Avalanche Advisory: Friday - Feb 2, 2018

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THIS AVALANCHE ADVISORY EXPIRED ON February 3, 2018 @ 6:31 am
Avalanche Advisory published on February 2, 2018 @ 6:31 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

Friday (2/2) - primary avalanche concern continues to focus on wet instabilities, on solar aspects (primarily SE-S-W) in the mid and upper elevations. Overnight temperatures again hovered near or above freezing with another round of unseasonably warm temperatures for today, the avalanche danger will remain at moderate for Loose Wet avalanches with the remote possibility of Wet Slabs. Monitor surface snow for excessive thawing. Avoid steep, rocky, and sunny slopes as temperatures climb through the day.

Below ~9,000’, natural and triggered releases are unlikely due to thin snow coverage (below threshold).​

2. Moderate

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Above Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

2. Moderate

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Near Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

1. Low

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Below Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
    Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
Avalanche Problem 1: Loose Wet
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Today (Friday) – once again, overnight lows remained very warm with only a few locations reporting below freezing temperatures. When combined with the forecasted highs that will climb to 10 to 20 degrees above average, anticipate the snowpack to thaw quickly during the day on solar aspects (SE-S-W). This will increase the potential for Loose Wet avalanches in the mid and upper elevations. Use extra caution in and around rock outcrops, below cliff bands, or where the snowpack is exceptionally shallow where triggered releases are more likely. Timing is critical for avoiding Loose Wet releases. As the day begins to warm, anticipate easterly aspects thawing first, followed by southerly, and then westerly as the sun moves across the sky. The lower elevations generally thaw more quickly than the higher elevations. Watch for signs of unstable snow: large roller balls, deep ski penetration, and small point releases. Small point releases can be a sign that larger avalanches are increasingly possible. Loose Wet avalanches typically involve the snow near the surface of the snowpack but can trigger larger deeper releases. Time your travels to be out of steep sunny terrain before the snow thaws from the heat of the day.

Loose Wet slides are dense and heavy, which can make it difficult to extract yourself, if entrained, and are capable of carrying a rider into hazardous terrain or lead to possible burial when combined with a terrain trap that favors deep burial.

Avalanche Problem 2: Wet Slab
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Northerly winds last weekend formed Wind Slabs on southerly alpine ridgelines and slopes. As these slopes heat up, the surface snow thaws and begins to loose strength while the melt-water percolates down into the snowpack, weakening the bonds to the underlying layers as demonstrated by a skier-triggered avalanche in the Virginia Lakes (1/28). This problem is primarily isolated to alpine ridgelines where these old Wind Slabs were deposited. Time your travels to be out of steep sunny terrain before the snow thaws from the heat of the day.

Below ~9,000’- below threshold, natural and triggered releases are unlikely due to low snow coverage.​ 

advisory discussion

Solar Aspects - High pressure remains parked off the coast of California and continues to deflect the storms to the north of the forecast region. This pattern is responsible for ushering in exceptionally warm air mass with daytime temperatures (10 to 20 degrees above average) and overnight lows struggling to fall below freezing in many locations. With the April-like weather, as the snowpack thaws during the day, the free water breaks down the bonds between snow grains and layers within the snowpack, loosing internal strength and cohesion. If the mass of the snow exceeds the snow strength, a Loose Wet or Wet Slab avalanche can occur. The threat of wet instabilities on solar aspects (SE-S-W) will be with us for the foreseeable future as long as the high-pressure ridge remains locked in place off the California coast. With the limited snow coverage, the exposed rocky terrain features can introduce tremendous amounts of heat into the snowpack and cause rapid localized thawing in cirques and gullies. Timing your travels to be out of steep sunny terrain before the snow thaws from the heat of the day is key to avoiding wet instabilities.

Northerly aspects - Patchy persistent weakness continue to lurk within the snowpack with weak faceted sugar layers down about ~30 to ~60cm (12” to 24”) from the surface. Distribution is primarily confined to NE-N-NW aspects from ~9500’ to as high as ~11,500’. Recent test results are somewhat mixed and relatively non-reactive. Despite this persistent weakness, there haven’t been any recent reports of this layer failing but a triggered release is possible, though unlikely. The snowpack on the northerly aspects remains relatively cool but is showing signs of warming, at least at the surface with some mid-elevations  beginning to warm and form a thin melt/freeze crust. Otherwise the snow is still somewhat soft in sheltered areas near treeline but aspect and shading is key. In steep sheltered terrain, isolated Loose Dry sloughs may be triggered where the snow is disturbed. Near treeline, where the persistent weakness exists, there is also the potential to encounter small isolated Wind Slabs that were deposited over facets, which could fail if a sufficient trigger is applied.   

Below ~9,000’, natural and triggered releases are unlikely due to thin and well-anchored snow coverage (below threshold).​

* Caution - Firm snow conditions on southerly aspects in the AM can produce slide-for-life conditions. A minor slide into hazardous terrain can have serious consequences.

Weather and CURRENT CONDITIONS
weather summary

Friday and beyond - High pressure remains firmly in place through the weekend and beyond resulting in unseasonable warmth and dry conditions. High temperatures will remain generally in the mid and upper 50s. Daily highs are about 15-20 degrees above season averages, more typically seen in mid-April. Winds will remain light through the weekend. Mostly clear skies are expected with some passing high clouds. Models show a wave dropping from north to south across the region as the ridge temporarily retrogrades to the west. This will increase cloud cover but most of the moisture is cut off from this feature with no precipitation. The ridge rebuilds into the forecast area starting late Tuesday and remains in place through Thursday. This will bring continued mild and dry weather. Temperatures will continue to run 10-15 degrees above normal throughout the extended period.    

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Friday Friday Night Saturday
Weather: Sunny Clear Sunny
Temperatures: 50 to 56 deg. F. 30 to 35 deg. F. 51 to 57 deg. F.
Wind Direction: Light winds. Light winds. North
Wind Speed: around 10 mph.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
Friday Friday Night Saturday
Weather: Sunny. Clear then becoming partly cloudy. Sunny.
Temperatures: 44 to 49 deg. F. 27 to 33 deg. F. 45 to 50 deg. F.
Wind Direction: North North North
Wind Speed: around 10 mph. around 10 mph. 10 to 15 mph.
Expected snowfall: 0 in. 0 in. 0 in.
Disclaimer
This Avalanche Advisory is designed to generally describe avalanche conditions where local variations always occur. This product only applies to backcountry areas located outside established ski area boundaries. 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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