An unusually late season Atmospheric River moved into the region Thursday afternoon and dropped 1.2” to 6” of water along the eastern Sierra by Saturday evening. Starting warm, with heavy wet snow above about 7500’, things cooled and lighter density snow fell as the system exited the region. Storm totals ranged from 19” at Tioga Pass to 37” at Mammoth Mountain. Snowfall was heaviest from June Mountain south. The system came in with moderate to strong Southwesterly flow (typical of these Atmospheric River storms) with winds gusting over 100mph at ridge tops. Wind Slabs that formed between Thursday and Sunday were reactive in some areas through Monday when natural avalanches were observed in Lundy canyon. In the Mammoth area, wind deposits have become more stubborn to trigger. Though with little data from elsewhere in the forecast zone, I wouldn’t be surprised if some isolated high elevation slopes remained sensitive through Wednesday.
Temperatures will again be spring like on Wednesday, with highs in the upper 40s to mid 50s. As temperatures climb, so will the risk of loose wet avalanches at lower elevations. Moderate to strong southwest winds cooling the snow surface may keep the danger relatively low. But as the sun heats the snowpack to its melting point it will become less stable. 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 that loose wet avalanches are becoming increasingly possible.
The active spring weather pattern continues, so be ready for another ride on the roller coaster. On Thursday, another moderate spring storm will push through our area bringing snow showers, and most importantly wind. Winds will be strong and sustained starting Wednesday and will increase going into early Thursday as the main cold front moves through. As is common with these kinds of spring storms, precipitation totals and snow levels can be difficult to predict. Though precipitation amounts are expected to be around 7”, remember that wind can deposit new snow onto leeward slopes 3 to 5 times faster than it can fall from the sky. So even smaller amounts of snow can create large wind slabs in just a short amount of time. It’s this kind of rapid loading that creates wind slabs sensitive enough for you to trigger. In areas where the most snow accumulates, just under ridge lines, convexities, and the side walls of gullies, you will most likely find hollow-sounding wind slabs. Areas north of Mammoth will see greater snow fall amounts. Blowing snow and cornice formation will point to where the avalanche danger is increasing. Use these observations to make conservative terrain choices that keep you out of harms way.