THIS AVALANCHE FORECAST EXPIRED ON March 18, 2020 @ 7:00 am
Avalanche Forecast published on March 17, 2020 @ 7:00 am
Issued by Andy Anderson - Tahoe National Forest - Sierra Avalanche Center

Feet of new snow and shifting winds created dangerous avalanche conditions in the backcountry at all elevations. Human-triggered avalanches involving large slabs of drifted snow in wind-exposed areas and slabs of new snow in sheltered areas are likely today. In some places, deep slab avalanches involving all 5+ ft of the new snow failing on weak old snow are possible. Avalanche danger is CONSIDERABLE at all elevations. Be conservative and cautious in your terrain selection.

3. Considerable

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

3. Considerable

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

3. Considerable

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Below Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
    Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
Avalanche Problem 1: Wind Slab
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While winds decreased some yesterday, they increased again overnight. They have also shifted directions from SW to more E and back to SW. These winds have remained strong enough to transport snow in many places and fresh slabs of wind-drifted snow likely exist on top of other wind slabs formed earlier in the storm. The largest of these wind slabs should exist on wind-loaded N-NE-E aspects and cross-loaded NW and SE aspects, but wind slabs could exist on any aspect due to the shifting winds. These wind slabs will remain fragile today. Large human-triggered wind slab avalanches that could bury a person or a car (D2-D3) remain likely. 

Identify cornices and pillows of wind drifted snow. Wind slabs are on the slopes below these features. Route find around these areas and do not linger below them. Avoidance is a better tactic today than trying to determine if wind slabs are stable or unstable.

 

Avalanche Problem 2: Storm Slab
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Changes during the storm like fluctuating temperatures and snowfall rates have created weaker layers under more slab-like layers within the storm snow. These weaknesses will likely remain sensitive to the additional weight of a person on top of the snowpack today. Human-triggered storm slab avalanches are still likely on steep slopes in sheltered areas. These avalanches could easily involve enough snow to bury a person. 

Use snow surface cracking triggered while breaking trail as a clue to the presence of this avalanche problem. Specifically choose terrain and routes that give you options for avoiding avalanche terrain if storm slab instability is present.

Avalanche Problem 3: Deep Slab
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Prior to this storm weak sugary snow existed just below the snow surface on many near and below treeline NW-N-NE aspects. The storm has deposited 3 to 5+ feet of new snow on top of this old weak snow. The old weak snow may not be able to support this sudden large load of new snow resulting in the possibility of large deep slab avalanches. While they may be more difficult to trigger than the avalanche problems mentioned above,  deep slab avalanches would involve all of the accumulated new snow, plus the additional depth of wind drifted snow if occurring near treeline. Very large avalanches may occur in below treeline areas on slopes without a history frequent avalanches.

Becoming caught in a deep slab avalanche near or below treeline may not be survivable. If you hear a whumph or experience snowpack collapse, unstable snow associated with this problem is present in the immediate area and an avalanche would have just occurred if the slope were steeper. Practical ways to avoid this avalanche problem are aspect avoidance or conservative terrain selection.

recent observations
Submit Observations

* Several avalanches were reported yesterday. One skier-triggered avalanche on a NE aspect at the top of Jakes Peak buried the top four switchbacks of the skin track in 2-3 ft of debris. Two intentionally triggered avalanches with 18 to 24 in crowns occurred on E and NE-ENE aspects on Andesite Peak. Snowmobilers reported a groomer-triggered avalanche with a foot deep crown on a road cut off of the Blue Lakes Rd. No one was injured or buried in any of these avalanches. Likely these represent a small sample of what occurred since many avalanches are not reported. 

* A skier reported shooting cracks on a wind-loaded test slope on Tamarack Peak that extended 10 ft away from the tips and tails of his skis. Other skiers on Rubicon Peak triggered smaller shooting cracks that measured up to six ft long when they stepped out of the skin track in protected low angle terrain. 

* Snowmobilers found old weak snow and crusts buried below several feet of new snow. This layer was identified with a formal snowpit and then seen again in several different areas while digging out stuck snowmobiles. Skiers on Rubicon Peak also reported about 4 ft of new snow on top of the old crusts. One party on Rubicon got a report of a whumpf from another group. 

* Observers reported continued snowfall yesterday with rates around 2 inches an hr at times on Donner Summit, on the West Shore, and in the Blue Lakes area.

Weather and CURRENT CONDITIONS
weather summary

More snow accumulated in the last 24 hours with some areas reporting up to 15 inches of additional accumulation since yesterday. So far 3 to more than 5 ft of new snow has fallen since Saturday. Things should start to calm down today. Still, the forecast calls for continued snow showers with light accumulations and light to moderate SW winds. Expect this showery weather to continue through Thursday.

CONDITIONS ALONG THE SIERRA CREST BETWEEN 8200 FT. AND 9200 FT. IN THE LAST 24 HOURS
Weather Station Map
Weather Station Table
6am temperature: 9 to 18 deg. F.
Max. temperature: 23 to 31 deg. F.
Average ridgetop wind direction: SW with a few hours of E winds yesterday afternoon and evening
Average ridgetop wind speed: 10 to 20 mph
Maximum ridgetop wind gust: 65 mph
New snowfall: 6 to 15 inches
Total snow depth: 67 to 100 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Tuesday Tuesday Night Wednesday
Weather: Mostly cloudy. Chance of snow. Snow levels below 7000 feet. Chance of precipitation is 35%. Mostly cloudy. Slight chance of snow through the night. Snow levels below 7000 feet. Chance of precipitation is 20%. Mostly cloudy. Chance of snow. Snow levels below 7000 feet. Chance of precipitation is 40%.
Temperatures: 25 to 30 deg. F. 13 to 18 deg. F. 31 to 37 deg. F.
Mid Slope Winds: Southwest around 15 mph with gusts to 30 mph. Light winds. Light winds.
Expected snowfall: 80% probability up to 2 inches. 20% probability of 2 to 4 inches. | SWE = less than 0.10 inch. 70% probability up to 1 inch. 30% probability of 1 to 2 inches. | SWE = less than 0.10 inch. 60% probability up to 1 inch. 40% probability of 1 to 3 inches. | SWE = less than 0.10 inch.
For 8000 ft. to 9000 ft.
Tuesday Tuesday Night Wednesday
Weather: Mostly cloudy. Chance of snow. Snow levels below 7000 feet. Chance of precipitation is 30%. Mostly cloudy. Slight chance of snow through the night. Snow levels below 7000 feet. Chance of precipitation is 20%. Mostly cloudy. Chance of snow. Snow levels below 7000 feet. Chance of precipitation is 40%.
Temperatures: 20 to 26 deg. F. 10 to 16 deg. F. 26 to 32 deg. F.
Ridge Top Winds: Southwest 15 to 25 mph. Gusts up to 45 mph decreasing to 30 mph in the afternoon. Southwest around 15 mph with gusts to 30 mph. Southwest around 15 mph with gusts to 25 mph in the morning becoming light.
Expected snowfall: 60% probability of 2 to 4 inches. 40% probability up to 1 inch. | SWE = less than 0.10 inch. 70% probability up to 1 inch. 30% probability of 1 to 3 inches. | SWE = less than 0.10 inch. 60% probability up to 1 inch. 40% probability of 2 to 4 inches. | SWE = less than 0.10 inch.
Disclaimer

This avalanche forecast is provided through a partnership between the Tahoe National Forest and the Sierra Avalanche Center. This forecast 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 forecast applies only to backcountry areas outside established ski area boundaries. This forecast describes general avalanche conditions and local variations always occur. This forecast expires 24 hours after the posted time unless otherwise noted. The information in this forecast is provided by the USDA Forest Service who is solely responsible for its content.

For a recorded version of the avalanche forecast call (530) 587-3558 x258