Avalanche Advisory: Thursday - Feb 22, 2018

THIS AVALANCHE ADVISORY EXPIRED ON February 23, 2018 @ 6:51 am
Avalanche Advisory published on February 22, 2018 @ 6:51 am
Issued by Clancy Nelson - Eastern Sierra Avalanche Center

As conditions deteriorate today the avalanche danger could rise to CONSIDERABLE. Human triggered wind slab avalanches could become likely and natural avalanches possible on leeward slopes the receive the most wind loading. Large avalanches may be triggered today just below ridgelines and on cross-loaded features.

Poor visibility and very cold wind chill values should be factored into your travel plans.

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

Westerly winds have increased steadily overnight above ~9000’ with ample snow available to build new wind slabs. As the weather deteriorates today, with more snow and strong winds forecast, the problem will worsen. The addition of 2-6” of new snow today could create new wind slabs 6-30” deep by this afternoon. The resulting avalanches could be large. Rapidly forming drifts will be sensitive to human triggering. Expect dense, rounded wind slabs to form quickly on leeward slopes near and above treeline today. Even steep favored features below treeline may develop touchy wind deposits. Watch for blowing snow and cornice formation as direct clues as to which slopes to avoid. Cracks shooting from your feet are an indication that you may already be in harms way.

advisory discussion

How quickly things can change in the Sierra! The primary story is not about a few inches of new snow, but about wind. After a small storm Sunday night and some moderate winds redistributing it off and on throughout the week there were still several inches available for transport on upwind slopes when westerly winds picked up again last night. Winds remain at prime slab building strength this morning as another burst of snow showers make their way towards our area.

Small soft wind slabs have been failing with the addition of a skier’s weight for the past 2 days. These slabs have been failing within the recent storm snow and sliding on old hard wind crusts in the tops of north facing alpine chutes. 2-6” of new snow today, with strong westerly winds, will build even bigger slabs across a wider variety of terrain. On some northerly slopes near 10,000’ weak faceted snow sits just under the surface and could become the weak layer causing even larger avalanches.

Continue to be aware of obstacles hidden just under the snow surface.

recent observations
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2/21- Virginia Lakes: Soft, Weak New Snow. Wind Loading. Mid-Pack Facets.

2/21- Mammoth Crest: Small Wind Slabs, Snow Available for Transport.

2/21- Small Skier Triggered Wind Slab Avalanche, Mammoth Area

Weather and CURRENT CONDITIONS
weather summary

...WINTER WEATHER ADVISORY IN EFFECT FROM 7 AM THIS MORNING TO 4 AM PST FRIDAY...

Below average temperatures will dominate today as low pressure remains over the West. The first in a series of storm systems will bring widespread light to moderate snowfall today. Snowfall rates could reach 2”/hr with RAPIDLY DETERIORATING CONDITIONS EXPECTED after 6AM. A secondary wave will drop along the Sierra which will result in another round of increased snow rates later in the day. Wind chills will drop to -20F in the high Sierra today.

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 in the morning, then widespread snow showers in the afternoon. Mostly cloudy. Widespread snow showers in the evening, then scattered snow showers after midnight. Mostly cloudy then becoming partly cloudy. Scattered snow showers.
Temperatures: 13 to 21 deg. F. 1 to 7 deg. F. 16 to 24 deg. F.
Mid Slope Winds: SW NW N
Expected snowfall: 70% probability...of 2 to 6 inches. 30% probability...of 6 to 8 in. 80% probability...of 1 to 5 inches. 20% probability...of 5 to 7 in. 60% probability...of 1 to 2 inches. 40% probability... of 0 in.
Over 10,000 ft.
Thursday Thursday Night Friday
Weather: Mostly cloudy. Chance of snow in the morning, then widespread snow showers in the afternoon. Mostly cloudy. Widespread snow showers in the evening, then scattered snow showers after midnight. Mostly cloudy then becoming partly cloudy. Scattered snow showers.
Temperatures: 4 to 9 deg. F. -5 to 1 deg. F. 8 to 13 deg. F.
Ridge Top Winds: SW shifting to W in the afternoon. NW N
Expected snowfall: 70% probability...of 2 to 6 inches. 30% probability...of 6 to 9 in. 80% probability...of 2 to 6 inches. 20% probability...of 7 to 10 in. 60% probability...of 1 to 2 inches. 40% probability...of 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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