An atmospheric river storm dumped up to 3’ of snow on top of an old melt-freeze crust between 4/6 and 4/9. Since then, a moderate storm between 4/12 and 4/14 added a thinner layer of newer snow in many areas, but the diurnal spring freeze-thaw cycle has largely strengthened the snowpack. Warm and sunny weather over the weekend created another melt-freeze crust on the surface, even above 12,000’ in some places. As a spring storm moved through the area on Monday afternoon, new wet snow began sliding on this more recent layer. Snow levels fluctuated during the storm and many areas received rain Monday before snow in the early morning hours on Tuesday. The initial rain soaked and weakened the old snow down to at least the crust that was buried on 4/6. Rain fell again up to almost 9,000’ Tuesday morning before a final shot of snow by Tuesday afternoon. Loose wet avalanches were prevalent during the storm at middle elevations, and while the snowpack was at its wettest slab fractures were observed in the Mammoth Lakes Basin. Temperatures were colder at higher elevations and the strong southwest winds accompanying the storm redistributed new snow onto leeward slopes above about 9,000’.
A hard freeze overnight on Tuesday should have helped to the underlying snowpack re-bond and its strength to rebound. As for the 3-10 inches of wind packed storm snow, it will take time to settle and strengthen. Remember that wind can deposit new snow onto leeward slopes 3 to 5 times faster than it can fall from the sky and even smaller amounts of snow can create large wind slabs in just a short amount of time. So these fast moving spring showers can create wind slabs sensitive enough for you to trigger. In areas where the most snow has accumulated, just under ridge lines, convexities, and the side walls of gullies, hollow-sounding wind slabs will be sensitive to your weight. Blowing snow and cornice formation will continue to point to where the avalanche danger has increased. Use these observations to make conservative terrain choices that keep you out of harms way.
Highs will creep up into the 40s Wednesday and Thursday. As temperatures climb, so will the risk of loose wet avalanches. As the sun heats the new snow to its melting point it will become less stable. Buried melt-freeze crusts like the one that we have now are good sliding surfaces for wet snow avalanches. Extra heat from cliffs, rock faces, and trees can result in additional localized thawing. Large roller balls originating from exposed rocks can be a good indicator the loose wet avalanches are becoming increasingly likely throughout the day.
