Avalanche Advisory: Friday - Feb 23, 2018

 
 
 
 
 
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THIS AVALANCHE ADVISORY EXPIRED ON February 24, 2018 @ 6:42 am
Avalanche Advisory published on February 23, 2018 @ 6:42 am
Issued by Clancy Nelson - Eastern Sierra Avalanche Center

The avalanche danger is MODERATE today near and above treeline where it may be possible to trigger small wind slab avalanches on specific terrain features or a larger avalanche on isolated features that promoted drifting yesterday on northerly aspects. The problem will become increasingly possible on southerly aspects today as the wind shifts to the N. Below treeline the avalanche danger is LOW but be aware of unstable wind slabs on isolated terrain features.

2. Moderate

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

2. Moderate

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

1. Low

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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
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  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

Limited observations yesterday show that turbulent winds deposited snow erratically near specific features on leeward slopes. Large openings in trees, convexities, just below ridges, and cross-loaded terrain on northerly aspects promoted the most drifting. Sheltered areas showed less wind effects or only developed very small slabs. This is not to say that steep alpine slopes with good sources of snow from upwind did not develop larger wind slabs which could result in larger avalanches. Large or small, recent drifts may still be sensitive to human triggering today thanks to the cold temperatures which tend to slow the bonding process. Make your own observations today looking for smooth, dense, rounded wind slabs just below cornices or in the tops of chutes. Convex rolls and the sidewalls of gullies are also features of concern. Be wary of shooting cracks or sudden collapsing in the snow under your feet. Watch for blowing snow which will point to new wind slab formation on more southerly aspects throughout today, especially above about 10,000’. The possibility of an inch or two of new snow today could give north winds new ammo for building slabs on southerly slopes.

advisory discussion

Ideal conditions for building new wind slabs include lots of soft snow available to blow around and steady moderate winds. While the new snow yesterday was more than enough to create large uniform slabs, the winds were strong and gusty making deposition more erratic. Wind slabs sensitive to a skier’s weight did form yesterday, but the distribution of those slabs was less widespread and more specific to ideal terrain features that promote drifting. Recently, wind slabs have been observed to fail on top of soft storm snow and to slide on hard, crusty, or board-like surfaces. In some more sheltered locations weak faceted snow has been found in the upper snowpack. Sugary facet snow often acts as the weak layer in avalanche failure, but it is unlikely that new wind slabs will lie directly over these layers. However, if a wind slab avalanche originating in alpine terrain were to run down to near treeline it could trigger these deeper weaknesses and cause an even larger slide.

Continue to be aware of obstacles hidden just under the snow surface and of wind scoured surfaces that may cause slide-for-life conditions increasing the consequences of a fall in steep terrain.

recent observations
  • Wind slabs that were observed yesterday in the Mammoth Area between 9000’ and 10,000’ were reactive but generally limited to specific terrain such as large openings in trees, favored convexities, and just leeward of ridges. These slabs were numerous, but relatively small by mid-day.

  • Other reports from the Mammoth Area near the end of yesterday pointed to some sensitive areas of wind deposited snow in exposed terrain, while more sheltered areas had unconsolidated storm snow about 9.5” deep.

  • Snow totals across the area as of 4AM this morning range from about 7” at Tioga Pass (9972’), to just over 11” at the Sesame Snow Study Site in Mammoth (9014’), 3” at Rock Creek Lake (9600’), and just 1” near Big Pine Creek (10,000’).

 

Weather and CURRENT CONDITIONS
weather summary

The trough that brought snow showers and powerful wind yesterday gradually drops southeast across southern Nevada and northern Arizona today with a chance for showers for Mono county through this afternoon. Otherwise today will be a break in the weather systems before the weekend with a brisk north flow in place.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Friday Friday Night Saturday
Weather: Mostly cloudy then becoming partly cloudy. Scattered snow showers. Clear. Sunny then becoming partly cloudy.
Temperatures: 16 to 22 deg. F. 3 to 11 deg. F. 28 to 34 deg. F.
Wind Direction: N NW W
Wind Speed: 10 to 15 mph with gusts to 30 mph. 10 to 15 mph in the evening becoming light. Gusts up to 40 mph. 15 to 25 mph with gusts to 50 mph.
Expected snowfall: 70% probability...up to 2 inches. 30% probability...up to 4 in. 0 in. 0 in.
Over 10,000 ft.
Friday Friday Night Saturday
Weather: Mostly cloudy then becoming partly cloudy. Scattered snow showers. Clear. Sunny then becoming partly cloudy.
Temperatures: 12 to 15 deg. F. 5 to 11 deg. F. 21 to 26 deg. F.
Wind Direction: N NW W
Wind Speed: 15 to 20 mph with gusts to 35 mph. 20 to 35 mph. Gusts up to 55 mph decreasing to 45 mph after midnight. 20 to 30 mph increasing to 30 to 40 mph in the afternoon. Gusts up to 65 mph.
Expected snowfall: 70% probability...of 1 to 3 inches. 30% probability...up to 4 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.

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