Avalanche Advisory: Thursday - Dec 17, 2020

THIS AVALANCHE ADVISORY EXPIRED ON December 19, 2020 @ 6:46 am
Avalanche Advisory published on December 17, 2020 @ 6:46 am
Issued by Chris Engelhardt - Eastern Sierra Avalanche Center

SNOWPACK SUMMARY:

A rapid burst of new snow (6”) as of 630am Thursday morning combined with SW-W winds will spike WIND SLAB hazard on NW-SE facing terrain today. Additionally, Thursday’s snowfall will add weight to weak underlying faceted snow and a concerning PERSISENT SLAB problem. Consequential human triggered avalanches are possible on steep N-E facing terrain where freshly deposited snow sits on top of our degraded and thin November snowpack. Evaluate snow and terrain carefully, not only for these avalanche problems, but for thin and obstacle-ridden conditions.

No Rating

?

Above Treeline

No Rating

?

Near Treeline

No Rating

?

Below Treeline
Avalanche Problem 1: Wind Slab
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

Rapid snowfall starting at 2am this morning (6" as of 630am) combined with strong to extreme SW-W winds will rapidly load leeward aspects. Be on the lookout for Wind Slab on NW-N-NE-E-SE aspects especially adjacent to ridgeline, steep midslope rollovers and other features that have convex slopes optimal for capturing snow on the leeward side. Shooting cracks, audible whoompfing or panels of firmer wind affected snow resting on top of looser, less cohesive snow will be good indicators that wind slab is present. Cold temperatures and changing wind direction coming from the North tonight and tomorrow will likely redistribute snow to ALL ASPECTS Friday. Maintain good travel protocols, such as traveling slopes one at a time (both up and down), anchoring up in safe spots, and maintaining good communication and consistent group plans and routes.

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

Up to this point, a shallow, weak, and concerning snowpack structure exists in a majority of our northerly facing terrain. Thursday mornings snowfall combined with this previous weekends new snow will add ever increasing weight to an upside down snowpack. We have loose, weak sugary faceted snow at the base of much of the snowpack, with now a firmer slab on top and more added weight falling today. This rapid addition of snow (6"snow .5" Snow Water Equilvalent as of 630am) may produce sensitive conditions for larger persistent slabs to initiate. Persistent slabs may be triggered from distant locations and travel over wide spaces of terrain if slopes have connected panels of snow. Shooting cracks and whoomphing are signs of unstable snow and terrain >30° would be best to avoid. Northerly-Easterly facing slopes are most suspect for this problem.

advisory discussion

Overall the snowpack throughout the forecast zone from Bishop Creek to Virginia Lakes is shallow to non-existent with early season conditions prevailing. Thursday mornings quick hitting storm will improve coverage while also increasing the risk of instability.

recent observations

12/16 – Mammoth Area – Sherwins – Thin snowpack, poor structure

12/15 – June Lake Area- White Wing – Spatial variability, thin snowpack

12/14 – VA Lakes – Thin variable snowpack, poor snowpack structure

12/14 –Mammoth Crest –Red Cone area –Shooting cracks, collapsing, propagating ECT

12/14 - 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.  

Weather and CURRENT CONDITIONS
weather summary

It will be a mostly cloudy day, with the heaviest snowfall accumulations on tap for this morning and chance of continued flurries through the afternoon. Moderate to Strong SW-Westerly winds will blow as this quick hitting storm passes over. Temperatures will be in the high teens to 20sF for the higher elevations and will hover just below freezing below 10000Feet. Snow is scheduled to taper off after today with a cold night in store with temperatures dropping into the single digits. Friday’s skies are slated to start out partly cloudy and become sunny with calmer conditions and a wind direction change to come from the N-NW as the system departs the area. Temperatures will warm a bit Friday with daytime hours hitting the high 30’sF below 10000ft and 23-29 for upper elevations.  Sunnier and drier conditions look to settle back into the area for this coming weekend.

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. Snow in the morning, then chance of snow in the afternoon. Snow levels below 7000 feet. Chance of precipitation is 90%. Mostly cloudy. Snow levels below 7000 feet. Chance of precipitation is 10%. Partly cloudy then becoming sunny. Snow levels below 7000 feet. Chance of precipitation is 0%.
Temperatures: 26 to 32. deg. F. 11 to 17. deg. F. 31 to 37. deg. F.
Mid Slope Winds: West 15 to 30 mph. Gusts up to 60 mph decreasing to 45 mph in the afternoon. North around 15 mph with gusts to 30 mph. Light winds.
Expected snowfall: 80% probability of 3 to 6 inches. 20% probability of 6 to 9 inches. | SWE = 0.20-0.45 inch. in. 0 in. 0 in.
Over 10,000 ft.
Thursday Thursday Night Friday
Weather: Mostly cloudy. Snow in the morning, then chance of snow in the afternoon. Snow levels below 7000 feet. Chance of precipitation is 95%. Mostly cloudy then becoming partly cloudy. Snow levels below 7000 feet. Chance of precipitation is 10%. Sunny. Snow levels below 7000 feet. Chance of precipitation is 5%.
Temperatures: 19 to 25. deg. F. 7 to 12. deg. F. 23 to 29. deg. F.
Ridge Top Winds: Southwest 35 to 50 mph becoming west 20 to 30 mph in the afternoon. Gusts up to 70 mph. North 15 to 25 mph with gusts to 35 mph. Northwest 15 to 25 mph with gusts to 35 mph.
Expected snowfall: 80% probability of 3 to 6 inches. 20% probability of 6 to 9 inches. | SWE = 0.30-0.55 inch. 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.

ESAC receives significant financial support from ...