Avalanche Advisory Archive Pre-2016
| Date Issued: | 2011-02-16 |
|---|---|
| Danger: | 3 |
| Trend: | 3 |
| Probability: | 4 |
| Size: | 3 |
| Problem: | 0 |
| Discussion: | The National Weather Service Forecasts- TODAY...SUNNY. LOCALLY WINDY. HIGHS 16 TO 22. NORTHEAST WIND 10 TO 20 MPH. NEAR DOWNTOWN JUNEAU AND DOUGLAS...NORTHEAST WIND 20 TO 30 MPH WITH GUSTS UP TO 50 MPH. TONIGHT...MOSTLY CLEAR. LOCALLY WINDY. LOWS 6 TO 16...EXCEPT THURSDAY...SUNNY...WINDY. HIGHS AROUND 20. NORTHEAST WIND 25 TO 35 MPH WITH GUSTS TO AROUND 45 MPH IN THE EARLY MORNING. WINDS DECREASING TO 10 TO 15 MPH IN THE AFTERNOON. WIND CHILL TO 20 BELOW. THURSDAY NIGHT...PARTLY CLOUDY. LOWS 14 TO 20. NORTHEAST WIND We have received over 10\" of light new snow in the last 72 hours at tram summit elevations. This new snow has now had a little time to settle and bond to the old snow surface. Temperatures have fallen by over 16C in the last 60 hours. And much more if you consider wind chill. This places stress on the snowpack by drawing out moisture and exchange of heat transfer. In the last 44 hours we have received over 18cm of wind transported snows at our Tram Summit Weather Site. This shows extreme windloading. The longer this wind event lasts the less snow there is available for transport. So in time danger starts to decrease do to less rapid loading and time for settlement and bonding. Winds are predicted to remain high today. The wind direction also directly favors wind slab development in our urban starting zones. Monday during the beginning of the wind storm there was enough wind transport to start natural avalanches in many areas along Thane Road such as Snowslide Creek and Cross Bay. Although some of the mass has now been reduced in those areas they can reload do to wind transport in a short period of time. Also in areas that have not seen any silde activity such as the Berhands Path and Chop Gulley the danger is still present and growing. Monday the slabs started out as 8-10\" of low density soft slab. As this wind event continues the slab depth can grow do to wind transport of new snows into the starting zones. Also slab densities will build as long as this wind event continues. So it becomes slightly harder to trigger yet the force of the avalanche is increasing should it be triggered. A greater concern comes in when we consider what this new wind slab is resting on. Ths storm cycle started on February 8th. Temperatures rose by more than 8 degrees during the first part of the storm. This created an upside down snowpack with low density snow under heavier, harder snows on the surface. This creates a weak layer. This layer has had more time to settle and bond and continues to gain strength yet remains present. Since the beginning of the cycle we have received 150mm of precipitation which rests in over 80cm of new snows(over 2 1/2')after settlement. That does not take into consideration the windloading as well so in areas the slab thickness could be much greater. There was quite a bit of natural activity in the first part of the storm removing some of the depth in certain starting zones. But in areas where there was no activity there is now a weak layer present over 2 1/2' deep without even considering the windloading in areas. So if we see the avalanche come in the top storm layers of 10+\" and windloading they will be fast avalanches creating a lot of additional stress on the snowpack under them. This has the possibility of then tearing down to the deeper weak layers in place. Recognize that although those deeper layers have had some time to settle and bond the weakness is still present. Should we see avalanche activity down to this deeper weak layer, slide sizes could be quite large. Recognize some of the factors leading to avalanche are present and danger continues due to windloading. AVALANCHE DANGER IS CONSIDERABLE AT THIS TIME> Natural avalanches are possible and human triggered avalanches are very likely. These are still dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding, and conservative decision making are the rules for the day if you choose to head out into the mountains to play. Recognize that most avalanche fatalaties in the mountains occur on days of Considerable danger. |
| Tip: | Wind Loading: Wind erodes snow from the windward (upwind) side of obstacles, such as a ridge, and deposits the same snow on the leeward (downwind) terrain. Wind loading is a common denominator in most avalanche accidents. And no wonder because wind can deposit snow 10 times more rapidly than snow falling from the sky. Moreover, wind-drifted snow is ground up by bouncing along the snow surface and when it comes to a rest it is often much denser than non-wind loaded snow. In other words, it not only adds significant weight on top of buried weak layers but it forms a slab that can propagate a fracture very easily. Wind can turn very safe snow into very dangerous snow in a matter of minutes. Wind is usually the most important weather factor in avalanche accidents. Luckily, we can easily recognize wind loaded slopes: Wind deposited snow/ Wind eroded snow What does it look like? Smooth and rounded, sometimes called ?pillows? chalky, dull appearance Sandblasted, etched look. Where does it form? Lee terrain (downwind of an obstacle such as a ridge). Often, a cornice overhangs the slopes Windward terrain (upwind side of an obstacle, such as a ridge). Often a cornice faces away from the slope What does it feel like? ?Slabby? or ?punchy,? meaning that denser and stiffer snow overlie softer snow. Rough, difficult to travel on. What does it sound like? Sometimes sounds hollow like a drum Noisy from the rough texture. It?s very important to memorize the look, feel and sound of wind loaded slopes. Always avoid steep slopes with recent deposits of wind drifted snow unless you are experienced, and have checked it out very carefully. Do you know the meaning of these avalanche terms??? Anchors To build your knowledge of avalanches and terms go to: http://www.fsavalanche.org/Encyclopedia.aspx |