Eastern Sierra Snowpack Summary - Apr 07 2016

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THIS ADVISORY EXPIRED ON April 9, 2016 @ 7:00 am
Avalanche Advisory published on April 7, 2016 @ 7:00 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: The threat of Loose Wet on solar aspects in the mid to upper elevations continues through Thursday. They will most likely be encountered mid-morning thru late afternoon, depending on aspect and localized heating. Generally, the hazard rises in the morning for E-SE aspects, SE-SW mid-day, and westerly aspects by afternoon. Plan to be off these slopes before the snow thaws excessively (e.g. above boot top, signs of fresh roller balls, wet releases on similar aspects and elevations). Use extra caution while traveling in complex terrain with multiple aspects where localized heating can produce unstable snow conditions. These slides may be small in size but can overtake a rider and possibly carrying them into hazardous terrain. Weak nightly freezes, along with strong daily warming at the lower elevations will permit more rapid thawing of the lower elevation snowpack, even northerly aspects. This scenario can potentially lead to isolated Loose Wet releases on all aspects, possibly to ground.

Friday (mid to upper elevations): As temperatures begin to cool and cloud cover increases, the threat of Loose Wet releases will diminish. Lower elevations (below ~9000’) with overnight temperatures hovering above freezing and the potential for rain showers Friday, the snowpack will trend toward isothermal, loosing supportiveness, with the threat of Loose Wet avalanches continuing thru Friday on all aspects at the lower elevations. 

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.

Friday: Forecasted new snow (~ up to 3”) and Southeast winds of 25-30 mph will possibly form fresh sensitive wind slabs on SW-NW-NE aspects, expanding to include E aspects as winds swing to the South by Friday afternoon. Wind slabs will likely be found leeward slopes, below ridgelines, around terrain features, or crossloaded gullies & depressions. Assess wind slab instability while ascending and prior to committing to big mountain faces with hazardous terrain. Quick hand pits can give you a good indication of how well fresh wind slabs are bonding to the adjacent snow interface. 

Snowpack Discussion

Primary avalanche issues for Thursday will be focused on Loose Wet avalanches on solar aspects as the temperatures rise and the snow thaws and very isolated wind slabs in the upper elevations. Friday’s avalanche concerns will depend on elevation and aspect: ~9000’ and below, wet loose on all aspects. Above~9000’, possible wind slab formation on leeward slopes (SW-NW-NE-E aspects) deposited on melt freeze crust on solar aspects or on firm windpack found on northerly aspects in the mid to upper elevations. Either interface can slow bonding and act as a good bedsurface.

Snowpack – The snowpack continues to transition toward spring with corn cycles setting up between spring snowfalls. At the mid and upper elevations, the snowpack consists of a relatively strong mid and upper snowpack with a snow surface of melt-freeze crust on solar aspects and firm windpack, wind board, with pockets of shallow soft snow on more northerly facing aspects. Lower elevations (~9,000’ and below), the snowpack is thinning quickly and consists of mostly granular melt/freeze snow. 

recent observations

Valentine Cirque (4/2/16) - Skinned up from Valentine Lake trailhead to half way up the Parachute and back.  Snow has melted at the bottom, so it was walking for the first half mile or so.  Skinning included a lot of walking over short stretches of dirt and rock on the way up the moraine.  By the time you get to lake level (8500 ft.) it is all skiing.  We ran a bit late and by the time we got to the cirque things were pretty wet.  Lots of recent, probably yesterday, rollerball activity on north and east aspects up high (starting in the 10,300 to 11,000 ft. range).  Some small wet slides from the last few days as well, these were on E and S aspects.  Boot penetration in the chute ranged from 5 to 60 cm.  There were pockets of hip deep post holing down low that were worrisome, higher it was more supportable.  We turned around at about 11,500 at 11:45 where the chute gets steepest and narrowest.  Snow was too soft and wet to travel well.  The whole chute was filled with rollerballs and made for mediocre heavy skiing.  Where it opened up below was good corn.  Snow was sticky in places around lake level on the way out.  Winds were light but consistent out of the SW, no blowing snow, temps were above freezing at all elevations but had obviously frozen last night.

 

weather

Thurs-Friday: Thursday will be the warmest day this year over most area, before temperatures begin to cool several degrees each day through Saturday. The limiting factor will be bands of cirrus spreading north Thursday afternoon. Friday, moisture (high precipitable water values near 0.75”) will surge northward across eastern CA-western NV, with shower activity increasing especially during the afternoon with a small possibility of isolated thunder. Friday night, upper level forcing will lead to increasing precipitation during the evening and overnight hours with snow levels starting above 9000 feet.

Saturday: the main upper low is projected to move inland over southern CA with an increased potential for afternoon-early evening thunderstorms. Snow levels will lower to near 8000-8500 feet by early Saturday morning, possibly drop to near 7000 feet at times where thunderstorms or heavier shower bands are present.

Sunday: a broad trough persists over the intermountain west with enough lingering moisture for showers across the Sierra. Showers should increase in coverage somewhat during the afternoons/evenings when instability is enhanced by daytime heating with snow in the higher elevations and passes of the Sierra.

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: PARTLY CLOUDY THEN BECOMING MOSTLY CLOUDY. CLOUDY. SLIGHT CHANCE OF SHOWERS AFTER MIDNIGHT. CLOUDY. CHANCE OF SHOWERS IN THE MORNING...THEN SHOWERS LIKELY IN THE AFTERNOON.
Temperatures: 56 TO 62 deg. F. 29 TO 36 deg. F. 48 TO 54 deg. F.
Wind direction: SOUTHEAST SOUTHEAST SOUTH
Wind speed: 15 TO 20 MPH. GUSTS UP TO 30 MPH IN THE AFTERNOON. 15 TO 25 MPH WITH GUSTS TO 35 MPH. 15 TO 20 MPH WITH GUSTS TO 30 MPH IN THE MORNING BECOMING LIGHT.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Thursday Thursday Night Friday
Weather: PARTLY CLOUDY THEN BECOMING MOSTLY CLOUDY. CLOUDY. SLIGHT CHANCE OF SNOW SHOWERS AFTER MIDNIGHT. CLOUDY. SNOW SHOWERS LIKELY IN THE MORNING...THEN SNOW SHOWERS IN THE AFTERNOON.
Temperatures: 46 TO 56 deg. F. 27 TO 33 deg. F. 39 TO 47 deg. F.
Wind direction: EAST SOUTHEAST SOUTHEAST
Wind speed: 20 TO 30 MPH WITH GUSTS TO 40 MPH. 20 TO 30 MPH WITH GUSTS TO 45 MPH. 25 TO 30 MPH WITH GUSTS TO 45 MPH BECOMING SOUTH 10 TO 15 MPH WITH GUSTS TO 25 MPH IN THE AFTERNOON.
Expected snowfall: 0 in. 0 in. 1 TO 3 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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