Eastern Sierra - Fri Feb 5 2010
Eastern Sierra avalanche advisory
Posted February 5, 2010 by Josh Feinberg
The new snow overnight has increased the avalanche danger rating for the entire forecast region. This morning a MODERATE danger rating exists with wind-loaded areas rated as CONSIDERABLE. As the day progresses and more snow accumulates, the danger rating is estimated to rise to CONSIDERABLE with wind loaded areas rising to HIGH.
Entire Forecast Area
The number one concern throughout the entire forecast region is the accumulation of new snow, especially in wind deposited areas. Winds can deposit snow up to 10 times faster than it can fall from the sky. The resulting dense slabs can release on their own or with the help of a human trigger. (Click here for a short education on wind slabs) The strong winds associated with this stormwill deposit snow below ridgelines and across slopes as well. Avoid open slopes and chutes that are steep enough to avalanche (usually >30 degrees, but 25 degree slopes can avalanche too). Careful route finding, expertise in recognizing avalanche terrain and snowpack assessment skills, and most importantly the will power to “just say no” to that beautiful looking powder field are all necessary for anyone venturing out into the backcountry during this time.
Additional Concern for Areas South of Mammoth
Another concern is focused on areas south of Mammoth at low to mid elevations where the bottom part of the existing shallow snow pack is made of up of rotten sugary snow. Anyone touring in these areas over the past couple of weeks probably have noticed their poles at times punching through the upper layer of denser snow and sinking effortlessly to the ground through this sugary snow. The snowpack so far has been able to support most skiers and boarders, but as the snow load increases, avalanches could release on top of this rotten layer. And the resulting avalanches could be large. The recent avalanche which occurred long after any storm on Kid Mountain outside of Big Pine is a good indication of the potential if this weak layer fails.
Additional Discussions
1.The old buried January 12th rain / melt/freeze layer is still being identified about mid-level in the snowpack in areas from June to Rock Creek. Tests yesterday near Patricia Bowl and previous tests this week in areas north reveal that it still has some limited potential to propogate in some area. However, it’s taken quite a bit of force. The extra weight of new snow could possibly trigger this layer in the most sensitive of areas, but the more likely scenario is that once this new snow settles, this layer will be even harder to trigger.
2. The sugary snow that has developed at the top of the snowpack over a wide variety of areas over the past week could act as a weak layer for the new snow load to slide on. In some open areas more than the top 20cm of the snowpack has become sugary
By Sue Burak
After days of clouds with no snowfall, snow returns to the Sierra today. The Southern Sierra is favored for the most snowfall, mainly Saturday and Saturday night, due to the southern trajectory of the jetstream.
For today, elevations from 8,000 to 9,500 ft in southern Mono county, including Mammoth and June Lake, could receive 8-12” of snow. Up to 2 feet of snow could fall along the Sierra Crest by early Saturday morning. High elevation bowls like the Negatives could get 18” of new snow plus a generous amount of wind.
Winds will be from the south from 10-20 mph with gusts up to 40 mph. Ridgetop winds will be strong with gusts to 75 mph. Daytime temperatures will remain cool with daytime highs in the upper 20’s. Elevations above 10,000 ft will be cold with highs in the upper teens to 24F.
There is a Winter Storm Warning in effect for the mountains west of Bishop to Mt Whitney. Two to four feet of snow could fall above 8,500 feet beginning last night and continuing through Saturday night. Winds will be relatively light from the southwest and south with gusts to 30 mph at the higher elevations. The southern Sierra will pick up more snow than the Mammoth area on Saturday due to the jet sagging south. It’s possible the Mammoth/June area may receive only snow showers on Saturday while the main storm action occurs farther south in Inyo County.
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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