That vast majority of loose wet instablity associated with the recent new snow is expected to have occured yesterday while the bond between the new snow and the underlying rain crust was weaker. Colder air temperatures today are expected to keep areas of loose wet snow instability to a minimum.
Unstable wind slabs that existed in some areas yesterday morning on N-NE-E aspects have been scoured by the wind shift from SW to NE last night. Redistribution of wind slabs from N-NE-E aspects to SE-S-SW-W-NW aspects has occurred. These new wind slabs are not expected to pose a significant threat to backcountry travelers at less than 1 foot deep and extending only a very short distance down slope. High consequence terrain such as areas above cliff bands and terrain traps could magnify the consequences of an avalanche that would otherwise be too small to bury or injure a person.
Observations made yesterday on Waterhouse Peak (Luther Pass area) and on Andesite Ridge (Donner Summit area) reveled significantly different amounts of snowpack instability associated with wind slabs during the morning hours. While no evidence of instability was observed in wind loaded N aspect areas along the summit ridge of Waterhouse Peak, on Andesite Ridge two small natural avalanches were observed above treeline on NE-E aspects at 7,800' to 8,400'. In both areas wind slab depth was noted at 4 to 12 inches. By 1 pm, formal and informal stability test indicated that wind slab instability had decreased significantly.
With cloud cover clearing to sunny skies much faster than forecast, wet snow instability was subsequently much more widespread than anticipated to occur with a forecast of the mostly cloudy skies through sunset. During the afternoon hours on Andesite Ridge, rapid warming was noted and natural and human triggered loose wet avalanches became widespread. The rain crust at the base of the new snow provided an efficient bed surface for avalanches large enough to injure a person to run full path. Human triggered wet loose avalanche activity was noted on all aspects on slopes over 30 degrees in slope angle below the ridge highpoint of 8,400'.
