Eastern Sierra Avalanche Advisory - 4/29/17

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THIS AVALANCHE ADVISORY EXPIRED ON May 2, 2017 @ 6:31 am
Avalanche Advisory published on April 29, 2017 @ 6:31 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center
bottom line

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

Loose Wet avalanches will be primarily a concern on NW-W-SW-S-SE-E aspects in the mid and upper elevations, all aspects for the low elevations as the surface snow thaws. Sat -Temperatures are forecasted to climb into the mid 40’s to mid 50’s below 10,000’, upper 30’s to mid 40’s above 10,000’, warming even further Sunday and Monday. A good overnight (Friday) freeze combined with moderate North to Northeasterly winds and mild daytime temps will limit the concern for Loose Wet releases Saturday. The threat will increase somewhat Sunday thru Monday as daily temperatures rise, overnight Lows moderate, and the winds diminish.

Sat – Low to Mid elevations, natural and triggered avalanches unlikely.

Sun thru Monday – Natural avalanches unlikely, triggered releases possible on steep slopes as the snow surface warms and thaws.

Caution – Potential Slide For Life conditions may exist due to firm spring snow conditions prior to thawing. A minor slide into hazardous terrain can have serious consequences.

How to read the advisory

Bottom Line

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

Loose Wet avalanches will be primarily a concern on NW-W-SW-S-SE-E aspects in the mid and upper elevations, all aspects for the low elevations as the surface snow thaws. Sat -Temperatures are forecasted to climb into the mid 40’s to mid 50’s below 10,000’, upper 30’s to mid 40’s above 10,000’, warming even further Sunday and Monday. A good overnight (Friday) freeze combined with moderate North to Northeasterly winds and mild daytime temps will limit the concern for Loose Wet releases Saturday. The threat will increase somewhat Sunday thru Monday as daily temperatures rise, overnight Lows moderate, and the winds diminish.

Sat – Low to Mid elevations, natural and triggered avalanches unlikely.

Sun thru Monday – Natural avalanches unlikely, triggered releases possible on steep slopes as the snow surface warms and thaws.

Caution – Potential Slide For Life conditions may exist due to firm spring snow conditions prior to thawing. A minor slide into hazardous terrain can have serious consequences.

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

Overnight temperatures were well below freezing overnight (Friday) throughout the region. A strong spring showing is on tap for the forecast period with mild daytime highs, cool nights, and decreasing winds. The combination of cool nights and mild days will set the stage for good corn skiing for Sat – Mon. The Loose Wet avalanche concerns will be limited Saturday but will rise somewhat Sunday – Monday as daytime temperatures continue to warm,overnight lows continue to moderate, and winds begin to diminish. Sun – Mon: As the snow surface thaws, triggered Loose Wet avalanches will be possible in steep terrain, natural will remain unlikely. Extra caution is recommended 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. Small point releases can be a sign that larger avalanches are increasingly possible. Loose Wet avalanches typically involve the snow near the surface of the snowpack but can trigger larger deeper releases.

Sat – Low to Mid elevations, natural and triggered avalanches unlikely.

Sun thru Monday – Natural avalanches unlikely, triggered releases possible on steep slopes as the snow surface warms and thaws.

- 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.

Caution - Firm snow conditions in the AM can produce slide-for-life conditions. A minor slide into hazardous terrain can have serious consequences.

 

advisory discussion

Spring continues to be the dominate theme this week with warm daily temperatures and cool nights interrupted by the occasional glancing blow from weak fast moving spring storm. The most recent spring system to moved through the region Wednesday (4/26) with moderate to strong winds, cooler temperatures, and increasing cloud cover (primarily Mammoth north) with light precipitation over the upper elevations forming very isolated and shallow Wind Slabs in the upper elevations primarily on N-E-S-SW aspects. The last significant storm to sweep though the region was 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. Since then, any Wind Slabs that formed have had several days to strengthen. 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 the typical spring Loose Wet avalanches.

 

 

weather

Sat thru Sunday - Gusty east to northeast winds at the ridge top level due to a thermal gradient with cooler air to the east and slightly warmer air to the west of the Sierra. As the upper level jet support moves east as a ridge builds the thermal gradient will lessen, decreasing the easterly winds over the ridges through the day. Temperatures warm today (Sat) and Sunday as the ridge builds. Highs are likely to reach the upper 50s to lower 60s in the Sierra valleys. By Sunday highs reach the mid 60s in the Sierra valleys with overnight lows at the ridge level warming a bit each day.

Mon thru Wednesday - The ridge is forecasted to retrograde slightly as another short wave trough carves its way southeast through the Great Basin and Intermountain West. This will support slightly cooler highs for Monday...cooler further Tuesday...and an increase in northwest flow over the region. By Tuesday the ridge will begin to amplify and shift over the western U.S. bringing light winds and rising temperatures through midweek. The ridge axis is most amplified over the southwest U.S. Wednesday, with high temperatures likely peaking Wednesday depending on how quickly the ridge begins to break down. Temperatures could peak about 10 degrees above average with highs in the low 70`s possible in the Sierra.

 

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
Temperatures: 46 to 54 deg. F. 30 to 36 deg. F. 55 to 63 deg. F.
Wind direction: Northeast Northeast Light winds.
Wind speed: 10 to 15 mph. Gusts up to 40 mph decreasing to 30 mph in the afternoon. 10 to 15 mph with gust to 25 mph in the evening becoming light.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Saturday Saturday Night Sunday
Weather: Sunny Clear Sunny
Temperatures: 38 to 46 deg. F. 30 to 36 deg. F. 48 to 56 deg. F.
Wind direction: North becoming northeast North Northwest
Wind speed: 30 to 45 mph with gusts to 65 mph, 20 to 30 mph with gusts to 45 mph in the afternoon. 10 to 15 mph with gusts to 35 mph in the evening becoming light. 10 to 15 mph with gusts to 30 mph.
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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