Eastern Sierra - Wed Feb 17 2010
Eastern Sierra avalanche advisory |
Today the avalanche danger is estimated to be mostly LOW. Over a week of dry weather, calm winds and mild temperatures has resulted in a relatively stable snowpack. Generally safe avalanche conditions exist, but continue to watch for: 1) Isolated shallow wind slabs along upper elevation ridgelines and in alpine terrain. 2) Areas of wet snow instability caused by surface melting on solar aspects (East – South). Use extra caution on slopes steeper than 35 degrees in shallow, rocky areas or steep rollovers. |
By Sue Burak and Josh Feinberg
The dry weather and gradual warming trend of the past few days has left us with generally stable avalanche conditions on most slopes throughout our advisory area. Due to the strong winds after the storm last week, most high elevation terrain is very variable from varying degrees of unpleasant breakable crusts, hard snow surfaces, to strips of soft snow with rippled surface texture.
With dry and warm weather continuing for the next two days, wet snow instability remains a concern. However, over the last two days, we are not seeing increasing wet snow activity despite strong mid February sun. Nights are cold and this may be keeping the snow surface colder for a longer period of time so surface melting is minimal. However, another day or so of clear skies could tip the balance in favor of wet snow, so be alert if you are traveling under cliffs or steep rocky slopes. A good example of terrain to watch out for are the eastern facing edges of gullies near shallow rock outcrops/ridges which can be seen above the long gradual approach to Old Man’s and Morrison Col in the Convict Lake area.
Some smaller wet snow point releases, and one slightly larger slab release have been observed on solar aspects (east to south), but appear to mostly have occurred at the beginning of this clear, calm, warm period (3-4 days ago). Snow stability is compromised by sudden changes to the snowpack, whether it be in the form of added weight and mass from snowfall and wind, or moisture released from surface melting.
The prospect for stormy weather is becoming less likely as the end of the week approaches. The ridge will get stronger and moves to the west allowing a weak low pressure system to undercut the ridge by Friday. In the classic El Nino pattern, the southern branch of the jet will drop into southern California, leaving most of the area under clouds and scattered snowshowers Friday afternoon into Saturday morning. With the jet position south of our area, southeast flow is a good pattern for upslope snow. The Sierra Crest could pick up 6-8” of snow. Little, if any snow will fall east of the crest. Winds will be light with this snowfall.
A more active pattern is expected by the middle of next week. Storms are expected to be wet with high snow levels.
Mild daytime temperatures will continue at the 8,000 to 9500 ft elevations with highs in the upper 40 and low 50’s. Higher elevations will reach the upper 30’s and low 40’s. Night time lows will be in the single digits and teens in narrow confined valleys at high elevation locations, otherwise lows will be in the mid to upper 20’s.
-Small wet snow point releases on solar aspects (east to south) from 3-4 days ago. (Back side of Mammoth Mountain, east facing terrain on Little Morrison and Big Morrison)
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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