Avalanche Advisory: Tuesday - Dec 15, 2020

THIS AVALANCHE ADVISORY EXPIRED ON December 17, 2020 @ 7:00 am
Avalanche Advisory published on December 15, 2020 @ 7:00 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

Snowpack Summary:  

A catch-22 situation exists right now in the Eastern Sierra.  The snowpack is very thin and is dangerous to descend in most areas due to barely covered obstacles, and where it actually is deep enough to ride, a scarry weak snowpack is likely to exist. Consequential human triggered Persistent Slab avalanches are possible on steep slopes where freshly deposited snow from this past week’s storms sits on top of old November snow that is faceted and sugary. Be on the lookout for fresh sensitive wind slabs created by today’s winds as well, especially at higher elevations.   

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Avalanche Problem 1: Persistent Slab
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We have one of the most concerning snowpack structures than we have seen in a long time right now.  A remotely triggered avalanche was reported yesterday in the Mammoth region with a crown over 100’ wide and over a meter high.  Snow from the November storms have been sitting and becoming more and more faceted and sugary. A significant slab of snow from this past week’s storms now sits on top of this weak snow.  While the snowpack is very variable across slope due to wind deposition, a human’s force in the right spot could trigger a dangerous avalanche that would likely release from above. Shooting cracks and whoomphing are signs of instable snow and should act as reminders to avoid terrain >30°, but you may not get one of these signs until it is too late.  Large talus, rocks and logs may act as anchors on slopes and keep them from sliding, but do you know what the ground under a slope looks like?  Play it safe and avoid steep slopes that could potentially have this structure.  Non-southerly facing slopes are most suspect for this problem.

Avalanche Problem 2: Wind Slab
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Moderate winds gusting into the 40s and higher at mid to upper elevations today out of the E to NE to NW (depending on elevation and terrain) will likely transport snow and lead to fresh wind slab development.  Be on the lookout for fresh denser wind deposits on the leeward side of ridges and mid-slope ribs. Even a small release could result in a nasty slide with the shallow snowpack that exists with many obstacles.  A small release could step down and cause a bigger avalanche to fail in underlying weak snow.   

advisory discussion

The snowpack depth throughout the forecast region from Bishop Creek to the south up to Virginia Lakes in the north is very variable.  South of Mammoth it is EXTREMELY thin and for the most part unrideable.  Mammoth north it is quite thin, but can be riden in certain areas with caution.     

*Daily avalanche advisories with Danger Ratings will begin hopefully after this next expected storm on Wednesday night.  

recent observations

-Report of remotely triggered avalanche on a northerly facing slope in the Mammoth region ~10,000'.  Avalanche was triggered as a skier approached a wind loaded feature mid-slope, and released ~100ft above.  Crown was over 100ft wide and up to over a meter tall.  Fortunately the person was not caught.  Wind loaded snow from this past week's storms failed in weak sugary faceted snow from November.  

-12/14: Mammoth Lakes Basin - Red Cone area: Shooting cracks, slope settlement, propogating ECT tests.

-12/14: Virginia Lakes: Thin variable snowpack with areas of concerning structure where new snow deposits exist on old November snow.

Weather and CURRENT CONDITIONS
weather summary

Conditions will remain dry until Wednesday night when a low-pressure system moves in bringing more snow into Thursday morning.  Expect seasonal temperatures with highs into the mid 30s around 10,000’ until then, with sunshine on Tuesday morning and then increasing clouds.  At higher elevations, winds should be moderate gusting into the 40s and 50s over ridgetops out of the NW shifting to the W.  

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: Sunny then becoming partly cloudy. Snow levels below 7000 feet increasing to 7000 feet in the afternoon. Chance of precipitation is 0%. Partly cloudy. Snow levels 7000 feet. Chance of precipitation is 0%. Partly cloudy. Snow levels below 7000 feet increasing to 7000 feet in the afternoon. Chance of precipitation is 0%.
Temperatures: 35 to 41. deg. F. 19 to 25. deg. F. 37 to 45. deg. F.
Mid Slope Winds: East to northeast 10 to 20 mph. Gusts up to 40 mph decreasing to 30 mph in the afternoon. Light winds. Light winds becoming southwest around 15 mph in the afternoon. Gusts up to 35 mph.
Expected snowfall: No accumulation. | SWE = none. in. No accumulation. | SWE = none. in. No accumulation. | SWE = none. in.
Over 10,000 ft.
Tuesday Tuesday Night Wednesday
Weather: Sunny then becoming partly cloudy. Snow levels below 7000 feet increasing to 7500 feet in the afternoon. Chance of precipitation is 0%. Partly cloudy. Snow levels 7500 feet decreasing to below 7000 feet after midnight. Chance of precipitation is 0%. Partly cloudy. Snow levels below 7000 feet increasing to 7000 feet in the afternoon. Chance of precipitation is 5%.
Temperatures: 27 to 33. deg. F. 16 to 21. deg. F. 29 to 35. deg. F.
Ridge Top Winds: Northwest to north 15 to 30 mph. Gusts up to 55 mph decreasing to 40 mph in the afternoon. Northwest 15 to 25 mph with gusts to 40 mph. West 15 to 25 mph with gusts to 45 mph.
Expected snowfall: No accumulation. | SWE = none. in. No accumulation. | SWE = none. 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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