Eastern Sierra Snowpack Summary - Apr 11 2016

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THIS ADVISORY EXPIRED ON April 14, 2016 @ 7:27 am
Avalanche Advisory published on April 11, 2016 @ 7:27 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.

The recent weather event ushered in warm temperatures and a mix of rain and snow at the lower elevations, wet snow in the mid to upper elevations.  The underlying snow was wet and warm, which the new snow in the mid to upper elevations has yet to fully bond to. The mild overnight temperatures are preventing the snowpack from freezing solidly between cycles and is rapidly thawing during the day. This has produced extensive Loose Wet releases throughout the mid to upper elevations. Rider triggered sloughing in steeper terrain where the new snow hasn't had sufficient time to form bonds to the underlying snow is likely. At the lower elevations, the weak nightly freezes continues to maintain the threat of isolated Loose Wet releases on steep slopes, especially solar aspects.   
 
Plan to be off steep solar exposed slopes before they soften to the point that a skier or rider could sink in up to their boot top. Once this happens, the likelihood for natural and human triggered wet point releases increases, which may have the potential to lead to larger slope failures. Even small releases can force a rider off balance and take them into undesirable terrain.  Look out for rollerball / pinwheel activity as well as signs that slopes are becoming less stable. Slopes will soften quicker at lower elevations, and later at higher 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.

Mid to Upper Elevations: the recent snowfall from Friday thru Saturday was confined to the mid to upper elevations with 5 to 15 cm of new (more in favored locations) with moderate Southerly winds, which produced fresh wind slabs on W-N-E aspects.  Mild temperatures since have helped the new snow to bond to the underlying snowpack but isolated tender wind slabs may persist in the mid to upper elevations. Wednesday winds are forecasted to increase in advance of an approaching storm system, which will likely form new sensitive wind slabs on NW-NE-SE aspects, which will most likely be encountered on 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 Monday-Wednesday will depend on elevation (~10500’ and below), Wet Loose avalanches on all aspects, especially solar aspects. From ~ 10500’ and above, isolated Wind Slabs on NW-NE-SE aspects deposited over a melt freeze crust on solar aspects or on firm windpack found on northerly aspects in the upper elevations. Either interface can act as a good bedsurface. As winds increase Wednesday, fresh Wind Slabs will likely form in the mid to upper elevations on NW-NE-SE aspects. 

Snowpack – The recent round of weather has added some new snow over the higher elevations with rain below ~9’000’. Mid and upper elevations received a shot of wet snow (5-15cm, more in favored locations) over alternating layers of melt-freeze crusts and snow. Lower elevations have undergone a series of melt-freeze cycles and has consolidated into granular melt/freeze snow (corn snow) and received significant rainfall from this last spring storm, which resulted in extensive melting and thinning with creeks opening up and snowbridges disappearing. 

recent observations

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 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. 
 
Laurel Mountain, Pinner Couloir (4/9/16): The southeast facing skin up is devoid of snow. Hit snow at 9800' or so, north face pretty scoured by previous wind event (4-6" of new snow). Not enough to cover all the rocks so we booted that too. Pinner- 6" of fresh on a crust. Got Mendenhall and another test slope to slide on the crust and got shooting cracks and other signs of instability on Pinner so we bailed and mostly booted back down. On the way down we set off wet slides and saw natural events. Was the only way down so we skied the slide. Snow that was sliding seems to be last night/today + wind loading that may have occurred. Avy conditions got much worse through the day today, despite being fogged in all day and not warm.

 

weather

Monday: expect to see another round of afternoon and evening showers, with isolated thunderstorms possible, as moist conditions remain entrenched in northeast California. The best chance for thunderstorms will be in the northern Sierra and northeast California for mid to late afternoon. 
 
By Tuesday: flow aloft turns to westerly as the upper low over southern California and Nevada exits into Arizona and the next round of Pacific energy approaches the Pacific Northwest with Mono and Mineral the best shot for showers Tuesday as flow converges off the Sierra in the afternoon.
 
Wednesday: winds will be on the increase as a vigorous system approaches the Pacific Northwest coast with strong downslope winds possible Wednesday night in the Sierra Front from Reno-Sparks down into Mono County as the system`s cold front nears and a 100+ kt jet passes overhead.  The system quickly exits the region, so precipitation amounts do not look overly high. 
 
Thursday: an upper trough starts to infiltrate into the region with snow levels falling to ~ 5000 feet early Thursday then rise during the day. The core of the coldest air now looks to be confined to far northern and northeast parts of NV with a secondary wave is forecasted to moving south on the back side of the trough. 
 

 

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. ISOLATED SHOWERS IN THE MORNING...THEN SCATTERED SHOWERS AND SLIGHT CHANCE OF THUNDERSTORMS IN THE AFTERNOON. MOSTLY CLOUDY THEN BECOMING PARTLY CLOUDY. SCATTERED SHOWERS AND SLIGHT CHANCE OF THUNDERSTORMS IN THE EVENING. PARTLY CLOUDY. SLIGHT CHANCE OF SHOWERS IN THE AFTERNOON.
Temperatures: 45 TO 53 deg. F. 26 TO 33 deg. F. 48 TO 56 deg. F.
Wind direction: LIGHT WINDS LIGHT WINDS LIGHT WINDS BECOMING SOUTHWEST
Wind speed: 10 TO 15 MPH IN THE AFTERNOON.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Monday Monday Night Tuesday
Weather: MOSTLY CLOUDY. ISOLATED SNOW SHOWERS IN THE MORNING...THEN SCATTERED SNOW SHOWERS AND SLIGHT CHANCE OF THUNDERSTORMS IN THE AFTERNOON. MOSTLY CLOUDY THEN BECOMING PARTLY CLOUDY. SCATTERED SNOW SHOWERS AND SLIGHT CHANCE OF THUNDERSTORMS IN THE EVENING. SUNNY THEN BECOMING PARTLY CLOUDY. SLIGHT CHANCE OF SNOW SHOWERS IN THE AFTERNOON.
Temperatures: 38 TO 44 deg. F. 25 TO 31 deg. F. 41 TO 47 deg. F.
Wind direction: LIGHT WINDS LIGHT WINDS LIGHT WINDS BECOMING SOUTHWEST
Wind speed: 10 TO 15 MPH WITH GUSTS TO 25 MPH IN THE AFTERNOON.
Expected snowfall: UP TO 2 INCH in. UP TO 1 INCH 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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