Eastern Sierra Snowpack Summary - 2016-02-25 07:02

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THIS ADVISORY EXPIRED ON February 27, 2016 @ 7:02 am
Avalanche Advisory published on February 25, 2016 @ 7:02 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center
Avalanche Character 1: Loose Wet
Loose Wet avalanches occur when water is running through the snowpack, and release at or below the trigger point. Avoid very steep slopes and terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.

A melt freeze crust has formed on E-S-W aspects, that will thaw and soften throughout the day as a result of sun exposure, with more easterly slopes softening by mid morning and westerly slopes softening later in the afternoon.  The forecasted light winds for today and Friday morning will not delay this softening, but the increasing winds Friday afternoon will likely result in a quicker refreeze and less softening on more westerly slopes.  Plan to be off steep solar exposed slopes before they soften to the point that a skier or rider could sink in up to their boot top.  Once this happens, the likelihood for natural and human triggered wet point releases increases, which may have the potential to lead to larger slope failures.  Even small releases can force a rider off balance and take them into undesirable terrain.  Look out for rollerball / pinwheel activity as well as signs that slopes are becoming less stable.  Slopes will soften quicker at lower elevations, and later at higher elevations.  At elevations below 8500’, the snow will warm rapidly on all aspects (even north) with risk of wet loose activity increasing on slopes >37 degrees by the afternoon.  Natural and human triggered wet loose avalanches maybe possible, starting on southeast slopes in the morning and moving around the compass toward southwesterly by afternoon. 

Avalanche Character 2: Wind Slab
Wind Slab avalanches release naturally during wind events and can be triggered for up to a week after a wind event. They form in lee and cross-loaded terrain features. Avoid them by sticking to wind sheltered or wind scoured areas.

In steep upper elevation exposed terrain, be on the lookout for the recently developed rare isolated small windslab that may be tender enough to be triggered by a human.  While these aren’t likely to be big enough to result in any kind of burial, they may be enough to knock a skier or boarder off balance.  Be especially aware of this in terrain where a fall would have bad consequences.  These will most likely be found near ridgelines on northerly to easterly facing slopes.  

Snowpack Discussion

The main avalanche concern for this period continues to be warm temperatures and sunny skies which increase the potential for wet releases on southerly/sunny aspects (E-SE-S-SW-W) beginning late morning and into the late afternoon depending on sun exposure.  Some avalanche concern exists for very isolated very small recently developed windslabs in steep upper elevation terrain near ridgetops on easterly to northerly facing slopes. 

High pressure continues to dominate the weather picture in our region through the weekend and into next week, with small disturbances in Northern CA on Friday and Sunday night resulting in increased winds for our region.  Expect sunny skies with occasional clouds, light winds and above average temperatures into the low 50s at low to mid mountain elevations for today (Thursday), and slightly cooler temperatures for Friday with winds increasing in the afternoon out of the southwest.  Night time lows are still expected to drop below freezing.  

A melt freeze crust has formed on E-S-W aspects, that will thaw and soften throughout the day as a result of sun exposure, with more easterly slopes softening by mid morning and westerly slopes softening later in the afternoon.  The forecasted light winds for today and Friday morning will not delay this softening, but the increasing winds Friday afternoon will likely result in a quicker refreeze and less softening on more westerly slopes.  Plan to be off steep solar exposed slopes before they soften to the point that a skier or rider could sink in up to their boot top.  Once this happens, the likelihood for natural and human triggered wet point releases increases, which may have the potential to lead to larger slope failures.  Even small releases can force a rider off balance and take them into undesirable terrain.  Look out for current rollerball / pinwheel activity as indicators that slopes are becoming less stable.  Slopes will soften quicker at lower elevations, and later at higher elevations.  At elevations below 8500’, the snow will warm rapidly on all aspects (even north) with risk of wet loose activity increasing on slopes >37 degrees by the afternoon.  Natural and human triggered wet loose avalanches maybe possible, starting on southeast slopes at mid to upper elevations in the late morning and moving around the compass toward southwesterly by afternoon.

In steep upper elevation exposed terrain, be on the lookout for the recently developed rare isolated small windslab that may be tender enough to be triggered by a human.  While these aren’t likely to be big enough to result in any kind of burial, they may be enough to knock a skier or boarder off balance.  Be especially aware of this in terrain where a fall would have bad consequences.  These will most likely be found near ridgelines on northerly to easterly facing slopes.  

Greater danger than avalanches at the moment is firm steep slopes where a fall could result in a slide-for-life.  Be especially cautious as you travel in steeper more complex terrain where a fall could likely result in injury or even death.  East to south to west facing slopes could be very firm and frozen before they begin to soften, and northerly facing slopes may have very firm wind-board.  Last Sunday, a skier pre-released from his Dynafit bindings at the top of a steep 2000’ couloir off the Dana Plateau, and tumbled out of control to the bottom and had to be evacuated with a helicopter.  If electing to travel in exposed steep terrain, make sure you have the proper equipment (ice axe, self-arrest grips, crampons, etc.), use extreme caution, and have a plan should something go wrong. 

recent observations

Skiers and riders throughout the region over the past few days have been finding mostly very stable snow conditions overall. 

Riders out of the Bishop area over the last few days report that high elevation east facing slopes have been softening ideally, given the sun/temperature/wind conditions, for smooth descents between 11-11:30 am.  A party out of Bishop yesterday also reported coming across a small windslab at the top of a ridge that resulted in shooting cracks. 

Skiers and riders in the Mammoth area over the past few days report soft wintery snow in northerly facing protected areas, and a mixture of variable snow surface conditions in more exposed areas. 

Skiers and riders throughout the region have been finding areas of very firm conditions above tree.  Greater potential danger than avalanches at the moment is a fall in steep terrain where these firm conditions exist.

weather

High pressure remains well in place over our region keeping temperatures well above average for the end of February, skies mostly sunny, and dry.  A quick moving small shortwave disturbance will move in over northern CA and OR Friday evening, with the only impact to our region being increased winds for a brief period Friday afternoon into Saturday, and slightly decreased temperatures. 

Expect sunny skies, light southwest winds and high temperatures into the upper 40s and low 50s at mid mountain elevations for today (Thursday).  Friday will be clear, high temperatures will be slightly less, and winds will increase to 20-30 mph with gusts up to 50 in higher elevations in the afternoon out of the southwest.

For the weekend and into next week temperatures will gradually cool, skies will remain mostly clear with some clouds, and winds will be breezy. 

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: SUNNY CLEAR SUNNY
Temperatures: LOW TO MID 50S deg. F. LOW TO MID 30S deg. F. UPPER 40S TO LOW 50S deg. F.
Wind direction: SOUTHWEST LIGHT SOUTHWEST
Wind speed: LIGHT 10-15 MPH LIGHT 15-20 MPH WITH GIUSTS TO 40 MPH
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Thursday Thursday Night Friday
Weather: SUNNY CLEAR SUNNY
Temperatures: MID 40S TO LOW 50S deg. F. MID 20S TO LOW 30S deg. F. LOW TO MID 40S deg. F.
Wind direction: SOUTHWEST SOUTH SOUTHWEST
Wind speed: 10-15 MPH WITH GUSTS TO 25 MPH 10-15 MPH, BECOMING LIGHT 20-30 MPH WITH GUSTS TO 50
Expected snowfall: 0 in. 0 in. 0 in.
Disclaimer

This snowpack summary applies only to backcountry areas outside established ski area boundaries. This snowpack summary only describes general avalanche conditions and local variations always occur. This snowpack summary expires 24 hours after the posted time unless otherwise noted. 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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