Eastern Sierra Snowpack Summary - 2016-02-11 06:16

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THIS ADVISORY EXPIRED ON February 13, 2016 @ 6:16 am
Avalanche Advisory published on February 11, 2016 @ 6:16 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center
Avalanche Character 1: Wet Slab
Wet Slab avalanches occur when there is liquid water in the snowpack, and can release during the first few days of a warming period. Travel early in the day and avoid avalanche terrain when you see pinwheels, roller balls, loose wet avalanches, or during rain-on-snow events.

High pressure will continue to dominate the weather through the week with clear skies and record breaking warm temperatures. As a result, E-SE-S-SW-W aspects will warm rapidly, at all elevations, with the surface melt-freeze crust thawing and becoming unsupportive during the day with Wet-Loose and Wet Slabs possible on steep slopes (>35 degrees). Plan to be off of these slopes early before they thaw.  Signs of rollerball (pin wheels) activity are an indicator of weakening bonds and rapid warming surface snow. Natural and human triggered wet avalanches are possible, on southeast slopes in the morning, southerly aspects mid morning to mid-day, and southwesterly by afternoon.  Lower elevations, the shallow snowpack will warm rapidly with the risk of wet slide activity on NE to N to NW will increase on slopes >35 degrees in the 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.

Recent moderate winds (variable direction) at upper elevations over the past couple of days have likely formed new isolated small sensitive windslabs in steep exposed terrain on a variety of leeward aspects. Forecasted light to moderate Southwest winds over the next two days may possibly create new windslabs on NW-N-SE aspects. Lookout for isolated pockets of dense hollow sounding snow.  Beware that small avalanches can be big enough to injure or carry a rider into hazardous terrain. 

Snowpack Discussion

The avalanche concern through the next few days remains focused on wet snow instability on solar aspects (E-S-W) at all elevations and isolated small windslabs in upper elevations, steep exposed terrain, on a variety of aspects.

As high pressure continues to dominate the weather picture through the week, with sunny skies and record-breaking temperatures forecasted along the Eastern Sierra front. Slopes (E-SE-S-SW) at all elevations will warm rapidly with firm supportive melt-crusts becoming unsupportive depending on aspect, and time of day. With record high temperatures, wet loose and wet slabs maybe encountered on steep (>35 degrees) slopes with the sun exposure.  If planning to travel in the backcountry, anticipate being off of these slopes prior to thawing beyond the first few inches. Rollerball (pin wheels) activity indicates weakening bonds between snow grains and rapidly warming snow surface with an increased potential for wet loose or wet slab avalanches that can lead to larger slope failures.  Natural and human triggered wet avalanches are possible on southeast slopes in the early AM, southerly mid-morning to mid-day, and southwesterly by afternoon. Lower elevations (below 8500’), the snowpack is showing signs of going isothermal in the afternoon, including northerly aspects. The risk of wet slide activity on these slopes (NE-N-NW-W) increases in the afternoon on slopes >35 degrees

The northerly mid to upper elevations remain cool with soft snow on NNW-N-NE-ENE aspects, this range maybe extend further, if extensively shaded.  Light southwest winds at upper elevations (10-20 mph) will continue for the next couple days, then veering toward the north-northeast over the weekend, which will bring somewhat cooler air into the region, likely forming new small sensitive windslabs in steep exposed terrain on a variety of leeward aspects.  Beware dense hollow sounding snow, obvious drifts, and sagging cornices. Be especially cautions in complex terrain where small avalanches can have high consequences.

recent observations

South of Mammoth Lakes, north of June Lakes: the snowpack remains considerably shallower (roughly 1.5 feet to just over 3 feet being reported) with facets dominating the mid to lower snowpack and as a result more effected by the recent warming. Southerly aspects in the low to mid elevations are going isothermal by afternoon. Stability tests continue to fail within the facet layer with the potential to propagate and trigger a larger avalanche.

Northerly aspects continue to remain cool and are holding onto their wintery snowpack with no CT failures noted and no concerning layers. Riding conditions reported to be: very variable on northerly facing slopes.

Mammoth/June: The snowpack is considerably thicker and shows signs of bridging-over the early season Depth Hoar. Northerly aspects and shaded slopes remain soft with no significant weak layers noted within the mid to up snowpack. Isolated windslabs are scattered about the upper elevations and have gained some strength with the recent warm temperatures and light winds. Easterly aspects near 10,000 have a weak melt-freeze crust and as you move south along the compass the melt-freeze crust is much more well-developed with some signs of recent rollerballs. 

weather

Thurs – Fri: The high pressure ridge off the coast of California temporarily flattens as a couple short waves move into the Pacific Northwest with increasing winds aloft, mixing is expected to be better Thursday and Friday. Slightly cooling temps aloft will also promote mixing cooling mountains slightly and bumping valley temps up into Friday. Record highs are possible in some areas the next two days, most notably along the highway 395 corridor with high temp Thursday forecasted to reach 59 (record for 2/11, 53) and 57 Friday (record for 2/12, 53) in Mammoth Lakes. Winds will remain light from the Southwest with temps reaching into the mid 40’s to low 50’s at 10,000 feet and above.

 Sat – Sun: The most prominent of the weak short waves is expected to move through Friday night with increased cloud cover and switch to north-northeast flow (5-15 mph), which is expected to bring a few degrees of cooling Saturday and Sunday, although temperatures will remain 5 to 10 degrees above normal with highs reaching the upper 30’s to mid 40’s at 10,000 feet and above. 

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: PARTLY CLOUDY. PARTLY CLOUDY. PARTLY CLOUDY.
Temperatures: 53 TO 59 deg. F. 27 TO 33 deg. F. 50 TO 56 deg. F.
Wind direction: SOUTHWEST SOUTHWEST SOUTHWEST
Wind speed: 15 TO 20 MPH WITH GUSTS TO 30 MPH. 15 TO 20 MPH WITH GUSTS TO 30 MPH. 15 TO 20 MPH WITH GUSTS TO 30 MPH.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Thursday Thursday Night Friday
Weather: PARTLY CLOUDY. PARTLY CLOUDY. PARTLY CLOUDY.
Temperatures: 46 TO 54 deg. F. 23 TO 29 deg. F. 42 TO 50 deg. F.
Wind direction: SOUTHWEST SOUTHWEST SOUTHWEST
Wind speed: 15 TO 20 MPH WITH GUSTS TO 30 MPH. 15 TO 20 MPH WITH GUSTS TO 30 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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