Eastern Sierra Snowpack Summary - Apr 04 2016

THIS SNOWPACK SUMMARY EXPIRED ON April 7, 2016 @ 7:04 am
Snowpack Summary published on April 4, 2016 @ 7:04 am
Issued by - 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.

As temperatures begin to rise over the next two days (Sierra Valleys low 70’s possible), it will increase the threat of Loose Wet on solar aspects in the mid to upper elevations. 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 releases on all aspects, possibly to ground.

A cautionary reminder: early morning spring snow conditions can be very firm and exceedingly icy with the potential for a minor slip and fall resulting in potentially high consequence slides. Traveling safely under these conditions in steeper complex terrain requires having the proper gear (ice axe, self arrest grips, and crampons) and good self-arrest skills.

 

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.

Isolated wind slabs are a concern in the mid to upper elevations with forecasted West winds (20 to 25 MPH) Monday veering to the Northeast (15 to 20 MPH) Tuesday favoring isolated wind slab formation on N-E-S aspects Monday, shifting to SE-SW-NW aspects Tuesday into Wednesday as recent snowfall is redistributed by the wind onto leeward slopes, below ridgelines, settles into drifts around terrain features, or cross load 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 issue is focused on Loose Wet avalanches on solar aspects as the temperatures rise and the snow thaws. Secondary concern is for recently formed isolated Wind Slabs deposited on the 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 is starting another corn cycle as the recent new snow begins to transform into corn snow. Mid and upper elevation snowpack is tending toward a spring snowpack with multiple melt-freeze crusts and a relatively strong upper and mid snowpack. The old/new snow interface is either a very firm melt-freeze crust on solar aspects or firm windpack, either makes a prime bedsurface. The Low that lingered for a good part of last week delivered 2-7” of snow throughout the region with minimal wind. Last week’s cool temperatures and occasional light snowfalls have helped slow the steady retreat of the snowline at the lower elevations (~7,700’ - ~9,000’) with no major layering other than the recent new snow. Low temperatures at night have been quite mild with the snowpack having less time to solidly refreeze. 

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

Mon-Tuesday: A shortwave trough will push across the Pacific Northwest coast today (Monday) producing increased high clouds and breezy north to northwest winds. Areas of weak instability showers maybe possible across portions of the Sierra in Mono County with light showers possible into early evening. Temperatures will remain mild Monday with highs in the low 60s for Sierra valleys. Tuesday, daytime temperatures will cool about 3 to 5 degrees as winds shift to the north-northeast behind the departing shortwave trough.

Wed-Thursday: This cool down will be short lived as an upper level ridge rebuilds by Wednesday with dry and mostly clear conditions. High temperatures will warm 10 to 15 degrees above normal by Wednesday afternoon with mid to upper 60's for Sierra Valleys. Thursday will continue to be warm as the ridge axis slides off to the east with a weak south flow aloft over the area with some Sierra valleys approaching 70. 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
  Monday Monday Night Tuesday
Weather: PARTLY CLOUDY PARTLY CLOUDY SUNNY
Temperatures: 53 TO 61 deg. F. 23 TO 30 deg. F. 51 TO 57 deg. F.
Wind direction: WES SOUTHWEST EAST
Wind speed: 10 TO 15 MPH 10 TO 15 MPH 10 TO 15 MPH
Expected snowfall: 0 in. 0 in. 0 in.
For 8000 ft. to 9000 ft.
  Monday Monday Night Tuesday
Weather: PARTLY CLOUDY PARTLY CLOUDY. ISOLATED SHOWERS IN THE EVENING SUNNY
Temperatures: 44 TO 54 deg. F. 23 TO 30 deg. F. 46 TO 52 deg. F.
Wind direction: WEST WEST BECOMING NORTHWEST NORTHEAST
Wind speed: 15 TO 20 MPH WITH GUSTS TO 35 MPH 20 TO 25 MPH WITH GUSTS TO 40 MPH BECOMING,10 TO 15 MPH WITH GUSTS TO 25 MPH AFTER MIDNIGHT. 15 TO 20 MPH WITH GUSTS TO 30 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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