Avalanche Advisory: Monday - Dec 21, 2020

THIS AVALANCHE ADVISORY EXPIRED ON December 22, 2020 @ 6:59 am
Avalanche Advisory published on December 21, 2020 @ 6:59 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

A thin atypical weak snowpack exists in many areas with sugary faceted snow buried at the base.  MODERATE avalanche danger exists at all elevations due to un-anchored pockets of deeper snow where this persistent weak layer could be triggered by a human.  Steep E to N to W facing slopes are the most concerning for this problem.  Small wind slabs are a secondary concern for exposed slopes at mid to upper elevations.  These factors combined with all the obstacles that exist mean that travel on lower angle terrain is recommended.  

2. Moderate

?

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

2. Moderate

?

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

2. Moderate

?

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: Persistent Slab
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

Just because no new snow has fallen since the 17th doesn’t mean that avalanches are not possible. Weak faceted snow “persists” at the bottom of the snowpack in many places.  Pockets exist on steep slopes with enough snow coverage, but without adequate anchors such as rocks and bushes, where a human could trigger a small avalanche that fails above them deeper in the snowpack than expected. This problem is most likely to be found on W to N to E facing slopes at all elevations.  While a resulting avalanche would likely not bury a person unless a terrain trap such as a gully was involved, it could certainly carry someone through nasty rocks and obstacles and over a cliff resulting in serious injury or worse. Our typical strong Sierra snowpack looks much more like an early-season weak snowpack expected to be found in Colorado. The whoomphing sound of a slope collapsing or shooting cracks are signs of an unstable slope. Take your shovel out and explore the snow yourself, and avoid steep consequential slopes where the right combination of buried weak layers and lack of anchors exist.  

Avalanche Problem 2: Wind Slab
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

Light winds over the last two days haven’t built any fresh wind slabs, however small isolated pockets of old wind slab sensitive to a human trigger may still exist. Pockets of small fresh sensitive wind slabs may build this afternoon and evening as winds pick up again out of the SW. Be wary of dense surface snow especially above consequential terrain where a small release could knock you off your feet and lead to a bad fall. Watch for clues such as blowing snow to help determine where new wind slabs may be forming.        

advisory discussion

The one thing that is consistent throughout the forecast region is a thin snowpack. What varies is just how thin. The Mammoth area has the deepest snowpack with the best chances of linking a few turns together without hitting a rock or a log, with those chances decreasing north toward VA Lakes, and tennis shoes being better travel tools than skis in the southern mountains above Bishop. The catch here is that the places with the most snow are also the places with a better chance of triggering an avalanche. Early season snow in November sat around for nearly a month turning into weaker and weaker faceted snow until it became buried by December’s snowfall.  The shallow snowpack with the plentitude of obstacles will make it hard to get into dangerous terrain, but not impossible.  Best to keep it mellow by keeping your speed down and sticking to lower angle terrain, and keep monitoring the snow pack for when more snow comes.        

 

recent observations
Submit Snowpack Observations
Submit Avalanche Observations

12/20- Mammoth Area - Red Cone: Weak snowpack, propogating test results, small sensitive windslabs.  

12/19- Virginia Lakes - Red Lakes Bowl: Snowpack

12/19- June Area - White Wing: Wind effects and snowpack

12/18- Mammoth Area, Thin coverage and poor structure in the Lakes basin

12/18- Mammoth Area, Human triggered avalanche, Red Cone approach

12/18- Mammoth Area, Poor snowpack structure on Earthquake dome

12/18- June Lake Area, Shallow snow coverage on Chicken Wing

Weather and CURRENT CONDITIONS
weather summary

Expect another sunny day today with above average temperatures reaching into the low 40s F around 10,000’ before a dry cold front moves in tonight resulting in increasing SW winds thru the afternoon with gusts reaching into the 50s. Today should be the last day for strong temperature inversion resulting in much colder valley temperatures before drop overall for the rest of the week.  Chances of snow returns for the weekend after Christmas, though quantities are looking light.   

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: Sunny. Snow levels 7000 feet. Chance of precipitation is 0%. Clear. Snow levels below 7000 feet. Chance of precipitation is 0%. Sunny. Snow levels below 7000 feet. Chance of precipitation is 10%.
Temperatures: 41 to 49. deg. F. 21 to 27. deg. F. 32 to 38. deg. F.
Mid Slope Winds: Light winds becoming southwest around 15 mph with gusts to 55 mph in the afternoon. Southwest 15 to 20 mph with gusts to 60 mph. West 15 to 20 mph. Gusts up to 65 mph decreasing to 45 mph in the afternoon.
Expected snowfall: No accumulation. | SWE = none. in. No accumulation. | SWE = none. in. No accumulation. | SWE = none. in.
Over 10,000 ft.
Monday Monday Night Tuesday
Weather: Sunny. Snow levels 7000 feet. Chance of precipitation is 0%. Clear. Snow levels below 7000 feet. Chance of precipitation is 0%. Sunny. Snow levels below 7000 feet. Chance of precipitation is 10%.
Temperatures: 33 to 41. deg. F. 18 to 23. deg. F. 24 to 30. deg. F.
Ridge Top Winds: Southwest 15 to 25 mph. Gusts up to 55 mph in the afternoon. Southwest 15 to 25 mph with gusts to 50 mph increasing to 30 to 50 mph with gusts to 80 mph after midnight. West 30 to 50 mph with gusts to 90 mph decreasing to 20 to 35 mph with gusts to 65 mph in the afternoon.
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.

ESAC receives significant financial support from ...