Eastern Sierra Snowpack Summary - Apr 21 2016

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THIS ADVISORY EXPIRED ON April 23, 2016 @ 6:54 am
Avalanche Advisory published on April 21, 2016 @ 6:54 am
Issued by Doug Lewis - 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.

Thursday, Low to Mid Elevations: Moderate freeze and warm daytime temperatures forecasted for Thursday will maintain the threat for Loose Wet releases, especially on E-S-W-NW aspects. Elevation, aspect, cloud cover, temperatures, and wind can greatly affect how much the snow surface thaws. In general, the snow surface will be frozen/firm early with easterly aspects softening mid morning, southerly aspects softening mid-day, and westerly slopes softening in the afternoon. Lower elevation slopes will soften more quickly with the risk of wet loose activity increasing by the afternoon on solar slopes >35 degrees. Plan your travel to avoid slopes before the snow thaws excessively. Punchy, manky snow past boot-top, and significant recent rollerball activity are indicators the threat of wet loose releases is increasing. Isolated natural and human triggered loose wet avalanches are possible at the mid to lower elevations, especially on solar aspects. While planning your travel in the backcountry take note of the daily High & Low temperatures of nearby remote weather observation stations to determine how well the snow froze over night, which can be a good indicator of how quickly the snow surface will thaw during the day and plan accordingly.

Friday, Mid to Upper elevations: up to 3” of wet snow is forecasted, which may not adhere well initially to the old snow surface. Rider triggered sloughing in steeper terrain (35 degrees and steeper) where the new snow hasn't had sufficient time to form bonds to the underlying snow is likely, natural avalanches may be possible.  Lower Elevations: rain is forecasted for the lower elevations, combined with a weak overnight freeze will increase the concern for Loose Wet releases. Natural and triggered releases may be possible on steeper terrain (~35 degrees and steeper).

Additional consideration (Thursday): Spring conditions prevail throughout the region with firm snow conditions during the morning where a slip could result in a serious slide for life. Use extra caution while traveling in or above hazardous terrain, have the necessary equipment to travel safely (i.e., crampons, ice axe, self-arrest grips), and practice self-arrest in a low risk environment prior to venturing into dangerous terrain. 

 

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.

Mid to Upper elevations: Moderately strong South to Southwest winds forecasted for Thursday into Friday combined with up to 3” of new snow will likely form tender Wind Slabs on NW-NE-SE aspects in the mid to upper elevations. These will most likely be encountered near ridgelines, in depressions and gullies, or adjacent to terrain features. Avoid freshly formed drifts and hollow sounding slabs on steeper terrain. Though these tender pockets likely wont be big enough to result in burial, they could entrain a rider and carry them into hazardous terrain with potentially high consequences.

Snowpack Discussion

Primary avalanche focus for Thursday – will depend on aspect and elevation. Mid to Lower Elevations - isolated natural and human triggered Loose Wet avalanches are possible at the mid to lower elevations, especially on solar aspects. Upper elevations - isolated pockets of Wind Slabs in the upper elevations primarily on N-NE aspects.

Friday – will see the potential for Loose Wet releases expand throughout the mid elevations, possible extending into the lower elevations with the addition of new wet snow, rain at the lower elevations.  Strong Southwesterly winds aloft will elevate the threat of tender Wind Slabs on NW-NE-SE aspects in the mid to upper elevations.  

Snowpack –The upper elevation northerly aspects are still holding onto to colder conditions with a variety of light crusts, windpack, wind board, facets, and soft snow. Southerly aspects are transitioning toward mature corn. Mid and Lower elevations, the snow surface has undergone extensive melt-freeze cycles and has consolidated into granular melt/freeze snow (corn snow) as the snowpack recedes. Creeks are opening up, snow bridges are sagging and possibly suspect. Friday, the next spring storm moves through the region with up to 3” of new snow forecasted for the higher elevations. The new snow forecasted will likely not bond well initially, elevating the concern for Loose & Loose Wet snow avalanches on steep terrain in the mid to upper elevations. Lower elevations will potentially see rain, which will elevate the Loose Wet potential on steep terrain (~ 35 degrees and steeper) on all aspects.

Additional consideration (Thursday): Spring conditions prevail throughout the region with firm snow conditions during the morning where a slip could result in a serious slide for life. Use extra caution while traveling in or above hazardous terrain, have the necessary equipment to travel safely (i.e., crampons, ice axe, self-arrest grips), and practice self-arrest in a low risk environment prior to venturing into dangerous terrain.

 

recent observations

Treasure Peak, East Face (4/20/16) - Started at the pack station around 0830.  Snow didn't feel like it had frozen hard the previous night, though it had frozen.  Started booting up the steeper terrain under Treasure around 1130 and found variable snow conditions ranging from 5 cm boot pen to hip deep (70 cm).  On true eastern aspects surface snow would entrain, moving to NE aspects it would not.  Above 12250 on NE aspects the snow had easily breakable wind crust over facets.  At 1215 we stopped around 12600 just below the summit ridge because of snow quality concerns.  We observed another party ski down at 1225, they triggered a small wet slide, it looked to be 10 feet wide but ran about 100 feet, in the rocks just below the summit ridge (12650).  Skiing was better than expected but heavy and wet.  Observing the other party's tracks they triggered at least two more small wet slides further down (11500) on steep east facing terrain, neither one ran nearly as far as the first.  Skiing on the way out was heavy and wet.  Lakes were still safely crossable with some care in the afternoon.  Temperatures were above freezing at all elevations by 1130.

Red Cone, East>Crystal Lake>TJ Bowl (4/20/16) – Snow surface thawed and wet throughout: easterly aspects 2-5 cm, southerly aspects 5-10cm. Temperature 47 degrees at 400pm (9,700’).  Skier triggered Loose Wet releases from the Crest (Crystal Chute area), ran ~200’. Looked to have been triggered 4/19/16. Signs of previous Loose activity as the last storm cleared on North thru East aspects. Numerous recent Loose Wet releases on easterly aspects below the crest with one small Wet Slab (~30’ x 100’) triggered by a Loose Wet release, elevation 10,300. Lakes are beginning to open, lake ice is becoming suspect. Creeks beginning to open up below ~9,000’. 

weather

Thurs-Friday: High-pressure ridge remains in place over the region through Thursday. Winds will become breezy Thursday afternoon, then strong and gusty Thursday night into Friday as an area of low-pressure moves onto the west coast with peak wind gusts of 40 to 45 mph. Models are having difficulty with its exact placement, which is common for springtime closed lows encountering ridges. While confidence in the exact details is low, be prepared for cooler and wetter conditions Friday with a slight chance for thunderstorms Friday afternoon. Snow levels, 7500-9000 feet Friday afternoon, expect snow levels to drop a bit further Friday night.

Sat-Sunday: The more northerly track of the storm means it will likely eject eastward quicker with the majority of the precipitation over by Saturday afternoon. Below normal temperatures will continue into Saturday with breezy northwest winds. A pattern of cool and cloudy days will continue thru the weekend as a series of weak shortwave lows drop across the region. Temperatures will fall around 5 to 10 degrees below season averages. Breezy winds are also possible with the best chances for gusty conditions expected by this Sunday afternoon.

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 THEN BECOMING PARTLY CLOUDY. PARTLY CLOUDY THEN BECOMING MOSTLY CLOUDY. SLIGHT CHANCE OF RAIN AFTER MIDNIGHT. SNOW AND RAIN
Temperatures: 54 TO 62. deg. F. 27 TO 34 deg. F. 42 TO 50 deg. F.
Wind direction: SOUTH SOUTH SOUTHWEST
Wind speed: 15 TO 25 MPH. GUSTS UP TO 30 MPH INCREASING TO 40 MPH IN THE AFTERNOON. 40 TO 50 MPH. GUSTS UP TO 65 MPH INCREASING TO 80 MPH AFTER MIDNIGHT 40 TO 50 MPH WITH GUSTS TO 80 MPH.
Expected snowfall: 0 in. 0 in. UP TO 3 in.
Over 10,000 ft.
  Thursday Thursday Night Friday
Weather: SUNNY THEN BECOMING PARTLY CLOUDY. MOSTLY CLOUDY SNOW
Temperatures: 46 TO 54 deg. F. 25 TO 31 deg. F. 36 TO 42 deg. F.
Wind direction: SOUTH SOUTH SOUTHWEST
Wind speed: 20 TO 25 MPH WITH GUSTS TO 40 MPH INCREASING TO 30 TO 35 MPH WITH GUSTS TO 50 MPH IN THE AFTERNOON. 45 TO 50 MPH WITH GUSTS TO 75 MPH INCREASING TO 55 TO 60 MPH WITH GUSTS TO 90 MPH AFTER MIDNIGHT. 55 TO 65 MPH WITH GUSTS TO 95 MPH.
Expected snowfall: 0 in. 0 in. UP TO 4 INCHES 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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