Issued by Doug Lewis - Eastern Sierra Avalanche Center
The avalanche concern for the next couple of days remains focused on wet snow instability affecting southerly/sunny aspects (E-S-W).
High pressure will continue to dominate the weather picture through Tuesday, with sunny skies and scatter clouds occasionally filtering through the Eastern Sierra front. Temperatures have cooled slightly since peaking earlier in the week but nighttime lows have creeped-up into the upper 20’s to mid 30’s, in some loactions, with the surface melt-freeze crust barely supportive in places. As a result, mid–upper elevation slopes (E-SE-S-SW) will warm rapidly with firm supportive melt-crusts becoming unsupportive quickly depending on aspect, and time of day. We have likely passed the peak for wet releases but they remain possible as surface crusts thaw with the underlying snow potentially becoming isothermal. With above normal high temperatures, wet loose maybe encountered on steep (>35 degrees) slopes with sun exposure, which could possibly trigger a wet slab release. If planning to travel in the backcountry, anticipate being off of these slopes prior to thawing beyond the first few inches. Natural and human triggered wet avalanches maybe possible, starting on southeast slopes in the morning and moving around the compass toward southwesterly by afternoon. Watch for signs of instability, such as Rollerball (Pin Wheels) activity. This indicates a weakening of snow grain bonds and a rapidly warming snow surface with an increased potential for natural and human triggered wet loose avalanches, which can lead to larger slope failures on southeast slopes in the early AM, southerly mid-morning to mid-day, and southwesterly by afternoon. At elevations below 8000’,the snowpack is disappearing rapidly but the remaining snow will warm rapidly on all aspects with risk of wet loose activity increasing on slopes >35 degrees by the afternoon.
The northerly mid to upper elevations, the N-NE-ENE aspects remain cool with occasional soft snow and windboard. The recent warm temps have helped settle and strengthen previously formed windslabs but isolated windslab pockets may exist in the upper most elevations on shaded northerly aspects. Light West winds veering to North at upper elevations (5-10 mph) will continue for the next couple days, which will bring somewhat cooler air into the region. Be especially cautions in complex terrain where small avalanches can have high consequences.
Sat-Sun: The high-pressure ridge along the west coast continues to be the main player with mid and high clouds filtering through occasionally. The above average temperatures and light winds will persist into Sunday. Sunday may be slightly cooler as a storm system drops into the eastern Great Basin with weak backdoor front and light northeast surface winds.
Mon-Tues: Models are in good agreement with high pressure and relatively quiet weather continuing into Monday thru Tuesday with well above normal temperatures. Models are coming into agreement with the blocking high-pressure ridge flattening, permitting storms to enter Northern California possibly Wednesday.
Continuing to receive reports of widespread loose wet activity on southerly aspects, mostly D1 isolated to the gullies and around rock bands. On more northerly terrain, variable, wind affected dry snow (textured powder, some supportable wind crusts, and wind board). Weak freezes at night, thin coverage, with widespread loose wet activity on solar aspects. Variable wind affected conditions on E-NW aspects with slight sun effect on Easterly aspects. Wet and heavy snow conditions in the trees near tree line. Light to moderate winds with no obvious new wind slabs.
High pressure continues to be the dominant player through the weekend with clear skies and above normal temperatures with high’s of 50 to upper 50’s reported above, 9,000’ and nightly lows remaining warm in the upper basins with low temperaures in the mid 20’s to low 30’s (the deeper valleys in some locations have trapped colder air with lows in the teens to mid 20’s). The melt-freeze crust at the surface on E-SE-S-SW-W aspects has little time to maintain a sustained freeze and set-up solidly before the next cycle. At all elevations, as the sun advances around the compass rose, the surface melt-freeze crust will thaw on the solar slopes and becoming unsupportive during the day. Wet-Loose avalanches are possible, which has the potential to trigger larger Wet Slabs on steep slopes (>35 degrees). Plan to be off of these slopes early before they thaw and become unsupportive. Signs of Rollerball (Pin Wheels) activity is an indicator of weakening snow grain bonds and a rapidly warming snow surface. Natural and human triggered wet loose avalanches are possible, on southeast slopes in the morning, southerly aspects by mid morning to mid-day, and southwesterly by afternoon. Use extra caution on large mountain faces with complex terrain and multiple aspects. Lower elevations (below 8000’), the snowpack is disappearing rapidly but the remaining snow will warm rapidly on all aspects with risk of wet loose activity increasing on slopes >35 degrees by the afternoon.
This snowpack summary applies only to backcountry areas outside established ski area boundaries. This snowpack summary only describes general avalanche conditions and local variations always occur. This snowpack summary expires 24 hours after the posted time unless otherwise noted. The information in this snowpack summary is provided by the Eastern Sierra Avalanche Center who is solely responsible for its content.