Avalanche Forecast published on March 30, 2015 @ 7:00 am
Issued by Andy Anderson - Tahoe National Forest - Sierra Avalanche Center

We have stopped issuing daily avalanche advisories for the 2014-2015 season due to significantly warmer than normal temperatures and below average precipitation this winter. Avalanche advisories will resume in the fall of 2015.

Avalanche activity can and most likely will continue to occur this spring as additional storm cycles impact the forecast area. Continue to monitor changing conditions and use caution when traveling in the backcountry. For general spring avalanche information please read the full spring avalanche statement.

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Avalanche Problem 1: Loose Wet
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Loose wet avalanche activity will continue to occur this spring. Usually this type of instability forms in response to daytime warming. With many previous melt-freeze cycles and rain events having already occurred this past winter and spring, free water drainage from the snowpack is well established. This is expected to keep the majority of wet snow avalanche activity limited to human triggered loose wet avalanches.

You can use the Snotel and National Weather Service sites to monitor hourly temperatures at many points throughout the forecast area. Cloudy skies overnight and air temperatures above freezing do not allow the snowpack to refreeze very well. When the snowpack does not refreeze overnight, other springtime activities that do not involve snow travel on steep slopes represent more prudent choices. Under clear skies, the top few inches of the snowpack will often refreeze despite near or slightly above freezing air temperatures. This superficial refreeze usually allows for a short period of good travel conditions during the early morning hours before surface wet snow instability becomes a concern. If a solid overnight refreeze occurs, getting out early and finishing in time to have a mid day barbecue should be your goal. Start with east aspects and follow the sun around to north aspects. Get off of your equipment on a regular basis and check boot penetration depth. Boot-top deep wet snow, significant roller ball activity, or any loose wet avalanche results from small test slopes all indicate that wet snow instabilities can occur. Moving to a different aspect with less sun exposure, terrain less than 25 degrees in slope angle without steeper terrain above, or simply heading over to the beach for a picnic all represent good choices for avalanche avoidance at that point. As a matter of etiquette, do not leave deep ruts in a slope that will freeze overnight and ruin the slope for others the next day.

Avalanche Problem 2: Wind Slab
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Natural and human triggered wind slab avalanches may occur during and immediately after any late season storms. Expect a period of snowpack instability during the storm itself, then a second cycle of avalanche activity as rapid warming occurs post storm. During the storm, watch for typical signs of mid winter instability such as recent avalanche activity, wind loading, collapse, audible whumpfing sounds, and/or shooting cracks. Post storm, new snow will be very sensitive to rapid warming and direct sunlight. Pay close attention to layer bonding within the new snow and to the old snow surface beneath it. It can lose strength rapidly as the day progresses causing a significant increase in avalanche danger.

Avalanche Problem 3: Wet Slab
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During periods of rapid warming after new late season snowfall, storm slabs and wind slabs can transition to wet slabs. The high angle sun in late April and May allows for significantly more incoming solar radiation to affect the snowpack on NW-N-NE-E-SE aspects than what occurs during December, January, and February. If an active weak layer exists at the base of a recently formed storm slab or wind slab, rapid warming of the snow surface can increase the deformation rate of the slab adding additional stress to the weak layer below. This can cause an increase in snowpack instability and in some cases cause natural avalanches to occur during periods of rapid warming post storm event under sunny skies.

Forecast discussion

Other hazards such as cornice collapse, moats, glide cracks, and open creeks exist. Stay well back from abrupt edges along ridgelines as human triggered cornice collapse will remain possible during the spring. Stay out from under cornice areas that are not well frozen, especially if you can see water dripping from the cornice. Areas of weak snow around rocks, vegetation, and along the base of cliff bands exist. Move carefully around these features as the thin bridges of snow could collapse under body weight allowing you to fall into a melted hole next to the feature. Exercise caution when traveling near or attempting to cross creeks as wet snow along the banks can collapse under the weight of a person.

As the season begins to change, the morning air becomes crisp and the days become shorter, check back on our home page for early season fundraising events for the 2015-2016 season. Enjoy your spring and summer and we will see you again right here in the fall.

Winter 2014-2015:

The winter of 2014-2015 will go down as one of the least snowy and warmest winters on record. As of March 29, snow water equivalent (SWE) measurements rested at an average of  ~2.5 inches compared to the average median snow water equivalent between 1981 and 2010 of ~22.6 inches. In other words as of March 29, this winter's snowpack came in at about of 9.6% of average for the Truckee River, Lake Tahoe, and Carson River Basins. While this winter may represent one of the least snowy winters on record, it was not the driest. Those same basins received about 51% of average precipitation amounts. Due to the warm temperatures, most of this precipitation fell as rain within the forecast area except at the highest elevations. The data from the snotel sensors at Mt. Rose (8801 ft.) and at the Central Sierra Snow Lab (6855 ft. ) illustrate this point. As of March 29, the Mt. Rose sensor showed 40% of median SWE and a snow depth of 37 inches, while the Central Sierra Snow Lab sensor shows 0% of median SWE and a snow depth of 0 inches. During the spring of 1977 after a winter that received less than 30% of average precipitation but colder than normal temperatures, the snowpack depths on March 29 still measured ~23 inches at the Central Sierra Snow Lab. In summary while less than average precipitation did hurt the Tahoe snowpack, it was the significantly warmer than average temperatures that had the most impact on snow coverage and snowpack depth.

For more information and data both current and historical, here are some interesting links:

1. Snotel Snowpack and Precipitation Report for March 29, 2015

2. Snotel Snowpack and Precipitation Report Creator

3. California Snow Water Equivalent Graphs

4. California Snow Water Equivalent Summary

5. California Climate Tracker

6. Central Sierra Snow Lab Facebook Page

7. Tahoe Climate Information Management System

8. Climate change and California drought in the 21st century by Michael E. Mann and Peter H. Gleick

9. Anthropogenic warming has increased drought risk in California by Noah S. Diffenbaugh, Daniel L. Swain, and Danielle Touma

6am temperature: Check the SNOTEL and NWS remote sensor sites mentioned above. The weather stations link to the left will show data from these stations. deg. F.
Max. temperature: deg. F.
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Average ridgetop wind speed: mph
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New snowfall: inches
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Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Monday Monday Night Tuesday
Weather: Check in with the Reno NWS at the link above for the latest weather forecasts.
Temperatures: deg. F. deg. F. deg. F.
Mid Slope Winds:
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For 8000 ft. to 9000 ft.
Monday Monday Night Tuesday
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Ridge Top Winds:
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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.