The avalanche danger is CONSIDERABLE at middle and upper elevations. Natural avalanches are possible and human-triggered avalanches are likely today. The combination of one to two feet of new snowfall and strong winds are creating widespread instability, overloading wind drifted snow from Friday and Saturday and buried weak layers at the bottom of the snowpack. . The last shot of snowfall impacts the higher elevations today and tonight.
CONSIDERABLE means dangerous avalanche conditions exist on all wind loaded terrain. Winds in excess of 100 mph have created unusual wind loading patterns. Careful snowpack evaluation and cautious routefinding is essential today. Avoid slopes steeper than 30 degrees and avalanche run out zones.
Likelihood ?CertainVery LikelyLikelyPossible
Size ?HistoricVery LargeLarge
Trend ?Increasing Danger
One to two feet of new wet snow fell over the last two days and more snow is expected along the Sierra Crest today. New snow and strong west and southwest winds are working together to create widespread instability in the past 24 hours. Air temperatures fluctuated a few degrees above and below 32 F yesterday and new snow ranged from 9 to 16% density. Strong winds formed instant slabs in wind exposed areas- slopes will be littered with an array of dense wind compacted pillows, drifts and cornices. These wind formed features will likely avalanche today under the added weight of more wind loading or a skier.
The snow and wind slabs are falling on a snowpack with a widespread persistent weak layer at the bottom. Thick facet and depth hoar layers are found on NW-N-NE aspects but could be more widespread on other aspects-there is no information from east, south and west aspects. Avalanches could start as small natural wind slabs overloaded by the wind but it’s possible the avalanches will step down to layers within the storm snow or last week’s storm. Small to medium sized avalanches could occur if avalanches step down to the buried facets and depth hoar layer at the base of the snowpack.
yesterday's mild temperatures, winds and thick cloud cover added alot of energy to the snowpack at the 8,500 to 9,000 ft elevations. Reports of small point releases came from the 9,000 ft elevation in the Mammoth Basin and I observed some small wet point releases on east aspects Snowpit results at the 9,500 ft elevation in Rock Creek suggested instability existed in the snow that fell two days ago. More information is needed before conclusions can be made. Swirling and drifting snow in open glades common at 9,000 to 9,500 ft. Whumpfing observed in Rock Creek and from a report from the Mammoth Basin.
The strongest of three waves of this wet subtropical storm will arrive later today. Elevations above 10,000 ft could pick up 6-10 inches of snow before the storm leaves the area by tomorrow morning. Mild temperatures in the 30’s are expected from the 8,000 to 10,000 ft elevations. Night time lows could drop to the low 20’s as cold air moves in as the storm moves east.
Strong, damaging west winds will continue through the day along the Sierra Crest. Sustained hourly wind speeds of 60-70 mph with gusts of 80 mph winds continue throughout the day, diminishing tonight.