Avalanche Advisory - Thu, Feb. 09, 2017

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THIS ADVISORY EXPIRED ON February 11, 2017 @ 5:12 am
Avalanche Advisory published on February 9, 2017 @ 5:12 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

bottom line:

High wind warning in effect from 6am thru 4pm Thursday.

Winter Storm Warning in effect from 4pm Thursday afternoon to 4am Saturday.

Thursday:  Elevated avalanche danger exists today with complex snow stability issues.  With very warm temperatures, recent heavy precipitation with rain level reaching above 9,000’ and heavy dense snow higher, and extreme SW winds, Natural and human triggered wet avalanches and wind slab avalanches will be likely depending on elevation.  Careful snowpack evaluation and cautious route-finding will be essential to avoid dangerous avalanche terrain.   
 

Friday:  With up to 1-2ft of new snow overnight with a dropping snowline, and continued snow throughout the day, and continued strong SW winds, Natural wind slab and storm slab avalanches will be likely and human triggered avalanches very likely, especially for northern areas which receive greater amounts of new snow where avalanche terrain should be avoided. 

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.

Thursday:  Extreme SW winds increasing throughout the day with gusts reaching 110mph below 10,000’, and upwards of 170mph over ridgetops will lead to widespread wind-stripping and wind slab development in all kinds of different areas.  Below 9,000’ the heavy-dense rained-upon snow will be hard to move, but above this elevation these strong winds will be able to move around the dense new snow which has fallen over the past several days.  Winds will be swirling and crazy, so assume anywhere you may travel to encounter wind-effected snow.  Where it is stripped, there won’t be danger of wind slab avalanches, but where it is deposited there will be.  Doing your own stability and snowpack assessments will be essential as will be making critical route-finding decisions based on these assessments to avoid dangerous avalanche terrain.  Natural and Human Triggered avalanches will be likely.

Friday: Strong SW winds with 1-2ft of new snow overnight and continued snowfall throughout the day with another foot possible will lead to widespread wind slab development.  Lesser amounts of new snow is expected further south toward Bishop, but significant wind slabs can develop with even a few inches of new snow with the right winds.  Sensitive windslabs are most likely to be found on NW-N-NE-S-SE facing terrain and around features that promote drifting such as gully sidewalls and leeward sides of ridges.  Natural avalanches will be likely, and human triggered avalanches very likely.  Avoid avalanche terrain, especially in areas with greater amounts of new snowfall. 

Avalanche Character 2: 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.

Thursday:  Very unseasonably warm temperatures today, combined with heavy precipitation yesterday with rain reaching 9,200’ in northern areas, has left a very moist if not wet snowpack below 10,000’.  Widespread rollerball and wet-point release activity on all aspects, as well as a number of wet-slab releases were seen yesterday, and will continue to be likely today.  With a saturated snowpack especially below 9000’, very large wet avalanches are possible on all aspects.  These may be slow moving, but will be extremely dense and destructive.  Further south where the sun could come out and less precipitation was received, this concern will be focused more on aspects with southern exposure. 

Friday:  It is quite possible that the new load of 1-3ft of new snowfall could trigger a deep release within the underlying saturated snowpack.  This concern will exist on all aspects, especially below 9,000’.  The force of a smaller natural or human triggered wind slab or storm slab avalanche could step down and cause a deeper wet release.

Avalanche Character 3: Storm Slab
Storm Slab avalanches release naturally during snow storms and can be triggered for a few days after a storm. They often release at or below the trigger point. They exist throughout the terrain. Avoid them by waiting for the storm snow to stabilize.

Thursday:  This concern will be more towards wet-slab today, as most areas that could remain wind sheltered will be low elevation where the snowpack is saturated.  An upside-down snowpack exists in many areas, as the storms earlier in the week dropped colder less-dense snow before very dense snow fell ontop.   This could mean that a storm slab avalanche could step down and become very large and destructive.  This concern exists for all aspects.

Friday:  As temperatures drop and winds decrease slightly, lower elevation sheltered areas will exist where storm slabs can develop.  These could be found on all aspects that are sheltered from the wind. 

Snowpack Discussion

It has been an intense week of weather so far, with back-to-back warming Atmospheric Rivers.  The latest bringing rainlines up to 9,200’, leaving a saturated snowpack below this elevation.  1-3ft of snow has fallen in the past few days, but the the warmth and rain has led to considerable snowpack settlement as well.  This combined with very strong SW winds have led to a variety of avalanche concerns from Loose wet, to wet-slab, to wind slab to storm slab. 

weather

High wind warning in effect from 6am thru 4pm Thursday.

Winter Storm Warning in effect from 4pm Thursday afternoon to 4am Saturday.

The final wave of this week’s series of Atmospheric Rivers will move into our area later today (Thursday) and persist thru Saturday morning, before dry conditions return for Sunday thru the middle of next week.

 Thursday:  SW winds will be on the increase all day to extreme levels, gusting up to 110mph below 10,000, and upwards of 170mph over ridgetops by the afternoon. Temperatures will be very warm reaching into the 40’s at 10,000’.  Mostly cloudy skies are expected throughout the day, with scattered showers with little accumulation expected in the southern regions near Bishop and up to 4” in the afternoon for Mammoth and north above 8500’.  Precipitation will increase in the evening, as snow line gradually drops below 8,000’ before dark.  Thursday night precipitation rates will increase dramatically, with 1-2” of water meaning up to 2’ of new snow in higher elevations.  Winds will continue to be very strong out of the SW and low temps are expected in the upper 20s at 10,000’. 

Friday:  Another 6 to 12” of snow is expected throughout the day for Mammoth and north, with only a couple of inches expected toward Bishop.  Snow levels will continue to drop through the day, reaching below 7,000’ by mid afternoon.  Strong SW winds will continue, although decreasing with gust “only” expected in the 70mph range above 10,000’.   High temperatures will be in the upper 30s around 10,000’ early in the day, and then will be dropping.     

Long-term:  High pressure gradually returns to our area through Saturday with clear skies, cold temps, and windy conditions for Sunday and early next week.

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 then becoming cloudy. Slight chance of rain and snow in the morning. Snow and rain in the afternoon. Snow level 9000 to 9500 feet Cloudy. Snow and rain. snow level 9000 feet lowering to 8000 feet overnight Cloudy. Snow
Temperatures: 36 to 46 deg. F. 27 to 33 deg. F. 28 to 38 deg. F.
Wind direction: SW SW SW
Wind speed: 25 to 35 mph increasing to 30 to 50 mph in the afternoon. Gusts up to 110 mph 30 to 50 mph with gusts to 110 mph decreasing to 25 to 35 mph with gusts to 95 mph after midnight 15 to 25 mph with gusts to 50 mph
Expected snowfall: Up to 4 in. 2 to 16 in. 6 to 11 in.
Over 10,000 ft.
  Thursday Thursday Night Friday
Weather: Mostly cloudy then becoming cloudy. Chance of snow Cloudy. Snow Cloudy. Snow in the morning, then snow likely in the afternoon
Temperatures: 27 to 35 deg. F. 21 to 28 deg. F. 23 to 29 deg. F.
Wind direction: SW SW SW
Wind speed: 60 to 80 mph. Gusts up to 145 mph increasing to 145 mph in the afternoon 60 to 75 mph with gusts to 140 mph decreasing to 40 to 60 mph with gusts to 125 mph after midnight 25 to 40 mph with gusts to 75 mph
Expected snowfall: 1 to 3 in. 8 to 16 in. 6 to 11 in.
Disclaimer

This Snowpack Summary 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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