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Thread: Building the Colorado snowpack
09-25-2006, 04:05 PM #1Registered User
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Building the Colorado snowpack
After the first couple storms blew through Colo. last week giving us an unusually early taste of what's to come I couldn't help noting several comments this week like "hopefully this new snow will melt" and "It's all gonna turn to depth hoar" and such.
This got me to thinkin' about "what if" this and that. Correct me if I'm wrong, but I just wanted to throw this up for discussion.
OK. To start out Colo. almost always has an unstable pack. Mostly due to the fact that it isn't very deep.
Early snowfall (like last weeks) might melt... or on the northerly aspects it might stick around. - These aspects seem to be the most suspect through the season.
The nights will be getting colder & colder causing a TG to be worse. Whereas I would think the ground (at least on the north or at least shaded) is pretty close to being as cold as it's gonna get. (speaking of ground that'll be buried in snow here).
So, here's my point. If instead of hoping the snow melts off, don't you think it would make for a better snowpack if we could continually keep getting a few inches every day or two until we had a decent depth to the pack now while the night time temps are only dipping into the twentys (or a few teens) vs. the usual thin pack in late Oct./ early to late Nov. when the lows drop below zero. This I tend to think would lessen the TG (Temp. Gradient) at the base giving us a much better pack.
I don't know much about the Maritime pack, especially the base layers, but does it's build do anything like this?
Seems to me a good late Sept./Oct. build could be much better than what we usually get. Or maybe absolutely nothing until late Nov. then comence the dumping... or a slower build at that late time?
What would be the ideal build on the snowpack with realation to the typical temps?
09-25-2006, 04:49 PM #2
My feeble grasp regarding instability is that a lot of it is a function of vapor transport in the snow at high vapor pressure gradients.
These high vapor pressure gradients can be the result of temperture fluctuations, which are typically greater at the beginning of the season. If things got cold and stayed cold, there wouldn't be as great of a Thermal Gradient.
So even if the snowpack built like it does here in the PNW, the difference is that the PNW doesn't get the big temperature swings and there's not as much vapor transport in the snow. Plus the snow here is slug snot, not the diaphanous fairy crystals that comprise the CO snowpack, so vapor transport here isn't as free.
Then again, this is just a lot of bookishness on my part, right?Merde De Glace
09-25-2006, 05:02 PM #3Registered User
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So, a thicker layer of snow (insulation) between these two extremes makes that process slow down & makes for a better pack.
So, No I think there would be a greater TG.
09-25-2006, 05:20 PM #4
In Silverton, for example, the ground freezes down below 6 feet. So I would have thought that the surface would be < 32f in the shade.
Anyway, I thought it had to do with the temperature fluctuation, meaning it getting warm in all that bloobirdy sunshine, caused a bunch of vapor transport up during the day and then down in the cold clear night.
So, really, an adjunct factor is that whe it snows, clouds are present and they generally stabilize the temperature. So in that view, lots of snow is good.
But discussing the merits of a lot of snow is like discussing the merits of a nubile breast. It's just a good thing and does lessen the TG.Merde De Glace
09-25-2006, 05:39 PM #5Registered User
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It's important to differentiate here between the base pack and the mid to late winter pack too.
In the base pack the ground would/(could) be the "warmer" end with the surface being the cold end. Whereas later, say after Dec when the depth is around 50", there probably is enough insulation and or layering to not be as big an influence at the base. Then it might be different where the air/surface temp. could be warmer than mid pack at times.
09-25-2006, 06:45 PM #6
I would say that you're both right, though I could certainly be wrong. I believe that the ground does stay right around 32. The amount of snow between the "warm" ground and the "cold" air factors into tg formation. so, if you had a 10 foot base, even if the air was getting to -20, there would be so much insulation from the ground to the air that the snow pack at the ground would not get sugary. Though the upper pack could have issues...
Thoughts on my year in Whistler...
The proximity to the sea and the northern latitude tend to make temps in Whis very stable and close to freezing. Temps always seemed to be 20-35. You would never get clear weather like we get in CO where the night can be below zero, but the sun can quickly warm things up above freezing. Add that to a deep, moist base, and tg is less of a problem. Rain, and rain crust bonding on subsequent snowfall, becomes a bigger concern.
As for your thoughts on the CO snowpack this year, you have a point. If it keeps snowing, we could be set up really nicely this season. Temps all this week seem to be 60something during the day and 20something at night, with no snow in the forecast. If this sept snow melts and we get snow later, that works too. The main thing is that when we get snow, we need to keep getting it without a long cold spell wrecking our pack at the base for the rest of the season. Last year was good like that- hardly any snow before early Nov, then a whole bunch of medium sized storms (at least here in CB). So, I'm hoping for continued snowfall, or a nice dry warm indian summer til late oct.
09-25-2006, 08:25 PM #7
I would much rather lose all the snow we have now then have it keep snowing and building. Why? It's only September. Temperatures rapidly fluctuate from day to day, night to night. Lot's of sunshine, 12+ hours of daylight, clear dry nights, and lowered humidity levels are all working against the current snowpack. Once late October-early November rolls around we have less direct sunlight, more consistent temps, higher humidity, and more frequent snowstorms. We could see the snowpack already breaking down as early as yesterday. The ground is still warm and holding heat, and lot's of transformation was going on down in that first couple inches next to the ground surface.
There's no way we would ever get the favorable weather from late September to early November to make the snowpack safer. There will be certain areas where this new snow won't melt and those areas are going to be a nuisance come late December into January. Right now is a good time to get out there, take tabs/notes on the various depths of the snowpack, do some obs, and keep that log handy for the next several months. Most areas will melt off entirely before the next predicted snowstorm, but those higher, shaded, heavily wind loaded, northerly aspects are gonna hold snow until the 'real' winter arrives. It's good to keep tabs on those spots now and observe the progress over the next several weeks.
09-25-2006, 09:12 PM #8
10 or 11 years ago we had a 6 foot base at treeline by october 20. That was the safest CO snowpack I've ever seen. It doesn't happen often, but it can and when it does, it's a good thing.
09-26-2006, 10:24 AM #9
Godammit, you're all being much too civil! Where's the juvenilie, cutting reparte' for which we are so well known?
So in summary:
0) The ground is consistently about 32F.
1) A bigger snowpack insulates better causing the TG to be smaller and hence less vapor transport.
2) Earlier in the year, there is generally drier weather and greater temperature oscillation, causing more vapor transport.
3) Years in which there was a big snowpack by Halloween were stable.
4) When there's a lot of snow, the air humidity is more consistently high and the cloud cover causes more stable temperatures.
It's not the snow on the ground per se that can improve the stability, but the effect of consistently snowing. Years in which stability had been good came about because of more consistent humidity and temperature.
If there's a lot of snow, but typical CO oscillations in temperature (can we use diurnal now, huh?) and humidity, that can contribute to an unstable lower layer of snow.Merde De Glace
09-26-2006, 10:50 AM #10
The key factor in depth hoar development is the mathematics of depth of snowpack in relation to temperature. Significant Temperature Gradient is defined as 1 deg Celsisus/10cm of snow pack. Thus if you have only 10cm of snow any temperature below -1 C will mean strong TG and therefore moisture transport and depth hoar development. This is why shallow snow pack here in Colorado = depth hoar (cause the ground is always right around 0 C and the fall/winter nights are cold and our snowpack is shallow). Notably our windy weather also means high variability in depth hoar development - where its deeply drifted it can be absent and 10' away the pack is all hoar.
The reason that some years (I think 1998/99 was the year mentioned) are much less dangerous re: depth hoar is that if sufficient quantity of snow falls then TG is either eliminated or at least minimized. Low wind can also help if the snowpack is both relatively deep and evenly distributed - at least we avoid the land mine factor. (1998/99 started with less wind that normal too)
So - best case - yeah if it snowed 24" every week for the next 3-4 weeks we'd have pretty minimal Depth Hoar.
Whats the liklihood of that? I'll give it a hopeful 20%.
09-26-2006, 10:57 AM #11
I think ISkiBC's point is that even with a big base, IF there's temperature oscillations or aridity, that will cause some gobletization to occur and this is more the rule than the exception.Merde De Glace
09-26-2006, 11:06 AM #12Registered User
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Just to throw storm amounts back in the mix... I think I remember Dale (who used to be over at CAIC) telling me once that smaller amounts like 4" were better for the build than big dumps. I think it had to do with bonding. But I also think he said they would have to be consistent - like every night or two.
Maybe I'm wrong but this seems to be what Wolf creek gets & I would go so far as to say that WC is one of Colo's safer packs typically. Plus The temps & wind are milder down that way.
09-26-2006, 11:09 AM #13
If there is a big base the mathematics make it more more difficult for Strong Temp Gradient to form and therefore Depth Hoar to form. For example compare two scenarios
I. Typical CO early season:
25 cm on the ground
Avg nighttime temps=-5C
A avg strong Temp Grad exists of 2 C/10 cm
After a couple weeks it'll probably be all depth hoar
II. Deep season
150cm of snow on the ground
Avg nighttime temps = -5C
A weak TG of .33 C/10cm exists
Therefore there will be little moisture transport and little if any depth hoar development.
Of course its always more complicated than that - but as a rule of thumb its true.
Iskibc is right - that its good to get out and observe and note where the snow is and what's happening. Also good to get out and get a couple turns while the danger is still relatively low (beware of snow snakes of course)
09-26-2006, 12:12 PM #14
Goblet is the term used to describe some of the shapes snow crystals take on when subjected to high vapor pressure.Merde De Glace
09-26-2006, 12:15 PM #15
Maybe I'm wrong but this seems to be what Wolf creek gets & I would go so far as to say that WC is one of Colo's safer packs typically. Plus The temps & wind are milder down that way.Merde De Glace
09-26-2006, 12:55 PM #16"It is not the result that counts! It is not the result but the spirit! Not what - but how. Not what has been attained - but at what price.
- A. Solzhenitsyn
09-26-2006, 12:59 PM #17Registered User
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Instead of rephrasing I'll just add some more questions for further discussion.
What would Colo's pack build best case scenario be? - Considering all the relavant stuff like we want to be turnin' as early as posible on through as late as possible (I'm sure we can pack spring, summer & fall into 3 months, right?), storm cycles, various average temps (like Sep-Oct vs. Nov-Dec), geograpical features...
What would Colo's pack build worst case scenario be? (ref 1976 prolly)
and I guess some comparisons to the "norm" are in order if you got 'em.
What makes some locations typically better (stable) than others? Like compare Wolf Creek to the To-hell-u-ride zone, or west slope some-place-or-other to east slope blah, blah, blah.
Lets hear your thoughts.
and yeah, I've been waitin' for Halsted too, but this seems to be going pretty good so far.
09-26-2006, 01:19 PM #18
Harrumphhh...you of all people....
Allow me to unbind my torsionally bundled knickers:
In one case, the snowpack could build up via a few large storms interspersed with dry sunny days and dry cold nights.
In another case, it could build up via a consistent series of small storms where the weather was consistently mild, stable temperatures and humidity.
I'd think the resulting snowpack would have different stability attributes.Merde De Glace
09-26-2006, 01:41 PM #19Merde De Glace
09-26-2006, 01:55 PM #20Registered User
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09-26-2006, 02:15 PM #21
2) Worst - as Buster said - NO snow. Or very little and cold temps.
3) What makes some locations more stable - ie Wolf Creek or Utah vs Colorado is 1) MORE snow, 2) Avg lower temps
09-26-2006, 05:47 PM #22
09-28-2006, 11:25 AM #23
Not sure I agree with your 1 and 3. Slow, consistent loading (6" a day) will give the pack more time to stabilize. Big dumps not only raise danger, but slides will occur, resulting in a shallow snowpack in the slide paths, and more TG in the slide areas. Sure, the flat, forested areas will have a deep, bomber pack, but who cares?
Wolfie typically has higher avg. temps than northern/central CO. Clear, cold, shallow snowpack is what makes the continental pack deadly.
Typically, a big dump in November followed by cold, clear, dry weather = rotten and deadly snowpack. Gradual cooldown and gradual snowpack accumulation will probably be most beneficial.
Last edited by homerjay; 09-28-2006 at 11:28 AM.
09-28-2006, 12:06 PM #24"It is not the result that counts! It is not the result but the spirit! Not what - but how. Not what has been attained - but at what price.
- A. Solzhenitsyn
09-28-2006, 12:26 PM #25Registered User
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Just to clear things up on behalf of ground temps. (or maybe muddy the waters more)
There's quite a bit of difference on ground temps depending on what's on top of it. Plowed streets where water & sewer lines are typically buried get really cold & frozen in the winter. They're plowed all the time plus having vehicles driving over it seems to "pound" that frozen zone even deeper. I forget what the correct term is for this frozen ground is... It's kinda like the term "permafrost" we've all heard.
Or in a place where we might run into this in the back country could be wind blown ridges - or maybe more importantly places (or aspects) that never get much depth for some reason. - think what happens here if the prevalent winds change a little & then start loading these previously shallow zones, or a big dump covers it just long enough to eat some unsuspecting gravity floater.
So, it makes sense where buster said
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