Eastern Sierra - Fri Apr 30 2010
Eastern Sierra avalanche advisory |
For Mammoth and nearby areas which received greater than 4” of new snow by yesterday (Thursday) morning there exists pockets of MODERATE avalanche danger above 10,000’ on steep exposed wind-loaded North to Northeast facing slopes below ridgelines. For all other areas the avalanche danger rating is estimated to be LOW. If cloud cover ends up being light, MODERATE danger could develop as the day progresses on steep slopes with solar aspects (E-S-W) at low to mid elevations (below 10,000'). Advisory Details. |
6 to 12 inches of new snow fell in the Mammoth area from Tuesday night into late Wednesday night. This was accompanied by strong SW winds. Thursday we woke up to bluebird weather, calm winds, chilly temperatures in the teens, and not a cloud in the sky. By mid-morning clouds were rolling in, and winds were blowing lightly out of the North-North-West. Periods of light flurries throughout the day and night didn’t amount to much new snow (mammoth weather site reported 1 inch last night). Areas outside of Mammoth received similar winds over the last several days, but much less snowfall (an inch or 2). For today (Friday) throughout the entire forecast area from June Lake to Bishop Creek expect partly cloudy skies with some chance of flurries, temperatures in the lower 30s near 10,000’ and breezy North-North-East winds gusting into the 20s and 30s mph range.
Observations made yesterday (Thursday) on Solitude ridge revealed that the recent snow fall accompanied by the strong SW winds was a recipe for soft sensitive wind slabs on steep (40 degree) north facing terrain. Ski cuts on steep slopes (>40 degrees) just below the exposed north facing solitude ridge consistently produced 10” deep soft slab avalanches, one of which propagated across the entire banana chute and ran the entire length (~1500’ vertical). These avalanches were running on a density change within the new snow (lighter snow on-top of denser snow that fell when temperatures were warmer at the start of the storm). Click here for further details.
For areas such as Mammoth which received greater than 4 inches of new snow this week, the main avalanche concern for today will be wind slabs that remain sensitive to skier triggering on exposed north to north-east facing slopes greater than 37 degrees just below ridgelines. These will be more sensitive to skier and boarder triggering over convexities. Soft slabs were found yesterday, but hard slabs could be possible.
Depending on the amount of cloud cover today, the secondary avalanche concern will be wet surface instabilities on solar aspects (Northeast – East – South – West), in particular for areas which received greater amounts of new snow. High cloud cover will overall increase the likelihood of wet surface instabilities for Northern facing slopes, while overall decreasing the likelihood of these instabilities on solar aspects. This will become a greater concern for Saturday as temperatures are projected to rise to 40 degrees at 10,000’ and skies are projected to be clear. Be aware of how soft and moist the snow you are travelling on is becoming, and look for signs of instabilities such as roller balls originating near rock outcrops.
Bottom Line
For Mammoth and nearby areas which received greater than 4” of new snow by yesterday (Thursday) morning there exists pockets of MODERATE avalanche danger above 10,000’ on steep exposed wind-loaded North to Northeast facing slopes below ridge lines. For all other areas the avalanche danger rating is estimated to be LOW. If cloud cover ends up being light, MODERATE danger could develop as the day progresses on steep slopes with solar aspects (E-S-W) at low to mid elevations (below 10,000').
It might be hard to tell but yesterday warmed up 8-10 degrees over Wednesday. Today will be warmer by 5-8F and by Sunday the 8,000 ft elevations could see 60. An occasional snow shower or two might occur today over the higher elevations but there will bemore sun poking through the clouds than yesterday. By Saturday, skies will be clear.
For today, Mammoth and the 8,000 ft elevation will warm up to 40F for the daytime high. Higher terrain will be in the low to mid 30’s. Gusty north and northwest winds will continue for the next two days with gusts to 40 mph expected today over the higher terrain.
The cold air aloft pushes to the east and snow showers decrease but the winds will pick up. The jet stream is still over Californiaand though it is not a strong jet, the winds will keep temperatures below normal for this time of year. By tonight, ridge top winds will pick up and continue through Saturday. Ridge top winds could reach the 50-60 mph range.
By Tuesday next week, cold temperatures and windy weather returns, though no precipitation is expected.
Bottom of Form
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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