Avalanche Advisory: Monday - Dec 28, 2020

THIS AVALANCHE ADVISORY EXPIRED ON December 29, 2020 @ 7:00 am
Avalanche Advisory published on December 28, 2020 @ 7:00 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

MODERATE avalanche danger exists above treeline in exposed areas where a WIND SLAB avalanche could be triggered. The modest amount of new snowfall and unusually light winds should keep this concern limited to just below ridgelines.  Higher amounts of lake-effect snowfall around the June area raise concern for LOOSE DRY avalanches. The ever-present weak snowpack keeps the unlikely but potentially consequential PERSISTENT SLAB avalanche problem a tricky concern. And again, don’t forget about all the obstacles lurking just under the shallow snow surfaces.     

2. Moderate

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

1. Low

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Near Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

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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Sensors are mostly showing 2-4” of new snow fall from yesterday thru last night, except for Gem Pass above June near 11,000’ which is showing 10” since noon yesterday.  This snowfall was accompanied by remarkably light winds except over higher elevation ridgetops where speeds reached into the 30mph range.  Similar additional snowfall amounts are expected today, with similar winds except shifted out of the N & NE instead of SW.  New sensitive wind slabs should be isolated to exposed areas above treeline just below ridgelines and sidewalls of gullies.  Old snow surfaces in these areas were mostly wind-stripped and firm, so new deposits may not bond well initially.  Use cornice formation and uneven surface conditions to help identify where denser sensitive wind deposited snow may be found.  Even a small slide could be consequential with all the obstacles that exist in our shallow snowpack right now.     

Avalanche Problem 2: Loose Dry
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In areas near June that may receive substantially greater snow amounts due to convection and the moisture source of Mono Lake, Loose Dry sloughs could be possible in steep terrain. While it may be difficult to access this terrain given the shallow snowpack, if you are on a powder-hungry mission be aware of the consequences of being swept off your feet especially with all the obstacles that exist.  A slough that you may normally be able to out-run could lead you careening speedily into a barely covered log, stump or rock.     

Avalanche Problem 3: Persistent Slab
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The one thing consistently found across our forecast region is a weak snowpack on shady aspects (NW-NE-E) where early season November snow sits sugary at the base.  Whoomphing and shooting cracks continue to be reported, and propagating test results are widespread.  Where the right combination exists of overlying slab on a steep enough slope, lack of underlying anchors, and a sweet trigger spot, a human triggered avalanche could be possible that could break above you.  As time passes without a substantial new snow load, triggering this type of avalanche is becoming less and less likely, but not impossible. Triggering a smaller wind slab could be the extra force needed to step down and result in a larger than expected persistent slab avalanche. Continue to be wary of steep shady terrain, and the low-likelihood but potentially high consequence of a persistent slab avalanche.  

advisory discussion

The mountains south of Mammoth still are not getting thrown a bone … and continue to remain bony.  A shallow but navigable snowpack exists Mammoth and north, but obstacles lurk, use caution!

recent observations
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-12/27 – June Area – White Wing: Wind effects and snowpack.

-12/27 – Mammoth Lakes Basin: Snowfall picking up!

-12/26 – VA Lakes: Small Windslab Avalanche triggered, weak snowpack, propagating test results.

-12/26 – Mammoth – Sherwins: Wind deposits including at lower elevations, whoomphing and shooting cracks, weak shallow snowack with propagating test results.

-12/26 – Mammoth Lakes Basin – Red Cone: ECTP

-12/26 – Mammoth Lakes Basin – Lake Mary Ridge: Whoomphing, propagating test results.

-12/26 – Mammoth Lakes Basin – Red Cone: Strong wind effects, ECTPs at multiple depths.   

Weather and CURRENT CONDITIONS
weather summary

A slow-moving low-pressure system is moving across Central California today bringing additional light snow, light winds and continued cold temperatures.  1-4” of new snow is expected, with potential for higher amounts near the Sierra Crest above Mono Lake. Temperatures should again reach the mid-teens around 10,000’, with mostly light N to NE winds gusting into the 30s over ridgetops.  North winds will be on the increase tonight into tomorrow.

Another fast-moving system should move through our area Wednesday night into Thursday with unimpressive snow totals, but models are currently showing a change in pattern for the first week of the New Year, with the Jet Stream shifting south toward CA and the potential for more significant snowfall.  

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Monday Monday Night Tuesday
Weather: Cloudy. Snow likely. Snow levels below 7000 feet. Chance of precipitation is 90%. Mostly cloudy with a slight chance of snow in the evening. Snow levels below 7000 feet. Chance of precipitation is 15%. Partly cloudy then becoming sunny. Snow levels below 7000 feet. Chance of precipitation is 0%.
Temperatures: 21 to 27. deg. F. 6 to 11. deg. F. 24 to 30. deg. F.
Mid Slope Winds: North around 15 mph with gusts to 30 mph. North around 15 mph. Gusts up to 30 mph increasing to 45 mph after midnight. North around 15 mph. Gusts up to 45 mph.
Expected snowfall: 80% probability of 1 to 4 inches. 20% probability of 3 to 6 inches. | SWE = up to 0.20 inch. in. Up to 1 inch. | SWE = less than 0.10 inch. in. No accumulation. | SWE = none. in.
Over 10,000 ft.
Monday Monday Night Tuesday
Weather: Cloudy. Snow in the morning, then chance of snow in the afternoon. Snow levels below 7000 feet. Chance of precipitation is 100%. Mostly cloudy. A slight chance of snow in the evening. Snow levels below 7000 feet. Chance of precipitation is 15%. Partly cloudy then becoming sunny. Snow levels below 7000 feet. Chance of precipitation is 0%.
Temperatures: 14 to 20. deg. F. 2 to 7. deg. F. 16 to 22. deg. F.
Ridge Top Winds: Northeast around 15 mph. Gusts up to 35 mph decreasing to 25 mph in the afternoon. North 15 to 30 mph. Gusts up to 25 mph increasing to 55 mph after midnight. North 30 to 45 mph decreasing to 20 to 35 mph in the afternoon. Gusts up to 70 mph.
Expected snowfall: 70% probability of 1 to 4 inches. 30% probability of 3 to 6 inches. | SWE = up to 0.20 inch. in. Up to 1 inch. | SWE = less than 0.10 inch. in. No accumulation. | SWE = none. 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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