Eastern Sierra Snowpack Summary - 2016-02-04 05:43

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THIS ADVISORY EXPIRED ON February 6, 2016 @ 5:43 am
Avalanche Advisory published on February 4, 2016 @ 5:43 am
Issued by Josh Feinberg - 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.

Although this past weekend’s storms began warm, they finished very cold.  Now as the sun comes out, and high temperatures continue to rise higher and higher each day, slope stability will weaken as they warm quickly, especially on steep slopes (>35 degrees) that have direct solar exposure (any kind of southern exposure – SE-S-SW) or receive indirect solar reflecting (slopes closely adjacent to these solar exposed slopes or rock bands).  Get on and off of these types of slopes early before they warm.  Be observant of the first signs of wet rollerball activity, and recognize that much larger slope failures could ensue.  Although wet slides move slower than dry snow svalanches, once someone is entrained, the snow is like flowing conccrete and it is difficult to get out of.  Natural and human triggered avalanches of this type will be possible and perhaps even likely during the time period when particular slopes rapidly warm (morning time for more easterly facing, afternoon for more westerly facing).

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.

Although lessening significantly with each day of calm winds, there still remains concern over windslabs being sensitive to human triggering.  Since the wind direction has been so variable during and after this past storm event, this concern exists for all aspects, on steep exposed slopes >35 degrees at mid and especially upper elevations.  Quick shallow hand pits in areas with dense surface snow can give you a lot of information as to how sensitive these windslabs still are.  In high elevation exposed terrain, isolated local wind may still be strong enough to transport snow and form new windslabs.  Although likely to be small, a resulting avalanche in the wrong spot could knock a skier or boarder off their feet and lead to injury or worse from a fall down nasty terrain.  At upper elevations, this type of avalanche is unlikely to occur naturally, but may be possible for a human to trigger.   

Snowpack Discussion

The avalanche concern over widespread storm slabs and windslabs from this past weekend’s significant snowfall and winds is subsiding quickly, and is being replaced with a new avalanche concern over rapid warming, and much more isolated windslabs.  

Although this past weekend’s storms began warm, they finished very cold.  Now as the sun comes out, and high temperatures continue to rise higher and higher each day, slope stability will weaken as they warm quickly, especially on steep slopes (>35 degrees) that have direct solar exposure (any kind of southern exposure – SE-S-SW) or receive indirect solar reflecting (slopes closely adjacent to these solar exposed slopes or rock bands).  Get on and off of these types of slopes early before they warm.  Be observant of the first signs of wet rollerball activity, and recognize that much larger slope failures could ensue.  Natural and human triggered avalanches of this type will be possible and perhaps even likely during the time period when particular slopes rapidly warm (morning time for more easterly facing, afternoon for more westerly facing).

Although lessening significantly with each day of calm winds, there still remains concern over windslabs being sensitive to human triggering.  Since the wind direction has been so variable during and after this past storm event, this concern exists for all aspects, on steep exposed slopes >35 degrees at mid and especially upper elevations.  Quick shallow hand pits in areas with dense surface snow can give you a lot of information as to how sensitive these windslabs still are.  In high elevation exposed terrain, isolated local wind may still be strong enough even during these calmer days to transport snow and form new windslabs.  Although likely to be very small, a resulting avalanche in the wrong spot could knock a skier or boarder off their feet and lead to injury or worse from a fall down nasty terrain.  At upper elevations, this type of avalanche is unlikely to occur naturally, but may be possible for a human to trigger.   

Recent storm and wind recap:

This past weekend’s storms came thru strong, with over 30” of new snow in some locations.  The storm was accompanied by strong winds out of the SW.  Widespread natural avalanches occurred during this storm, both as a result of windslabs failing on wind deposited slopes (northeasterly facing) and also light low-density storm slabs failing in steep wind-protected areas.  North winds turned on strong for a day (Monday) as the storm system exited the area, stripping snow from many north facing slopes at upper elevations and depositing fresh windslabs on more southerly facing slopes.  On Tuesday, the strong winds shifted again from out of the west, creating a whole new set of sensitive windslabs.  The winds were finally calm yesterday (Wednesday), generally giving these new windslabs a real chance to begin stabilizing.  Continue be on the lookout for denser surface snow on top of less dense snow, particularly on exposed slopes at mid to upper elevations, and do your own quick stability tests to assess how well these windslabs have bonded. 

recent observations

Several parties observed wet avalanche activity yesterday (Wednesday) as a result of rapid warming.  A party on Mt. Tom’s East face witnessed rollerballs and a small natural D1-1.5 wet avalanche at 9500’ on the ENE facing slope of a gully (60-70cm crown).  A party mid-morning witnessed wet avalanches flowing down the sidewalls, into, and down the east facing Pinner Couloir on Mt. Laurel.   

Observers testing the stability of recent windslabs that formed on Monday and Tuesday in various locations (Sherwins, Mt. Tom) reported that the windslabs are gaining strength.  On Tuesday, windslabs failed upon isolation in hand pits (McGee Creek, Baldwin Cirque), and yesterday (Wednesday) windslabs took moderate force before shearing. 

Parties throughout the region reported widespread evidence of natural avalanche activity that took place during the snowfall and wind during this past week’s storm.  Notable was an R3-D3 slide off of Dream Peak behind June Mtn (north facing), and countless others throughout the region. 

weather

A strong high-pressure ridge will remain over our region through the weekend and well into next week.  Dry conditions will prevail, with light winds and temperatures returning to normal, and above normal by Saturday, with cold temperature inversions in the valleys.

Expect sunny skies and high temperatures in the mountains today (Thursday) in the mid 20s to low 30s, and light to moderate westerly winds shifting to northeast by afternoon (15-30mph, gusts to 50 up high).  Friday will be sunny, 5-10 degrees warmer than today, with continued light easterly winds becoming southerly later in the afternoon.  Temperatures will rise into the 40s by Saturday. 

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 SUNNY PARTLY CLOUDY MOSTLY SUNNY
Temperatures: UPPER 20S TO MID 30S deg. F. SINGLE DIGITS TO MID TEENS deg. F. LOW TO UPPER 30S deg. F.
Wind direction: WEST SHIFTING TO NORTHWEST IN THE AFTERNOON SOUTHEAST SOUTHEAST
Wind speed: 15 TO 25 MPH, GUSTS UP TO 35 MPH 10 TO 15 MPH, GUSTS TO 25 MPH 10 TO 15 MPH
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Thursday Thursday Night Friday
Weather: MOSTLY SUNNY PARTLY CLOUDY MOSTLY SUNNY
Temperatures: MID 20S TO MID 30S deg. F. SINGLE DIGITS TO LOW TEENS deg. F. UPPER 20S TO MID 30S deg. F.
Wind direction: NORTHWEST NORTHEAST NORTHEAST SHIFTING TO SOUTHWEST IN THE AFTERNOON
Wind speed: 15 TO 50 MPG, WITH GUSTS UP TO 50 MPH, DECREASING TO 40 MPH IN THE AFTERNOON 15 TO 20 MPH, WITH GUSTS UP TO 30 MPH IN THE EVENING 10 TO 15 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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