Eastern Sierra Snowpack Summary - Apr 25 2016

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THIS ADVISORY EXPIRED ON April 28, 2016 @ 7:11 am
Avalanche Advisory published on April 25, 2016 @ 7:11 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center
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.

Sunday saw a brief period of light to moderate snowfall with an accumulation of trace to 3” of new snow in the Mid to Upper Elevations. Forecasted North to Northwest winds thru Tuesday may possibly form tender Wind Slabs on NE-SE-SW-W facing slopes in the Mid to Upper Elevations. Natural releases are unlikely, triggered releases are possible. Tender Wind Slabs will most likely be encountered below ridgelines, in gullies/depressions, and adjacent to terrain features that promote drifting. 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.

 

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.

Recent new storm snow has yet to fully bond to the underlying snow and may warm quickly under the intense spring sun on solar aspects as skies clear and temperatures begin to rise during the day. As the snow warms and the snow softens it will elevate the concern for Loose Wet avalanches in the Lower to Mid Elevations on slopes of ~ 35 degrees and steeper, especially on solar aspects. Any new snow, where present, may easily slough off the underlying snow if disturbed. Plan your travels to avoid slopes before the snow heats-up excessively. Punchy, manky snow past boot-top, and significant recent rollerball activity are indicators the threat of wet loose releases is increasing. Natural releases are unlikely but human triggered Loose Wet avalanches are possible in the mid to lower elevations, especially on solar aspects.

 

 

Snowpack Discussion

Primary avalanche focus for Monday – Wednesday: Isolated Wind Slabs in the mid to upper elevations on NE-SE-SW-W aspects. Mid to Lower Elevations - triggered Loose Wet avalanches are possible on solar aspects, especially Tuesday as temperatures warm.

Snowpack

Upper Elevations: prior to Friday’s snowfall (4/22/16), the upper elevation snowpack primarily on solar aspects was transitioning toward mature corn on East, South, and West aspects with North to Northeast aspects still holding onto to colder snow conditions with a variety of light crusts, windpack, wind board, and some soft facets. Friday’s storm delivered 2”-8” of new snow in the mid to upper elevations. Saturday the weather cleared and temperatures rose to form a thin breakable melt-freeze crust on all aspects but NW-NE aspects to ~ 10,500’. NW-NE aspects remained cool with soft snow over a spring snowpack. Friday’s storm snow has bonded well to the warm underlying snowpack with minimal signs of natural avalanche activity. Sunday saw additional showers with Trace to 3” of new snow in the mid to upper elevations with moderate West to Northwest winds.

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 at the lower elevations. Friday’s storm delivered light wet snow to light rain showers with little to no accumulation to the lower elevations.

Friday’s storm brought another round of wet spring snow with 2” to 8” being reported in the Mid to Upper Elevations with moderate to strong Southwest winds. The storm came in warm and the new snow bonded well to the underlying snowpack. Temperatures climbed into the 30’s and low 40’s Saturday thru Sunday forming a breakable melt –freeze crust on all aspects but NW-NE. Sunday saw additional snow showers over the higher elevations with Trace to 3” accumulation with light to moderate winds. Overall Sunday’s new snow should bond well to the underlying snowpack. Forecasted North to Northwest winds may form tender Wind Slabs on NE-SE-SW-W aspects in the Mid to Upper Elevations, especially in wind prone terrain. Lower to Mid Elevations: on solar aspects the snow may warm enough under the intense spring sun where triggered Wet Loose may be possible on terrain ~35 degrees and steeper. 

recent observations

Red Cone Bowl>McLeod Lake>Twin Lakes (4/23/16) – Snow surface 2’-8” of new snow over melt-freeze crust. At 1300, elevation 8600’, temperature 6.3C, new snow very moist on all aspects but steep open slopes on N-NE aspects. Extensive skier traffic with tracks visible in TJ Chute/Bowl and Red Cone Bowl. Snow triggered releases visible, signs of previous Loose activity visible that occurred during the storm. Noted a few rollerballs on a westerly aspect from a skier traversing a steep west facing slopes, 9,800’ elevation. New snow seems to have bonded well to the underlying snow with test slopes yielding no results. Lakes are beginning to open, lake ice is becoming suspect, especially around inlets and outlets. Creeks beginning to open up to ~9,000’.

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.

weather

Monday: An upper level low is currently centered across Nevada. Current water vapor and IR satellite show drier air aloft moving into northern California Monday confining light shower activity to along and west of the Hwy 395 corridor. Brisk northwest winds will continue Monday with steady 15-25 mph winds expected with gusts around 30-35 mph possible. These northerly winds may produce some terrain enhanced snow showers across Mono County, diminishing by Monday evening.

Tues - Wednesday:  Drier conditions expected for Tuesday as shortwave ridging briefly builds aloft followed by the next upper level Low which is expected to arrive Tuesday night into Wednesday. The system will be a little warmer with limited significant accumulations and isolated embedded thunderstorms possible Wednesday afternoon.

Thurs-Friday: The upper level Low begins to slowly slides east on Thursday, with additional showers and a few thunderstorms possible through Thursday with peak instability in the afternoon. Snow levels will remain around 7000 feet, although some stronger showers could bring snow levels down to around 6000 feet at times. By Friday, clearing skies and temperatures rising into upper 50s in the Sierra. 

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: MOSTLY CLOUDY. SCATTERED SNOW SHOWERS. PARTLY CLOUDY. ISOLATED SNOW SHOWERS IN THE EVENING. SUNNY THEN BECOMING PARTLY CLOUDY.
Temperatures: 28 TO 36 deg. F. 13 TO 20 deg. F. 41 TO 47 deg. F.
Wind direction: NORTHWEST NORTHWEST NORTHWEST
Wind speed: 20 TO 30 MPH WITH GUSTS TO 40 MPH. 15 TO 25 MPH WITH GUSTS TO 40 MPH. 15 TO 20 MPH WITH GUSTS TO 30 MPH.
Expected snowfall: UP TO 2 INCHES in. 0 in. 0 in.
Over 10,000 ft.
  Monday Monday Night Tuesday
Weather: MOSTLY CLOUDY. SCATTERED SNOW SHOWERS. PARTLY CLOUDY. SCATTERED SNOW SHOWERS IN THE EVENING. SUNNY THEN BECOMING PARTLY CLOUDY.
Temperatures: 20 TO 26 deg. F. 11 TO 17 deg. F. 35 TO 41 deg. F.
Wind direction: NORTHWEST NORTH NORTHWEST
Wind speed: 20 TO 30 MPH WITH GUSTS TO 45 MPH. 20 TO 30 MPH WITH GUSTS TO 45 MPH. 15 TO 20 MPH WITH GUSTS TO 35 MPH.
Expected snowfall: UP TO 2 INCHES 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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