Eastern Sierra - Thu Feb 4 2010
Eastern Sierra avalanche advisory
Posted February 4, 2010 by Sue Burak
The avalanche danger rating is estimated to be LOW with areas of MODERATE south of Convict Lake to Bishop Creek in steep, shallow, rocky terrain below ridgetops and the sides of gullies. A variety of localized conditions exist. Click here for further discussion.
Report just in: On Sunday a snowboarder triggered just such a pocket above 11,500' on Four Gables and was taken aprox. 700' down the couloir.
Bishop Creek to Rock Creek
The avalanche danger rating for this southern sub-region is estimated to be LOW with areas of MODERATE in steep, shallow, rocky terrain below ridgetops and the sides of gullies.
On Sunday a snowboarder triggered just such a pocket above 11,500' on Four Gables and was taken aprox. 700' down a couloir. After making 3 ski cuts down the top of the couloir, he began making turns, the 3rd of which was "on a wall of snow that had drifted onto the rocks on the side of the chute." This is where he triggered the avalanche and was enveloped and swept down the chute. Luckily he ended up unburied and uninjured, much to the surprise of his partners. This shallow area formed by winds blowing snow across the slope and depositing it shallowly over the rocks and thin snow on the edge of the gully. This kind of shallow area is exactly the kind of area to watch out for. Please see a full account under "Discussions". We should all take this as a reminder not to let our guard down, especially when it comes to wind loading, even during times when MOST areas are considered stable.Thank goodness he was ok.
Several days of mild temperatures, light winds and very little precipitation have strengthened the snowpack. However, the snowpack dragon south of Rock Creek blinked a few days ago. A large natural avalanche occurred on the north facing slopes of Kid Mountain in the Big Pine Creek drainage. Though Big Pine Creek is not part of our forecast area, the snow doesn’t know the difference. This avalanche is a HUGE red flag that persistent weak snow at the bottom of the snowpack can suddenly be brought back to life by a human trigger, windloading or rapid warming.
The avalanche started in steep north facing terrain at about 10,500 ft in a shallow, wind stripped area above a wind loaded bowl. The avalanche ran almost 3,000 vertical feet to the runout zone near Big Pine Creek.
Even though avalanche conditions are generally safe, unstable faceted snow is found at low elevations from 7,000 to around 8,500 ft. Snowpit tests at these elevations often collapse on the ground while isolating a column. As elevations increase, the snowpack generally gets a little deeper and the layer of facets and depth hoar gets a little thinner and less reactive. However, as the Big Pine avalanche demonstrates, there is unstable snow in windscoured areas or places where the snow is thin. While one can safely assume there is weak snow at the bottom of the snowpack in most areas from Rock Creek to the southern areas- we don’t know where this layer will react to added loading of a skier or rider. For this reason, the avalanche danger rating is LOW with areas of MODERATE in steep, shallow terrain below ridgetops and the sides of gullies.
Convict Lake to South of Mammoth
The avalanche danger rating for this central sub-region is estimated to be LOW for all aspects and elevations. Human triggering and natural avalanches are unlikely, but not impossible. Continue to use caution when in the backcountry.
Top concerns in order of importance for this sub-region include:
1. Widespread weak structure of the bottom third or more of the snowpack at all elevations, especially lower to mid elevations. Snowfall prior to the “big” Martin Luther King snow storm week was progressively less than Mammoth the farther south of Mammoth you go. This shallow snow pack existed for quite some time before the MLK storms. The longer a shallow snow pack exists with relatively cold air temperatures, the more the bottom part of that pack turns into weak sugary snow, with increased concentrations around bushes and rocks. Now there exists a heavier denser load on top of this fairly “rotten” snow. Tests done on northwest facing slopes around 8000’ above Convict Lake confirmed this weak snow pack structure. Columns of this dense snow repeatedly slid on top of the weak snow underneath from simply cutting the sides of the columns out, with no additional pressure. However, no snowpack settlement or “woomphing” was felt when touring through these areas. This doesn’t mean this obvious pour structure should be ignored. Stick your pole through the snow, feel the layers. Use caution when in steeper terrain, especially on a convex roll.
2. Isolated pockets of wind-deposited snow on North-East slopes near Alpine ridges. The relatively light recent winds and lack of available snow for transport means that most of the wind deposits that exist have been there for at least a few days and have had time to settle. This doesn’t mean that it is impossible to find a shallower area of an older slab in a steeper area where a small slide could be triggered. A small slide is all it could take to result in serious injury or worse.
Mammoth to June (including extension to Lee Vinning Canyon)
The avalanche danger rating for this northern sub-region is estimated to be LOW for all aspects and elevations. Human triggering and natural avalanches are unlikely, but not impossible. Continue to to use caution when in the backcountry.
Top concerns in order of importance for this sub-region.
1. Isolated pockets of wind-deposited snow on North-East slopes near Alpine ridges. The relatively light recent winds and lack of available snow for transport means that most of the wind deposits that exist have been there for at least a couple days and have had time to settle. This doesn’t mean that it is impossible to find shallower areas of older slabs near ridgelinesin steep areas where a slides could be triggered.A small slide is all it could take to result in serious injury or worse.
2. Old buried crust/weak layer in low to mid elevations. A Range of snow pits in the Mammoth Area, June Lake area and above Lee Vining Canyon at low to mid elevations have revealed a fairly widespread old rain/ melt/freeze layer which exists as a crust in some areas and in others as simply a thin weak layer. This is being termed the Jan 12th layer (the date when new snow covered a surface layer that was created during warm temperatures and/or rain at lower elevations around Jan 9th). Tests focusing on this layer reveal that it still has the potential to fail and propagate, but that it is difficult to trigger It’s average depth is a meter or more below the snow surface under fairly dense snow. However unlikely, it could still be possible to trigger this weak layer in isolated steep convex rolls where the overlying snow is thin.
Additional Discussion:
Surface faceting and thin surface hoar continue to be observed in a variety of locations. This is the sugary “dry” feeling snow you might feel at the surface. The calm winds, and past relatively clear / high cloud cover and relatively high relative humidity has favored this crystal growth and transformation. In some areas such as mid slope up V-bowl out of Lee Vinning Canyon in open areas away from trees there are areas where more than the top 20cm of the snowpack is sugary faceted snow. This makes for good skiing now, but makes for a good weak layer for future snow loads to slide on later.
The much hyped week of stormy weather and significant snowfall is only a memory. The systems are weak and the main jet is diving south with the northern jet pushing into the Pacific Northwest. Storms reach the West Coast and split- “Split City” as one Reno meteorologist described the weather pattern.
Expect an unsettled weather pattern all week with periods of snow showers, clouds and normal temperatures for this time of year. If you want snow, the long range forecast is bleak. This is a typical winter pattern for a moderate El Nino. Southern California, the southwest and southeastern states get above average precipitation with the Sierra being occasionally but not consistently favored.
For today, skies will be cloudy with snow showers. Winds will blow from the southwest from 8,000 ft to the Sierra Crest. Winds will be light, ranging from 10-20 mph. Higher terrain will see southwest gusts to 30 mph. Daytime highs will hover around freezing with higher elevations reaching the mid 20’s to 30F.
-Sunday, Jan 31st: Snowboarder triggered and caught in avalanche on Four Gables, was swept down 700 feet. Luckily, he was uinjured.
-Large D3 natural off north slopes of Kid Mountain, Big Pine Canyon
Please note that the avalanche danger rating in this advisory expires in 24 hours. This advisory is our best interpretation of snow pack conditions and NWS forecasts issued today. Backcountry travelers should be aware that elevation and geographic distinctions are approximate and that a transition zone exists between upper and lower elevations. Avalanches do not happen by accident and most human involvement is a matter of choice not chance. Most avalanche accidents are caused by slab avalanches that are triggered by the victim of member of the victim's party. Even small slides can be dangerous. Always practice route finding skills and carry avalanche rescue gear. Remember that avalanche danger ratings are only general guidelines. Distinctions between geographic areas, elevations, slope aspects and slope angles should be made.
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