Eastern Sierra - Wed Feb 24 2010
Eastern Sierra avalanche advisory |
Updated February 24, 2010 - 1:11 pm
| For areas south of Mammoth The avalanche danger rating for today is estimated to be MODERATE for wind loaded areas at upper elevations on slopes greater than 35 degrees. For areas near Mammoth and June Mountain that received over a foot of new snow a rating of MODERATE exists with areas of CONSIDERABLE in wind loaded areas at upper elevations especially on North to East facing slopes below ridge lines and cross loaded gullies. |
South of Mammoth to Bishop Creek
Wind slabs are our main concern for the next two days and will be deeper and more widespread than the first part of the week, thanks to 6-8" of new snowfall and howling winds. Avoid smooth pillows of snow on north to east aspects. Human triggered avalanches are possible in steep terrain and cross loaded gullies below ridgelines.
The old snow surface is a mix of fresh snow from the weekend storm and wind affected surfaces, ranging from thin breakable crusts to small hard deposits of wind drifted snow. Yesterday, in many areas exposed to the sun and warm temperatures from last week, a 2-3 inch suncrust hidden under 5-6 inches of new snow from the weekend storm, broke under my weight and made travel tedious on the flats and traversing steep slopes.
Weak faceted snow still remains at the bottom of the shallow snowpack in Rock Creek and Bishop Creek. This layer ranges from one inch to over 4 inches. While the warm temperatures of last week helped strengthen and round the facets, continued cold temperatures could reverse the trend of increasing strength. On Tuesday, one compression test failed with hard force when the facet layer collapsed. Two other compression tests in the same area had no results demonstrating what we all should know- the snow can change in a very short distance on any slope. Even rounded, coarse textured facets are not to be trusted. Triggering an avalanche deeper in the snowpack on the facet layer is unlikely on most slopes, but remains a risk in steep, rocky terrain. With new storms adding load to an already compromised snowpack, there are two avalanche concerns for today- wind loading is the main concern followed by a slight chance of triggering a deeper slide, especially in steep rocky terrain.
Mammoth Area North to June Lake
10 -14” of new snow fell last night with moderate SW winds gusting into the 60 mph range. Another 6- 8" fell this morning along with very strong west and southwest winds gusting over 70 mph. This means fresh windslabs will be the main concern for today and tomorrow. On North to East slopes, expect wind slabs below ridgelines. Be wary of cross loading at a wide range of elevations, especially on the windward side of any kind of gully or chute-like feature.
Observations made on Punta Bardini and Red Cone over the last 2 days revealed an overall fairly well bonded snow pack. One isolated area of concern was found Monday on Punta Bardini. It was an area where a thin weak wind slab existed with a foot of fresh unconsolidated snow on top. Compression tests and Extended column tests revealed easy clean propagating fractures below this thin wind slab over faceted snow. If this weak layer interface hasn’t stabilized over the last 2 days, last night and today’s additional snow load could be enough to make this interface a concern in isolated areas.
The National Weather Service ended the Winter Weather Advisory this morning. Skies are clearing and strong west winds will veer to the north later today. Storm snowfall amounts exceeded NWS forecasts with 20" of snow recorded near Main Lodge.
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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