Avalanche Advisory: Thursday - Dec 31, 2020

THIS AVALANCHE ADVISORY EXPIRED ON January 1, 2021 @ 6:53 am
Avalanche Advisory published on December 31, 2020 @ 6:53 am
Issued by Chris Engelhardt - Eastern Sierra Avalanche Center

MODERATE Avalanche danger exists today for fresh WIND SLAB above and near tree line on ALL ASPECTS. Look for cross- loaded ribbons of snow, fresh pillows adjacent to ridgeline, and convexities and other terrain features conducive to capturing wind transported snow.  Remain cognizant of PERSISTENT SLAB conditions at all elevations on Northerly-Easterly terrain.  Evaluate your exposure to terrain traps or lurking obstacles where even a small avalanche could be consequential.

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 possible to trigger on ALL ASPECTS today, primarily in the Upper elevations and Near Tree Line. Winds transitioned around mid-day yesterday to a SW flow and blew through the night only to switch again this morning back to a Northerly pattern. Pay particular attention in the Northern part of the forecast zone (June Lk Area- VA Lakes) where upwards of a foot of snow fell on Dec 28th. 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 gullies. Stay focused on your exposure to down slope hazards, terrain traps, and cliffs where even a small slide could prove to be hazardous. 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 at all elevations. 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 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. Steeper slopes above 30degrees and above exposure or terrain traps are of most concern. Persistent slabs may be remotely triggered from a distance and propagate over wide spaces of terrain. 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.

advisory discussion

There has been a lot of discussion about our unusually weak, thin snowpack and persistent slab problems. Although this issue has proved to be unreactive the past week and there has not been a documented persistent slab avalanche in our forecast zone since Dec 22nd, it doesn’t mean the upside down, weak structure of our thin snowpack has gone away. It’s just sitting there, on sunnier aspects potentially strengthening a bit, and in the shady dark confines continuing to show potential for slab avalanches to propagate. The main point is to not lose sight of this issue, as potentially in our near future more significant storms with increased water content will be laid upon this degraded wafer like snowpack. Future storms not only bring everybody’s wish for better riding conditions, but may also be the force to tip the scale… in that likely re-awakening of the avalanche dragons residing in our shallow and scabby Nov/Dec snowpack.

 

Weather and CURRENT CONDITIONS
weather summary

There was snow falling at Mammoth Mountain Ski Area this morning with minimal accumulations as of writing at 6am. Today should be another cloudy day albeit cooler than yesterday. Temperatures will be in the 20sF for the lower mountain elevations and remain cold up high in the teens to low 20sF above 10000ft. We have had a back and forth pattern of Northerly-Southerly winds the past few days, and after W-SW winds picked up yesterday afternoon through last night they have transitioned back to a Northerly flow with speeds of 15-30mph and gusts of 50mph at ridge top on tap for today. After a cold night with single digits for New Year’s Eve, sunnier and warmer conditions return for New Years day.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Thursday Thursday Night Friday
Weather: Mostly cloudy then becoming partly cloudy. Partly cloudy then becoming clear. Sunny then becoming partly cloudy.
Temperatures: 22 to 28. deg. F. 9 to 14. deg. F. 33 to 39 deg. F.
Mid Slope Winds: Northwest 15 to 20 mph with gusts to 40 mph North around 15 mph in the evening becoming light. Gusts up to 35 mph. Light winds. Gusts up to 25 mph in the morning.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
Thursday Thursday Night Friday
Weather: Mostly cloudy then becoming partly cloudy. Partly cloudy then becoming clear. Sunny then becoming partly cloudy.
Temperatures: 15 to 21 deg. F. 6 to 11 deg. F. 27 to 33 deg. F.
Ridge Top Winds: North 15 to 30 mph with gusts to 50 mph Northeast 15 to 30 mph with gusts to 50 mph North 15 to 20 mph. Gusts up to 35 mph in the morning.
Expected snowfall: 0 in. 0 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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