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Hot Topic (More than 30 Replies) Above the snowline in the South Island (Read 38885 times)
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Re: Above the snowline in the South Island
Reply #30 - Jul 2nd, 2010 at 5:06pm
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02.07.2010
The Beginning of winter during the month of June got off to a good start with two good snowfalls from the southerly quarter. Snow is still lying on the ground to low levels, about 700m in the central south Island. At higher altitudes there is a good base, of a meter or more. The whole month remained overall cold so that snow has as yet remained fairly constant in depth. Only on the last days has strong solar radiation had its chance to work its magic in settling down the snowpack on sunny faces. However again, very cold temperatures will be hindering any real loss of snow depth at higher altitudes.
As a broad rule of thumb, it’s often said that a deep snowpack at the beginning of winter boads well and will be less problematic as the winter progresses.  And, a thin snowpack at the winter begin is less healthy and will tend to form unstable, weakly bonded layers within the thin snowpack.
Overall the risk of avalanche is moderate. No major slide has been observed and the layers are for the most part bonding well to each other. No natural avalanche occurrences have been reported as of today.  
Caution should still be exercised in slopes lee to the south west where new snow has accumulated due to wind transport.
The cold temperatures will be producing frost on the snow surface in shady places. These Hoar Frost crystals’ will form a weak bond between the present snow pack and any new snow that falls. Warm weather and wind may destroy them That remains to be seen.
Hoar Frost crystals growing over several days can reach depths of 6 inches and more in shady areas. They produce a very weak layer for the next snowfall to lie on.


  

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Re: Above the snowline in the South Island
Reply #31 - Jul 18th, 2010 at 1:18pm
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All the new snow and avalanche predictions warn of weakening bonds between the layers in the snowpack. There are also areas where surface hoar (frost) will fail when new snow arrives and settles on top of it. The surface hoar can be found in shady slopes.

Current reports can be read here.
http://www.avalanche.net.nz/index_aa.asp#mc


The danger is still low, because there is no "loading" on those weak bonds. That will change quickly though if we receive any new snow and especially if that new snowfall is accompanied by wind, which will carry the snow into lee slopes. Caution should be exercised if there is any new snow between now and your next trip into snowy regions.  

For the technically curious...

The bonds are weakening because of weeks of cold weather and calm clear nights chilling the surface layers. The snowpack is undergoing changes as it cools. The layers nearer the ground will remain more stable in temperature at around zero. The surface layers will be coldest. This produces what is called a temperature gradient in the snow.

This temperature gradient is morphing the snow grains.

Snow metamorphism

"Snow metamorphism refers to how snow crystals change once they are part of the snow pack, which affects how they bond or don't bond to other crystals based on their shape and density, which in turn influences the stability of the snow slope.  The major factors affecting snow metamorphism are temperatures within the snow pack, the temperature gradient between the bottom of the snow pack and the surface, the amount of moisture that is present, and pressure due to weight (Daffern 1983).  The process whereby the snow pack gains strength is called "sintering", which forms necks between grains of snow.  This typically occurs when metamorphism creates rounded grains due to a relatively low (flat) temperature gradient with regard to snow depth. In contrast, sintering does not occur when metamorphism creates faceted grains due to a relatively high (steep, or big temperature difference between base and surface) temperature gradient with regard to snow depth.  "

Surface Hoar
Heres a pic of surface hoar on the snow surface.

And this pic shows Surface Hoar that has been snowed on. The new snow on the top looks pretty stabil and well sintered, but its sitting on the surface hoar which is acting as a weak bridge between the top and bottom of the snowpack. The whole slope is ready to roll with enough disturbance, such as a hunter or skier moving over it.

Quote of the day.

"Three things we look for in the snowpack to determine the likelihood of an avalanche are poor STRUCTURE, lack of STRENGTH and available ENERGY. Currently, we’re lacking only an overlying slab of snow to provide the energy."
« Last Edit: Jul 18th, 2010 at 8:32pm by headcase »  

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Re: Above the snowline in the South Island
Reply #32 - Sep 3rd, 2010 at 10:16am
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And yet again.



Overall, watch the shady faces.  Cycles of sun and warmth have firmed up slopes with northern aspects (sunny), with a few exceptions where cross loading  and southerly winds have transported snow at higher altitudes.

Why is it that shady slopes are overalll the least stable? Ill be writing a short explanation of this in the coming week, so if you interested watch this space.  
« Last Edit: Jul 11th, 2011 at 3:41pm by headcase »  

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Re: Above the snowline in the South Island
Reply #33 - Sep 25th, 2010 at 9:11am
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This one is worth reading if you are looking for Tahr in the higher regions. Strong winds and snow have changed the avalanche danger for the worse. East facing slopes (morning sun) appear to be the most affected, but all aspects at altitude should be treated with caution.

There are reports of avalanches running down to the valley floor in the Mt Cook region. This is always a good indicator of big wind transport and cross loading at higher levels.


« Last Edit: Jul 11th, 2011 at 3:42pm by headcase »  

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Re: Above the snowline in the South Island
Reply #34 - Jul 11th, 2011 at 3:43pm
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Dont forget to visit

http://www.avalanche.net.nz/Forecasts/

if you a planning a backcountry trip this winter.  Smiley
  

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Re: Above the snowline in the South Island
Reply #35 - Jul 11th, 2011 at 9:19pm
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headcase wrote on Jul 11th, 2011 at 3:43pm:
Dont forget to visit

http://www.avalanche.net.nz/Forecasts/

if you a planning a backcountry trip this winter.  Smiley


That new site is defintiely a step up
  
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Re: Above the snowline in the South Island
Reply #36 - Jul 12th, 2011 at 1:31am
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TG wrote on Jul 11th, 2011 at 9:19pm:
That new site is definitely a step up


Sure is, the new graphic way of displaying which slopes, at which height and aspect, have which level of danger is awesome. World class.. Perhaps better.  Cheesy

Over four meters of new snow has fallen over the last days on the Tasman Glacier, so there is definitely a big increase in snow depth the further one gets up against the main divide.
  

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Re: Above the snowline in the South Island
Reply #37 - Jul 15th, 2011 at 9:31pm
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That site is awesome!
Love the "avalanche danger 4. High/Don't go"

Very good work by those folks and should be effective also.

4m up the Tasman Shocked Shocked Shocked Shocked
  
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Re: Above the snowline in the South Island
Reply #38 - Jul 16th, 2011 at 12:19pm
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Winter 2011   
This is the first of a series of
short comments I’ll be making about the forces at work
in the backcountry this winter. Written to help the
curious understand what makes snow tick and expand
ones understanding of avalanches.

Here we find ourselves again at the beginning of a winter. It’s a time when many might hang up their hunting gear
and wait it out for more comfortable times when the
spring is upon us, but others see the winter backcountry
as a challenge and opportunity to hunt almost alone.

Our first big storm cycle has just passed, and covered the higher areas all over New Zealand in a blanket of snow. All the indicators are now on high that a significant avalanche danger exists. Anyone heading into the backcountry so
soon after the cessation of the storm cycle, should be aware of the danger in the area they choose to visit.
Time and weather will allow the snowpack to settle and
the situation will become clearer. There have been, in the past couple of days, several significant events recorded during avalanche control work on Southern and Northern
ski areas, so using that as an indicator, back country
travel at the moment should only be undertaken with caution.

As always, depending on the direction the storm was predominantly coming from, some areas received more of the white stuff, and others less. The media coverage would have us believe that huge amount fell everywhere but looking at the reports objectively, it’s apparent that higher areas toward the East received significantly less that similar areas close in to the Main divide. Ill stick to the South Island for the moment so as not to confuse the issue. Mt Cook village at 720m has 50cm of snow lying on the ground today, but Lake Tekapo Village, also 720 m and   only 20km distant as the crow flies, has no snow at all.

What does that have to do with the price of fish anyway? Well it tells us that some areas have started the season with a thick blanket of snow and others have only a thin layer, which will be quickly degraded by warm temperatures and sun. It’s pretty easy to gain an idea of how much snow is in any particular area by careful observation on a fine day. A thin snowpack will be quickly melted on the sunny faces and bare patches will be visible everywhere. The rocks and the detail in the terrain will be easily visible under the snow, tussock scrub and rock may stand out quite clearly if one looks carefully. Most ridges will be snow free as in the pic below



Or at a distance it may look similar to this.



At this time in the winter its early days, if you’re walking up ridges on a slopes as above, your hardly doing to get into trouble but what significance does a thin snow pack as pictured above have for the future.

First let’s look at what a deep early winter snowpack will look like as a comparison. At a distance it will resemble this picture, in the higher regions the terrain features are softened, few if any ridges are visible and the vegetation and rock are hidden by a thick blanket of snow.



  At a closer distance the slopes have a similar appearance as below.



Only the largest rocks and terrain features are visible under the snowpack. . Sunny faces and exposed ridges at lower altitudes may well lose their snow cover over time but the landscape is predominantly packed up like a bug in rug in a warm insulating snow cover. This insulation from the weather allows the ground temperature to remain at a pretty constant zero degrees through the whole winter. A significant fact, in influencing snowpack metamorphism as the winter progresses.

So here we have two different situations, a thin snowpack, and a thick snowpack, that may exist in the same general area at the same moment in time. They may only be a few kilometres apart, of just a range of mountains away. It might only be the difference between a shady wind sheltered place on one side of a ridge,  and a sunny face on the other. The combination of snow and terrain are many. 

Sun and shade, the snow pack will have cooler overall temperatures than the sunny face. 

What significance does a thin snowpack have against a thick snowpack at the beginning of the winter?

Easy one would think.  By any reasonable logic, just a little snow would be mean less avalanche danger as winter progresses, and a lot of snow would mean a big avalanche danger.
In fact, as general rule of thumb, a thin, marginal snowpack at this early time has the potential to produce an avalanche prone winter in that area, and a thick deep snowpack would tend toward a relatively stable and avalanche free winter. A gross generalisation of course, but nevertheless the overall tendency will be a stabile snowpack, if the snow is deep from the outset.
 
Why is this? The classic explanation is that a thin snow layer will be more influenced by low temperatures than a thick, deep layer. Low air temperatures and cold clear nights allow the snowpack to cool and what better time for low temperatures from early winter through to mid winter. Cold snowpack temperatures’ are relevant to how the stability of the snow layers increase or decrease.

The influence of temperature on the stability of the snowpack.
The snowpack is not uniform in temperature. Broadly speaking, at the beginning of winter, though to spring, the bottom snow layer(s), closest to the ground are warmer than the top layers, closer to the sky, and the influence of the weather. If we imagine that the ground, insulated from the cold nights and weather, by the snow itself, will stay at a relatively constant temperature, but, the surface of the snow, exposed to the clear night skies and cold weather will be much cooler.

The net effect is that at the bottom of the snowpack we have zero degrees and on the surface we have a minus temperature, for example - 10c after a clear cold night. If we dig a pit in the snowpack and measure the temperature range from top to bottom, say every 10cm in depth, we can plot a graph of the different temperatures at increasing depth, a temperature gradient or profile, a graph as pictured below.

This gradient in temperature effects the way the snow grains break down, or grow, at different levels in the snowpack. More to this process at a later date. At this point it’s sufficient to note there can be a strengthening or a weakening of the bonds between each grain of snow, and each layer, influenced by the steepness of the plotted temperate gradient, and the amount water vapour moving inside the snowpack.
Colder temperatures at the beginning of winter, influencing the snow to become loose and less stabile;;  and warmer temperatures in the snowpack influencing the snow crystals to break down and bond together. This is a pretty simple explanation and there are dozens of variable that will produce varying results, but in broad terms, any area that has a thin snowpack a the beginning of winter will be more avalanche prown as the winter progresses, and more snow accumulates over the poorly bonded cold lower layers.
Areas that start the winter with plenty of snow tend to settle and consolidate, the temperature gradient is flatter and the snow grains will tend to form better bonds to one another.  That also affects the way different layers, (every new snowfall produces a new layer) bond as well.



Here we see the various layers accumulated after many snowfalls. The various layers can be recorded, the size and type of snow crystals, the temperatures and the stability, and bonding of the layers. Many of those visible layers would have been deeper when the snow fell, but time, metamorphosis or the grains, and pressure as new snow builds up on the surface reduce the overall thickness of any one snowfall. (Storm cycle)
Whether the layers morph into a loose and unstable snowpack, or into a tightly knitted and bonded snowpack is highly dependent on the weather, depth of snow,  the temperature gradient, the terrain , and time,. Cold, open skies at night, and lack of sun all tend to cool the snowpack, producing overall unstable conditions until mid winter has passed. Warm weather = Warm winds, strong sun, or cloud cover at night all tend to warm the snowpack, producing a settling, and a stronger bonding of the layers,,  if the warm periods are interspaced with some colder temps. eg night and day with clear nights.  
What now happens after our first big snowfall is still in the hands of the Gods, but the weather over the next week or two will influence to a great extent the avalanche danger for the rest of the winter.


« Last Edit: Jul 18th, 2011 at 8:22pm by blackbunny »  

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Re: Above the snowline in the South Island
Reply #39 - Jul 16th, 2011 at 12:23pm
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HiTop wrote on Jul 15th, 2011 at 9:31pm:
That site is awesome!
Love the "avalanche danger 4. High/Don't go"

Very good work by those folks and should be effective also.

4m up the Tasman Shocked Shocked Shocked Shocked

Those folks have sure upped their game. Your right, its world class.
  

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Re: Above the snowline in the South Island
Reply #40 - May 27th, 2012 at 9:29am
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Its that time again. New snows already here in  most places around the tops, and new snow arriving with every front.

The Tahr Rut now underway, some will be venturing up to and well above the snowline.

Have a look at this when planning a trip to a specific area, and keep looking at it as the time comes to actually hunt that area.

its all here..

http://www.avalanche.net.nz
  

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Re: Above the snowline in the South Island
Reply #41 - Jun 7th, 2012 at 9:14am
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With big new snowfalls through the Southern Alps its worth having a good look at the area you are planning on going to this weekend and comparing it to the Avalanche Forecast. This is the first big snow of the year and things are uncertain as to what will happen. The first warming of the snow pack should bring some consolidation, but temperatures are to remain cold, and more snow is forecast, so safe conservative decisions are called for ..


Quote.. for the Arthers Pass area

Over a meter of new snow has fallen above 1200m in a short time (less than 24hrs). This will likely be sensitive until it has time to settle. Light
triggers such as a single person's weight could be sufficient to start avalanches in steep areas, so stick to lower angled terrain untill the new snow
has had time to settle.
  

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Re: Above the snowline in the South Island
Reply #42 - Jun 8th, 2012 at 10:11am
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Here we are, just 24 hours later, and there has been a significant change. High winds around the tops will have moved snow into the lee faces, and a significant avalanche danger will remain around the higher exposed areas/ridges.. .

Snow transport

The result.

A strong North Westerlies airflow over the alps has brought a significant warming, to zero and above in the lower regions and valleys. Even some sporadic rain showers. This warming will have an stabilizing effect on the snow, allowing it to settle and compact into a harder layer. This stabilisation process will continue especially as temperatures cool again.

BUT today still, as the snowpack warms and settles a significant moment will occur when the snow pack actually becomes more avalanche prone. It looses stability as it starts to settles. this moment may last for an hour or a day. that depends on , the weather trend.

This is a common occurrence after any snow storm cycle, and may be a simple raise in air temperature, a light rain shower, or the first rays of sun hitting a snow laden slope as the weather clears,

Today, I expect we will  observe  in the middle and higher regions some spontaneous avalanches as the warming process continues.
« Last Edit: Jun 9th, 2012 at 10:15am by headcase »  

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Re: Above the snowline in the South Island
Reply #43 - Jun 19th, 2012 at 10:06am
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Re: Above the snowline in the South Island
Reply #44 - Jun 25th, 2012 at 8:20am
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For those planning a trip anywhere near/behind Mt Hutt, and Craigeburn Areas

http://www.avalanche.net.nz/Forecasts/region.asp?a=1

The Arrowsmith mountains have had upwards of 50cm of new snow in the past 24hrs at 2000 meters. Below 1700 meters the snowpack has
been rain soaked initially and now has up to 20cm of new snow on this interface. The June 20th crust interface remains a concern at higher
elevations although we expect a widespread natural avalanche cycle to have occurred around this layer. In the Mount Hutt Backcountry up to 35cm
of new snow is bonding to the old interface. Winds are picking up and wind slabs are forming on aspects lee to the westerly quarter.
  

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