Eastern Sierra Snowpack Summary - 2016-02-29 07:03

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THIS ADVISORY EXPIRED ON March 2, 2016 @ 7:03 am
Avalanche Advisory published on February 29, 2016 @ 7:03 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.

High pressure continues through the week, keeping conditions dry with above average temperatures. Expect a mix of high clouds and blue skies, with light W to SW winds (increasing Tuesday afternoon).  Melt freeze crusts on E-S-W aspects will be frozen and firm early, and will thaw and soften through the day as the sun moves across the horizon.  You can expect easterly slopes to soften by mid morning, southerly aspects softening by mid-day, and westerly slopes softening by the afternoon.  Elevation, Cloud cover and wind will affect the exact timing of this thaw.  Slopes can turn quickly from firm and supportable to mucky and prone to wet-slide instability.  Safe enjoyable travel on these slopes is all about timing.  Make plans to be up and down off of these slopes before they warm too much.  Good indicators of overly warm slopes include punchy or manky snow resulting in sinking past boot-top and significant rollerball activity.  These indicators mean the likelihood for natural and human triggered wet point releases is increasing, which may have the potential to lead to larger slope failures.  Even small releases can have consequences in potentially hazardous terrain.  Lower elevation slopes will soften quicker (below 8500’) with the snow warming rapidly on all aspects and the risk of wet loose activity increasing on slopes >35 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.

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.

If traveling in high elevation, steep exposed terrain where a fall could have high consequences, be on the lookout for the rare very isolated small recently formed windslab that may be tender enough to be triggered by a rider.  Though these wont be big enough to result in a burial, they could be enough to knock a skier or boarder off balance, resulting in a slide into terrain with potentially high consequences.  These will most likely be found near ridgelines on northerly to easterly facing slopes. 

Snowpack Discussion

Our mid-winter thaw continues through this week with spring-like conditions and small wet-loose avalanche concerns due to the warm temperatures, mild nightly freezes, and sunny skies. Concern for wet releases exists on southerly/sunny aspects (E-SE-S-SW-W) beginning late morning and into the late afternoon depending on sun exposure.  Additionally, there is slight concern over the potential for isolated small fresh windslabs in upper elevation terrain near ridgetops on easterly to northerly facing slopes. Forecasted cloud cover may slow the thawing of the surface snow. 

High pressure continues through the week, keeping conditions dry with above average temperatures. Expect a mix of high clouds and blue skies, with light W to SW winds (increasing Tuesday afternoon).  Melt freeze crusts on E-S-W aspects will be frozen and firm early, and will thaw and soften through the day as the sun moves across the horizon.  You can expect easterly slopes to soften by mid morning, southerly aspects softening by mid-day, and westerly slopes softening by the afternoon.  Elevation, Cloud cover and wind will affect the exact timing of this thaw.  Slopes can turn quickly from firm and supportable to mucky and prone to wet-slide instability.  Safe enjoyable travel on these slopes is all about timing.  Make plans to be up and down off of these slopes before they warm too much.  Good indicators of overly warm slopes include punchy or manky snow resulting in sinking past boot-top and significant rollerball activity.  These indicators mean the likelihood for natural and human triggered wet point releases is increasing, which may have the potential to lead to larger slope failures.  Even small releases can have consequences in potentially hazardous terrain.  Lower elevation slopes will soften quicker (below 8500’) with the snow warming rapidly on all aspects and the risk of wet loose activity increasing on slopes >35 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.

Additionally, if traveling in high elevation, steep exposed terrain where a fall could have high consequences, be on the lookout for the rare very isolated small recently formed windslab that may be tender enough to be triggered by a rider.  Though these wont be big enough to result in a burial, they could be enough to knock a skier or boarder off balance, resulting in a slide into terrain with potentially high consequences.  These will most likely be found near ridgelines on northerly to easterly facing slopes. 

More concerning than unstable snow at this time, are the very firm snow conditions that can be found widespread on all aspects.  Be especially cautious as you travel in steeper more complex terrain where a fall could result in injury or even death. East to south to west facing slopes are firm and frozen prior to thawing, northerly facing slopes may have very firm wind-board that can make it difficult to arrest a fall. If traveling in exposed terrain, make sure you have the proper equipment (ice axe, self-arrest grips, crampons, etc.), know how to use them effectively, use extreme caution, and have a plan should something go wrong.

recent observations

A very faceted snowpack was found on a NE aspect at 11,800’ on Sunday east of McGee creek on the Nevahbe Ridge.  An area with a 1 meter total snow depth was found to have facets just below the surface all the way to the ground, and test column failed upon isolation just above the ground.  This could be particularly concerning if substantial snow loading occurs ontop of this weak sugary snowpack. 

Observations made over the past few days have been showing variable timing on the warming/softening/melting of snow surfaces on E-SE facing terrain due to the mixture of variable high cloud cover and variable winds from calm to moderate.  At upper elevations above 10,500’, if E to SE facing slopes were going to soften, 11-11:30am has shown to be a good descent time before these slopes warm too much.  Firm conditions have been found on many aspects, and crampon use is recommended on steeper slopes, and ski crampons useful even on less steep slopes.    

weather

High pressure continues through the week, keeping conditions dry with above average temperatures. Expect a mix of high clouds and blue skies, high temperatures in the 40s at elevations above 10,000’, and light west to southwest winds, increasing Tuesday afternoon (40-45mph with gusts to 65 at higher elevations) as a weak system crosses through OR and Northern CA.

Excitement lies on the horizon!  Forecast confidence is increasing for substantial winter storms to affect our region Sunday into Tuesday next week, as an atmospheric river makes landfall bringing moisture from the pacific and the potential for back to back storms.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Monday Monday Night Tuesday
Weather: PARTLY CLOUDY THEN BECOMING SUNNY PARTLY CLOUDY PARTLY CLOUDY
Temperatures: HIGHS IN THE UPPER 40S TO UPPER 50S deg. F. LOWS IN THE UPPER 20S TO MID 30S deg. F. HIGHS IN THE LOW TO UPPER 50S deg. F.
Wind direction: WEST SOUTHWEST SOUTHWEST
Wind speed: 10 TO 15 MPH 10 TO 15 MPH 15 TO 20 MPH WITH GUSTS TO 30 MPH INCREASING TO 25 TO 30 MPH WITH GUSTS TO 45 MPH IN THE AFTERNOON
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Monday Monday Night Tuesday
Weather: PARTLY CLOUDY THEN BECOMING SUNNY PARTLY CLOUDY PARTLY CLOUDY THEN BECOMING SUNNY
Temperatures: HIGHS IN THE LOW TO MID 40S deg. F. LOWS IN THE LOW TO UPPER 20S deg. F. HIGHS IN THE LOW TO UPPER 40S deg. F.
Wind direction: WEST SOUTHWEST SOUTHWEST
Wind speed: 15 TO 20 MPH WITH GUSTS TO 30 MPH 15 TO 20 MPH WITH GUSTS TO 30 MPH 25 TO 30 MPH WITH GUSTS TO 40 MPH INCREASING TO 40 TO 45 MPH WITH GUSTS TO 65 MPH IN THE AFTERNOON
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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