Eastern Sierra Snowpack Summary - Apr 02 2016

THIS ADVISORY EXPIRED ON April 4, 2016 @ 5:33 am
Avalanche Advisory published on April 2, 2016 @ 5:33 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.

As temperatures begin to rise over the next two days (Sierra Valleys in the 60’s), there will be an increased threat of Loose Wet on solar aspects. They will most likely be encountered mid-morning to mid-day, 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). Use extra caution while traveling in complex terrain with multiple aspects where localized heating can produce extensive loose slides. These slides may be small in size but can overtake a rider and possibly carrying them into hazardous 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.

Isolated wind slabs are the key concern in the mid to upper elevations with forecasted Southwest winds Sunday favoring isolated wind slab formation on NW-N-SE aspects where the recent new snow is transported onto leeward slopes, below ridgelines, settles into drifts around terrain features, and can crossload gullies & depressions. Assess Wind Slab instability while ascending and prior to committing to big mountain faces with hazardous terrain.


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 very icy and makes a prime bedsurface. The Low that lingered for a good part of last week delivered 2-7” of snow throughout the region with the new snow still forming bonds to the underlying snow interface. 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.

recent observations

- Treasure Lakes Basin via South Lake Road (March 30, 2016), objective - Mt. Johnson but retreated due to poor visibility. South Lake TH was approximate. 30-minute skin from where we parked below Parchers Resort. Summer trail had thin snow cover immediately above the lake but coverage improved quickly. Cloud ceiling was in and out of the Treasure Lakes basin throughout the morning and the clouds finally settled in the South Fork drainage around 1:00 p.m. Snowfall intensity increased from periodic showers (S-1) to steady snow (S1) by 2:00 p.m. Winds were North throughout the day. Calm to Light in the a.m. increasing to Moderate in the afternoon. 3-4" light new snow below treeline. 6-10" new snow above treeline. Beneath new snow was primarily a firm old snow surface (MfCr and old wind pack).We observed several recent D1 Loose Dry avalanches on North aspects in very steep terrain, especially beneath rock-bands and originating in small couloirs. Active light snow transport by North winds above treeline. Great powder skiing on North aspects with very little wind effect.

- Tour- June Lake Ski Area>Hourglass>Fern Creek>Yost Lake>June Lake Ski Area (3/29/16) 1200, Skies clear, Early melt cycle snow on E-S-W aspects, ~10,000’ and below. Ski penetration 2-4 cm. NE-N aspects, ~ 9700’ still cool with mix of windboard, wind scallops, soft facets, and firm wind slab. Upper Hourglass light melt freeze crust forming, temps remain cool, light melt-freeze crust over 2-4cm new snow, patchy in places due to wind, snow wind loaded onto North-Northeast aspects. Old melt-freeze crust on Southeast to South aspects. Summit ridge (1400) Partly Cloudy, clouds building from the SW, light to moderate SW winds, and upper descent into Fern Creek mixture of Sastrugi, ice, and patches of freshly transported snow, down to ~10,200. Below well formed ribbons of soft snow (5-10cm) over very icy/firm base to Fern Lake. Fern Lake (1430), mostly cloudy, snow flurries forming over the higher elevations. 1-3cm of new snow over breakable melt-freeze crust over 4-8cm loose dry to moist snow underneath, depending on aspect from Fern Lake to ski area boundary. Test slopes yielded no results. Light snow transport noted during the day on N-NE aspects. 


Sat-Sunday: Upper level ridge will maintain unseasonably warm temperatures this weekend. High temperatures will rise 10-15 degrees above seasonal averages this afternoon with high temperatures reaching the low 60's for Sierra valleys. Winds will be light through Sunday morning, with Zephyr breezes peaking around 20 mph. The upper ridge will become a little less amplified as a weak shortwave trough passes north of the region today with some cumulus development this Saturday afternoon with low prospects for precipitation. Sunday increasing high clouds should hinder surface based instability and will likely again just see some cumulus development during the afternoon hours.

Mon-Tuesday: Monday, a approaching fast moving shortwave is tracking along the Oregon border with the bulk of the colder air staying in Oregon. Most of next week looks dry with above average temps as high pressure off the west coast builds into the northern Great Basin. Tuesday, light NE-E surface winds will keep temps a bit cooler (although still above average), with highs mainly in the mid-upper 60's for the lower elevations. 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Saturday Saturday Night Sunday
Temperatures: 53 TO 59 deg. F. 25 TO 32 deg. F. 53 TO 59 deg. F.
Wind speed: 10 TO 15 MPH
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Saturday Saturday Night Sunday
Temperatures: 43 TO 53 deg. F. 24 TO 30 deg. F. 42 TO 52 deg. F.
Expected snowfall: 0 in. 0 in. 0 in.

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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