Eastern Sierra - Fri Apr 23 2010

With new snow totals ranging from 1-2 feet at the higher elevations and the first day of clear skies and significant warming, the avalanche danger rating is estimated to be LOW this morning. By this afternoon, the avalanche danger could increase to MODERATE as surface snow melts and wet snow slides are possible. Greatest concern exists for steep rocky terrain facing north, east and south.

 

After two days of cold winter weather, skies are clear and morning lows are already 5-10 degrees warmer than Thursday morning. Another few inches of snow fell overnight, bringing storm totals over the last few days to 1 to 2 ft in the Mammoth area. Rock Creek and Bishop Creek picked up about 8-12’ of new snow above 10,000 ft.

Estimating when the avalanche danger will increase from LOWto MODERATE

today is difficult because there is new snow that will get a huge dose of the sun’s energy this morning. The avalanche danger on different slope aspects and elevations will vary with the time of day. Snow strength & avalanches are a function of heat entering the snowpack. Strength and stresses are reasonably balanced this morning but will change as the sun’s energy weakens the snow surface.

Last weekend, strong sun and warm temperatures created a stout melt freeze crust. Since then, new snow and below freezing temperatures brought winter conditions back. The main avalanche concern will be wet slides. The sun is coming out for the first time in several days and it is a strong end of April sun. Once the sun starts to melt the snow and weaken it, avalanches can occur in steep to very steep terrain. This can happen fast, typically over a few hours.

Slopes that had a thick melt freeze layer before the storm came in now have a slick sliding layer. East, south and west aspects below about 10,500 ft will require careful attention today.

Windloaded slopes in north and east facing slopes in the Mammoth area that had 1-2 ft of new snow over the last few days likely don’t have a strong crust for a sliding layer, but the avalanche danger still exists and could begin as roller balls or surface releases that pull out the new snow and end up in deep wet debris piles.

The low pressure system that brought cold and snow has moved south and east. High pressure building over the West Coast today brings sun, north winds and possibilities of thunderstorms to Mono County.

Mid elevations from 8,000 to 9500 ft could reach 50 today and the mid 50’s by Saturday. North winds are expected, 15-20 mph with gusts to 30 mph. Higher elevations will warm 10 degrees to the mid 30’s today with north winds 15-20 with gusts to 30 mph. The higher elevations will be warm over the weekend with highs in the low to mid 40’s. Saturday’s highs will be in the low to mid 40’s.

Nights will warm up also with lows near 30F for the next few days.

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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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