. "Resets" are definitely possible, but you shouldn't let such generalised thinking form the basis of forecasts for specific slopes.

[Meathelmet]

good stuff..

What kind of surface layers would you consider "resettable",if you would remove the time-to-exposure variable?

Surface hoar? Depth hoar? Graupel?
More persistent ones (firnspiegel,sun/rain crust) one could thing "un-resetable", or?
Is it even remotely possible to sort out snowtypes under some french lines?

[CookieMonster]

Thanks for the kind words.

Layers that are destroyed during the avalanche, such as surface hoar and graupel, are in theory more "reset-able" than harder crusts or thick layers of facets.

BUT.

Such general information may or may not be useful or applicable at any particular place or time, unless you have made extensive observations of the snowpack for a specific area, and/or have extensive operational forecasting knowledge. General information with dubious applicability can have a dangerous and unwarranted influence on your beliefs about the presence of instability and its parameters.

That's why, even if some crystal forms are more suitable for "resets" than others, it's best to avoid using such generalised thinking in backcountry avalanche forecasting.

"Resets" do not mean a specific slope is safe to descend. More than anything, over the long term a "reset" such as a wide-area rain event will change the character of the danger rather than eliminate it. There really are no rules of thumb, but if you need to use simplified rules, here are several:

Avalanches remove weak snow from some, but not all, slopes.
Avalanches may or may not remove all the weak snow from a specific slope.
Bed surfaces may or may not reload.
Incremental changes to the snowpack are a primary source of uncertainty.

[mogul5480]

Thanks for your help, Cookiemonster. I guess the goal of starting this thread was actually more effectively articulated by Meathelmet - I was looking to see if there were any rules of thumb around the "resetting" of the snowpack. Completely understood on the unpredictability of the forecast (though I was hoping naively for an easy rule of thumb and a good prognosis for my trip in early March).

Thx again for your help on this.

[Bunion 2020]

CM is pretty close to the what is going on in SW Montana, WY and Eastern ID.

Its not as simple as a layer that needs to "go away"

We (as in the Royal We) have a sandwich that has 2 weak layers and 2 potential bed surfaces .

Some slides are taking out the upper layer and running on a bed surface that is the crust above the 2nd layer of facets.

I have seen uncountable slides over the last 3 days that have cleaned out either layer.

Within the ski area we are resetting several areas that we will ski the rest of the season provided we get some more snow and can affect them.

In the BC, we are in for another sketchy year. It will be a game of either digging on any slope that you want to ski or guessing as to what is lurking.

Here is an example of slides on Fan Mtn, near Big Sky. Lots of areas that haven't slid. With an additional load, would you ski them?

[CookieMonster]

Thanks to Bunion for being kind enough to provide some very specific information about conditions in the area.

For the OP: You may have to work around the s**t sandwich Bunion describes in his post, but you can still have a fantastic trip.

[mogul5480]

I eat s**t sandwiches for breakfast!!! (the economy being what it is, it's a great cost-saver)

Bunion - that's good area-specific info. Thanks to both you guys for the help!

[homemadesalsa]

Hey Bunion can you send me a big jpg of the photo above, with a good short story, for the next TAR please please?

And Cookie Monster my friend, wanna recap the above for the same issue?

Shameless, aren't I?

[LarsLarson]

Hey Ms. Salsa, did a little walk about on the pass today and saw some interesting things. Mostly the usual NE stuff, but all natural as far as I could tell. An interesting one on Windy Ridge -- hope no one was involved.

[homemadesalsa]

Great to get more in-depth and thoughtful answers/ directions on the questions lots of us are wrestling with this winter.

Anybody else wanna play? Your thoughts? What heals well? Critical values such as SWE (an inch of water in a 24-hr period?) or wind? Layering concerns? Management techniques? Do you play it conservatively until your shit sandwich is a meter and half down? Do tell...

[cmor]

Our biggest problem with cycles similar to this is the large scale spatial variability game. Not everything will rip out. And its a waiting game to see if the old bed surfaces become reactive, if spots that haven't slid stabilize with greater depths, or vise versa. And no matter how hard you try to track what has gone and what hasn't, its hard to make good larger scale terrain judgments when new snow blankets the old fracture lines and you're hopping between two different snowpacks on the same aspect/incline/elevation.

[cmor]

A quick example: Big rain event ripped out a lot of terrain but left a few spots. Nothing broke down below this event in the future, but the differences in the the top layer led to differences in bed surfaces after some new snow. Had a result on the non-avalanched bed surface on lower angle terrain with no results on higher angle terrain adjacent to it on the avalanched bed surface:

Before:

Text: "It is interesting to note that the earlier chutes on the throne did not avalanche to ground (Chutes A, B, C, etc.) There is a small fracture down low which is indicative of rain events in this area, but near the ridge the snow held. A few weeks later a large avalanche was released with explosives across the area that did not slide during this event. "

Result few weeks later:

Not a great photo. but SS-AE-R3-D3-O

Whole event in excruciating detail:
http://www.alpinedescent.com/crystalweather.pdf