THIS AVALANCHE FORECAST EXPIRED ON January 2, 2021 @ 6:59 am
Avalanche Forecast published on December 31, 2020 @ 6:59 am
Issued by Andy Anderson - Tahoe National Forest - Sierra Avalanche Center

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. 

2. Moderate

?

Above Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

2. Moderate

?

Near Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

2. Moderate

?

Below Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
    Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
Avalanche Problem 1: Persistent Slab
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

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.

Avalanche Problem 2: Wind Slab
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

Winds shifted back to the SW on 12/30 and remained strong during the night. While 1 to 3 inches of new snow may not sound like much, wind transport can form deeper slabs of drifted snow on wind-loaded slopes. In addition to this new snow, some old snow also remains available for wind slab building. With the winds forecasted to shift to the N and E 12/31/2020 and 1/1/2021, wind slabs will migrate back across the ridges and reform on previously scoured slopes. These shifting winds and blowing snow mean that wind slabs could exist on any wind-exposed aspect 12/31/2020 and 1/1/2021. For the most part, these wind slabs should remain small, but some larger ones could form on the most heavily wind loaded slopes. Human triggered wind slabs will be possible. 

Newly developed cornices, wind pillows, and wind created surface textures can all help to identify which slopes may hold wind slabs. Identifying where these wind slabs may exist only represents part of the strategy right now since the persistent slabs mentioned above may exist in the more sheltered terrain.  

recent observations
Submit Observations

* 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. 

Weather and CURRENT CONDITIONS
weather summary

A small, fast system moved into the area during the night. It brought 1 to 3 inches of snow and strong SW winds with it. In most places, snowfall had tapered off by this morning and winds had started to decrease. The forecast calls for clearing skies on 12/31 with cooler temperatures and winds shifting to the north and east. New Year's day should remain dry, but cloud cover should begin to increase. By night on 1/1 the forecast area should see a small weak storm move through with mostly increased cloud cover and wind. Snowfall amounts should remain limited with this system. Expect more storms to follow this small system with the potential for some larger winter storms next week. Check-in with the Reno NWS for more details. The remote weather data for the last 24 hours and Friday's (1/1/2021) weather forecast were updated on 1/1/2021 at 7:00 am.

CONDITIONS ALONG THE SIERRA CREST BETWEEN 8200 FT. AND 9200 FT. IN THE LAST 24 HOURS
Weather Station Map
Weather Station Table
6am temperature: Dec 31: 17 to 24 deg F | Jan 1: 23 to 30 deg. F.
Max. temperature: Dec 31: 28 to 35 deg F | Jan 1: 27 to 32 deg. F.
Average ridgetop wind direction: Dec 31: SW | Jan 1: NE
Average ridgetop wind speed: Dec 31: 25 to 38 mph | Jan 1: 25 to 45 mph
Maximum ridgetop wind gust: Dec 31: 96 mph | Jan 1: 72 mph
New snowfall: Dec 31: 1 to 3 inches | Jan 1: 0 inches
Total snow depth: Dec 31: 30 to 37 inches | Jan 1: 30 to 35 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Thursday Thursday Night Friday
Weather: Mostly cloudy becoming sunny. Snow levels below 7000 ft. Chance of precipitation is 5%. Partly cloudy. Snow levels below 7000 ft. Chance of precipitation is 0%. Sunny then becoming mostly cloudy. Snow levels below 7000 ft. Chance of precipitation is 0%.
Temperatures: 28 to 33 deg. F. 14 to 20 deg. F. 37 to 42 deg. F.
Mid Slope Winds: North around 15 mph with gusts to 30 mph. Light winds. Light winds.
Expected snowfall: No accumulation. | SWE = None No accumulation. | SWE = None No accumulation. | SWE = None
For 8000 ft. to 9000 ft.
Thursday Thursday Night Friday
Weather: Mostly cloudy becoming sunny. Snow levels below 7000 ft. Chance of precipitation is 5%. Partly cloudy. Snow levels below 7000 ft. Chance of precipitation is 0%. Sunny then becoming mostly cloudy. Snow levels below 7000 ft. Chance of precipitation is 0%.
Temperatures: 23 to 29 deg. F. 13 to 19 deg. F. 34 to 39 deg. F.
Ridge Top Winds: North 15 to 25 mph with gusts to 45 mph. East 20 to 35 mph. Gusts up to 65 mph decreasing to 45 mph after midnight. East around 15 mph with gusts to 35 mph shifting to the southwest with gusts to 25 mph in the afternoon.
Expected snowfall: No accumulation. | SWE = None No accumulation. | SWE = None No accumulation. | SWE = None
Disclaimer

This avalanche forecast is provided through a partnership between the Tahoe National Forest and the Sierra Avalanche Center. This forecast 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 forecast applies only to backcountry areas outside established ski area boundaries. This forecast describes general avalanche conditions and local variations always occur. This forecast expires 24 hours after the posted time unless otherwise noted. The information in this forecast is provided by the USDA Forest Service who is solely responsible for its content.

For a recorded version of the avalanche forecast call (530) 587-3558 x258