Natural and human triggered Loose Wet avalanches will become increasingly possible as slopes warm throughout the day. For Wednesday: Heightened avalanche conditions may exist after substantial warming and near rocky terrain features. Thursday: Generally safe avalanche conditions will exist, but watch for signs of unstable snow such as large roller balls, deep ski penetration, and small point releases on isolated terrain features. Pay special attention near rock bands, outcroppings, and exposed vegetation.
-Hard snow surfaces and slide-for-life conditions may be found early in the day or as gusty winds and cloudy skies prevent daily melting.-
Likelihood ?CertainVery LikelyLikelyPossible
Size ?HistoricVery LargeLarge
Spring conditions will continue to dominate the area through Thursday afternoon. High pressure and mid-level clouds will keep temperatures a good 5-10 degrees above average Wednesday. Skies will remain mostly cloudy during the forecast period. Clouds can prevent the sun from warming slopes as quickly, but help keep solar radiation from escaping back into space. This means that once slopes do warm they may warm more quickly and intensely. Clouds at night may keep the snow from re-freezing as thoroughly. The deeper the snowpack warms, the larger wet avalanches can be. But even small point releases are heavy and can be hard to escape, and they can be a sign that larger avalanches are possible. As the snow surface warms to its melting point throughout the day loose wet avalanches will be increasingly possible, especially around rock outcrops and below cliff bands. Time of day is critical for wet point releases. East aspects receive the first radiation of the day, then south, then west as the sun moves across the sky. Cloudy skies reflecting the sun’s rays may allow northerly aspects to heat up as well. Lower elevations will warm more quickly than higher elevations. Watch out for pinwheels rolling down the slopes around and above you.
*Small isolated wind slabs may be encountered at upper elevations by Thursday night, depending on the timing of an Atmospheric River storm set to land in the area by Friday morning. Though unlikely, small isolated Wind Slabs prone to human triggering may be possible if you are still in the mountains by evening.*
Spring has been in full swing in the Sierra. The snowpack has mostly consolidated and is characterized by Melt-Freeze crusts separated by rounds or decomposing grains closer to the snow surface. A minor disturbance raced through the northern half of the forecast zone on Sunday night with rain levels up to almost 9,000’ for a short period. Low temperatures were exceptionally mild overnight and the snowpack warmed quickly on Monday. Lows in the 20s Monday night helped set up a good freeze going into the daily melting cycle on Tuesday.
Wednesday, temperatures are forecast to reach well above average. Rocky outcrops, trees, and mostly cloudy skies will trap the heat and reflect in back down onto the snow, elevating the hazard of loose wet avalanches. Large pinwheels or ski penetration of boot top or greater are signs the snow is loosing strength and becoming unstable. Starting early and being out of steep, rocky terrain before things get too warm is the best strategy for avoiding loose wet avalanches.
Winds will be moderate and gusty as the ridge that has been dominating our weather pattern for the past several days begins to push east. On Thursday, winds will increase with southwest flow aloft as an unusually late Atmospheric River storm makes its way towards the California coast. Small Wind Slab avalanches may become possible Thursday evening depending on the timing of an incoming storm. 1-3” of snow are forecast overnight on Thursday with most accumulation expected after midnight. Predicting the timing and snow levels for spring storms can be tricky, so pay attention to changes and trends if you plan to be out after dark.
Wednesday and Thursday morning, and as winds cool the snow surface during the day, firm, slide-for-life conditions will exist. Be careful of your exposure to a slip and fall on steep terrain where consequences are high.
Dry and warmer conditions are expected Wednesday, though clouds will be abundant. Winds will begin to increase Wednesday afternoon with periods of stronger winds Thursday followed by a sharp change to a cooler and wetter pattern through the weekend. Warm air advection Thursday ahead of the main storm will bring isolated areas of light rain and up to 1 inch of snow to the mountains, with the main AR moisture push coming Thursday night into Friday.