Eastern Sierra Snowpack Summary - 2016-02-27 06:56

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THIS ADVISORY EXPIRED ON February 29, 2016 @ 6:56 am
Avalanche Advisory published on February 27, 2016 @ 6:56 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.

A melt freeze crust on E-S-W aspects, which will thaw and soften throughout the day with sun exposure (easterly mid morning, southerly aspects mid-day, and westerly slopes later in the afternoon). The forecasted moderate winds Saturday will delay thawing, especially in the upper elevations. 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. As the snow thaws 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.  Additional red flags: rollerball/pinwheel activity. Slopes will soften more quickly at lower elevations while higher elevations later in the day.  At elevations below 8500’, the snow will warm rapidly on all aspects (even north) with risk of wet loose activity increasing on slopes >35 degrees by the afternoon. Natural and human triggered wet loose avalanches maybe possible, starting on southeast slopes in the morning and moving around the compass toward southwesterly by afternoon.

 

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.

Upper elevations: be on the lookout for isolated small fresh windslabs on steep exposed terrain, which may be prone to triggering. While isolated and relatively small, may be enough to knock a skier or boarder off balance and resulting in a fall into hazardous terrain. These will most likely be found near ridgelines on northerly to easterly facing slopes. 

 

Snowpack Discussion

The main avalanche concern for this period continues to be wet loose releases due to the warm temperatures, mild nightly freezes, and sunny skies which increases the potential for wet releases on southerly/sunny aspects (SE-S-SW-W) beginning late morning and into the late afternoon depending on sun exposure. Additionally, there is the potential for isolated small fresh windslabs in upper elevation terrain near ridgetops on easterly to northerly facing slopes. Forecasted cloud cover may slow the thawing of the surface snow. 

The High pressure ridge that’s governed the weather picture in our region this past week is showing a signs of slight weakening, which will allow a small disturbance to move into Northern CA Sunday night with increasing clouds for the region. Expect partly cloudy to mostly cloudy skies, winds decreasing and above average temperatures of upper 30’s to 40’s at mid and upper elevations, mid 40’s to low 50’s in the lower elevations, nightly lows in the 20’s to low 30’s.

A melt freeze crust has formed on E-S-W aspects will thaw and soften through the day as the sun moves across the horizon. You can expect easterly slopes to soften by mid morning, southerly aspects softening by mid-day, and westerly slopes softening by the afternoon. Saturday, moderate Westerly winds (20 to 30 mph, gusts 40’s, in the afternoon) and cloud cover will generally slow the softening above 10,500’. However, isolated shelter locations may thaw more quickly. Sunday, winds are forecast to subside and a weak freeze is expected, which could result in more rapid thawing and increase the potential for wet loose releases.

Plan to be off steep solar exposed slopes before they thaw. A good indicator is: if a skier or rider sinks up to their boot top, the risk of wet loose releases is rising and the likelihood for natural and human triggered releases is increasing, which may have the potential to lead to larger slope failures.  Remember, small releases may not be large enough to bury a rider but it can force a rider off balance, resulting in a slide into hazardous terrain. Red Flags: recent rollerball / pinwheel activity. Lower elevation slopes will soften quicker (below 8500’) with the snow warming rapidly on all aspects and the risk of wet loose activity increasing on slopes >35 degrees by the afternoon.  Natural and human triggered wet loose avalanches maybe possible, starting on southeast slopes at mid to upper elevations in the late morning and moving around the compass toward southwesterly by afternoon.

Upper elevation exposed terrain: lookout for the recently formed isolated small windslabs that may be tender enough to be triggered by a rider, though not big enough to result in a burial, again may be enough to knock a skier or boarder off balance, resulting in a slide into terrain with potentially high consequences. These will most likely be found near ridgelines on northerly to easterly facing slopes. 

Additional considerations: firm snow conditions on steep slopes where a fall could result in a slide-for-life.  Be especially cautious as you travel in steeper more complex terrain where a fall could result in injury or even death. East to south to west facing slopes are firm and frozen prior to thawing, northerly facing slopes may have very firm wind-board that can make it difficult to arrest a fall. If traveling in exposed steep terrain, make sure you have the proper equipment (ice axe, self-arrest grips, crampons, etc.), know how to use them effectively, use extreme caution, and have a plan should something go wrong.

recent observations

McGee Creek, Baldwin (2/25/16) - Lower elevations melting out quickly, especially where exposed to direct sun. Minor rockfall from southerly facing cliffs. No natural rollerball activity but moderate ski-induced rollerballs up to 2' diameter.  Timing descent is key to mitigating wet instability and less ski penetration on decent. Skiing/boarding was still very good soft spring quality. 

Sherwin Forest, Punta Bardini (2/23/16) - 8800', WNW aspect: surface snow verging on dry in the shade. Small rollerballs. Some thin (<15cm thick) wind slabs cracking with heavy jumping but not very reactive. 8800’, NE aspect, surface snow moist with small rollers. 9700', NE aspect, surface snow verging on dry. Again jumped on some wind deposits and had the same result as down lower. 9,700’, WNW aspect, surface snow dry. Snow on the way down the chute ranged from soft and moist to an unsupportable crust. Small sloughs in the soft snow ran about 100' down. Sticky spring snow in the bottom of chute.

June Lake, Negatives (2/21/16) - Warm temperatures, ambient air temp above freezing. Minor rollerball activity on SE-S aspects. Negative 1-4, top very firm windslab, lower down mixed windboard with patches of soft facets. Solar Bowl, summit ridge - melt-freeze crust, not fully formed with variable supportiveness. Mid elevations, melt freeze crust still maturing, not quite corn conditions. Large crown visible under summit rockband (south side of cirque) more than 200' wide. Snow on approach very moist. 

 

weather

Sat-Sun: A weak system moved across the northern California Friday ushering in very light precipitation north of Lake Tahoe while Mono county and south saw little more than high clouds, windy conditions, and slightly cooler air behind the front, though still be 5 to 8 degrees above normal. Winds will begin to subside Saturday afternoon into the evening. A secondary weak system is possible Sunday night following a similar path along the Oregon border ushering in slight cooling and scattered to partly cloudy skies.

Mon-Tues: Monday will be trending a few degrees cooler than Sunday.  However, a shortwave ridge is forecast to return Tuesday forcing a weak system north along the Oregon/ California border late Tuesday with minimal impact other than clouds and some minor cooling for the eastern Sierra Front. Models are trending towards a more active pattern by late next week.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Saturday Saturday Night Sunday
Weather: PARTLY CLOUDY THEN BECOMING MOSTLY CLOUDY. PARTLY CLOUDY. PARTLY CLOUDY THEN BECOMING MOSTLY CLOUDY.
Temperatures: 47 TO 55 deg. F. 21 TO 28 deg. F. 21 TO 28 deg. F.
Wind direction: SOUTHWEST SOUTHWEST SOUTHWEST
Wind speed: 15 TO 20 MPH WITH GUSTS TO 30 MPH. 10 TO 15 MPH WITH GUSTS TO 25 MPH. 10 TO 15 MPH.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Saturday Saturday Night Sunday
Weather: PARTLY CLOUDY THEN BECOMING MOSTLY CLOUDY. PARTLY CLOUDY. PARTLY CLOUDY THEN BECOMING MOSTLY CLOUDY.
Temperatures: 41 TO 47 deg. F. 17 TO 24 deg. F. 41 TO 49 deg. F.
Wind direction: WEST BECOMING SOUTHWEST WEST WEST
Wind speed: 35 TO 40 MPH WITH GUSTS TO 60 MPH, DECREASING TO 25 TO 30 MPH WITH GUSTS TO 45 MPH IN THE AFTERNOON. 20 TO 30 MPH WITH GUSTS TO 45 MPH. 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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