Avalanche Advisory: Tuesday - Jan 21, 2020

THIS AVALANCHE ADVISORY EXPIRED ON January 22, 2020 @ 6:35 am
Avalanche Advisory published on January 21, 2020 @ 6:35 am
Issued by Chris Engelhardt - Eastern Sierra Avalanche Center

Moderate avalanche danger exists at Upper and Middle Elevations on Northerly-Easterly aspects. Southerly winds and a bit of fresh snow today may develop new Wind slab.  Be vigilant in evaluating complex terrain for residing isolated wind slabs in leeward catchment zones.

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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    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

Snow transport and shallow, fresh wind deposits were observed in the forecast area yesterday and this trend will continue today.  Northerly-Easterly aspects at Upper to Middle elevations will be of most concern. Cross loaded terrain at Tree Line has been found to harbor the deepest wind deposits from last Thursday’s storm and in isolated areas there is the possibility of stubborn firmer wind slabs resting on older weak snow surfaces.  Although there have not been any reported natural or human-triggered avalanches in the area, there have been numerous concerning results found with stability tests in regards to crust/wind board-facet combos beneath the new snow. Although unlikely, a triggered wind slab adding weight to underlying weakness could cause an avalanche to “step down” and produce a larger more destructive avalanche. Identify features of concern, such as fat pillows adjacent to ridgeline, rock outcroppings or cliff bands and avoid riding over terrain traps.

advisory discussion

There have been no recorded natural or human triggered avalanches the past 4 days post-storm which deposited 6-12” across the range. In the big picture the new snow has bonded and settled rapidly with recent warm temperatures and relative lack of wind until Monday when our customary SW flow kicked in. The facts that have given the forecaster group pause are the poor structure in the old snow (crust/facet combos) and numerous easily propagating results with Extended Column tests found in the central part of the zone. The uncertainty and lack of confidence with these underlying weaknesses possibly being reactive to skier triggers are the factors contributing to keeping Danger Rating levels in the Moderate category.  It is difficult to list  “Persistent Slab” as an “Avalanche Problem” since there has been no avalanches associated with this condition, but it is something we all should track and be aware of to see how things develop in the future. Although unlikely, the small possibility of larger avalanches in isolated terrain keeps us a bit wary. Additionally, since the storm, there have been limited observations from the highest elevations in regards to the extreme spatial variability in snowpack depth, structure and stability.  As I stated yesterday, the winter season started out well in regards to consistent snow fall and right side up snowpack structure, but those initial conditions have degraded with a lengthy lull in precipitation in January, violent eroding winds and strong temperature gradients that have weakened residing old snow. A variety of crust/facet combos are present within the upper part of the snowpack and weaker loose snow can be found around most rocky areas and on shallow alpine slopes. Most of these shallow exposed alpine areas are composed of loose weaker snow encapsulated by firm wind board and slab from the extreme January winds.

Weather and CURRENT CONDITIONS
weather summary

SW winds will continue today with moderate to stronger velocities (25-40mph) above 10,000ft. Gusts are predicted to reach 60mph again over the high country. Mostly cloudy conditions will prevail with up to 1” of snow falling today and perhaps another skiff tonight. Dry and a bit warmer temperatures return for Wednesday through the end of the week.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Tuesday Tuesday Night Wednesday
Weather: Mostly cloudy. Chance of snow in the afternoon. Snow levels below 7000 feet. Chance of precipitation is 25%. Mostly cloudy. Chance of snow in the evening. Snow levels below 7000 feet. Chance of precipitation is 25%. Mostly cloudy. Snow levels below 7000 feet. Chance of precipitation is 5%.
Temperatures: 32 to 40. deg. F. 21 to 26. deg. F. 36 to 44. deg. F.
Mid Slope Winds: Southwest 15 to 25 mph with gusts to 50 mph. Southwest 15 to 25 mph. Gusts up to 50 mph decreasing to 40 mph after midnight. Southwest around 15 mph with gusts to 35 mph in the morning becoming light.
Expected snowfall: 80% probability up to 1 inch. 20% probability of 1 to 3 inches. | SWE = less than 0.10 inch. in. 90% probability up to 1 inch. 10% probability of 1 to 3 inches. | SWE = less than 0.10 inch. in. 0 in.
Over 10,000 ft.
Tuesday Tuesday Night Wednesday
Weather: Mostly cloudy. Chance of snow in the afternoon. Snow levels below 7000 feet. Chance of precipitation is 35%. Mostly cloudy. Chance of snow in the evening, then slight chance of snow showers after midnight. Snow levels below 7000 feet. Chance of precipitation is 30%. Mostly cloudy. Snow levels below 7000 feet. Chance of precipitation is 10%.
Temperatures: 24 to 30. deg. F. 16 to 21. deg. F. 29 to 35. deg. F.
Ridge Top Winds: Southwest 25 to 40 mph with gusts to 60 mph. Southwest 30 to 45 mph decreasing to 25 to 35 mph after midnight. Gusts up to 70 mph. Southwest 20 to 30 mph with gusts to 45 mph shifting to the west 15 to 20 mph with gusts to 30 mph in the afternoon.
Expected snowfall: 80% probability up to 1 inch. 20% probability of 1 to 3 inches. | SWE = less than 0.10 inch. in. 90% probability up to 1 inch. 10% probability of 1 to 3 inches. | SWE = less than 0.10 inch. in. 0 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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