Eastern Sierra Snowpack Summary - Apr 14 2016

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THIS ADVISORY EXPIRED ON April 16, 2016 @ 6:56 am
Avalanche Advisory published on April 14, 2016 @ 6:56 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.

Moderately strong Southwest to West winds forecasted for Thursday into Thursday night may form isolated Wind Slabs near ridgelines, in depressions and gullies, or adjacent to terrain features in the upper elevation on NW-NE-SE-S aspects initially. As winds shift to the North Friday, drifting will shift to E-S-W aspects. If traveling in the upper elevations, watch for recently formed tender Wind Slabs. These tender pockets likely wont be big enough to result in a burial but they could entrain or cause a slide into hazardous terrain with potentially high consequences. 

Additional considerations: Cool temperatures forecasted thru Friday will limit the snow surface thawing in the mid to upper elevations, which may leave the surface firm and an elevated risk of slide for life conditions. Use extra caution while traveling in the mid to upper elevations, especially above exposed terrain, have the necessary equipment (i.e., crampons, ice axe, self-arrest grips), and practice self-arrest in a low risk environment.  

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.

Cool and windy weather will limit Loose Wet avalanche concerns to lower elevations and very protected mid-elevation terrain where the sun can thaw the snowpack sufficiently. Melt freeze crusts on E-S-W aspects will be frozen and firm early, thawing and softening through the day. Easterly aspects may soften by mid morning, with southerly aspects softening by mid-day, and westerly slopes softening by the afternoon. Elevation, cloud cover and wind can greatly affect how much the snow surface thaws. Lower elevation slopes will soften more quickly (below 8500’) with the risk of wet loose activity increasing by the afternoon on slopes >35 degrees. Plan your travel to avoid slopes before the snow thaws too much. Punchy, manky snow past boot-top, and significant recent rollerball activity are indicators the threat of wet loose releases is increasing. Natural and human triggered wet loose avalanches maybe possible at the lower elevations and very protected mid-elevations on easterly aspects in the late morning, southerly aspects midday, and westerly by afternoon.

 

Snowpack Discussion

Primary avalanche issues for Thursday thru Friday will focus on isolated Wind Slabs in wind prone areas in the mid to upper elevation forming on NW-NE-SE aspects Thursday, N-E-S aspects Thursday night as winds shift to the west, W-S-E aspects Friday as winds veer to the North. Secondary concern is for isolated Wet Loose avalanches on solar aspects (E/S/W aspects), primarily in the lower elevations but may include isolated protected slopes in the mid-elevation.

Snowpack – Last weekend’s round of weather brought limited 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. In the higher elevations, a very narrow range of N-NE aspects are still holding onto colder snow conditions. Mid-elevations the snow surface is transitioning toward corn (especially solar aspects) while the Lower elevations have undergone extensive melt-freeze cycles and has consolidated into granular melt/freeze snow (corn snow) as the snowpack recedes with creeks opening up. 

Additional considerations: Cool temperatures forecasted thru Friday will limit the snow surface thawing in the mid to upper elevations, which may leave the surface firm and an elevated risk of slide for life conditions. Use extra caution while traveling in the mid to upper elevations, especially above exposed terrain, have the necessary equipment (i.e., crampons, ice axe, self-arrest grips), and practice self-arrest in a low risk environment.  

 

recent observations

Mt. Locke / Wahoo Gullies (4/10/16) - Widespread natural shallow (surface - 6" depth) wet slides were observed on most East/Northeast aspects yesterday. Small point-releases and roller balls accumulating were the trigger. Debris piles were several hundred feet high, but not very deep, nor very set up/frozen (actually were skinable/skiable).

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 natural 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.

 

weather

Thurs-Friday: Expect breezy conditions today into Friday with very isolated snow showers with a trace to an inch limited to the higher elevations, favoring northern Mono County. Winds will remain breezy today mainly out of the west with gusts generally up to 40 mph and winter like temperatures of teens for Sierra Valleys and single digits for the higher elevations can be expected. Winds peak this afternoon with the passage of a surface cold front; there could be isolated gusts up to 50 mph during frontal passage especially for more wind prone locations. Becoming more northerly tonight and Friday, conditions remain brisk with gusts falling a bit. 
 
Sat-Sunday: Ridging begins build by Saturday heralding a drying and warming trend as winds abate. While surface flow remains more easterly, temperatures will begin to rebound with the higher April sun angle. Expect temperatures in the Sierra reaching into 60s, which will increase snowmelt runoff this weekend.

 

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: SUNNY THEN BECOMING PARTLY CLOUDY. SCATTERED SNOW SHOWERS IN THE MORNING. ISOLATED SNOW SHOWERS IN THE AFTERNOON. PARTLY CLOUDY THEN BECOMING MOSTLY CLOUDY. ISOLATED SNOW SHOWERS IN THE EVENING. MOSTLY CLOUDY THEN BECOMING PARTLY CLOUDY. SLIGHT CHANCE OF SNOW SHOWERS IN THE MORNING.
Temperatures: 34 TO 42 deg. F. 13 TO 20 deg. F. 32 TO 40 deg. F.
Wind direction: SOUTHWEST WEST BECOMING NORTHWEST AFTER MIDNIGHT. NORTH
Wind speed: 40 TO 45 MPH WITH GUSTS TO 70 MPH BECOMING WEST 30 TO 35 MPH WITH GUSTS TO 55 MPH IN THE AFTERNOON. 35 TO 40 MPH WITH GUSTS TO 60 MPH, 25 TO 30 MPH WITH GUSTS TO 50 MPH AFTER MIDNIGHT. 30 TO 40 MPH. GUSTS UP TO 50 MPH INCREASING TO 60 MPH IN THE AFTERNOON.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Thursday Thursday Night Friday
Weather: SUNNY THEN BECOMING PARTLY CLOUDY. ISOLATED SNOW SHOWERS. PARTLY CLOUDY THEN BECOMING MOSTLY CLOUDY. ISOLATED SNOW SHOWERS AFTER MIDNIGHT. PARTLY CLOUDY. SLIGHT CHANCE OF SNOW SHOWERS IN THE MORNING.
Temperatures: 27 TO 35 deg. F. 7 TO 14 deg. F. 22 TO 28 deg. F.
Wind direction: SOUTHWEST BECOMING WEST WEST NORTH
Wind speed: 55 TO 60 MPH WITH GUSTS TO 80 MPH, DECREASING TO 45 TO 50 MPH WITH GUSTS TO 75 MPH IN THE AFTERNOON. 35 TO 45 MPH. GUSTS UP TO 70 MPH DECREASING TO 60 MPH AFTER MIDNIGHT. 30 TO 35 MPH WITH GUSTS TO 55 MPH INCREASING TO 40 TO 45 MPH WITH GUSTS TO 70 MPH IN THE AFTERNOON.
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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