Avalanche Advisory: Friday - Dec 25, 2020

THIS AVALANCHE ADVISORY EXPIRED ON December 26, 2020 @ 6:34 am
Avalanche Advisory published on December 25, 2020 @ 6:34 am
Issued by Steve Mace - ESAC

MODERATE avalanche danger exists today at all elevations. Wind slabs will be the primary concern on Northerly and Easterly aspects near and above tree line. Particularly in areas at the mid elevation band with loose surface snow available for transport. In addition, a concerning Persistent slab problem exists on Northerly and Easterly aspects at all elevations. Evaluate snow and terrain carefully if you head into the backcountry today and be wary of the very thin obstacle ridden conditions.

We expect a quick moving storm to impact the area around midnight tonight. Avalanche danger will rise quickly with the arrival of additional precipitation.

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.

2. Moderate

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Below Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
    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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Expect wind speed to increase dramatically today as an incoming low-pressure system approaches the area. Strong to gale force ridge top winds are expected today with gusts approaching 90 mph his afternoon. Upper elevations are not holding a substantial amount of loose snow at the moment. Solar aspects have been cooked over the past week of sunny warm weather and recent strong winds have already stripped much of the alpine. However, strong winds can distribute snow in unexpected ways. Don’t be surprised to find wind deposits at lower elevations and further down slopes than you may typically expect to find them.  Areas that still hold loose surface snow in the mid elevations band may be most problematic today. Use surface clues such as blowing and drifting snow, snow with a hollow drum-like feel, and uneven snow surfaces to help you identify and avoid areas of recent wind deposit. Do your own localized assessments and be suspect of terrain features that encourage drifting such as the leeward sides of ridgelines, gully features, and cross-loaded depressions.  

Avalanche Problem 2: Persistent Slab
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Field observations continue to identify signs of instability in the lower snowpack including slope scale collapsing, recent avalanche activity, and propagating test results. A very poor snowpack structure dominates on northerly and easterly aspects throughout the forecast area. Persistent slab issues test our patience and require continuous evaluation. Don’t be afraid to dig in and get a look at the lower layers of the snowpack. Remember that persistent slab avalanches often propagate across and beyond terrain features that would otherwise confine surface instabilities such as wind or storm slab and in some cases persistent slab avalanches can be remotely triggered from adjacent slopes. Give yourself a wide safety margin, be wary of hazard that may reside above you, and use terrain choice to limit your exposure.

advisory discussion

Evaluating this persistent slab issue has led to many conversations amongst the forecaster staff. The very shallow nature of the snowpack has in many ways been beneficial from an avalanche standpoint.  Sparse coverage broken by exposed areas of bare ground has limited the areas with enough contiguous snow cover to produce a sizable avalanche. In addition, the coverage is so thin in many areas that the current “slab” is broken by small trees, rocks, and bushes that are effectively working as anchors to the snowpack.  However, this fine balance is likely to change with the addition of a significant new load. Recent observations have shown the very poor structure remains and in fact may be degrading as the weak sugary snow persists and the overriding slab has settled and gained strength.  Stability tests in the Mammoth area and in Virginia lakes have confirmed this trend, and recent avalanche activity reported in the Virginia lakes area highlights the potential for the scales to tip with the addition of a new load.

Make no mistake we do not have a typical sierra snowpack at the moment. Instead, we have a snowpack much more similer to something you would expect to find in Colorado or Utah.  As we continue to do our snow dance and hope for some significant snowfall in the near future, it will be important for us all to remain vigilant. A new load of snow may just tip the scales and kick off a cycle of instability.

recent observations
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12/24-Mammoth Area- Poor structure, propagating test results below tree line

12/24- Virginia lakes- Very thin coverage, poor structure, and slope scale collapsing

12/23- Mammoth Area- Poor structure and variable surface conditions near the Mammoth crest

12/23- Mammoth Area- Snow conditions near Duck pass

12/22- Virginia lakes- Natural avalanche activity in Virginia lakes

12/22- Mammoth Area- Sensitive wind slabs and poor structure in the Sherwins

Weather and CURRENT CONDITIONS
weather summary

Cloudy skies and mild temperatures are expected today. The big change will be the increasing winds out of the SW which are likely to ramp up throughout the day approaching 90 mph on ridgetops overnight as a quick moving storm approaches the area. Expect Strong winds even at mid elevations today. Precipitation is not likely to start until this evening with the bulk of the snow arriving after midnight tonight. Models suggest a quick burst of snow totaling 1-6”, the bulk of which is expected to impact the northern part of the forecast area.

This weekend may bring cool temperatures and increasing clouds before another storm system moves into the region Sunday night or Monday morning.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Friday Friday Night Saturday
Weather: Partly cloudy. Snow levels below 7000 feet increasing to 7000 feet in the afternoon. Chance of precipitation is 0%. Partly cloudy. Chance of snow. Snow levels 7000 feet. Chance of precipitation is 25%. Mostly cloudy then becoming partly cloudy. Chance of snow. Snow levels below 7000 feet. Chance of precipitation is 35%.
Temperatures: 33 to 41. deg. F. 22 to 27. deg. F. 27 to 35. deg. F.
Mid Slope Winds: Southwest 15 to 20 mph. Gusts up to 45 mph in the afternoon. Southwest 15 to 30 mph with gusts to 80 mph. Southwest 15 to 35 mph with gusts to 55 mph.
Expected snowfall: No accumulation. | SWE = none. in. 60% probability of 1 to 3 inches. 40% probability of 3 to 6 inches. | SWE = up to 0.25 inch. in. 60% probability up to 2 inches. 40% probability of 2 to 4 inches. | SWE = less than 0.10 inch. in.
Over 10,000 ft.
Friday Friday Night Saturday
Weather: Partly cloudy. Snow levels below 7000 feet increasing to 7500 feet in the afternoon. Chance of precipitation is 5%. Partly cloudy then becoming mostly cloudy. Slight chance of snow in the evening, then snow likely after midnight. Snow levels 7500 feet. Chance of precipitation is 60%. Mostly cloudy. Chance of snow. Snow levels below 7000 feet. Chance of precipitation is 50%.
Temperatures: 26 to 32. deg. F. 17 to 22. deg. F. 19 to 27. deg. F.
Ridge Top Winds: Southwest 15 to 30 mph. Gusts up to 45 mph. No accumulation. Southwest 40 to 60 mph with gusts to 90 mph. Southwest 30 to 50 mph with gusts to 80 mph decreasing to 25 to 40 mph with gusts to 60 mph in the afternoon.
Expected snowfall: | SWE = none. in. 60% probability of 1 to 4 inches. 40% probability of 3 to 6 inches. | SWE = up to 0.25 inch. in. 60% probability up to 2 inches. 40% probability of 2 to 4 inches. | 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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