Natural and human triggered loose wet avalanches will become more likely as extensive solar radiation saturates the snow. The majority of these avalanches should be small, but larger avalanches may be possible where the snowpack retains more heat. Timing of these avalanches will depend on aspect, elevation, winds, and cloud cover. East facing and low elevation slopes become unstable earlier. South and West facing slopes, and higher elevations, become more dangerous later in the day. Winds cooling the snow surface will create firm, “slide for life” conditions. Be careful of your exposure to a slip and fall on steep terrain. Evaluate snow and terrain carefully.
Likelihood ?CertainVery LikelyLikelyPossible
Size ?HistoricVery LargeLarge
Air temperatures will remain above normal through Thursday. Southwest winds will cool the snow surface and could keep things firm until later in the day, but cloudy skies this afternoon and tonight may keep daytime heat from leaving the snowpack. This situation could create conditions favorable to larger wet avalanches at middle and lower elevations by Thursday. This balance between extended solar warming and cooling from increasing winds may make conditions trickier to predict for backcountry travelers. Remember that even small point releases show that slopes are becoming unstable. These smaller slides can knock you off balance and can carry you into dangerous terrain if you aren’t careful. Pay attention as the snow surface warms, especially around rocky outcrops, in cirques and gullies, and on treed slopes. Sinking into wet snow up to your boot top, pinwheels, and small point releases are good indicators of instability. Time of day is critical for loose wet avalanches. East aspects receive the first radiation of the day, then south, then west in the late afternoon.
One phrase has come to mind over the past week (besides “Hawaiian shirt” and “flip flops”): “Melt-Freeze.” Temperatures have been trending ever warmer in the Sierra and turning our former powder into creamy corn snow by mid day on most slopes. The melt-freeze process – with the spring sun warming the snow throughout the day and cold nights refreezing it – generally makes the snowpack more stable over time. Prolonged warming, however, can start to weaken the snowpack on a deeper level. On slopes with tree cover and in rocky gullies where solar radiation doesn’t dissipate as readily as on open slopes, melt water has been saturating the snow down to a foot deep or more. As snow temperatures stay warmer, and water content is higher, loose wet and wet slab avalanches become more likely. Tuesday, South and East aspects warmed early. Surfaces became wet by 11am at elevations up to about 11500’. Slopes also warmed up to alpine elevations. Skiers triggered some small wet slabs at 13000’ on Mt. Tom yesterday. As skies become cloudy this afternoon and into this evening, solar radiation will have an even harder time escaping. Loose wet avalanches may become more widespread and larger, especially near rocks, cliffs, and on slopes with forest cover. Small point releases and roller balls indicate that wet snow is becoming unstable.
Light winds from the south have been cooling the snow at upper elevations. Firm wind board can be dangerous in it’s own way since a slip and fall on steep terrain could make for a long slide down and have higher consequences. Winds are set to be stronger from the southwest today through Thursday morning. This could result in slopes taking longer to soften.
A fast moving low pressure system moving through the Pacific Northwest will bring increasing winds and clouds to the Sierra today. High pressure builds back into the region Thursday with lighter winds by Friday. Cold air remains well to the north of our area, so high temperatures will still be well above normal for mid-March.