Eastern Sierra Avalanche Advisory

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Avalanche Advisory published on January 21, 2017 @ 6:59 am
This advisory expires in 1 day, 4 hours, 30 minutes
This advisory is valid for 72 hours
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

bottom line:

*Winter Storm Warning in effect from now until 4am Monday

Saturday:  With 1-2ft of additional new snow since Friday morning along with moderate to strong SW winds, and the possibility of continued light snowfall today with continued moderate SW winds especially at upper elevations, the greatest avalanche concern will exist in the form of wind slabs.  Human triggered avalanches will be likely on slopes greater than 35 degrees where sensitive windslabs exist.  These will most likely be found on NW-N-NE-E-SE facing slopes below ridgetops, leeward sides of ridgelines, and in and around terrain features that promote drifting and cross loading.  Evaluate snow and terrain carefully.   

Sunday:  Intense snowfall and high winds throughout the day will form widespread sensitive windslabs and storm slabs at all elevations.  Natural and human triggered avalanches will be very likely, if not certain.  Traveling on or under avalanche terrain is not recommended! Large destructive avalanches will be likely. 

Monday:  Avalanche danger will begin to decrease today as snowfall diminishes and winds decrease.  Human triggered avalanches will remain very likely throughout the day, and isolated natural avalanches possible. 

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.

Saturday:  With 1-2ft of new snowfall since Friday morning and Moderate to strong SW winds, sensitive windslabs will have formed below ridgetops, leeward sides of ridgelines, and in and around terrain features that promote drifting and cross loading.  Some of these areas of wind slab will begin to stabilize today, however, moderate SW winds today will continue to load some of these areas, especially at higher elevations, keeping windslabs sensitive.  Natural avalanches will be unlikely at mid to low elevations today, but could be possible at higher elevations where continued wind-transport exists.  Areas will exist where human triggered avalanches will be likely.  These will most likely be found on slopes greater than 35 degrees facing NW-N-NE-E-SE in areas exposed to the wind at mid to upper elevations, with increased concern with increased elevation.

Sunday:  Intense snowfall and high winds throughout the day will form widespread sensitive windslabs in leeward areas not sheltered from the wind at all elevations.  Natural and human triggered avalanches will be very likely, if not certain!  Avoid traveling on or under any potentially wind-loaded slopes.

Monday:  Windslabs will continue to form today in areas exposed to wind below ridgetops, leeward sides of ridgelines, and in and around terrain features that promote drifting and cross-loading.  Human triggered avalanches will remain likely, and isolated natural avalanches still possible. 

Avalanche Character 2: 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.

Saturday:  Observations yesterday throughout the region found the new 1-2ft of snow bonding well to the underlying old snow surface, and falling fairly upright with denser snow toward the bottom.  While some subtle density changes within the snow caused some failures in tests, the limited new snowfall today should allow storm snow in sheltered areas to continue to gain strength.

Sunday:  Intense snowfall throughout the day, with up to 5 additional feet possible, will make natural and human triggered avalanches very likely on all slopes at all elevations.  Traveling on or under wind-sheltered avalanche terrain is not recommended!

Monday:  Up to 5'+ of new snow is likely to have fallen in the past 2 days.  As snowfall is expected to diminish today snow in wind-sheltered locations will begin stabilizing today, especially with the up-right snow structure that is forecasted (less dense snow falling on top of more dense snow), but this will take some time and human triggered avalanches in terrain that has been sheltered from the wind will still be likely especially at convex points on slopes greater than 35 degrees.  Do your own careful stability assessments to see just how fast this sheltered snow is stabilizing. 

Snowpack Discussion

10-20” of new snow has fallen in the forecast area since early Friday morning, which is on top of 10-20” snow that fell Wednesday night and Thursday.  Snowpack depths in the Mammoth and June area are over 3meters deep at 10,000’.  Observations over the past several days from Bishop to June have shown that storm snow has been falling fairly up right (less dense snow on top of more dense snow), and stabilizing quickly in wind-sheltered areas.  However, moderate to strong SW winds have been observed throughout the forecast area transporting snow, creating sensitive wind slabs, which are of much greater concern.  Very little natural avalanche activity has been observed, however visibility has been limited. 

weather

*Winter Storm Warning in effect from now until 4am Monday

For today (Saturday) a relative lull is expected in storm conditions with only light to moderate snow showers and lighter winds before much more powerful storms enter our region with intense snowfall beginning late Saturday night around midnight lasting until early Monday morning.  A cold air mass will move in Sunday afternoon turning the snow much less dense.  Snow levels should stay under 5,500’ throughout the entire period. 

Saturday:  Expect mostly cloudy skies, with some snow showers with accumulation up to 4”, West winds in the 20-35mph range with gusts into the 60s at higher elevations, and temperatures in the upper teens to low 20s around 10,000’.

Sunday:  Intense heavy snowfall beginning late Saturday night will continue until early Monday morning, with well over 5’ of additional new snow possible at mid-mountain elevations around Mammoth (with slightly lesser amounts north and south).  Winds will be strong out of the SW in the 40-60mph range with gusts reaching 125mph over ridgetops in the afternoon.  High temperatures will reach the upper teens to mid 20s late morning before starting to drop in the afternoon. 

Monday:  Snow showers will taper off early Monday morning, with only a couple additional inches expected.  High temps will be cold in the teens, with continued breezy conditions.   

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: CLOUDY. SCATTERED SNOW SHOWERS. CLOUDY. CHANCE OF SNOW IN THE EVENING, THEN SNOW AFTER MIDNIGHT. CLOUDY. SNOW.
Temperatures: 23 TO 29 deg. F. 19 TO 24 deg. F. 25 TO 30 deg. F.
Wind direction: W S S
Wind speed: 15 TO 25 MPH WITH GUSTS TO 50 MPH 15 TO 25 MPH. GUSTS UP TO 45 MPH INCREASING TO 60 MPH AFTER MIDNIGHT. 20 TO 35 MPH WITH GUSTS TO 85 MPH
Expected snowfall: up to 5 in. 5 to 10 in. 12 to 24 in.
Over 10,000 ft.
  Saturday Saturday Night Sunday
Weather: CLOUDY. SNOW SHOWERS IN THE MORNING, THEN SCATTERED SNOW SHOWERS IN THE AFTERNOON. CLOUDY. CHANCE OF SNOW IN THE EVENING, THEN SNOW AFTER MIDNIGHT. CLOUDY. SNOW.
Temperatures: 17 TO 23 deg. F. 14 TO 20 deg. F. 19 TO 24 deg. F.
Wind direction: W SW SW
Wind speed: 20 TO 35 MPH WITH GUSTS TO 60 MPH 20 TO 30 MPH WITH GUSTS TO 60 MPH INCREASING TO 30 TO 45 MPH WITH GUSTS TO 90 MPH AFTER MIDNIGHT. 45 TO 60 MPH. GUSTS UP TO 110 MPH INCREASING TO 125 MPH IN THE AFTERNOON.
Expected snowfall: UP TO 4 in. 5 TO 10 in. 15 TO 25 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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