Eastern Sierra Avalanche Advisory - 5/9/17

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THIS AVALANCHE ADVISORY EXPIRED ON May 11, 2017 @ 7:04 am
Avalanche Advisory published on May 9, 2017 @ 7:04 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center
bottom line

Primary avalanche problems for Tuesday thru Wednesday will increasingly focus on Loose Wet while Wind Slabs concerns begin to subside.

Loose Wet will be primarily a concern on NW-W-SW-S-SE-E aspects in the Mid elevations, all aspects for the Low elevations (where snow is present) as the new snow warms today (Tuesday) AM. The threat will increase somewhat Wednesday as daily temperatures warm further in the Mid to Upper elevations as the sun begins to warm the snow surface. 

Tues thru Wednesday – natural avalanche possible, triggered releases likely. Mid – E-SE-S-SW-W-NW, Upper Elevations – NE-S-NW, especialy south of Mammoth. Low Elevations - All aspects, natural releases unlikely, triggered releases possible.

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.

Wind Slabs - Latest storm system to pass through the region (5/6) deposited up to 8” in some locations with moderate winds forming Wind Slabs in the Mid to Upper elevations on W-S-E aspects.

Tues thru Wed – natural avalanche unlikely, triggered releases possible in the Mid to Upper elevations Tuesday trending to natural and triggered releases unlikely for Wednesday. 

How to read the advisory

Bottom Line

Primary avalanche problems for Tuesday thru Wednesday will increasingly focus on Loose Wet while Wind Slabs concerns begin to subside.

Loose Wet will be primarily a concern on NW-W-SW-S-SE-E aspects in the Mid elevations, all aspects for the Low elevations (where snow is present) as the new snow warms today (Tuesday) AM. The threat will increase somewhat Wednesday as daily temperatures warm further in the Mid to Upper elevations as the sun begins to warm the snow surface. 

Tues thru Wednesday – natural avalanche possible, triggered releases likely. Mid – E-SE-S-SW-W-NW, Upper Elevations – NE-S-NW, especialy south of Mammoth. Low Elevations - All aspects, natural releases unlikely, triggered releases possible.

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.

Wind Slabs - Latest storm system to pass through the region (5/6) deposited up to 8” in some locations with moderate winds forming Wind Slabs in the Mid to Upper elevations on W-S-E aspects.

Tues thru Wed – natural avalanche unlikely, triggered releases possible in the Mid to Upper elevations Tuesday trending to natural and triggered releases unlikely for Wednesday. 

Avalanche Problem 1: Loose Wet
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Spring has been the dominant weather feature until Saturday when an upper level low brought the region 4” to 8” of snow above 9000’. Overnight temperatures varied from slightly above freezing north of Mammoth to mid 20’s south. Mammoth north may see the surface snow warm more quickly as a result. As temperatures rapidly climb today (Tuesday) the new snow and surface snow will warm quickly increasing the potential for Loose Wet avalanche thru the day. In the Mid and Upper Elevations, as the snow surface warms the threat of triggered Loose Wet avalanches will become likely in steep terrain, natural releases are possible. Lower elevations, natural Loose Wet avalanches are unlikely but triggered releases are possible, especially in steep terrain. 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.

Tues thru Wednesday – Mid and Upper Elevations, natural possible, triggered avalanches likely on steep slopes. Low Elevations, natural unlikely, triggered possible in steep terrain.

- 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 ~10000’ elevate the concern for Glide Avalanches. Where encountered, give them wide berth and avoid riding on or under slopes where they are present. 

Avalanche Problem 2: Wind Slab
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This weekend’s storm deposited up to 8” of new snow with moderate winds, which likely produced shallow isolated Wind Slabs in the Mid to Upper elevations. Northerly winds are forecasted to remain at or just below threshold (15 – 25 mph) for the duration of the forecast which may continue to form additional shallow isolated Wind Slabs on leeward slopes (E-SE-S-SW-W-NW aspects). Anticipate Wind Slabs in the mid to upper elevations above ~ 9500’. Riders will likely encounter these below ridgeline, adjacent to terrain features that promote drifting, along 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.

 

advisory discussion

The spring weather was interrupted briefly by a weather system passing through the region Saturday (5/6), which brought cooler temperatures and 4” to 8” of snow over the high country with light to moderate winds that stated out southerly then shifted to northerly as the system began to move off to the east. The heaviest snowfall was from Mammoth north. The new snow bonded relatively well to the underlying melt/freeze crust and little internal weakness.

Prior to the latest storm, 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 has resulted in the upper snowpack becoming isothermal, full-depth where the snowpack is shallow. Glide cracks were observed on solar aspects below 10,000’ 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. 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 typical spring Loose Wet avalanches.

 

 

weather

Tues thru Wednesday - Warming and drying trend remains on track for early this week as shortwave ridging ramps up across California. Temperatures are expected to warm-up into upper 60s to near 70 through Wednesday afternoon for Sierra valleys. Temperatures will peak on Wednesday as the ridge axis moves across the Sierra before cooler conditions return for late in the week and into the weekend with potential elevated stream flows. By Wednesday as high temperatures peak with the ridge axis over the region, there is the possibility for enough instability to produce some showers for the Mono-Mineral convergence zone, specifically from Ebbetts Pass to as far south as Mt Whitney.

Thurs thru Sat - The shortwave ridge begins to breakdown and shift east as another storms approaches the Pacific Northwest. Increasing southwest flow aloft will result in gusty winds by Thursday afternoon with gusts reaching into the 40 to 45 mph range. A trough over the Pacific Northwest will usher in cooler temperatures for the weekend. The front will also bring breezy winds Friday, but winds are not expected to be as strong as on Thursday. Below normal daytime highs with clear skies and light wind are forecasted.

 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Tuesday Tuesday Night Wednesday
Weather: Sunny Clear Sunny then becoming partly cloudy. Slight chance of showers in the afternoon.
Temperatures: 54 to 60 deg. F. 33 to 38 deg. F. 60 to 66 deg. F.
Wind direction: Light winds becoming North North West
Wind speed: 10 to 15 mph with gusts to 25 mph in the afternoon. 10 to 15 mph with gusts to 25 mph in the evening becoming light. 10 to 15 mph with gusts to 30 mph in the morning becoming light.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Tuesday Tuesday Night Wednesday
Weather: Sunny Clear Sunny then becoming partly cloudy. Slight chance of showers in the afternoon.
Temperatures: 46 to 54 deg. F. 28 to 34 deg. F. 52 to 58 deg. F.
Wind direction: Northeast North Northwest
Wind speed: 15 to 25 mph with gusts to 45 mph. 10 to 15 mph with gusts to 30 mph. 15 to 20 mph with gusts to 35 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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