Eastern Sierra Snowpack Summary - Mar 24 2016

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THIS ADVISORY EXPIRED ON March 26, 2016 @ 6:55 am
Avalanche Advisory published on March 24, 2016 @ 6:55 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.

Mostly clear skies, moderate warm temperatures, and light winds over the next 2 days will make loose-wet instability a possibility as slopes warm throughout the day as they receive sunshine.  This concern will begin with easterly facing slopes in the late morning, move to southerly facing slopes mid day, and westerly facing slopes in the later afternoon.  The cool upper elevation temperatures will confine this concern to mid elevation slopes (mostly below 11,000’) and lower.  Cool nights with clear skies will make for a solid refreeze of any slopes with a solar aspect, as well as northerly facing slopes at lower elevations that warmed during the day.  Pay attention to signs of instability such as boot penetration beyond boot-top and new rollerball activity as these slopes melt and soften through the day.  Plan to be off of them before this happens, and be open to changing your plans if you find yourself on slopes exhibiting these signs. 

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.

Light to breezy SW winds over ridge tops have persisted over the past couple of days and are projected to continue for the next couple of days.  These winds could result in the formation of some small new windslabs that could be sensitive to human triggering.  These could possibly be found on steep exposed high elevation slopes just below ridgelines, and on the sidewalls of gullies.  Be on the lookout for hollow sounding dense snow, and do your own handpits do assess how sensitive these slabs may be.  Realize that even a small avalanche could result in injury if it knocks a skier or boarder off balance in steep rocky terrain.    

Snowpack Discussion

Limited avalanche concern exists for today and tomorrow (Thursday and Friday).  The concern that does exist is over loose-wet instability on solar aspects (SE-S-SW-W) and will be focused on steep mid-elevation slopes as they heat up and soften throughout the day.  Some slight concern exists over small isolated pockets of sensitive windslabs on upper elevations steep exposed slopes with a focus on N-NE-E-SE facing aspects.

Mostly clear skies, moderate warm temperatures, and light winds over the next 2 days will make loose-wet instability a possibility as slopes warm throughout the day as they receive sunshine.  This concern will begin with easterly facing slopes in the late morning, move to southerly facing slopes mid day, and westerly facing slopes in the later afternoon.  The cool upper elevation temperatures will confine this concern to mid elevation slopes (mostly below 11,000’) and lower.  Cool nights with clear skies will make for a solid refreeze of any slopes with a solar aspect, as well as northerly facing slopes at lower elevations that warmed during the day.  Pay attention to signs of instability such as boot penetration beyond boot-top and new rollerball activity as these slopes melt and soften through the day.  Plan to be off of them before this happens, and be open to changing your plans if you find yourself on slopes exhibiting these signs. 

The new wind deposits and windslabs that resulted from Monday night’s snowfall and high winds have had time to settle and stabilize.   These deposits ranged in depth from 8-10” at upper elevations where the most snow fell near Mammoth, to 4-6” deposits in nearby areas north and south, to dramatically less in areas further away.  Limited isolated natural avalanches occurred during this storm, and moderate avalanche activity resulted from control efforts on Mammoth Mountain Tuesday morning.  Winds predominated out of the SW during this storm, but switched around from the north for a period on Tuesday afternoon evening, resulting in some wind deposits on a variety of aspects.  Light to breezy SW winds over ridge tops have persisted over the past couple of days, which could result in the formation of some small new windslabs that could be sensitive to human triggering.  These could possibly be found on steep exposed high elevation slopes just below ridgelines, and on the sidewalls of gullies.  Be on the lookout for hollow sounding dense snow, and do your own handpits do assess how sensitive these slabs may be.  Realize that even a small avalanche could result in injury if it knocks a skier or boarder off balance in steep rocky terrain.    

Perhaps of greater safety concern than avalanches at the moment is very firm snow surfaces in the form of still frozen melt-freeze crusts and very firm wind-board.  Be prepared for these slide-for-life conditions.  Use of crampons and ice ax is recommended, and realize that a fall maybe very hard to self-arrest. 

recent observations

Observations made out of independence out of Onion Valley yesterday (Wednesday) revealed totally clear skies, calm winds except for light SW breeze over high ridge tops, and moderate to cool temperatures (just below freezing above 12,000’ in the early afternoon).  A little bit of new snow fell Monday night, as seen by a couple centimeters below 10,000’ and isolated areas of 1-4” wind deposits above 11,500’.  No evidence of recent rollerball activity or loose wet activity within the last few days.  There was a small crown visible on a steep westerly facing slope at 11,500’ between cliff bands (~4-6” crown, 30-40’ wide).  This likely occurred toward the end of Monday night’s storm as a result of wind load.  No areas of sensitive windslabs found.  Aspects with some southerly exposure had good surface softening yet remained very supportable (good corn) even at 3:00pm from 10,500’ to 9,200’.   

Observers on the Nevahbe Ridge out of Crowley yesterday (Wednesday) noted clear blue skies, moderate above freezing temperatures, and some breezy conditions up high resulting in some visible snow transport.  They found old windslabs 3-6” thick failing with rough shears as a result of moderate force in hand pits.  No note of wet instability, or any other instability.  3-6” of new wind deposited snow in gully at mid to upper elevations from Monday night’s storm, above variable wind and melt-freeze crusts.

On Tuesday morning on Mammoth Mountain, 4.5” of new snow was recorded at 9000’.  Avalanche control work resulted in moderate results at mid and upper mountain.  4-6” soft slab crowns mid mountain and 8-10” soft slab crowns on the upper mountain.  On June mountain some small 3-4” sloughs occurred from avalanche control work, and only ~2” of new snow recorded mid mountain. 

weather

High pressure will remain over the region into the beginning of the weekend.  For today and tomorrow (Thursday and Friday) expect mostly clear skies with some clouds moving in Friday, high temperatures in the mid 40s at 10,000’, and breezy winds from the West in the 20-30 mph range gusting into the 40s on Thursday and into the 50s on Friday.  Nighttime lows will be in the 20s, with some clouds.

A low-pressure system is expected to move into the area Sunday night, bringing increased winds, much cooler temperatures, and some chances of light snow.

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 PARTLY CLOUDY THEN BECOMING CLEAR SUNNY
Temperatures: HIGHS IN THE UPPER 40S T MID 50S deg. F. LOWS IN THE MID 20S TO LOW 30S deg. F. HIGHS IN THE UPPER 40S TO MID 50S deg. F.
Wind direction: WEST NORTHWEST WEST
Wind speed: 15 TO 25 MPH WITH GUSTS TO 40 MPH 20 TO 30 MPH WITH GUSTS TO 40 MPH WEST 20 TO 30 MPH WITH GUSTS TO 40 MP
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Thursday Thursday Night Friday
Weather: SUNNY PARTLY CLOUDY THEN BECOMING CLEAR SUNNY THEN BECOMING PARTLY CLOUDY
Temperatures: HIGHS IN THE LOW TO UPPER 40S deg. F. LOWS IN THE LOW TO UPPR 30S deg. F. HIGHS IN THE LOW TO UPPER 40S deg. F.
Wind direction: WEST NORTHWEST WEST
Wind speed: 20 TO 30 MPH WITH GUSTS TO 40 MPH 20 TO 30 MPH WITH GUSTS TO 45 MPH 20 TO 30 MPH WITH GUSTS TO 50 MPH
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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