THIS AVALANCHE ADVISORY EXPIRED ON December 12, 2014 @ 6:54 am
Avalanche Advisory published on December 11, 2014 @ 6:54 am
Issued by Andy Anderson - Tahoe National Forest

The avalanche danger will remain LOW until the precipitation starts to fall. Once the snow begins accumulating the avalanche danger will increase quickly and could reach HIGH on slopes steeper than 32 degrees during the night. Natural avalanches will become likely tonight and early tomorrow morning. Wind slab, storm slab, and deep persistent slab avalanches will all become problems during the next 24 hours. Travel in and around avalanche terrain is not recommended during the storm.

1. Low

?

Above Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

1. Low

?

Near Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

1. Low

?

Below Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
    Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
Avalanche Problem 1: Persistent Slab
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

Triggering a persistent slab avalanche will remain unlikely until new snow starts to accumulate this evening and during the night. At some point the weight of the new snow will likely overload this weak layer resulting in destructive deep persistent slab avalanches that break near the ground. NW-N-NE aspects above 9000-9500 ft. where this persistent weak layer of loose sugary snow (facets) still lingers represent the best places to trigger deep persistent slab avalanches. Sometime tonight natural and human triggered deep persistent slab avalanches will become likely.

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

New snow and wind during this storm will form wind slabs on wind-loaded NW-N-NE-E-SE aspects. Due to the strength of the winds associated with this storm wind slabs may form at all elevations and in areas that typically remain sheltered from the winds. These winds slabs may start forming today, but triggering a significant wind slab will remain unlikely until more snow accumulates tonight. During the night natural and human triggered wind slab avalanches will become likely.

Avalanche Problem 3: Storm Slab
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

Changing conditions during the storm will create weaknesses within the storm snow that could result in storm slab avalanche activity. Like the other avalanche problems, storm slabs will remain unlikely until enough snow accumulates on top of the current snowpack. Sometime during the night natural storm slab avalanches could become possible and human-triggered storm slabs could become likely in areas with enough new snow on them.

advisory discussion

Snow covered mountains, improving weather, and fresh tracks tomorrow will make it easy to forget about the deep persistent slabs, wind slabs, and storm slabs that form during the night. Slopes that don't slide tonight will be problematic tomorrow especially in areas where the persistent weak layers exist. A slope breaking around a person and avalanching often represents the only warning that persistent slabs give people. For some advice on dealing with persistent slabs, wind slabs, and storm slabs check out this page.

recent observations

Observations from the Mt. Rose backcountry yesterday continue to show a persistent weak layer near the base of the snowpack on NW-N-NE aspects above 9000-9500 ft. Tests on this layer in the Relay Peak area and on Tamarack Peak indicated that once this layer breaks the resulting fractures can travel along the weak layer. While data around the forecast area indicates a currently stable snowpack, the same data also points to a snowpack that will not be able to support the additional snow load that the forecasted storm is supposed to deliver over the next 24 hours.

Weather and CURRENT CONDITIONS
weather summary

Strong southwest winds began hammering the forecast area last night with average speeds along the ridgelines around 50 mph and gusts around 109 mph. These winds should continue to increase in ferocity today ahead of the approaching storm. They will remain strong through Friday morning, but they should begin a steady decrease once the precipitation starts to fall. Temperatures should start out warm today with snow levels around 8000 ft. Snow levels and temperatures should lower significantly over the next 24 hours. By tomorrow morning the forecast calls for snow level to have dropped below 6000 ft. Precipitation could begin over the forecast area this afternoon with some rain and snow falling before 4 pm. The precipitation will start in the north and move south as the storm progresses. The most intense periods of precipitation will occur tonight with snowfall rates of 2-3 inches per hour above 7000 ft. Snowfall should start to taper off during the day tomorrow. Overall the forecast calls for 1 to 2 ft of new snow above 7000 ft. with some areas along the Sierra Crest receiving closer to 3 ft. by the end of the day tomorrow. Below 7000 ft. expect a mix of rain and snow with 7 to 10 inches of new snow by the end of the storm.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 35 to 43 deg. F.
Max. temperature in the last 24 hours: 40 to 49 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 48 mph
Maximum wind gust in the last 24 hours: 109 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: 12 to 27 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Thursday Thursday Night Friday
Weather: Cloudy with a chance of rain and snow. Snow level should start around 7500 to 8000 ft. and fall throughout the day. Cloudy with rain in the evening and snow during the night. Snow level dropping below 6000 ft. during the night. Snow likely in the morning becoming cloudy with snow showers likely in the afternoon
Temperatures: 42 to 48 deg. F. 22 to 29 deg. F. 26 to 33 deg. F.
Wind Direction: Southwest South Southwest
Wind Speed: 60 to 70 mph with gusts to 105 mph 55 to 60 mph with gusts to 90 mph decreasing to 40 to 45 mph with gusts to 70 mph after midnight 30 to 35 mph with gusts to 50 mph decreasing to 15 to 20 mph with gusts to 30 mph in the afternoon
Expected snowfall: up to 3 in. 8 to 14 in. 4 to 8 in.
For 8000 ft. to 9000 ft.
Thursday Thursday Night Friday
Weather: Cloudy with a chance of rain and snow. Snow Snow in the morning. Then snow showers in the afternoon
Temperatures: 36 to 42 deg. F. 20 to 27 deg. F. 22 to 29 deg. F.
Wind Direction: Southwest South Southwest
Wind Speed: 75 to 80 mph with gusts to 120 mph increasing to 90 to 95 mph with gusts to 140 mph in the afternoon 80 to 85 mph with gusts to 130 mph decreasing to 65 to 70 mph with gusts to 105 mph after midnight 50 to 55 mph with gusts to 80 mph decreasing to 20 to 25 mph with gusts to 40 mph in the afternoon
Expected snowfall: up to 4 in. 8 to 16 in. 4 to 8 in.
Disclaimer

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.