Avalanche Advisory Archive Pre-2016
Date Issued: | 2010-12-29 |
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Danger: | 2 |
Trend: | 3 |
Probability: | 1 |
Size: | 1 |
Problem: | 0 |
Discussion: | The National Weather Service Forecasts- TODAY...PARTLY CLOUDY. HIGHS 25 TO 28. EAST WIND TO 10 MPH. TONIGHT...MOSTLY CLOUDY. CHANCE OF SNOW. LOWS 19 TO 22. EAST THURSDAY...SNOW. SNOW ACCUMULATION UP TO 3 INCHES. HIGHS AROUND 32. SOUTHEAST WIND 10 MPH. THURSDAY NIGHT...SNOW AND RAIN. LOWS 28 TO 34. SOUTHEAST WIND We received several new inches of snow yesterday. This is laying on top of several inches of snow from the previous 2 days. During the last 48 hours temps have fallen slightly overall. This is a good trend for stability. There has been isolated windloading in areas near ridgetop and summits. This windloading in places has placed additional weight load on slopes that have not seen much new snow in quite some time. Before the new snow storms there were still several weak layers present in the snowpack. From the earth surface all the way up to the snowpacks surface. The snowpack is still quite shallow in places, yet the snowpack is becoming quite deep in areas near ridge top and in isolated crossloaded areas.. During our period of clear cold weather some faceting was noticed in weak layers throughout the snowpack yet there is quite a bit of spatial variability at this time. As new load is added via new snow and windload be cautious as it is unclear how the weak layers in place will support new snow load at this time. Currently they are holding load well but as Fridays expected precip arrives, continue to monitor these lower layers for stability. Avalanche Danger is Moderate at this time in the Urban Avalanche Zones. Natural Avalanches are Unlikely. Yet Human Triggered Avalanches are Possible. Be cautious in the backcountry as you enter windloaded areas. Be cautious in steeper terrain where snow will want to sluff off to releive itself. Yesterday some natural sluffing activity was spotted on the Thane road starting zones. This helps relieve stress and load in steep unanchored areas. Yet many areas of instability still exist. Be looking for the presence of wind affect and slabs forming in places near ridgeline as well as other wind affected areas. Great day to remember the basics. |
Tip: | Decisions, decisions, decisions? * A paper writtne by:Pascal H?geli 1 and Grant Statham 2 You and your buddies are on a backcountry ski trip and after hours of hiking through a beautiful winter landscape you?re there ? you just made it to the top. You?ve been dreaming of this run all week, and since your friends don?t know much about avalanches, the decisionmaking is pretty much up to you. Before leaving this morning you checked the avalanche bulletin, and at the trailhead you noticed about 10 cm of fresh snow on the trees. On the way up you observed some old avalanche debris and during a water break you even did a quick snowpack test: Compression Test Moderate down 25cm ? clean shear on surface hoar. You now have all these different pieces of information, but how does it all fit together to help you make a decision? This article focuses on the decisionmaking process and examines how to connect all of this information. Risk Framework It?s an unfortunate irony of the backcountry that some of the very best and most exciting skiing and snowboarding is found in perfect avalanche terrain. We all know there is risk Risk is often described as the probability of an event happening weighed against the severity of the consequences. Traditional avalanche education has focused primarily on the probability, or chance of initiating an avalanche. However, the initial triggering of an avalanche is only one side of the equation & equally important is what happens once the avalanche is initiated. The following questions are designed to help you assess avalanche risk more comprehensively, and to stimulate your thinking about what risk means to you. What is the main avalanche issue today? Ask yourself this question right from the beginning when you start thinking about a backcountry trip. Depending on the kind of instability, you can immediately make conclusions about the distribution of avalanche hazards and plan your route accordingly. For example, if you?re dealing with a thaw instability on solar aspects, your trip plan should avoid south and west facing terrain in the afternoon. The same Avalanche bulletin text provides information on the predominant kinds of instability, and forecasters will describe where these can be found this will give you a huge head start on understanding the ?character? of the avalanche hazard. Your job during the day is to verify this information is the bulletin accurate or is everything different in the drainage you are skiing? How easily can the instability be triggered? Once you have verified the instabilities, you need to determine how easily you might trigger one. In other words, what will it take to initiate an avalanche? Sometimes it is This part of decisionmaking receives a lot of attention through the use of snowpack tests, which can be used to quantify the strength of a weakness. These tests provide valuable information, but remember that assessing the strength of an instability is only one piece. Several potential routes through alpine terrain, each offering different exposure to avalanche terrain. Your risk will be different depending on which route you select. Of the decisionmaking puzzle it is important to figure out how these test results fit into the big picture. How far is an initial fracture going to propagate? Your next step is to visualize the potential size of a resulting avalanche, and with this we shift our focus to the consequence part of risk. Fracture propagation describes how easily and how far an initial shear fracture can travel along the weak layer, and is critical in determining consequence as it influences the size of a potential avalanche. Consider the following examples: You?re skiing a steep face after a fresh dump of snow and with each turn you trigger little sluffs in the top 10 cm ? this is an example of high instability and low fracture propagation. The resulting consequences of these small avalanches are minimal, keeping the avalanche risk relatively low and allowing an awesome run. Contrast this with a low strength surface hoar instability down 60 cm, where initial shear fractures can travel fast and far. The consequences of triggering this instability are far more serious. The avalanche danger is probably high and you can reliably anticipate large avalanches. How do you assess fracture propagation? Use your eyes first & large or remotely triggered avalanches are sure signs. However, these signs are not always obvious and so you need What terrain options do I have to work with? Terrain affects consequences in several ways. First, terrain has an effect on the size of potential avalanches. You might be traveling in undulating terrain with many small features, where the landscape is simply not capable of producing large avalanches. At the same time, catastrophic avalanches might be occurring in wide open bowls across the valley. Your personal avalanche risk is directly related to where you are positioned in the terrain. Second, consider terrain traps and what might happen if you get caught in an avalanche above Consider the earlier example of skiing that steep face after new snow. It was reasonable when you were on a large open slope, but what about if there was a cliff below you Everything just changed ? small sluffs matter and your avalanche risk just skyrocketed. Always consider your position in the terrain and recognize that every time you move, even a few meters, your avalanche risk changes. Terrain is undeniably your best tool for How will I manage the terrain appropriately? Deciding to ski avalanche terrain is one thing, but figuring out how to actually do it and where exactly to go is another. Where you travel in relation to the terrain is of huge importance ? the subtleties matter. Centre punching the start zone is your most committing move, but what about skirting the edges? Can you reduce your risk by snowboarding beside the start zone until you are lower down and below the most likely triggering locations? Staying high on a piece of terrain is a good habit ? being above the majority of the slab is a huge advantage. The lower you go into the start zone, the more committed you can become. Also, which direction are you traveling? Are you wearing skins and setting an uptrack or are you carving your board downhill? You are much less mobile when touring up, and how you manage the terrain will be different depending on which direction you?re heading. The final decision: Am I comfortable with the level of risk? Managing risk is all about your position in the terrain. When the complexity of the snowpack threatens to overwhelm you, use More Information |