Eastern Sierra Avalanche Advisory - 5/25/17

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THIS AVALANCHE ADVISORY EXPIRED ON May 27, 2017 @ 6:32 am
Avalanche Advisory published on May 25, 2017 @ 6:32 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 very isolated Wet Slabs.

Loose Wet will be a concern for all elevations and aspects (where snow is present). The risk for Loose Wet releases may become somewhat more widespread as overnight Lows remain near or above freezing and daytime temperatures remain above seasonable. Be especially cautious in complex terrain where sheltered rocky features can promote warming (chutes, couloirs, etc) and inhibit overnight freezing.

Thursday thru Friday – All elevations and aspects - Natural Loose Wet avalanches possible, triggered are likely on steep terrain greater than 35 degrees.

Wet Slab - Weak overnight freezes and unseasonably warm days are forecasted, which can cause the snowpack to thaw quickly in the AM and become saturated with water during the day, especially where the snow is thin or where free water runs along an ice lens undermining the snowpack above, producing a Wet Slab release when it fails. These are generally very isolated in nature and size but can be very dangerous if a rider becomes entrained. Use extra caution where the snowpack is thin and/or punchy (unsupportive), ie., Cliff bands, rocky features, shallow snowpack resting on grassy slopes, treed slopes, etc.

Thursday thru Friday – All elevations and aspects - Natural Wet Slab avalanches unlikely, triggered are possible. 

How to read the advisory

Bottom Line

Primary avalanche problems for Thursday thru Friday will focus on Loose Wet avalanches and very isolated Wet Slabs.

Loose Wet will be a concern for all elevations and aspects (where snow is present). The risk for Loose Wet releases may become somewhat more widespread as overnight Lows remain near or above freezing and daytime temperatures remain above seasonable. Be especially cautious in complex terrain where sheltered rocky features can promote warming (chutes, couloirs, etc) and inhibit overnight freezing.

Thursday thru Friday – All elevations and aspects - Natural Loose Wet avalanches possible, triggered are likely on steep terrain greater than 35 degrees.

Wet Slab - Weak overnight freezes and unseasonably warm days are forecasted, which can cause the snowpack to thaw quickly in the AM and become saturated with water during the day, especially where the snow is thin or where free water runs along an ice lens undermining the snowpack above, producing a Wet Slab release when it fails. These are generally very isolated in nature and size but can be very dangerous if a rider becomes entrained. Use extra caution where the snowpack is thin and/or punchy (unsupportive), ie., Cliff bands, rocky features, shallow snowpack resting on grassy slopes, treed slopes, etc.

Thursday thru Friday – All elevations and aspects - Natural Wet Slab avalanches unlikely, triggered are possible. 

Avalanche Problem 2: Wet Slab
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    Certain
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  • Size ?
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With weak freezes overnight and above seasonable day time highs, the snowpack can become saturated during the day, especially where the snow is thin or where free water runs along an ice lens undermining the snowpack above produing a Wet Slab release. These are generally isolated in nature and size but can be very dangerous if a rider becomes entrained. Use extra caution where the snowpack is thin and/or punchy (unsupportive), i.e., Cliff bands, rocky features, shallow snowpack resting on grassy slopes, treed slopes, etc. 

advisory discussion

Late Spring weather continues to be the dominant weather feature this week with above seasonable highs and overnight lows remaining above freezing in most locations. This has created a classic spring snowpack with consolidated melt freeze snow grains with the occasional ice lens marking the winter storms of the season. The recent warming trend combined with above freezing overnight temperatures has limited the corn skiing to a few short hours in the AM before becoming saturated. As the snow becomes saturated the risk for Loose Wet releases can rise rapidly during the day. Loose Wet releases can overload the underlying snowpack as they descend, triggering Wet Slab releases, especially where the snowpack is thin or saturated with water.

Spring took a brief backseat to Winter when a quick shot of winterish weather 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 sufficient time to bond to the underlying but as it warms under the Sierra sun can become saturated and prone to Loose Wet release. Frontal passage winds remained Westerly to Northwesterly producing shallow isolated Wind Slabs on N-E-S-SW aspects. Winds then shifted becoming more Northerly and remained at threshold (15-25 mph) thru Thursday night transporting the dryish new snow on Northerly aspects onto W-S-E aspects, producing an additional round of shallow isolated Wind Slabs in the upper elevations. The Wind Slabs formed during the brief storm cycle have strengthened. Since the storm’s passage, temperatures have begun a steady climb with highs forecasted to reach the mid 70’s in the Sierra Valleys over the next few days and overnight Low will remain at or above freezing, which will increase the threat of Loose Wet releases through the forecast period.

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 - A weak surface frontal boundary is passing eastward with breezy West to Northwest winds this morning and afternoon with gusts of 25-30 mph. Temperatures will cool a few degrees today and Friday but will still remain a few degrees above seasonal averages. Expect highs mainly around mid to upper 60s will prevail for Sierra valleys through Friday with low 70s possible by Saturday. Overall, the pattern looks to remain relatively dry with the exception of some isolated convection across the Sierra south of the Tahoe Basin on Friday afternoon.

Sunday thru Wednesday - The end of the Memorial Day weekend and most of next week will feature a period of late spring heat with potential for enhanced melting of the high elevation (8000 feet+) Sierra snowpack. The heat along with light winds aloft will allow for several days of convective activity and increasing chances for afternoon and evening thunderstorms. First day of activity could be Memorial Day and mainly along the Sierra Crest. Shower and thunderstorm potential should continue into Tuesday/Wednesday. By late in the week, models are showing a trough approaching the west coast. This would likely increase the afternoon and evening winds off the Sierra and decrease the potential for showers and thunderstorms.

  

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: Partly cloudy then becoming sunny. Clear. Sunny then becoming partly cloudy.
Temperatures: 56 to 66 deg. F. 33 to 38 deg. F. 56 to 66 deg. F.
Wind direction: Southwest Southwest Light winds.
Wind speed: 10 to 15 mph with gusts to 35 mph. 15 to 20 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: Partly cloudy then becoming sunny. Clear. Sunny then becoming partly cloudy.
Temperatures: 46 to 54 deg. F. 28 to 34 deg. F. 41 to 49 deg. F.
Wind direction: Southwest West Light winds.
Wind speed: 20 to 30 mph. Gusts up to 50 mph decreasing to 40 mph in the afternoon. 15 to 20 mph. Gusts up to 35 mph decreasing to 25 mph after midnight.
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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