Backcountry Avalanche Forecast Central Sierra Nevada
Sierra Avalanche Center
www.sierraavalanchecenter.org
- Issued
- Sunday, December 3, 2023 - 6:52AM
- Expires
- Monday, December 4, 2023 - 6:52AM
- Author
- Brandon Schwartz
The Bottom Line
Wind drifted snow has formed into cohesive slabs near ridgelines and thin slabs of recent new snow exist on top of older, weaker sugary snow. This has created avalanche concerns for all snow covered areas. Move carefully around slopes that are below cornices or wind drifts and any areas where snow surface cracking, snowpack collapse, or whumpfing sounds are occurring.
Avalanche Danger
Sunday, December 3, 2023
Monday, December 4, 2023
- Above Treeline2 - Moderate
- Near Treeline2 - Moderate
- Below Treeline2 - Moderate
 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
The first wind slab avalanche of the season was intentionally triggered on Dec 1 on the east face of Mt. Judah near Donner Summit (more info). Additional snowfall and SW wind over the past 24 hours has kept this avalanche problem ongoing with additional drifting and snow surface cracking observed yesterday.
In some areas, wind slabs may exist on top of a very weak, sugary (faceted) snowpack. Triggering lower on the slope than is common for a wind slab avalanche could easily occur anywhere a slab exists on top of weak faceted snow and the avalanche could fail wider than expected. In some areas, an easily missed, thin layer of faceted snow exists sandwiched between the recent new snow and a firm rain crust below it.
Use clues such as visible drifting snow, cornice formations, wind pillows, or other signs of snow distribution on the ground to identify areas that hold wind slabs. Exercise increased caution around areas where wind drifted snow has been deposited.
Problem #2: Persistent Slab
Problem Type
Aspect/Elevation
Likelihood
Size
Thin slabs, 4 to 8 inches thick composed of recent new snow have formed on top of a very weak snowpack in some areas, mainly on NW-N-NE aspects. In more isolated areas, weak faceted snow also exists on E and W aspects. There is a high amount of uncertainty here. Relatively small, incremental loads were added to the snowpack over the last few days. There is a high degree of variability in recent new snow amounts and the structure of the existing snowpack around the forecast area, making it difficult to highlight the locations where a persistent slab avalanche is most likely to occur.
Any snowpack collapses, whumpfing sounds, or shooting cracks encountered are signs of an unstable snowpack. If you are encountering any of these signs of instability, avoid NW-N-NE aspect avalanche terrain in the area.
Forecast Discussion
Throughout the forecast region, 1 to 2 feet of snow can be found on NW-N-NE aspects above 8000'. Continuous snow cover starts around 7,200' to 8,000' depending on location. There is a lot of variability in the structure of the existing snowpack depending on elevation and location. Looking beneath the recent new snow, the snowpack on NW-N-NE aspects holds concerning weak layers in some areas. Some locations are thick rain crust either with or without an easily missed thin layer of facets and sometimes surface hoar on top of the crust. Some locations are a mix of layers of rain crusts and faceted snow. While other areas are all faceted snow and depth hoar without any crusts. There is generally little to no rain crust within the snowpack above 8,800' to 9,000'.
Previous NE wind events scoured the upper portion of many wind exposed slopes down to rock, dirt, and/or ice before the recent snowfall. Despite this scouring, weaker faceted snow persisted on the lower portions of many slopes. A much thinner snowpack or bare ground exists across E-SE-S-SW-W aspects.
While the snowpack has become more usable for over the snow travel, there is still the potential to hit many obstacles (rocks, logs, etc.) that are barely covered. Areas with a supportable rain crust below the recent new snow are the most usable. If you are traveling on snow for recreation, you should come up with a daily travel plan based on the current conditions and associated hazards. 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
- Sunday, December 3, 2023 - 6:52AM
- Author
- Brandon Schwartz
High pressure will start to build over the region today. Rain and snow showers are expected to decrease as the day progresses. Ridgetop winds have started to decrease in speed this morning, but are still strong out of the SW. Clear skies and warming air temperatures are forecast for Monday and Tuesday. The next weather system to bring precipitation to the forecast area is expected on Wednesday.
| 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 | Cloudy skies with rain and snow. Snow level rose from 6000 feet to 7700 feet. | Cloudy. Slight chance of rain and snow through the day. Snow levels 7500 feet. Chance of precipitation is 25%. | Mostly cloudy. Slight chance of snow. Snow levels 7000 feet. Chance of precipitation is 20%. | Partly cloudy then becoming sunny. Snow levels 7000 feet increasing to 8000 feet in the afternoon. Chance of precipitation is 5%. |
| Temperatures 7000 to 8000 ft. | @ 5am: 30 to 33 ° F | Max: 31 to 35 ° F | 37 to 43. ° F | 25 to 31. ° F | 43 to 48. ° F |
| Temperatures 8000 to 9000 ft. | @ 5am: 29 to 31 ° F | Max: 29 to 33 ° F | 33 to 39. ° F | 24 to 29. ° F | 38 to 44. ° F |
| Mid Slope Winds | SW average 7 mph, max gust 27 mph. | Southwest around 15 mph. Gusts up to 45 mph decreasing to 30 mph in the afternoon. | Light winds. | Light winds. |
| Ridgetop Winds | SW average 56 mph, max gust 97 mph. | Southwest 20 to 35 mph with gusts to 65 mph. | Southwest 15 to 20 mph with gusts to 35 mph in the evening becoming light. | Light winds. |
| Snowfall 7000 to 8000 ft. | 1 to 4 inches | SWE 0.1 to 0.4 inches | No accumulation. | SWE = Less than 0.10 inch. | No accumulation. | SWE = Trace amounts. | No accumulation. | SWE = None. |
| Snowfall 8000 to 9000 ft. | 1 to 4 inches | SWE 0.1 to 0.4 inches | No accumulation. | SWE = Less than 0.10 inch. | No accumulation. | SWE = Trace amounts. | No accumulation. | SWE = None. |
| Total Snow Depth between 8200 and 8700 ft. | 7 to 13 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.