Avalanche Advisory: Thursday - Feb 1, 2018

Please help!  Post your observations to our site under the "Participate" Tab... Click Here to Find Out How

THIS AVALANCHE ADVISORY EXPIRED ON February 2, 2018 @ 6:32 am
Avalanche Advisory published on February 1, 2018 @ 6:32 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

Thursday (2/1) - primary avalanche concern continues to focus on wet instabilities on solar aspects (E-S-W) in the mid and upper elevations. Overnight temperatures were again hovering near or above freezing with another round of unseasonably warm temperatures 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

?

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

2. Moderate

?

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

1. Low

?

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
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

Today (Thursday) - overnight lows were once again very warm with only a few locations reporting below freezing temperatures. When combined with the forecasted unseasonably warm temperatures for today, you can anticipate the snowpack to thaw quickly during the day on solar aspects (E-S-W), which will increase the potential for Loose Wet avalanches in the mid and upper elevations. Extra caution is recommended in and around rock outcrops and below cliff bands where triggered releases are more likely, especially where the snowpack is exceptionally shallow. Timing is critical for avoiding Loose Wet releases. Easterly aspects thaw first, followed by southerly, and then westerly as the sun moves across the sky. Lower elevations thaw more quickly than higher elevations. Watch for signs of unstable snow such as 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.

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

 

Avalanche Problem 2: Wet Slab
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

Recent (1/27) northerly winds formed Wind Slabs on southerly alpine ridgelines and slopes. As these slopes heat up, the surface snow thaws and begins to loose strength and 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. This problem is primarily isolated to alpine ridgelines where these old Wind Slabs have formed. 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.​ 

Avalanche Problem 3: Persistent Slab
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

Patchy Persistent Slab issues 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’. Test results continue to be mixed and relatively non-reactive. Despite the persistent weakness, there haven’t been any recent reports of this layer failing but a triggered release is possible, though unlikely. Whumphing and shooting cracks are signs that sudden collapse is possible. Perform your own stability tests to assess the snowpack of the slope you want to ride.

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

 

advisory discussion

Solar Aspects - High pressure off the coast of California continues to deflect storms to the north of the forecast region and usher in exceptionally warm daytime temperatures (10 to 20 degrees above average) while overnight temperatures struggle to fall below freezing in many locations. With the spring like weather, the snowpack is experiencing daily thaws where the free water breaks down the bonds between snow grains and layers within the snowpack. As this process continues, the snow looses internal strength and cohesion; if the mass of the snow exceeds the snow strength, a Loose Wet or Wet Slab avalanche occurs. The threat of wet instabilities on solar aspects (E-S-W) will be with us for the foreseeable future as long as the high-pressure ridge remains locked in place of 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 - the primary avalanche problem is the persistent weakness from lingering facets that formed early season and are now buried ~30 to ~60cm (12” to 24”) from the surface. On the northerly aspects the snowpack remains cool but is showing signs of warming, at least at the surface with some mid elevations warming and forming 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 or a small isolated suspended Wind Slab 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

Thur thru Sunday - Upper level ridging will amplify off the west coast heading into the weekend with the dry and warm weather as high pressure remains in control. Will see unseasonably high temperatures reaching as much as 15-20 degrees above average with near record high temperatures through the week. Generally 40s to mid to upper 50s for Sierra into the weekend. A few high clouds streaming across the region today, thinning by Friday afternoon with mostly clear conditions into the 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: Mostly cloudy. Partly cloudy then becoming clear. Sunny.
Temperatures: 48 to 54 deg. F. 30 to 36 deg. F. 49 to 55 deg. F.
Wind Direction: Light winds Light winds Light winds
Wind Speed:
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
Thursday Thursday Night Friday
Weather: Mostly cloudy. Partly cloudy then becoming clear. Sunny.
Temperatures: 40 to 46 deg. F. 27 to 33 deg. F. 42 to 47 deg. F.
Wind Direction: Northwest North Northwest
Wind Speed: around 10 mph. 10 to 15 mph in the evening becoming light. 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.

ESAC receives support from ...