Avalanche Advisory: Monday - Jan 22, 2018

 
 
 
 
 
-- placeholder --
 
 
 
THIS AVALANCHE ADVISORY EXPIRED ON January 23, 2018 @ 6:29 am
Avalanche Advisory published on January 22, 2018 @ 6:29 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

The primary avalanche concern for Monday (1/22) will focus on Wind Slabs above treeline. With plenty of transportable snow and light to moderate Southwesterly to Westerly winds throughout the night and forecasted to continue through the day, anticipate Wind Slab development in the upper elevations, primarilly on NW-N-NE-E-SE-S aspects where the avalanche danger will remain MODERATE. Watch for blowing snow and recent cornice formation on leeward slopes that could suggest new wind slabs.

Mammoth Area – the persistent weak layer is showing signs of possible failure in isolated locations between 10,500’ and 9,000’, though the possibility remains LOW: Natural and triggered avalanches are unlikely but not impossible and the resulting avalanche could be large. Whumphing and cracking are signs layers can suddenly fail.

Below ~9,000’, natural and triggered releases are unlikely due to thin snow coverage (below threshold).​

 

2. Moderate

?

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

1. Low

?

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

1. Low

?

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

The recent storm (1/19 – 1/20) delivered a welcomed 4” to 7” of snow for Mammoth and 6” to 8” for June and Virginia Lakes. The new snow was quite low-density with minimal wind effect and uniform distribution leaving Windward slopes primed with plenty of transportable snow. Light to moderate Southwesterly to Westerly winds forecasted overnight and through today may have created isolated Wind Slabs in favored locations above treeline primarilly on NW-N-NE-E-SE-S aspects. Anticipate Wind Slabs on the Leeward side of ridgelines, under cornices, and cross-loaded terrain features in steeper upper elevation terrain.  

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

Patchy Persistent Slab issues continue to lurk within the snowpack. Recent cold temperatures have reinforced the temperature gradient within the upper snowpack and as a result the upper snowpack continues to loose strength over time, leaving the Persistent Slab weakness teetering just below failure in some locations, while other areas require additional loading for failure. This was highlighted by recent reports of Whumphing (9150’) above Lake George with tests again showing fracture propagation across the slope. After a brief hiatus, there are new signs of instability in this persistent weak layer. Limited observations indicate this needs to be watched for further degradation. Perform your own stability tests to assess the stability of the slope you want to ride. Large Wind Slab avalanches could step-down into the snowpack as it over-loads the persistent weakness deeper in the snowpack. 

Below ~9,000’- below threshold, natural and triggered releases are unlikely due to low snow coverage.​ 

 

advisory discussion

A cold air mass has settled into the region with light to moderate winds with a few lingering showers over the higher elevations as a weak weather system passes to the north. The recent storm (1/19 -1/20) deposited 4” to 7” of snow in the Mammoth region and 6” to 8” June Lake and north with little wind leaving a uniform blanket of light transportable snow. Mammoth Mountain winds - Southwesterly – Westerly/20 to 30 mph and are forecasted to continue though the day as a weak disturbance passes to the north of the forecast region with snow showers and a Trace to 1” of new snow, 2” possible in favored areas. Moderate winds will continue build wind slabs until downwind fetches are exhausted. Watch for signs of blowing snow and wind loading (snow banners, recent cornice formation, and fresh drifts). Caution is recommended in steep alpine terrain.

Treeline and below, the persistent weak layer that has plagued us through much of the early part of the season may be showing signs of further weakening in the Mammoth area, renewing concern for potential deep releases, if this trend continues. A new report of whumphing in the Mammoth Lakes Basin highlights this problem layer and the potential for failure. Whumphs (sudden collapse) are a strong sign of instability. Do your own stability assessments, especially as you enter steeper or complex terrain.

Below ~9,000’ the new snow will not be enough to cover the rocks and brush poking up from the surface.

 

Weather and CURRENT CONDITIONS
weather summary

Mon thru Tuesday - Weak shortwave passing through northern California with shallow moisture will keep light snow going across the eastern Sierra this morning (Monday), with precipitation decreasing by afternoon. Additional accumulations of less than 1 inch, locally up to 2 inches possible.

Monday night thru Tuesday: flat ridging will keep dry conditions across the region with increasing ridge level winds by Tuesday night. High temperatures in the lower 50s.

Wed and beyond: a fast moving cold front with strong winds, primarily Wednesday afternoon and evening, followed by moderate to heavy precipitation (mainly snow) Wednesday night. Crest winds gusts of over100 mph with forecast 700 mb flow of 50-60 kt. The short duration of the heaviest precipitation will again be the limiting factor to total snow amounts Wednesday night into early Thursday, snowfall rates could briefly peak at 2-3" per hour for the eastern Sierra. Snowfall amounts in eastern CA above 7000 feet, 6-12”. 

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: Mostly cloudy. Isolated snow showers in the morning Partly cloudy Partly cloudy
Temperatures: 34 to 40 deg. F. 16 to 22 deg. F. 38 to 44 deg. F.
Mid Slope Winds: Southwest Light winds Light winds
Expected snowfall: up to 1 in. 0 in. 0 in.
Over 10,000 ft.
Monday Monday Night Tuesday
Weather: Mostly cloudy. Scattered snow showers in the morning. Partly cloudy Partly cloudy
Temperatures: 27 to 33 deg. F. 13 to 18 deg. F. 31 to 37 deg. F.
Ridge Top Winds: West Southwest West
Expected snowfall: up to 1 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 Snowpack Summary is provided by the Eastern Sierra Avalanche Center, who is solely responsible for its content.

ESAC receives support from ...