I am bored it is cold as hell, let us discuss this Snow Pack scenario

[icelanticskier]

isothermic=32 degrees throughout. colder=rotten/sugar?

rog

[rideit]

Right...but...literally, 'isothermic' would just mean 'one temperature'.

[icelanticskier]

an isothermal snowpack is one with water present. maybe if one huge dump fell at one temperature you could call that isothermal, temporarily, but the snow near the ground will almost always be warmer (32 degrees) than above and at the surface until spring/summer.

rog

[Bunion 2020]

Right...but...literally, 'isothermic' would just mean 'one temperature'.

Correct.

an isothermal snowpack is one with water present. maybe if one huge dump fell at one temperature you could call that isothermal, temporarily, but the snow near the ground will almost always be warmer (32 degrees) than above and at the surface until spring/summer.

Isothermal snow packs have general characteristics as Rog had described.

They are close to or at 0 deg. C throughout with no temperature variations, temps much below 0 deg. C usually will exhibit some variations in Temp.

It is the equality of Temp through the pack rather than with liquid H2o present that causes the pack to be labeled Isothermal, liquid H2o may or may not be present.

It is more a descriptive term than a scientific term.

[TBS]

Isothermal = one temperature.

There's an article Feb 2012 TAR edition about the 2011 "Martin Luther Crust" in the PNW. That snowpack went pretty much isothermal...air & surface temps were -0.5c, then 0 at 10 cm and below.

How do I know the conditions? I was digging pits in it for a L1 refresher, wishing I'd brought a chainsaw and m80s

[klar]

Slight tangent here.
I have always heard the term isothermic to mean, essentially, above freezing all the way through.
Can snow be 'isothermic' at, say, 10 degrees all the way through the pack?
(I know it would have to be fairly shallow)

Snow, even if very wet, is never warmer than 0 ° C.

[rideit]

Obviously.
I meant 10F.

[TBS]

Analyze This

This is a less chilly version of Bunion's original question.

Location = Central Oregon. Skiing mostly occurs between 6-9K feet
After highs in the 50's/lows in the 20s during the week of Thanksgiving, it's been colder than a stainless steel toilet seat all last week. Single digit highs and in the -10-15F range for lows. Friday we got 12" of glorious blower pow at temps between 5 & 12F, the first we've had since early November. Yesterday had a high of Zero to +5F. Last night's low was -15-20F. Except for a moderate breeze during the snowstorm on Friday, winds have been calm/light.

Dug a quick pit this PM after skiing inbounds. Air temp was 12F (-10C). NE aspect at 6300'. ouldn't find my loupe & crystal card so couldn't get deeply into crystal types or size.


We have a warm moist front coming in this week. Hi temps near freezing by Thursday, then falling a few degrees. About 4" new snow is forecast Thurs-Saturday.

Discussion Topics
- what, if any, hope is there that the heavier, warmer snow expected this coming week will help equalize the pack?

- What conditions (other than a big rain storm or a slide) will turn this pack right-side up again?

[klar]

Obviously.
I meant 10F.

Right. Sorry. So confusing.


Telebobski,
Any thoughts why the lowest layer is so hard?

[TBS]

Telebobski,
Any thoughts why the lowest layer is so hard?

All but the top two layers(and maybe the topmost rounded layer) have been there a while. The bottom layer is probably on the ground too - was running out of light and desire, so didnt dig to turf.

the Area got some early season snow, followed by several weeks of unseasonably warm temps with a little rain. Last week it got increasingly cold/dry, and a bunch of light dry snow fell last Thurs/Friday while temps continued to plummet.

[icelanticskier]

so yer dealing with mostly new snow instabilities.^^^^^^^^^^^tricky

rog

[TBS]

so yer dealing with mostly new snow instabilities.^^^^^^^^^^^tricky
rog

Congrats on your fifth grade reading skill. Yup, just like i wrote, recent cold dry snow on top of month-old layers, damn cold temps. Faceting in older layers caused by the cold. We don't see this kind of pack very often in the cascades. And a more normal flow moving in with temps in the high 20s - low 30s and more snow.

So, answer the question Chauncey, what's it gonna take for this pack to heal in the near term when the warm front arrives? Tell us how it all be OK if everyone just waits until 72 hrs from the last snowfall and stays on low angle slopes.

[icelanticskier]

You said earlier that the early season snowpack got rained on then it got cold with no new snow and cold until recently when it started to snow again with continued cold, correct. Now there's a warmer system moving in which will add a lot of weight, correct?

Ro

[icelanticskier]

So, answer the question Chauncey, what's it gonna take for this pack to heal in the near term when the warm front arrives? Tell us how it all be OK if everyone just waits until 72 hrs from the last snowfall and stays on low angle slopes.

Time. Time heals. Maybe just 48 hours but i like 72 mo bettah.

Rain followed by cold is the best healer tho

Rog

[TBS]

Just to recap here

Analyze This

- what, if any, hope is there that the heavier, warmer snow expected this coming week will help equalize the pack?

- What conditions (other than a big rain storm or a slide) will turn this pack right-side up again?

Time. Time heals. Maybe just 48 hours but i like 72 mo bettah.

Rain followed by cold is the best healer tho

Rog

Note the highlighted portion of the question. Rain storms equalize packs = Duh!

So you think two existing layers of buried facets, plus the temp diff in the top layer and the stuff to fall shortly will equalize in 72 hours?

Anybody with a clue about continental pack conditions care to chime in? I would seriously like to learn something about situations I don't get to see that often.

[Bunion 2020]

I am a bit busy at the moment. Get back to you tomorrow.

Powder day (of sorts) today.

[klar]

@telebobski

around here the hoar layers would more or less stay around indefinitely unless the whole pack warms up alot (apart from rain I suppose a prolonged stretch of above freezing and cloudy nights might do it). As there is quite a bit of snow on top of your bad layers I doubt the weight of the new snow will do much, at least I wouldn't expect it to here. Attached are 2 profiles from last season in november and december with typical buried crap. I asked about the hard layer at the bottom because I would have expected depth hoar there as well after a cold spell.

does the horizontal dash in your profile mean melt form?

[Lindahl]

I'm not a snow science guy, so most of my information is experiential...

Freeze crust will act as a vapour barrier so give it a little time (~day) and you will get a layer of hoar below the freeze crust, i.e. either on the old snow, in the new snow, or both. Voilá!

Don't you mean above? We see faceting above crusts out here, since most of our crusts form just above the first couple major storms of the year, when we get a warm up around November. In fact, it seems lots of our big avalanches in late Dec/Jan aren't to ground, but rather, to a crust. Sometimes people see facets and assume they go all the way to the ground though (depth hoar).

Our late year problems are often E/SE aspects that have a suncrust on them, followed by cold/calm night of surface hoar, followed by a relatively big storm that follows the cold front. Toss a load on there, surface hoar collapses and bingo, lots of 1-3' crowns.

Those are generally the two categories of avalanches I've see out here.

The 72 hour rule means to hope what's there settles a bit to be so what useful for somewhat safely getting around till shit does in fact change or shit hits the fan. Rain to ridges with deep freeze would be best as I stated.

Problem is, in a continental snowpack, you need more than 72 hours, especially for bigger storms. A week is often a pretty good idea, and even then, you still need to know where the wind is moving snow, as that's a constant imbalancing effect as well.

We currently have a mild form of Bunion's conditions in the PNW - way different from the norm. These offbeat situations are good for thinking outside the normal box - esp since its gonna warm up a bunch over the next week.

Let me know how that works. I'm curious to see how a ton of warmth changes things. We usually don't get enough warmth down here to get an idea of what happens. From poking around at shallow late season snowpacks though, we see the facets refreeze which can stabilize things (at least in the short term). But then, you can get the suncrust/surface hoar/dumpage combo that I mentioned in the beginning of this post, which is no bueno.

what's it gonna take for this pack to heal in the near term when the warm front arrives?

Deep faceting generally doesn't stop being a problem out here until they get sufficiently buried in Mar/Apr (Jan/Feb for Utah?). Though, once late season rolls around, a huge storm, followed by really warm temperatures can loosen the facets back up again and we can get HUGE slides, especially during low snow years - when the bridging is relatively thin. This was a big problem here last April. We had a pretty large cycle that was sending things to ground. Rain can break facets down though (we don't get rain, so not sure how much/long that would take).

[klar]

Don't you mean above? We see faceting above crusts out here, since most of our crusts form just above the first couple major storms of the year, when we get a warm up around November. In fact, it seems lots of our big avalanches in late Dec/Jan aren't to ground, but rather, to a crust. Sometimes people see facets and assume they go all the way to the ground though (depth hoar).

In that case I did mean below because I was thinking about cold new snow on warm older snow (or a temperature drop during snowfall). In theory the temperature gradient would be stronger over the whole pack and vapour transport would be happening from the ground up, eventually hitting the barrier of the crust. If the new snow is warmer than the pack the gradient would be reversed (at least for a while) and facets would be above the crust. Obviously these are mostly theoretical musings, in practice we almost always find facets both above and below the crusts if they have been in the pack for a while.

Deep faceting generally doesn't stop being a problem out here until they get sufficiently buried in Mar/Apr (Jan/Feb for Utah?). Though, once late season rolls around, a huge storm, followed by really warm temperatures can loosen the facets back up again and we can get HUGE slides, especially during low snow years - when the bridging is relatively thin. This was a big problem here last April.

In 07/08 there was a construction site on the way to one of the resorts here and we had been wondering why they would build a tunnel in the middle of nowhere. april brought a lot of late season snow on a previously shallow pack consisting mostly of early season depth hoar. we figured out the point of the tunnel

[icelanticskier]

"Problem is, in a continental snowpack, you need more than 72 hours, especially for bigger storms. A week is often a pretty good idea, and even then, you still need to know where the wind is moving snow, as that's a constant imbalancing effect as well."

thanx lind. I use the 72 hour thing as a symbol for a need to wait and let time heal before going out and about for a look/poke around. that's why I used words like "bit" "somewhat".

here, maritime, it's closer to 24-48, intermountain, 48-72+, and continental 72++.

just one of my lil things that gets some "geeks" in a tizzy

rog

[Bunion 2020]

Bobski from my experiences it takes much less time for cold weather to facet a layer than it does for warm temps to heal a layer, particularly if that layer is deeper and buried by a good amount of new snow.

I always hope that "this warmer weather is doing that crappy layer some good" and end up disappointed.

[TBS]

Klar, Lindahl, Bunion - thanks for your comments. I sorta figured the facets will stay for a while but nice to hear what people who see this all the time have to say.

Klar, that bottom layer is ice - breaking into fist size chunks. I started singing 'Chain Gang' at that point.

Its been warming up this week. Next few days are gonna see high temps above freezing (in the 0-4*C range) and lows in the -5 to 0*C range, with about 6" of new expected. Will dig again early next week after the newer, wetter stuff has a chance to settle and bond (or not) to the layer of cold blower we got last Fri-Sat.

Bunion, thanks for starting this thread. Hope others post their abnormal situations for review as well.

[Bunion 2020]

Bunion, thanks for starting this thread. Hope others post their abnormal situations for review as well.

No worries, I have never dealt with a maritime regime so reading your observations as well as those from CO helps me to reference the vast differences.

And for all of Rogs BS at times, an EC perspective is interesting as well.

For the SW MT area at this time; Lots of weak snow near the ground from south of Bozeman to West Yellowstone. Some failures at that layer and some failures higher in the pack that then step down.

It is sunny, warmer and the wind has started to blow. Play safe this weekend, these are the ingredients that make for a snowy torrents write up.

[Z]

Definitely an interesting year for SW MT from what I have seen. I've been traveling around alot and haven't been around in early December. Besides the last two at Bridger luckily! But, I'm interested in this thread as I know that you (Bunion) have a ton of experience in this area, and I know I could learn from you.

My thoughts...

The Bridger Range and the Big Sky zone/Northern Madison seems to have a drastically different snowpack. It's interesting to see a fairly "average" intermountain snowpack in the Bridgers/North of BB, and what looks like a fairly shallow to average continental snowpack in the N. Madison. Interesting to note the huge difference between the snow cover, even with them being so close and without ocean influence. The cold temperatures seem to have done WAY less damage in the Bridgers. We are dealing with a new/old interface problem in the upper pack, and related wind slabs as always. I don't think I have seen or observed a full depth slide in the Bridgers, where as they are widespread in the Madison. And Cooke City it seems but I haven't been down there. Cooke City seems like a different story.

It doesn't seem like it's as simple as a deeper pack in the Bridgers has allowed for less faceting at the ground. The pack was plenty shallow here for the entire month of November. Cold here too, but maybe less temperature variation (day/night)? No rain that I know of in the Bridgers here so not that. What else? If this recent cold snap did damage to everyone, then we should have seen significant activity, or atleast ONE full depth slide in the Bridgers seeing how Big Sky got half the moisture and tons of movement. Instead, we got every bit of 25 inches of POW and fairly isolated avalanche activity. I'm not saying anything that slid in the Bridgers was insignificant, but if the Big Sky zone got 2 feet of snow, I would probably ride the lifts and expect class 4 avalanches to run naturally. Or similiar.

Really interesting me to see the drastic differences so close by. It seems that normally, large discrepancies in snowpack happen in Coastal areas. As in a quick changes - coastal to inter. to cont. - as you travel inland and away from ocean influence. That's a generalization of course, but you know what I mean.

I went off on a tangent here it seems, but interesting stuff. I think that saying that all of SW MT is dealing with the same avalanche problem/metamorphosis isn't accurate. There are major differences between ranges, and the cold snap/Bunion's initial question - has affected them differently.

[Lindahl]

It doesn't seem like it's as simple as a deeper pack in the Bridgers has allowed for less faceting at the ground. The pack was plenty shallow here for the entire month of November. Cold here too, but maybe less temperature variation (day/night)?

Was it dry too? More frequent snowfall can make a big difference, as fresh snowfall keeps the top of the snowpack warmer, and thus reduces the gradient. The amount of fresh snowfall (not just frequency) can also make a noticable difference. Heck, even cloud coverage can make a difference (a clear sky is a huge heat sink). Also, you say it was plenty shallow, but certainly not as shallow?

Also interested in what Bunion thinks...

Really interesting me to see the drastic differences so close by. It seems that normally, large discrepancies in snowpack happen in Coastal areas. As in a quick changes - coastal to inter. to cont. - as you travel inland and away from ocean influence. That's a generalization of course, but you know what I mean.

I'm surprised you're shocked by large differences in snowpack stability in areas of relatively close proximity. Didn't you used to live in CB? Surely there's a pretty big stability difference between the Keblar/Ruby area which gets significantly larger snowfalls than the eastern side of the region? We see it here out in Summit/Eagle as well. Parts of Vail Pass and the Gores can be significantly more stable than Summit and the rest of Eagle. This year, the Aspen and CB area has gotten hammered pretty hard, and those areas have generally been a lot more stable than the rest of the state as a result. I know more than a few people that have taken trips down that way just to get away from typical Summit shit. Of course, our most recent large storm destabilized things everywhere quite a bit.

In complex mountain ranges, like out here in CO, orographics can create huge differences in frequency and amount of snowfall in very small geographic areas. Not sure what orographics and ranges are like in MT though.