THIS AVALANCHE ADVISORY EXPIRED ON March 30, 2014 @ 6:12 am
Avalanche Advisory published on March 29, 2014 @ 6:12 am
Issued by Brandon Schwartz - Tahoe National Forest

Avalanche danger will increase to CONSIDERABLE today and eventually to HIGH danger late this afternoon or evening near and above treeline on NW-N-NE-E-SE aspects on slopes 35 degrees and steeper. For all other areas avalanche danger is MODERATE on slopes 35 degrees and steeper.

3. Considerable

?

Above Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

3. Considerable

?

Near Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

2. Moderate

?

Below Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
    Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
Avalanche Problem 1: Wind Slab
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

Following the onset of high intensity snowfall today, new wind slabs will rapidly build near treeline and above treeline on NW-N-NE-E-SE aspects. Human triggered avalanches will become possible to likely today with natural avalanches becoming possible by late this afternoon or this evening. New wind slabs 2+ feet thick are expected to have formed by the late afternoon or evening hours.

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

Periods of high intensity snowfall hold good potential to create unstable storm slabs in below treeline areas on all aspects today. This avalanche problem will be highly dependent on what weak layers form within the storm snow as the day progresses. Human triggered shooting cracks in wind protected areas below treeline are an excellent indication of the presence of this avalanche problem.

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

Near the end of this storm cycle following the accumulation of 1 to 2 feet of new snow, unstable persistent slabs may form on NW-N-NE aspects above 8,000'. The weak layer of near crust facets that exists just below the melt-freeze crust at the base of the recent storm snow has the potential to become over loaded by new snowfall. This weak layer has been observed in many locations around the forecast area and is most well developed in near treeline and below treeline areas above 8,000' on NW-N-NE aspects. Any avalanches that fail on this weak layer will be large, involving all of the snow accumulated since March 25th.

recent observations

Observations made yesterday on Andesite Ridge (Donner Summit area) and on Powderhouse Peak (Luther Pass area) both gave some indications of a potential developing persistent slab avalanche problem on NW-N-NE aspects above 8,000'. Near crust facets just below the melt-freeze crust at the base of the recent storm snow are a weak layer that in some areas have shown the ability to propagate collapse in formal snowpit tests. This weak layer can currently support the snow load above it, but that may change as significant new snow accumulation occurs over the next few storm cycles.

On Andesite Ridge, wind slabs that had formed near and above treeline over the past few days appeared to have gained strength with no signs of instability observed.

Overall, the snowpack below 8,000' is in good condition to handle new snow loading. The snowpack above 8,000' on NW-N-NE aspects is in poor conditions to handle new snow loading due to the presence of near crust facets that has been observed as a weak layer in some areas.

Weather and CURRENT CONDITIONS
weather summary

A major storm system will move into the forecast area today with 1 to 2 feet of new snow expected over the next 24 hours. High intensity snowfall with rates up to 2 inches per hour are expected. The storm system will fill in from north to south across the forecast area today. Areas north of Hwy 80 will see high intensity snowfall starting mid morning. Areas from Hwy 50 south will not see high intensity snowfall begin until the mid to late afternoon hours. Snow levels are forecast at 6,500' to 7,000' this morning, falling to around 4,000' to 5,000' by this evening. Maximum daytime air temperatures are expected to reach the low 30s to low 40s today for areas above 7,000'. Ridgetop winds remain out of the southwest this morning and are increased to strong to gale force in speed. Winds are expected to begin to decrease tonight and become moderate in speed by tomorrow morning.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 30 to 33 deg. F.
Max. temperature in the last 24 hours: 32 to 39 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 34 mph
Maximum wind gust in the last 24 hours: 62 mph
New snowfall in the last 24 hours: Trace to 2 inches
Total snow depth: 36 to 54 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Saturday Saturday Night Sunday
Weather: Cloudy skies with snow. Cloudy skies with snow. Mostly cloudy skies with a slight chance of snow throughout the day.
Temperatures: 37 to 43 deg. F. 15 to 22 deg. F. 27 to 34 deg. F.
Wind Direction: Southwest Southwest Southwest
Wind Speed: 25 to 35 mph with gusts to 50 mph. 10 to 20 mph. Gusts to 35 mph decreasing to 25 mph after midnight. 10 to 15 mph with gusts to 25.
Expected snowfall: 6 to 12 in. 4 to 8 in. Trace in.
For 8000 ft. to 9000 ft.
Saturday Saturday Night Sunday
Weather: Cloudy skies with snow. Cloudy skies with snow. Mostly cloudy skies with a slight chance of snow throughout the day.
Temperatures: 33 to 39 deg. F. 12 to 19 deg. F. 27 to 33 deg. F.
Wind Direction: Southwest Southwest West
Wind Speed: 50 to 60 mph with gusts to 90 mph. 25 to 35 mph. Gusts to 55 mph decreasing to 45 mph after midnight. 15 to 25 mph. Gusts to 40 mph decreasing to 30 mph in the afternoon.
Expected snowfall: 6 to 12 in. 4 to 8 in. Trace 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.