December 19, 2016 – MODERATE avalanche danger may exist due to possible wind slabs resting on a variety of weak layers on all wind-loaded aspects. Some of these wind slabs are small and some could be large. Human triggered wind slab avalanches may be possible today. A human triggered avalanche that may have been a wind slab was reported in the Mt. Rose backcountry yesterday. Triggering a deep slab avalanche on NW-N-NE aspects above 9000 ft. has become unlikely but not impossible.
Shifting winds during the last few days means that wind slabs could exist on almost any near and above treeline aspects. These wind slabs rest on a mix of frozen rain crusts and soft unconsolidated snow sitting on frozen rain crusts. Cold clear weather has allowed that soft snow near the rain crust to weaken and it may be on its way to becoming another persistent weak layer on some aspects. In some areas, it may have already grown too weak to support the lingering wind slabs above it. As a result, human triggering of these winds slabs may be possible again today in some areas. Since data concerning this problem remains very limited (a snowpit test a few days ago on Tamarack and an avalanche report yesterday from Bronco Chutes), a great deal of uncertainty remains concerning these wind slabs and whether or not they may fail at or below the 12/15 rain crust.
Wind slabs on the W-SW-S aspects should remain smaller and more limited. On the NW-N-NE aspects as well as a few areas on E and SE aspects wind slabs could be larger, stiffer, and could break above the person who triggers them. They would also involve much more snow and could propagate wide distances.
Clues like blowing snow, cornices, snow surface scouring, firm but hollow sounding snow, and wind pillows can help identify which slopes may be wind loaded and where wind slabs may exist. Giving potential wind slabs a wide berth could be a wise choice today.
Triggering a deep slab avalanche has become unlikely due to the strength of the snow above the deeply buried persistent weak layers that still exist on NW-N-NE aspectsabove 9000 ft. However, in the unlikely event that this layer does break, fractures can still travel along the layer and the resulting avalanche would be large, destructive, and likely unsurvivable. While triggering these deeps slabs has become unlikely, it is not impossible for some very large trigger or a trigger on just the right trigger point to break the PWL and release a deep slab on some isolated terrain. Some of the wind slabs mentioned above could possibly serve as large enough triggers to release a deep slab in a few isolated places.
Due to the consequences associated with the large destructive avalanches that could occur if the persistent weak layer fails and the high degree of uncertainty associated with it, this unlikely but not impossible avalanche problem still warrants extra caution.
Yesterday, a party reported an avalanche on a NE aspect in the Bronco Chutes area near Relay Peak in the Mt. Rose backcountry. Details on this avalanche remain unclear, as does the nature of the weak layer that it failed on. It did take out an existing skin track.
Across the Lake observations from on Waterhouse Peak (Luther Pass) and Rubicon Peak (West Shore) showed limited snow below 8100-8300 ft. with much more snow above those elevations. The upper snowpack consisted of 3 to 8 inches of soft weak snow (near surface facets) sitting on top of the frozen 12/15 rain crust between 8100 ft. and 9000 ft. with closer to 15 inches of softer less faceted snow above the crust above 9000 ft. on Waterhouse Peak. At the north end of the Sierra Crest on Anderson Ridge, Andesite Peak, and Castle Peak, east winds had scoured exposed above treeline E facing slopes back to the rain crust and redeposited that snow on the SW-W-NW aspects. 3 to 6 inches of soft weak snow existed on top of the rain crust in areas sheltered from the E winds above 7800 ft. in these areas. While weak snow existed on the surface in all three of these areas: Luther Pass, Rubicon, and Donner Summit, observations and snowpit tests did not reveal significant signs of instability below 12/15 rain crust.
Colder air has settled into the valleys with remote sensors reporting lows in the single digits and teens below 7000 ft. At the upper elevations, much warmer temperatures existed this morning with sensors showing morning temperatures in the mid 20’s to low 30’s between above 8000 ft. This inversion should remain in place due to the light winds associated with the high pressure over the region. The forecast calls for much warmer temperatures in the mountains today with daytime highs reaching the upper 30’s to low 40’s above 7000 ft. The high pressure should also keep skies mostly clear this morning with light winds expected at all elevations through tonight. Some clouds may start to develop over the region this afternoon and tonight as a small cold front approaches the region. This will also cause moderate southwest winds to return on Tuesday which should allow more warming on Tuesday ahead of the cold front.
This avalanche advisory is provided through a partnership between the Tahoe National Forest and the Sierra Avalanche Center. This advisory covers the Central Sierra Nevada Mountains between Yuba Pass on the north and Ebbetts Pass on the south. Click here for a map of the forecast area. This advisory applies only to backcountry areas outside established ski area boundaries. This advisory describes general avalanche conditions and local variations always occur. This advisory expires 24 hours after the posted time unless otherwise noted. The information in this advisory is provided by the USDA Forest Service who is solely responsible for its content.
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