Avalanche Advisory: Saturday - Dec 26, 2020

THIS AVALANCHE ADVISORY EXPIRED ON December 27, 2020 @ 6:48 am
Avalanche Advisory published on December 26, 2020 @ 6:48 am
Issued by Chris Engelhardt - Eastern Sierra Avalanche Center

MODERATE Avalanche danger exists today for fresh WIND SLAB above and near tree line on W-SE ASPECTS adjacent to ridgeline, convexities and other terrain features conducive to capturing wind transported snow. As of 630am 2" of new snow had fallen at 9000ft. Avalanche conditions could rise if snowfall totals this morning exceed forecasted expectations of another 2" during morning hours. A concerning PERSISENT SLAB problem remains at all elevations on Northerly-Easterly terrain. Evaluate your exposure to terrain traps or lurking obstacles where even a small human triggered avalanche could be consequential. Fresh snow today will be just enough to disguise a plethora of nasty obstacles just under the surface.

2. Moderate

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Above Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

2. Moderate

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Near Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

1. Low

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Below Treeline
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.
Avalanche Problem 1: Wind Slab
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Fresh wind slabs will be found on W-SE slopes today, primarily in the Upper elevations and near tree line. Although there was only 1-2 inches reported at local weather stations at the time (630am) of this writing, transporting SW winds combined with this fresh snowfall will continue to build fresh deposits in leeward aspects today. Human triggered wind slab avalanches will be possible in specific areas such as slopes adjacent to ridgeline, deposition zones under cliff bands, and steeper convex terrain such as rollovers and cross loaded gully features. Stay focused on your exposure to down slope hazards, terrain traps, and cliffs where even a small slide could prove to be hazardous if you were caught. There is a lot of exposed rock and other obstacles that even a small ride will be dangerous. Be on the lookout for fresh pillows of snow and fat looking drifts to gauge the distribution of fresh wind slab

Avalanche Problem 2: Persistent Slab
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Up to this point, a shallow, weak, and concerning snowpack structure exists in a majority of our northerly-easterly facing terrain. Observations throughout the forecast zone continue to show propagating results in faceted layers near the base of our thin snowpack. There was also a natural persistent slab avalanche  in VA lakes Dec 22nd on a northerly aspect near 10600ft. This avalanche occurred the last time we had significant transporting winds and available snow to move. We will have similar conditions today with strong to gale force SW winds slated to blow most of the day Saturday. All elevations currently have these persistent slab conditions, so continuous evaluation is warranted with snow pits, probing, and solid awareness of your position in relation to terrain traps, such as ravines, or gullies where a small slide could potentially bury you. Steeper slopes above 30degrees are of most concern. Persistent slabs may be triggered from distant locations and travel over wide spaces of terrain if slopes have connected panels of snow. Triggering a fresh wind slab today may be just enough to cause a persistent slab to step down and cause a deeper more destructive avalanche. Avoid travelling on or underneath N-E slopes in the alpine today where weak Persistent slabs may be residing and becoming more sensitive to wind loading from today’s fresh snow and wind transport. Shooting cracks and audible collapsing are sure signs of unstable snow.

advisory discussion

Warm temperatures and time has helped much of the thin snowpack strengthen a bit through settlement, but overall, the basic weak upside down structure of a firmer slab above looser slightly faceted November snow at the base remains. Areas of most concern remain northerly-easterly slopes that held onto November snows and are now capped with the firmer slab from the few minimal storms we have had in December. In some places it is weak enough where even a small load, a skier, or a building wind slab from wind deposited snow may trigger a small Persistent slab. A significant storm would be really advantageous to help initiate an avalanche cycle to clean this thin scab of a degraded pack off the slopes, but we have yet to see anything of note this season thus far. Yes, it may only be December, so patience is key, but these short days and cold temps around winter solstice are when you really want some snow to be laid down, before solar angle increases and sunny aspects get warmer again. Overall, the depth of the snowpack throughout the forecast zone remains thin, sporadic to non-existent composed of a stripped landscape with a few scattered snowfields. The deepest zone, which still is far from being considered good coverage, is in the Mammoth area. 

recent observations
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12/25 – Mammoth Area – Lakes Basin – poor structure, propagating results

12/25 – Mammoth Area – Minaret Vista – strengthening snowpack, overall thin conditions

12/24 – VA Lakes – Shallow coverage and weak snowpack structure

12/23 – Mammoth Area- Mammoth Crest – shallow coverage, variable conditions

Weather and CURRENT CONDITIONS
weather summary

Saturday should start out snowy in the mountains with mostly cloudy conditions, falling snow and extreme SW winds at ridge top. SW winds should decrease as the day progresses and are slated to average 25-40mph with 60mph gusts by afternoon. Temperatures will range from 19-27F in the upper elevations to reaching above freezing in the mid 30sF below 10000ft. Snow is scheduled to taper off this morning with a cold night in the teens followed by continued cloudy conditions and another chance of snow for Sunday. Forecasted snow totals have decreased since initial reports before the holiday.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Saturday Saturday Night Sunday
Weather: Mostly cloudy then becoming partly cloudy. Chance of snow in the morning. Snow levels below 7000 feet. Chance of precipitation is 45%. Mostly cloudy. Snow levels below 7000 feet. Chance of precipitation is 0%. Mostly cloudy. Chance of snow. Snow levels below 7000 feet. Chance of precipitation is 35%.
Temperatures: 27 to 35. deg. F. 13 to 19. deg. F. 26 to 32. deg. F.
Mid Slope Winds: Southwest 15 to 25 mph with gusts to 55 mph. West around 15 mph with gusts to 30 mph in the evening becoming light. Light winds becoming south around 15 mph in the afternoon. Gusts up to 25 mph.
Expected snowfall: 80% probability up to 2 inches. 20% probability of 2 to 4 inches. | SWE = less than 0.10 inch. in. 0 in. 0 in.
Over 10,000 ft.
Saturday Saturday Night Sunday
Weather: Mostly cloudy then becoming partly cloudy. Chance of snow in the morning. Snow levels below 7000 feet. Chance of precipitation is 55%. Mostly cloudy. Snow levels below 7000 feet. Chance of precipitation is 0%. Mostly cloudy. Chance of snow. Snow levels below 7000 feet. Chance of precipitation is 45%.
Temperatures: 19 to 27. deg. F. 9 to 14. deg. F. 18 to 24. deg. F.
Ridge Top Winds: Southwest 30 to 50 mph with gusts to 80 mph decreasing to 25 to 40 mph with gusts to 60 mph in the afternoon. South 15 to 20 mph with gusts to 35 mph. South 15 to 25 mph with gusts to 40 mph.
Expected snowfall: 90% probability up to 2 inches. 10% probability of 2 to 4 inches. | SWE = less than 0.10 inch. in. 0 in. Up to 1 inch. | SWE = less than 0.10 inch. in.
Disclaimer
This Avalanche Advisory is designed to generally describe avalanche conditions where local variations always occur. This product only applies to backcountry areas located outside established ski area boundaries. The information in this Avalanche Advisory is provided by the Eastern Sierra Avalanche Center, who is solely responsible for its content.

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