Backcountry Avalanche Forecast Central Sierra Nevada
Sierra Avalanche Center
www.sierraavalanchecenter.org
- Issued
- Thursday, December 7, 2023 - 6:49AM
- Expires
- Friday, December 8, 2023 - 6:49AM
- Author
- Brandon Schwartz
The Bottom Line
Wind drifted snow and new snowfall loaded onto a weak existing snowpack have created dangerous avalanche conditions. The hazard is complicated by the potential to trigger avalanches low on the slope, well below traditional avalanche start zones. There is a high likelihood of serious trauma for anyone caught in an avalanche in the currently rocky, shallow snowpack. If in doubt or faced with uncertainty, choose not to travel in or below avalanche terrain.
Avalanche Danger
Thursday, December 7, 2023
Friday, December 8, 2023
- Above Treeline3 - Considerable
- Near Treeline3 - Considerable
- Below Treeline3 - Considerable
 1 - Low |  2 - Moderate |  3 - Considerable |  4 - High |  5 - Extreme | |
|---|---|---|---|---|---|
| Travel Advice | Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features. | 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. | Very dangerous avalanche conditions. Travel in avalanche terrain not recommended. | Extraordinarily dangerous avalanche conditions. Avoid all avalanche terrain. |
| Likelihood of Avalanches | Natural and human-triggered avalanches unlikely. | Natural avalanches unlikely; human-triggered avalanches possible. | Natural avalanches possible; human-triggered avalanches likely. | Natural avalanches likely; human-triggered avalanches very likely. | Natural and human-triggered avalanches certain. |
| Avalanche Size and Distribution | Small avalanches in isolated areas or extreme terrain. | Small avalanches in specific areas; or large avalanches in isolated areas. | Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas. | Large avalanches in many areas; or very large avalanches in specific areas. | Very large avalanches in many areas. |
Avalanche Problems (2)
Problem #1: Wind Slab
Problem Type
Aspect/Elevation
Likelihood
Size
An example of wind slab clues such as cornice, wind drifts, and snow surface cracking. 12/03/2023 near Castle Peak, CA
Photo: SAC
Gale force SW winds have drifted and deposited new snow into cohesive wind slabs. This avalanche problem exists mainly above treeline and near treeline across NW-N-NE-E-SE aspects. Unstable wind slabs may also have formed below treeline in some areas due to the strength of the recent winds.
Look for clues indicating the presence of wind slabs such as visible drifting snow, cornice formations, wind drifts, wind textured snow surfaces, and scouring vs deposition patterns on the ground. Consider avoiding areas where wind drifting snow has deposited, especially if weak faceted (sugary) snow exists in the snowpack below.
Problem #2: Persistent Slab
Problem Type
Aspect/Elevation
Likelihood
Size
New snow has loaded on top of a weak, faceted snowpack in some portions of the forecast area on NW-N-NE aspects. The structure of the snowpack varies widely by location and by elevation. Regardless, numerous problematic weak layers exist. None of them are expected to handle new snow loading very well or to stabilize quickly post storm.
While wind slab is the most widespread avalanche problem today, persistent slab is the more difficult problem to manage. Signs of instability such as recent avalanches, snow surface cracking, snowpack collapse, or whumpfing sounds are indications to avoid NW-N-NE aspect avalanche terrain in the area. Keep in mind that previous NE wind events may have scoured weak snow out of the upper portion of a slope, laying a trap of weak snow on the lower half of the slope that is just waiting for a trigger. Anticipate wider avalanches that are triggered lower on the slope than what commonly occurs for wind slabs.
Forecast Discussion
More on the persistent slab problem: In most areas, the weak faceted (sugary) snowpack exists above about 8,000', but could be encountered as low as 7,200' in isolated locations. Look for an easy to miss thin layer of faceted snow on top of the uppermost rain crust in the snowpack (photo below). As elevation increases expect to find layers of faceted snow in between rain crusts. Above about 8,500' or so, rain crusts disappear and the snowpack becomes nearly all weak faceted snow. Generally speaking, the higher up you go on wind protected NW-N-NE aspects, the weaker the snowpack.
There is still the potential to hit many obstacles (rocks, logs, open water, bushes, etc.) that are barely covered (or not) by the shallow snowpack. Areas with a supportable rain crust in the 7500’ to 8500’ elevation range provide the most usable snowpack. Anticipate encountering a different snowpack structure and a different set of travel conditions if you are moving from one place to another around the forecast area.
Media
Mountain Weather
- Issued
- Thursday, December 7, 2023 - 6:46AM
- Author
- Brandon Schwartz
The first of two weather systems that brought snowfall to the forecast area yesterday afternoon and evening, departed the region overnight. A lull in precipitation is expected this morning. The second weather system is forecast to bring additional snowfall to the area starting around midday and lasting through the afternoon hours. Clearing conditions are expected tonight as the second weather system exits the region. Ridgetop winds were strong to gale force over the past 24 hours. Strong SW winds are forecast to continue today. A wind shift from SW to NE is expected to occur tomorrow.
| Central Sierra Nevada | Conditions in the last 24hrs | Weather Forecast for Today Produced in partnership with the Reno NWS | Weather Forecast for Tonight Produced in partnership with the Reno NWS | Weather Forecast for Tomorrow Produced in partnership with the Reno NWS |
|---|---|---|---|---|
| Weather | Increasing cloud cover with snowfall starting in the early afternoon. | Partly cloudy then becoming mostly cloudy. Chance of snow in the afternoon. Snow levels Below 5500 feet. Chance of precipitation is 50%. | Mostly cloudy then becoming partly cloudy. Chance of snow in the evening. Snow levels below 5500 feet. Chance of precipitation is 55%. | Sunny. Snow levels Below 5500 feet. Chance of precipitation is 5%. |
| Temperatures 7000 to 8000 ft. | @ 5am: 23 to 26 ° F | Max: 40 to 45 ° F | 27 to 32. ° F | 11 to 17 ° F | 27 to 33. ° F |
| Temperatures 8000 to 9000 ft. | @ 5am: 19 to 23 ° F | Max: 36 to 40 ° F | 23 to 29. ° F | 9 to 14 ° F | 23 to 29. ° F |
| Mid Slope Winds | SW average 10 mph, max gust 49 mph. | Southwest 15 to 30 mph with gusts to 50 mph decreasing to 15 to 20 mph with gusts to 35 mph in the afternoon. | West around 15 mph in the evening becoming light. Gusts up to 30 mph. | Light winds. |
| Ridgetop Winds | SW average 36 to 55 mph, max gust 70 to 112 mph. | Southwest 30 to 50 mph with gusts to 85 mph. | West 30 to 45 mph with gusts to 80 mph decreasing to 15 to 30 mph after midnight. | West 15 to 25 mph shifting to the northeast in the afternoon. |
| Snowfall 7000 to 8000 ft. | 6 to 12 inches | SWE = 0.6 to 1.0 inch | 70% probability up to 4 inches. 30% probability of 3 to 6 inches. | SWE = Up to 0.30 inch. | 50% probability up to 2 inches. 50% probability no accumulation. | SWE = Less than 0.10 inch. | No accumulation. | SWE = None. |
| Snowfall 8000 to 9000 ft. | 6 to 12 inches | SWE = 0.6 to 1.0 inch | 60% probability of 3 to 6 inches. 40% probability of 6 to 10 inches. | SWE = Up to 0.50 inch. | 70% probability up to 2 inches. 30% probability no accumulation. | SWE = Less than 0.10 inch | No accumulation. | SWE = None. |
| Total Snow Depth between 8200 and 8700 ft. | 9 to 21 inches |
This information is provided by the U.S.D.A. Forest Service and describes general backcountry avalanche hazard and conditions.
It does not apply to ski areas and highways where avalanche mitigation is conducted.