Eastern Sierra Avalanche Advisory - 5/20/17

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THIS AVALANCHE ADVISORY EXPIRED ON May 23, 2017 @ 6:55 am
Avalanche Advisory published on May 20, 2017 @ 6:55 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center
bottom line

Primary avalanche problems for Saturday thru Monday will focus on Loose Wet avalanches.

Loose Wet will be the concern in the Low (where snow is present) on all aspects. In the Mid to Upper elevations (especially south of Mammoth), Loose Wet avalanches will primarily be a concern on NW-W-SW-S-SE-E aspects as the snow surface warms. The risk for Loose Wet releases may become a bit more widespread on Sunday thru Monday as overnight Lows remain near or above freezing and daytime temperatures continue to climb. Upper elevations, be especially cautious in sheltered locations that promote excessive warming (chutes, couloirs, etc).

Sat thru Monday - Low Elevations: Natural Loose Wet avalanches unlikely, triggered are possible on terrain 35 degrees and steeper. Mid and Upper Elevations – natural avalanches unlikely, triggered releases possible primarily on NW-W-SW-S-SE-E aspects with limited activity possible on N-NE aspects, especially Sun thru Monday. 

How to read the advisory

Bottom Line

Primary avalanche problems for Saturday thru Monday will focus on Loose Wet avalanches.

Loose Wet will be the concern in the Low (where snow is present) on all aspects. In the Mid to Upper elevations (especially south of Mammoth), Loose Wet avalanches will primarily be a concern on NW-W-SW-S-SE-E aspects as the snow surface warms. The risk for Loose Wet releases may become a bit more widespread on Sunday thru Monday as overnight Lows remain near or above freezing and daytime temperatures continue to climb. Upper elevations, be especially cautious in sheltered locations that promote excessive warming (chutes, couloirs, etc).

Sat thru Monday - Low Elevations: Natural Loose Wet avalanches unlikely, triggered are possible on terrain 35 degrees and steeper. Mid and Upper Elevations – natural avalanches unlikely, triggered releases possible primarily on NW-W-SW-S-SE-E aspects with limited activity possible on N-NE aspects, especially Sun thru Monday. 

Avalanche Problem 1: Loose Wet
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The forecast is for sunny skies, continued warming under the intense May sun, while overnight Lows will remain near or above freezing thru Monday, which will increase the threat of Loose Wet releases as the sun heats the snow surface.The risk for Loose Wet releases may become a bit more widespread on Sunday thru Monday as temperatures continue to climb and overnight lows remain near or above freeing. 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. Upper elevations, be especially cautious in sheltered locations that promote excessive warming (chutes, couloirs, etc). Watch for signs of unstable snow such as: avalanches, large roller balls, deep ski penetration, and small point releases.

Sat thru Mon – Low Elevations: Natural Loose Wet avalanches unlikely, triggered releases are possible on slopes of 35 degrees and steeper on all aspects. Mid and Upper Elevations - Natural Loose Wet avalanches are unlikely but triggered releases are possible on slopes of 35 degrees and steeper primarily on NW-W-SW-S-SE-E aspects with limited activity possible on N-NE aspects, especially Sun thru Monday.

- 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.

advisory discussion

Spring continued to dominate the weather this week 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. 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

Sat thru Monday - Temperatures will become unseasonably warm this weekend as highs in the Sierra will head into the low 70's, increasing each day through the middle of next week. There are some low chances of general variety thunderstorms for Mono County on Sunday. Monday, convective build-up will be possible and may result in a few isolated showers along the Sierra crest, but thunderstorm chances will greatly diminish.

Tuesday thru Friday - Weak upper low approaching the west coast and upper ridge over the Great Basin will allow for some limited moisture to ride up the Sierra Tue-Wed as afternoon temperatures push 10-15 degrees above normal. Isolated thunderstorms will develop along the Sierra from Lake Tahoe southward. Storms will be slow moving and capable of locally heavy rain, especially Wednesday. Temperatures both days will push well into mid-upper 70's Sierra valleys. Temperatures will cool down for the very end of the week. 

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: Sunny. Clear. Sunny then becoming partly cloudy. Slight chance of showers and thunderstorms in the afternoon.
Temperatures: 61 to 67 deg. F. 36 to 42 deg. F. 58 to 66 deg. F.
Wind direction: Light winds. Light winds. Light winds.
Wind speed:
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Saturday Saturday Night Sunday
Weather: Sunny Clear Sunny then becoming partly cloudy. Slight chance of showers and thunderstorms in the afternoon.
Temperatures: 50 to 58 deg. F. 30 to 36 deg. F. 50 to 58 deg. F.
Wind direction: East Light winds. Light winds.
Wind speed: 10 to 15 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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