Eastern Sierra Avalanche Advisory - 2/22/17

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THIS AVALANCHE ADVISORY EXPIRED ON February 24, 2017 @ 6:28 am
Avalanche Advisory published on February 22, 2017 @ 6:28 am
Issued by Clancy Nelson - Eastern Sierra Avalanche Center
bottom line

The primary avalanche concern for Wednesday will be small to very large, widespread wind slabs on leeward slopes at all elevations. Storm slab avalanches will also remain possible in steep, sheltered terrain. On slopes steeper than about 35 degrees, natural avalanches with be likely, and human triggered avalanches very likely today. Travel in avalanche terrain is not recommended.

For Thursday, natural wind slab and storm slab avalanches will be possible and human triggered avalanches likely. Identify possible wind loaded areas such as below ridge lines and along the side walls of gullies. Due to the strong winds and local variability, these slabs may be encountered on unusual aspects, in normally sheltered areas, as well as further down slope than usual. Careful snowpack evaluation, cautious route-finding, and conservative decision-making will be essential.

How to read the advisory

Bottom Line

The primary avalanche concern for Wednesday will be small to very large, widespread wind slabs on leeward slopes at all elevations. Storm slab avalanches will also remain possible in steep, sheltered terrain. On slopes steeper than about 35 degrees, natural avalanches with be likely, and human triggered avalanches very likely today. Travel in avalanche terrain is not recommended.

For Thursday, natural wind slab and storm slab avalanches will be possible and human triggered avalanches likely. Identify possible wind loaded areas such as below ridge lines and along the side walls of gullies. Due to the strong winds and local variability, these slabs may be encountered on unusual aspects, in normally sheltered areas, as well as further down slope than usual. Careful snowpack evaluation, cautious route-finding, and conservative decision-making will be essential.

Avalanche Problem 1: Wind Slab
  • Character ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small
  • Trend ?
    Decreasing Danger

Winds across the region have been strong and wind slabs have been building on many aspect and all elevations. Potentially dangerous avalanche conditions will continue on wind loaded terrain at all elevations today. For Thursday, winds will be in the 15-35 mph range. This is a good speed for transporting snow. Sensitive wind slabs will likely remain with us throughout the forecast period. Be wary of hollow, drum-like sounding snow or cracks shooting out from your feet. Better yet, watch for blowing snow and cornice formation to rule out hazardous terrain before you get too close.

Avalanche Problem 2: Storm Slab
  • Character ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small
  • Trend ?
    Decreasing Danger

The winter storm warning will remain in effect until 10am today. We received several more inches of precipitation overnight, and this will increase your likelihood of encountering storm slabs today. Cold temperatures will slow bond formation within the new snow and storm slabs will linger in sheltered terrain on slopes of about 35 degrees or steeper.

For Thursday, storm slabs will still be possible, but less likely. Careful snowpack evaluation, cautious route-finding, and conservative decision-making will be essential.

 

advisory discussion

Another Atmospheric River moved into the region Monday with strong winds and heavy snow above ~7000’. The storm began Sunday night when temperatures were colder allowing for some lighter winter snow to fall. By Monday, however, the sub-tropical nature of the system brought warm temperatures and heavy dense snow. Remember that stronger, more resistant snow over a weaker layer is the recipe for slab avalanches. Mammoth Mountain Ski Patrol reported many results from control work on Monday with the same paths sliding multiple times throughout the storm.

Storm Totals as of 5 AM today:

Virginia Lakes: 16”

Tioga Pass: 21"

Gem Pass: --

Agnew Pass: --

June Mtn. Weather Plot: 22”

Mammoth Pass: --

Sesame Snow Study Plot: 22”

Rock Creek: 9”

South Lake: --

Sawmill: 10”

The strong Southwesterly flow aloft associated with this system formed Wind Slabs throughout the region through Tuesday. June Mountain Ski Patrol reported large and extremely sensitive wind slabs in the J7 area during Tuesday’s control work. One skier who ventured beyond the Mammoth Mountain Ski Area boundary triggered an avalanche large enough to bury or kill a person. Luckily he suffered only minor injuries. Due to the strong winds and local channeling, wind slabs may be encountered in unusual locations today and in normally sheltered areas, possibly lower in elevation than usual.

Last night the area received another 2 to 6 inches of snow. This will keep small storm slabs a problem for today. Temperatures will dip much colder across the region today and tonight and snow is forecast to taper off before this evening. Winds, however, will remain in the 15-35 mph range with ridge top gusts in the 60s. New snow and continuous winds will keep sensitive wind slabs forming throughout the forecast period. So while sunnier skies and new snow may make backcountry travel very tempting, we will still be stuck with elevated avalanche danger. Remember, the first sunny day after a storm is often when people get caught in slides. Stay alert!

weather

...WINTER STORM WARNING IN EFFECT UNTIL 10 AM THIS MORNING…

After the convective snow bands diminish this morning, chilly and unsettled conditions will continue today through Thursday as weak disturbances within cold northwest flow aloft will keep the chance for light snow showers going at times. Then by Thursday afternoon, snow showers look to increase in coverage again along the Sierra. With the exception of isolated locations where heavier snow bands persist, most areas will receive additional snowfall of 1 inch or less. Thursday night through Friday night should be mostly dry as more stable air mass with limited moisture moves overhead.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Wednesday Wednesday Night Thursday
Weather: Mostly cloudy then becoming partly cloudy. Scattered snow showers in the morning, then isolated snow showers in the afternoon. Partly cloudy. Partly cloudy. Isolated showers in the afternoon.
Temperatures: 21 to 27 deg. F. 1 to 11 deg. F. 21 to 26 deg. F.
Wind direction: W W W
Wind speed: 15 to 20. Gusts to 45. 15 to 20. Gusts to 40. 10 to 15. Gusts to 30.
Expected snowfall: 1 to 3 in. 0 in. Up to 1 in.
Over 10,000 ft.
  Wednesday Wednesday Night Thursday
Weather: Partly cloudy. Scattered showers in the morning, then isolated showers in the afternoon. Partly cloudy. Partly cloudy. Isolated showers in the afternoon.
Temperatures: 15 to 21 deg. F. 1 to -5 deg. F. 15 to 21 deg. F.
Wind direction: W W NW
Wind speed: 20 to 35. Gusts to 65, decreasing to 50 this afternoon. 20 to 30. Gusts to 50. 15-25. Gusts to 45
Expected snowfall: 1 to 3 in. 0 in. Up to 1 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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