AVALANCHE FORECAST PUBLISHED ON DECEMBER 31, 2020 @ 6:59 AM

Triggering avalanche on steep slopes still remains possible especially in wind affected terrain where new slabs of wind drifted snow exist or in more sheltered terrain where the old snow below the surface remains weak. MODERATE avalanche danger exists at all elevations. Conservative terrain choices are recommended when traveling in the backcountry. This forecast will be updated on Jan. 2 by 7 am. 

Over the last few days, data has indicated that the Dec 11 facets have started to adjust to the load above them and that the likelihood of triggering an avalanche has decreased. The 1 to 3 inches of snow that fell overnight on 12/30 should not provide enough new weight to overload this weak layer in most places. That being said, this layer is not strong by any measure. Persistent slab avalanches will remain possible, they just may not be as widespread. Signs of instability will become more sporadic with whumpfing, cracking, and unstable snowpack test results in some areas and few or no signs of instabilities in other areas. In some cases, a large avalanche may be the first and only clue that the slope is unstable. Persistent slabs behave in unpredictable ways. Wide propagation, large avalanches, mid-slope avalanches, and avalanches on slopes that already have tracks could occur. 

Terrain choices represent a great way to manage persistent slab problems. Travel plans that target more conservative terrain or terrain without the weak layer can provide fun recreation opportunities with significantly higher safety margins. In many cases, the snow conditions on terrain less steep than 30 degrees are better than on the steeper terrain right now. Be aware of connected terrain above, below, or to the side of steeper slopes since remote triggering remains possible. Be patient, the bigger, steeper terrain will still be here after the persistent slab problem goes away. After all, it is a persistent slab problem, not a forever slab problem.

Issued by Andy Anderson – Tahoe National Forest – Sierra Avalanche Center

Observations Sent into Avalance Center:

  • On 12/30 observers on Webber Peak (Little Truckee Summit), Red Mountain (NE of Donner Summit), Mt. Judah (Donner Summit), and Chickadee Ridge (Mt Rose backcountry) all found the Dec 11 facet layer buried in the snowpack. However, no one reported obvious signs of instability or unstable snowpit test results on this layer. 12/30 was the first day without reports of obvious instability since 12/25.  
  • Surface hoar existed on sheltered slopes on Webber Peak, Red Mountain, and Mt. Judah. It has been reported in other areas over the last few days as well. Prior to 12/30 reports of buried surface hoar, 1-2” below the surface had come in from several areas around the forecast region.
  • * Blowing snow started drifting onto NE – E facing slopes during the day on 12/30.
  • * Observers reported soft cold snow on northerly and easterly aspects on 12/30 with some warmer wetter snow on sunny more southerly aspects

Images and reports from the https://www.sierraavalanchecenter.org/

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