Eastern Sierra Snowpack Summary - Apr 16 2016

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THIS ADVISORY EXPIRED ON April 18, 2016 @ 7:07 am
Avalanche Advisory published on April 16, 2016 @ 7:07 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.

A strong over night freeze and slowly warming temperatures forecasted for Saturday will limit concern for Loose Wet releases to southerly aspects in lower elevations and protected mid elevation slopes. Sunday, as temperatures rebound toward seasonable, the threat of Loose Wet releases in the mid to lower elevations will rise and include East and West aspects from the additional heating. Elevation, aspect, cloud cover and wind can greatly affect how much the snow surface thaws. Generally, the surface snow will be frozen and firm early with easterly aspects softening mid morning, southerly aspects softening mid-day, and westerly slopes softening 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. Saturday, isolated natural and human triggered loose wet avalanches are possible at the mid to lower elevations on southerly aspects. Sunday, warming temperatures will expand the threat of Loose Wet releases to include easterly aspects in the late morning, southerly aspects midday, and westerly by afternoon.

Additional considerations: Spring conditions prevail throughout the region with firm snow conditions during the morning and an elevated risk of slide for life conditions. 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 environments.  

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.

Moderately strong SW-N winds Thursday into Saturday likely formed isolated Wind Slabs near ridgelines, in depressions and gullies, or adjacent to terrain features in the upper elevation on W-NW-NE-SE-S aspects. If traveling in the upper elevations, watch for recently formed tender Wind Slabs. These tender pockets likely wont be big enough to result in a burial but they could entrain or cause a slide into hazardous terrain with potentially high consequences. 

Snowpack Discussion

Primary avalanche focus for Saturday thru Sunday depends on aspect and elevation. Mid to Lower Elevations - Saturday, isolated natural and human triggered Loose Wet avalanches are possible at the mid to lower elevations on southerly aspects. Sunday, warming temperatures will expand the threat of Loose Wet releases to include easterly and westerly aspects. Upper elevations - isolated pockets of Wind Slabs in the upper elevations primarily on W-N-E-SE aspects or in exceptionally wind prone areas in the mid-elevations.

Snowpack – Last weekend’s storm system (4/7-4/8) deposited 3”to 6” of new snow over the higher elevations with rain below ~9’000’. Due to the unseasonably cool temperatures this past week, the new snow continues to consolidate and bond to the underlying melt-freeze crust and has slowed the surface melt-freeze corn cycle, especially at the upper elevations. In the higher elevations, N-NE aspects are still holding onto colder snow conditions with pockets of soft, wind board, and wind pack. Mid-elevations the snow surface continues to transition toward corn (especially solar aspects) while the Lower elevations have undergone extensive melt-freeze cycles and has consolidated into granular melt/freeze snow (corn snow) as the snowpack recedes.

Additional considerations: Spring conditions prevail throughout the region with firm snow conditions during the morning and an elevated risk of slide for life conditions. 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 environments. 

recent observations

Mt. Locke / Wahoo Gullies (4/10/16) - Widespread natural shallow (surface - 6" depth) wet slides were observed on most East/Northeast aspects yesterday. Small point-releases and roller balls accumulating were the trigger. Debris piles were several hundred feet high, but not very deep, nor very set up/frozen (actually were skinable/skiable).

Negatives (4/10/16): June Lake ski resort 3” new snow @ 9,200’ 4/9/16. Partly cloudy (scattered cumulous), temps above freezing, weak overnight freeze (Low briefly hit 28 @ June snow study site) light winds from the South-Southeast. On approach 5-10cm new wet snow on SW-SE aspects, 10,000’.  Observed a number of natural Loose Wet slides on ENE-SE-S aspects. New snow not adhering to the underlying melt-freeze crust. Melt-freeze crust not well frozen with moist snow underneath. Skier triggered Loose Wet on Negative 1 ran ~1000’. Test slopes produced small Loose Wet releases on 40-degree slopes. Extensive rollerball activity from previous day.

 

weather

Sat-Monday: Benign weather is forecast for the weekend into the beginning of next week with temperatures above seasonal averages. Northeasterly to easterly flow will keep Saturday`s highs around seasonal averages (mid 50’s for the Sierra) to slightly above average. Flow switches southwesterly by Monday with more typical warm-season thermal circulations for the afternoons. As a result, temperatures surge 10 degrees above average Sunday and up to 15 degrees above on Monday.

Tuesday:  Sharply amplified ridge sets up over western Nevada between a low over the Front Range of the Rockies and a trough trying to push into northern California. The models show it moving onshore Tuesday/ Wednesday with a few showers possible over the Sierra. This may open the door for additional storm systems to move inland next weekend. 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Saturday Saturday Night Sunday
Weather: SUNNY CLEAR SUNNY
Temperatures: 47 TO 53 deg. F. 24 TO 31 deg. F. 54 TO 60 deg. F.
Wind direction: NORTH NORTHEAST NORTHEAST
Wind speed: 25 TO 30 MPH WITH GUSTS TO 45 MPH. 15 TO 20 MPH WITH GUSTS TO 30 MPH. 10 TO 15 MPH.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Saturday Saturday Night Sunday
Weather: SUNNY CLEAR SUNNY
Temperatures: 39 TO 47 deg. F. 24 TO 30 deg. F. 45 TO 53 deg. F.
Wind direction: NORTH NORTHEAST NORTHEAST
Wind speed: 25 TO 35 MPH WITH GUSTS TO 45 MPH. 15 TO 25 MPH WITH GUSTS TO 35 MPH. 10 TO 15 MPH. GUSTS UP TO 25 MPH IN THE MORNING.
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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