Snowpack Summary - Thu, Dec. 29, 2016

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THIS ADVISORY EXPIRED ON December 31, 2016 @ 6:58 am
Avalanche Advisory published on December 29, 2016 @ 6:58 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

bottom line:

Overall fairly stable snow conditions will persist through the forecast area for the next 48 hours (Thursday and Friday).  For Thursday some concern exists in steep alpine terrain on slopes facing NW-N-E for small isolated windslabs that may remain sensitive to human triggering from wind loading that occurred from moderate SW winds on Tuesday.  Also for Thursday concern will rise for the possibility of human triggered loose wet sloughs that could enlarge as then entrain more snow as they flow downhill on solar exposed slopes steeper than 37 deg near rock bands that face SE-S-SW as the snow softens and moistens as it receives increased sun exposure in combination with warmer than normal air temperatures.

For Friday, avalanche concern may develop in the form of isolated small new windslabs forming at upper elevations on steep slopes as moderate westerly winds return Thursday night into Friday.  This concern will be focused mostly on N-E-S facing slopes in the limited areas where loose snow is still available for transport. 

Avalanche Character 1: 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.

For Thursday, some concern exists in steep alpine terrain on slopes facing NW-N-E for small isolated windslabs that may remain sensitive to human triggering from wind loading that occurred from moderate SW winds on Tuesday.  Other than Tuesday, winds have remained mostly light since Christmas day.  Although unlikely, it may still be possible for a rider or skier to trigger one of these small old wind slabs on Thursday.

For Friday, new windslabs will likely form at upper elevations as moderate westerly winds return Thursday night into Friday.  This concern will be focused mostly on N-E-S facing steep slopes in the limited areas where loose snow is still available for transport.  These new windslabs where found will surely be much more sensitive than the older windslabs that formed on Tuesday.

While a resulting avalanche from any of these wind slabs failing would likely be small, it could be enough to knock someone off balance and lead to a bad fall in dangerous terrain.  Be on the lookout for smooth firm hollow sounding snow, and do your own quick assessments of these patches when you encounter them, such as isolating a small column and see how easily the upper firm layer fails as you try and push it off. 

Avalanche Character 2: 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.

For Thursday concern will rise for the possibility of human triggered loose wet sloughs that could enlarge as then entrain more snow as they flow downhill on solar exposed slopes steeper than 37 deg near rock bands that face SE-S-SW as the snow surface softens and moistens as it receives increased sun exposure in combination with warmer than normal air temperatures.  Rock bands heat up faster than snow both from the direct sun and the warm air temperatures, and the heat form these rock bands will warm the adjacent snow more than snow further away.  Be on the lookout for signs of wet instability such as roller balls.  While a resulting avalanche is likely to be small, it could knock a skier or rider off balance, and although they move slowly, the thick nature of wet slides makes it hard to escape.  Of particular concern is where one of these steep slopes ends in a terrain trap where even a small amount of snow could pile up deep.  This will not be a concern Friday as temperatures cool, clouds return, and winds increase. 

Snowpack Discussion

The new snow and windslabs that formed during the Christmas Eve storm that dumped 1-2ft of new snow over the mountains in our region have stabilized fairly well.  On Tuesday winds increased out of the SW to strong enough levels to transport snow, and new isolated small sensitive wind slabs formed at mid to upper elevations.  Calm winds since Tuesday have given these newer windslabs time to heal, and while it will be hard to find one that will be sensitive to human triggering, it is not impossible.  However, winds are forecasted to increase again to moderate levels Thursday night into Friday out of the W, so new windslabs will again be more of a concern on Friday.  Skies have remained mostly sunny since Christmas, and cold temperatures through Monday kept the surface of the snowpack faceting making the skiing in sheltered areas better and better.  Temperatures went on the increase dramatically Tuesday and Wednesday, and loose wet sloughs became a small concern on solar aspects, as a small incident involving these warming conditions on the backside of Mammoth Mountain on Tuesday demonstrates.  There still exists buried weak layers in the form of facets near ice crusts from the warm October storms, and a thinner harder to find one from the Dec 9th storm that can be found in many areas between the elevations of 9,500’ and 11,200’ on NE-N-NW facing slopes.  While many avalanches have failed on this upper October layer during past storm events, there are still slopes where it exists and hasn’t failed yet.  It is highly unlikely that a human could trigger this weak layer, but another significant storm and snow load could re-awaken it.  This is likely more of a concern in areas where the snowpack is especially thin at these elevations (Rock Creek south to Bishop).  It is still worth poking around and keeping an eye on this deeper layer.          

weather

High pressure will remain with us one more day today (Thursday) in the Eastern Sierra mountains, keeping skies clear, temperatures a good bit warmer than normal, and winds light.  Expect highs again in the mid 40s F even above 10,000’.  A splitting low-pressure system is on course to move towards us from the north on Friday, bringing back normal seasonal cool temperatures again, an increase in winds out of the west with gusts up to 30mph, and partly sunny skies becoming cloudy by evening with a slight chance of light snow showers with up to an inch of accumulation in the mountains possible Friday night.

High and low Temperatures will continue to drop through the weekend and into next week as a strong cold front drops into our area, with greater chances of cold Canadian storms to bring snow after the New Year. 

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 THEN BECOMING PARTLY CLOUDY MOSTLY CLOUDY. SLIGHT CHANCE OF SNOW SHOWERS
Temperatures: 41 TO 46 deg. F. 17 TO 25 deg. F. 35 TO 40 deg. F.
Wind direction: LIGHT WEST WEST
Wind speed: LIGHT WINDS LIGHT WINDS 10 MPH WITH GUSTS UP TO 25 MPH AFTER MIDNIGHT 10 TO 15 MPH. GUSTS UP TO 30 MPH
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Thursday Thursday Night Friday
Weather: SUNNY CLEAR THEN BECOMING PARTLY CLOUDY MOSTLY CLOUDY. SLIGHT CHANCE OF SNOW SHOWERS THROUGH THE DAY
Temperatures: 40 TO 46 deg. F. 19 TO 24 deg. F. 28 TO 36 deg. F.
Wind direction: LIGHT WEST WEST
Wind speed: LIGHT SOUTHWEST 10 TO 15 MPH WITH GUSTS TO 25 MPH WEST 15 TO 20 MPH. GUSTS UP TO 35 MPH DECREASING TO 25 MPH IN THE AFTERNOON
Expected snowfall: 0 in. 0 in. 0 in.
Disclaimer

This Snowpack Summary 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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