Issued by Doug Lewis - Eastern Sierra Avalanche Center
The avalanche focus is shifting toward the southern aspects due to spring like temperatures and surface warming while northerly terrain and shaded slopes the focus is on isolated windslabs from the previous snowfall (1/29-2/1).
As we roll into February, the sun is well overhead and the snow on southerly aspects is showing the effect with signs of strong warming during the day. Mid and upper elevations will warm rapidly with crusts going from supportive to unsupportive depending on underlying snow, aspect, and time of day. Possible wet loose and wet slabs maybe encountered on steep (>35 degrees) southerly aspects. Be especially aware on large southerly faces, complex terrain, and large rocky faces where strong localized warming can occur. Plan to be on and off of these types of slopes early before they warm too much. Signs of rollerball (pin wheels) activity is a indicator of weakening bonds and rapid warming of the surface snow, which can lead to larger slope failures. Natural and human triggered avalanches maybe possible during the AM on southeasterly aspects and moving around the south-end of the compass toward west by afternoon.
Northerly aspects (NW – N – NE – NNE) in the mid to upper elevations remain cool and as the upper elevation winds have eased, new sensitive windsabs are decreasing in these areas but isolated pockets prone to triggered release certainly exist. With variable post-frontal winds, windslabs are possible for all aspects, on steep exposed slopes >35 degrees at mid and especially upper elevations. In high elevation exposed terrain, isolated local winds may be strong enough to transport snow and form new windslabs. Remember, even small avalanches in the wrong spot can injury or force you into dangerous terrain. Continue be on the lookout for denser surface snow over less dense snow, particularly on exposed slopes at mid to upper elevations, and do your own quick stability tests to assess how well these windslabs have bonded.
Saturday thru Sunday - High pressure continues to build over the region with a few mid and high clouds passing over northern CA as a shortwave moves into the Pacific Northwest. Temperatures will continue to warm over with valley inversions and dry conditions. Warming temperatures in the mountains should lead to a slight increase in runoff in the rivers and streams.
Monday thru Tuesday – The high-pressure ridge off the coast of CA will then strengthen and expand inland to the great basin by Monday, keeping clear skies and light winds in place, with strong valley inversions and temperatures 5-10 degrees above average. Light southwest-west flow developing at ridge level behind the ridge axis, which should improve mixing & potentially warmer temps.
Observations in the Mammoth Lakes region show a strong positive trend overall in snow stability with mild temperatures and lite winds are allowing the snowpack to settle and gain strength, aiding stability. In general, the mid-snowpack here has gained sufficient strength to compensate for the Depth Hoar that has been reported throughout the season. The growing concern is the southerly aspects, which are undergoing strong warming during the day. Aspect and timing is key when traveling on sunnier slopes. Northerly aspects continue to strengthen overall but isolated windslabs continue to be a concern on steep exposed terrain 35 degrees and steeper. Good riding can be found in sheltered treed areas.
Outside of the Mammoth Lakes Basin, the snow pack remains relatively shallow and variable with several slides observed that occurred during the previous storm, some failing deep into the snowpack. This indicates that the deeper weaknesses are still a concern; especially shallow areas were a rider could possibly trigger a deeper release. Additional observations of minor sloughing on southerly aspects due to PM heating have been reported. One observer reported extensive collapsing and whoompfing over a large area with well-developed Depth Hoar above the ground below Red Mountain at 9,800’, near Tom’s Place.
SnoTel sites are recording temperatures well into the upper 30’s and mid 40’s above 9,000’ with Gem Pass (10,750’) reporting a high of 46 on 2/5. Below 9,000’, reports of mid 50’s. Areas that receive strong solar radiation during the day can expect to see even warmer temperatures.
Daily temperatures continue to rise with strong solar gain on Southerly exposures (SE-S-SW-W), weakening the snowpack, especially on steep slopes (>35 degrees) that have direct solar exposure or receive indirect solar heating from adjacent to exposed slopes or rock bands. Time your travel to be off of these slopes early before they thaw. Be aware of wet rollerball activity or wet loose release, which can be an indicator that larger slope failures are possible. Although wet slides move slower than dry snow avalanches, once entrained, it can be difficult to get out and can possibly carry a person into dangerous terrain. Natural and human triggered avalanches are possible and even likely when slopes rapidly warm. Time your travel to stay ahead of the sun and snow thaw.
This week’s mild weather pattern and minimal winds over the past few days has significantly helped reduce the concern for windslabs overall. However, isolated human triggered windslabs are still possible for all aspects, on steep exposed slopes >35 degrees at mid and especially upper elevations. Regularly assess windslab sensitivity as you travel, especially as you change aspects or move into higher terrain where the wind may be strong enough to form fresh sensitive windslabs.
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.