Avalanche Advisory: Thursday - Mar 19, 2020

THIS AVALANCHE ADVISORY EXPIRED ON March 20, 2020 @ 6:48 am
Avalanche Advisory published on March 19, 2020 @ 6:48 am
Issued by Steve Mace - ESAC

CONSIDERABLE avalanche danger persists at middle and upper elevations today, and MODERATE at lower elevations. 7.5" of new snow fell overnight in the Mammoth area, accompanied by moderate to strong ridgetop winds. Fresh and sensitive wind slabs will be the primary concern today on northerly and easterly aspects at mid and upper elevations. It will also be important to keep in mind that significant weight has been to a weak underlying snowpack over the last few days.  Persistent slab avalanches may be possible on northerly and easterly aspects at all elevations.  Cautious route-finding and conservative decision making will be essential if you travel into the backcountry today.  

*Avalanche Danger will be more concerning around Mammoth and areas to the north which received notably more snow than the southern reaches of the forecast area.*

3. Considerable

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Above Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

3. Considerable

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Near Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

2. Moderate

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Below Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
    Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
Avalanche Problem 1: Wind Slab
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As of 6 am this morning 7.5” of new snow and .4” of SWE has been recorded at the base of Mammoth Mountain. This new snow was accompanied by moderate to strong ridgetop winds. The size and sensitivity of wind slabs will vary in size and sensitivity today, fresh and sensitive wind slabs are likely residing on top of older and more stubborn deposits. Wind speeds are predicted to drop significantly today but we may see continued slab development on leeward slopes near and above treeline if wind speeds exceed expectations. Wind slabs sensitive to human trigger remain likely on NW-N-E-SE aspects at mid and upper elevations. Do your own localized assessments and be suspect of terrain features that encourage drifting such as the leeward sides of ridgelines, gully features, and cross-loaded depressions.  Surface clues such as blowing snow, large drifts, dense hollow feeling snow, recent cornice growth, and uneven snow surfaces can help you identify and avoid areas of recent wind deposits.

Avalanche Problem 2: Persistent Slab
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Prior to this weak, the old snow surface was variable across the range, composed in many areas of a verity of crust facet combos or week sugary snow. While avalanches resulting in these deeper weak layers may be hard to trigger, a resulting avalanche may include all of the recent storm snow significantly increasing the consequences. It will be important to give the snowpack time to adjust to this new load and realize that even a small wind slab avalanche may step down and result in a more sizable slab avalanche. This poor structure will be most prevalent on Northerly and easterly aspects where snow coverage was more contiguous before the recent input of new snow.  Solar aspects and many areas at lower elevations were bare ground before this most recent storm and are far less concerning. Don’t be afraid to dig in to evaluate these lower layers.

Remember that persistent slab avalanches often propagate across and beyond terrain features that would otherwise confine surface instabilities such as wind or storm slab and in some cases persistent avalanches can be remotely triggered from adjacent slopes. Give yourself a wide safety margin, be wary of hazard that may reside above you, and use terrain choice to limit your exposure.

advisory discussion

While more than 7” of new snow fell in the Mammoth area last night the distribution of new snow is variable across the range. Elevated avalanche danger may be equally variable. Evaluate conditions carefully and be suspect of areas with higher new snow totals.

 Field observations have indicated last week’s storm totals range from 1-3 ft throughout the forecast area with the most impressive accumulations centered in the Mammoth area.  Mild weather of the last couple of days has led to a healing trend keeping our focus on surface instabilities. However, it is prudent to think about how the underlying snowpack is accepting this new load. Don’t be afraid to dig in and evaluate these lower layers. It is also worth remembering that despite the recent storm overall the coverage remains very thin. Keep an eye out for shallowly buried rocks and trees and move cautiously through the landscape, a personal injury at this point in time may have negative implications for the larger community.  

Weather and CURRENT CONDITIONS
weather summary

Expect mostly cloudy skies and scattered snow showers today.  Temperatures will remain cold with highs below freezing at lower elevations and in the low 20°F at upper elevations. Light to moderate winds will continue out of the southwest this morning dropping throughout the day. 

Unsettled weather should continue through the end of the week bringing scattered showers and continued cold temperatures. Another low-pressure system is likely to impact the area early next week, bring strong winds the potential for more impressive snow totals.

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. Chance of snow. Snow levels below 7000 feet. Chance of precipitation is 50%. Partly cloudy. Slight chance of snow in the evening. Snow levels below 7000 feet. Chance of precipitation is 15%. Partly cloudy. Slight chance of snow in the afternoon. Snow levels below 7000 feet. Chance of precipitation is 15%.
Temperatures: 21 to 29. deg. F. 8 to 14. deg. F. 26 to 34. deg. F.
Mid Slope Winds: Light winds. Light winds. Light winds.
Expected snowfall: 70% probability up to 2 inches. 30% probability of 2 to 4 inches. | SWE = less than 0.10 inch. in. Up to 2 inches. | SWE = less than 0.10 inch. in. No accumulation. | SWE = trace amounts. in.
Over 10,000 ft.
Thursday Thursday Night Friday
Weather: Mostly cloudy. Snow likely. Snow levels below 7000 feet. Chance of precipitation is 55%. Mostly cloudy then becoming partly cloudy. Slight chance of snow in the evening. Snow levels below 7000 feet. Chance of precipitation is 15%. Partly cloudy. Slight chance of snow in the afternoon. Snow levels below 7000 feet. Chance of precipitation is 15%.
Temperatures: 12 to 20. deg. F. 1 to 7. deg. F. 17 to 25. deg. F.
Ridge Top Winds: Southwest around 15 mph with gusts to 30 mph in the morning becoming light. Northwest around 15 mph. Southeast around 15 mph.
Expected snowfall: 70% probability of up to 2 inches. 30% probability of 2 to 4 inches. | SWE = up to 0.15 inch. in. Up to 2 inches. | SWE = less than 0.10 inch. in. No accumulation. | SWE = trace amounts. 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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