Backcountry Avalanche Forecast
Central Sierra Nevada
This product is expired
Issued Saturday, March 20, 2021 - 6:59AM
Expires Sunday, March 21, 2021 - 6:59AM
Author Andy Anderson

THE BOTTOM LINE

Unstable slabs of wind drifted snow (wind slabs) still exist in near and above treeline terrain. Human-triggered wind slab avalanches remain possible. If today is sunnier than forecast, the intense March sun could make these wind slabs easier to trigger and could cause some wet loose instability on sun-exposed slopes despite the cold temperatures. MODERATE avalanche danger exists near and above treeline with LOW danger below treeline.

Avalanche Danger
Saturday, March 20, 2021
Above Treeline Moderate (2)
Near Treeline Moderate (2)
Below Treeline Low (1)

Avalanche Problem #1
Wind Slab
All aspects except W-SW At and Above Treeline
Possible
Small (D1) - Large (D2)

Winds continued to load leeward aspects yesterday. The slabs of wind drifted snow that formed near ridgelines, in gullies, and on open slopes exposed to the winds may still remain problematic today. Winds have shifted direction in some places and wind slabs may exist in some unusual spots. Large fragile cornices also loom above many wind-loaded slopes. Human-triggered wind slab avalanches large enough to bury or injure a person (size D2) remain possible today on steep wind-loaded slopes. If the sun comes out today, it could make triggering one of these wind slabs easier on sun-exposed slopes. 

Look for cornices above a slope, wind pillows, snowdrifts, ripples on the snow surface, and other wind-created features to help identify where unstable wind slabs may exist. Avoiding steep wind-loaded areas in favor of more sheltered terrain will help limit your exposure to wind slab problems and could provide more consistent snow conditions. 

Avalanche Problem #2
Wet Loose
W-S-E At and above treeline
Unlikely
Small (D1) - Large (D2)

Cloud cover and cool temperatures should limit the formation of wet snow today. However, if the cloud cover is less widespread, the intense March sunshine on the new snow could provide enough warming to cause wet loose instabilities on steep sunny slopes.

Since cloud cover is hard to forecast, pay attention to the weather and snow conditions as you travel and be willing to change plans as conditions change. If you are in a sunny area and start to see signs of wet snow like roller balls, pinwheels, or wet sticky snow, move to a colder aspect where the snow remains dry and soft or switch to lower angle slopes.

Forecast Discussion

* Skier-triggered wind slab avalanches were reported on Elephant's Hump (Carson Pass), Fireplug/Galena Peak (Mt. Rose backcountry), and on Andesite Peak (Donner Summit). These all failed in the new snow with crown depths of 4 to 12 inches. No one was injured. 

* The small wind slab on Andesite released when a cornice broke farther back than the person kicking it expected. This cornice failure took him onto the wind-loaded slope below where the small wind slab released. Fortunately, it was not large enough to bury or injure the person.

* Large cornices existed above wind-loaded slopes. On Castle Peak and Andesite Ridge, cornices broke easily and well away from their edges. Some of the cornice pieces triggered cracking or small wind slabs and others just caused some sluffing in the new snow. 

* Observers on Echo Summit, Donner Summit, and Carson Pass reported 10 to 14 inches of new snow on top of the old snow surfaces above 7000 ft yesterday with significantly less new snow at the lower elevations. Less new snow existed in areas east of Lake Tahoe. In sheltered areas, the new snow was heavier and wetter near the base of the new snow and got lighter near the surface.

Weather Forecast

Snow showers continued over the last 24 hrs with another 2 to 6 inches of accumulation. Temperatures have cooled off significantly. The forecast calls for today's highs to remain in the mid 20's to mid 30's above 7000 ft. The forecast area could continue to see isolated snow showers in some places today but accumulations should remain limited. As for cloud cover, expect partly cloudy skies with some periods of sun. Light westerly winds should continue today before shifting to the NE and E overnight. Clouds should clear up tomorrow and temperatures should get a bit warmer.