Eastern Sierra Avalanche Advisory - 5/18/17

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THIS AVALANCHE ADVISORY EXPIRED ON May 20, 2017 @ 6:56 am
Avalanche Advisory published on May 18, 2017 @ 6:56 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center
bottom line

Primary avalanche problems for Thursday thru Friday will focus on Loose Wet avalanches and isolated Wind Slabs in the Mid to Upper elevations.

Loose Wet - Will be a concern in the Low (where snow is present) and Mid to Upper elevations, especially south of Mammoth, as the new snow warms during the day, primarily on NW-W-SW-S-SE-E aspects. The risk for Loose Wet releases may become more widespread on Friday as temperatures continue to climb with the full force of the sun taking its toll on the new snow and overnight lows moderate. Upper elevations, the concern will rise in sheltered locations that promote warming (chutes, couloirs, etc).

Thurs thru Friday - Low Elevations: Natural Loose Wet avalanches possible, triggered are likely on steep on NW-W-SW-S-SE-E aspects, all aspects by Friday. Mid Elevations - primarily on NW-W-SW-S-SE-E aspects with limited activity possible on N-NE aspects. Upper Elevations – primarily W-S-E aspects.

Wind Slabs - Northerly winds are forecasted thru Friday with the potential light snow transport forming isolated shallow Wind Slabs primarily on exposed W-S-E aspects above ~ 9500’. Natural avalanches unlikely, triggered releases possible below ridgelines, adjacent to terrain features that promote drifting, and in gullies and shallow depressions.

Thurs thru Friday – Mid and Upper elevations, natural avalanche unlikely, isolated triggered releases possible on steep terrain 35 degrees and steeper along ridgelines, near terrain features that promote drifting and wind loading.

How to read the advisory

Bottom Line

Primary avalanche problems for Thursday thru Friday will focus on Loose Wet avalanches and isolated Wind Slabs in the Mid to Upper elevations.

Loose Wet - Will be a concern in the Low (where snow is present) and Mid to Upper elevations, especially south of Mammoth, as the new snow warms during the day, primarily on NW-W-SW-S-SE-E aspects. The risk for Loose Wet releases may become more widespread on Friday as temperatures continue to climb with the full force of the sun taking its toll on the new snow and overnight lows moderate. Upper elevations, the concern will rise in sheltered locations that promote warming (chutes, couloirs, etc).

Thurs thru Friday - Low Elevations: Natural Loose Wet avalanches possible, triggered are likely on steep on NW-W-SW-S-SE-E aspects, all aspects by Friday. Mid Elevations - primarily on NW-W-SW-S-SE-E aspects with limited activity possible on N-NE aspects. Upper Elevations – primarily W-S-E aspects.

Wind Slabs - Northerly winds are forecasted thru Friday with the potential light snow transport forming isolated shallow Wind Slabs primarily on exposed W-S-E aspects above ~ 9500’. Natural avalanches unlikely, triggered releases possible below ridgelines, adjacent to terrain features that promote drifting, and in gullies and shallow depressions.

Thurs thru Friday – Mid and Upper elevations, natural avalanche unlikely, isolated triggered releases possible on steep terrain 35 degrees and steeper along ridgelines, near terrain features that promote drifting and wind loading.

Avalanche Problem 1: Loose Wet
  • Character ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
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    Very Large
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  • Trend ?
    Increasing Danger

With 1” to 6” of new snow reported throughout the region. The threat of Loose Wet releases will depend on how much new snow was received and aspect. The forecast is for sunny skies and warming under the intense May sun. The risk for Loose Wet releases may become more widespread on Friday as temperatures continue to climb and the full force of the sun takes its toll on the new snow and overnight lows moderate. Use extra caution in and around rock outcrops and below cliff bands where triggered releases are more likely. Timing is critical for avoiding Loose Wet releases. Easterly aspects thaw first, followed by southerly, then westerly, and finally northwesterly aspects as the spring 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.

Thurs thru Friday – Low Elevations: Natural Loose Wet avalanches possible, triggered are likely on steep on NW-W-SW-S-SE-E aspects, all aspects by Friday. Mid Elevations - primarily on NW-W-SW-S-SE-E aspects with limited activity possible on N-NE aspects. Upper Elevations – primarily W-S-E

- 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 terrain traps.

- Glide Slides – Recent sightings of Glide Cracks on SE aspects below ~11000’ elevate the concern for Glide Avalanches. Where encountered, give them wide berth and avoid riding on or under slopes where they are present.

- Cornices are beginning to show signs of weakening and sagging. Give them plenty of room while traveling along ridgetops and avoid riding on or under corniced slopes. 

Avalanche Problem 2: Wind Slab
  • Character ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
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  • Trend ?
    Decreasing Danger

Monday thru Tuesday (5/15 -5/16) a quick moving spring system moved into the region with moderate winds, and 1”- 6” of new snow over the higher elevations 5/15 -5/16. Snow amounts varied greatly depending on location due to the squall like convective showers. The new snow has had time to bond to the underlying snow but will likely peel off the old snow surface easily either as it begins to warm under clear skies or by a rider. Post-frontal passage winds have remained Westerly to Northwesterly producing shallow Wind Slabs on N-E-S-SW aspects. The Wind Slabs formed during the brief storm cycle have likely strengthened but winds are forecasted to become more Northerly today (5/18) and remain at threshold (15-25 mph) thru Thursday night, which will likely transport the dry new snow from Northerly aspects onto W-S-E aspects producing new sensitive Wind Slabs in the upper elevations. These will likely be shallow in nature and confined to near treeline and above (~ 9500 and above).

Wind Slabs - Riders will likely encounter these below ridgeline, adjacent to terrain features that promote drifting, crossloaded gullies and depressions. Be wary of hollow, drum-like sounding snow or shooting cracks.​ Changing wind directions can alter windloading and making it difficult to discern previous drifting patterns. 

Thurs thru Friday – Mid and Upper elevations, natural avalanche unlikely, isolated triggered releases possible on steep terrain 35 degrees and steeper along ridgelines, near terrain features that promote drifting and wind loading.

advisory discussion

Spring weather has returned after taking a brief backseat to a quick shot of winterish weather that moved into the region Monday (5/15) with moderate winds, and 1”- 6” of new snow over the higher elevations 5/15 -5/16. Snow amounts varied greatly depending on location due to the squall like convective showers. The new snow has had time to bond to the underlying snow but will likely peel off the old snow surface easily either as it begins to warm under clear skies or by a rider. Post-frontal passage winds have remained Westerly to Northwesterly producing shallow Wind Slabs on N-E-S-SW aspects. The Wind Slabs formed during the brief storm cycle have likely strengthened but winds are forecasted to become more Northerly and remain at threshold (15-25 mph) thru Thursday night, which will likely transport the dry new snow on Northerly aspects onto W-S-E aspects producing new sensitive Wind Slabs in the upper elevations. These will likely be shallow in nature and confined to near treeline and above (~ 9500 and above).

The last significant weather system passed through the region Saturday (5/6) bringing: cooler temperatures, 4” to 8” of snow over the high country, and light to moderate southerly winds, which then shifted to northerly as the system began to move off to the east. The heaviest snowfall was from Mammoth north with a consistent 6” to 8”, south precipitation was a bit more spotty. The new snow bonded relatively well to the underlying melt/freeze crust with little internal weakness and has transitioned rapidly to corn or near corn snow as the storm moved east and sunny skies returned with temperatures rapidly rebounding to well above normal.

Prior to the latest storm (5/6), spring has been the dominant theme with overnight temperatures struggling to fall below freezing and daily highs climbing well above seasonable with upper 50’s and low 60’s recorded in many locations above 9000’. The combination of weak freezes and unseasonably warm temperatures  resulted in the upper snowpack becoming isothermal where the snowpack is shallow. Glide cracks have been observed on solar aspects below 11,000’ on steep slopes with smooth underlying terrain.

A weak spring storm moved through the region Wednesday (4/26) with moderate to strong winds, cooler temperatures, increasing cloud cover (primarily Mammoth north) and light precipitation over the upper elevations forming very isolated and shallow Wind Slabs in the upper elevations primarily on N-E-S-SW aspects.

A significant storm swept though the region Tuesday (4/18/17) with 3” to 12” inches of new snow reported across the forecast area above ~8500’. However, snow levels fluctuated considerably during the storm with many areas receiving rain Monday before turning to snow in the early morning hours on Tuesday. Snow levels rose once again during the day, Tuesday, with rain up to ~ 9,000’ then easing back down to ~8000’ by Tuesday PM. Loose Wet avalanches were prevalent during the storm throughout the mid elevations as the surface snow becoming saturated with water and internal bonds began to dissolve. Moderate to strong SW winds during the storm formed Wind Slabs in exposed locations throughout the mid and upper elevations, primarily above ~ 9000’ on NW-NE-SE aspects with several avalanches observed throughout the forecast region. Mammoth Mountain Ski Patrol reported significant results from avalanche control work on Wednesday morning. Most of these avalanches were triggered in Wind Slabs with small hand charges and ski cutting. Westerly winds continued thru Wednesday and Thursday (4/19-4/20) with snow banners and localized drifting observed from Mammoth south to Rock Creek, forming a new round of Wind Slabs throughout the upper and mid elevations, primarily on N-E-S aspects.

Moderate SW winds in upper elevations Sunday (4/23) formed another round of isolated shallow Wind Slabs in the upper elevations on NW-NE-SE aspects where upwind fetches still had snow available for transport. Since then, there have been several days for the Wind Slabs to strengthen and heal, redirecting the avalanche threat toward typical spring Loose Wet avalanches.

 

weather

Thurs thru Saturday - High pressure builds over the West Coast today (Thursday) beginning a significant warming trend. Light easterly flow will keep high temperatures somewhat tamped down today, upper 50s to around 60 for Sierra valleys, before the pressure gradient weakens further into the weekend.

Sun thru Wednesday - Ridge of high pressure continues to amplify on Sunday along the Pacific Northwest coast, while a weak shortwave trough dropping through the northern Rockies splits off over CA. The ridge will keep temperatures warm, around 5-10 degrees above average with afternoon highs in the low/mid 70s for Sierra valleys by Sunday. The weak low pressure dropping over CA will bring a few degrees of cooling aloft that will help to increase instability over the Sierra. This will bring chances of thunderstorms for the Sierra Sunday through Wednesday, mainly driven by afternoon heating and terrain convergence over the High Sierra. Temperatures will continue to rise into next week as high pressure dominates. Highs will reach into the mid 70s in the Sierra. Snowmelt will increase with the well above normal temperatures. Rises are expected for creeks, streams, and mainstem rivers coming out of the Sierra through 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: Sunny Clear Sunny
Temperatures: 47 to 53 deg. F. 29 to 34 deg. F. 52 to 58 deg. F.
Wind direction: Light winds becoming north North Light winds
Wind speed: 10 to 15 mph with gusts to 25 mph in the afternoon. 10 to 15 mph with gusts to 30 mph in the evening becoming light.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Thursday Thursday Night Friday
Weather: Sunny Clear Sunny
Temperatures: 38 to 44 deg. F. 24 to 30 deg. F. 43 to 51 deg. F.
Wind direction: North North North
Wind speed: 15 to 20 mph with gusts to 40 mph. 15 to 25 mph with gusts to 40 mph. 10 to 15 mph with gusts to 25 mph in the morning becoming light.
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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