Eastern Sierra Avalanche Advisory - 5/11/17

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THIS AVALANCHE ADVISORY EXPIRED ON May 13, 2017 @ 5:18 am
Avalanche Advisory published on May 11, 2017 @ 5:18 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center
bottom line

Primary avalanche problem for Thursday thru Friday will focus on Loose Wet.

Loose Wet will be a concern on all aspects in the Mid and Lower elevations (where snow is present) as the new snow thaws today (Thursday) AM. The threat will decrease a bit Friday as daily temperatures cool somewhat in the Mid to Upper elevations as a cold front brushes by northern CA.

Thursday thru Friday – Low and Mid elevation natural avalanche unlikely, triggered releases possible on steep terrain 35 degrees and steeper on all aspects. Upper Elevations – NW-W-SW-S-SE-E aspects, natural avalanche unlikely, triggered releases possible on steep terrain 35 degrees and steeper.

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.

- 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 solar aspects below ~11000’ elevate the concern for Glide Avalanches. Where encountered, give them wide berth and avoid riding on or under slopes where present.

 

How to read the advisory

Bottom Line

Primary avalanche problem for Thursday thru Friday will focus on Loose Wet.

Loose Wet will be a concern on all aspects in the Mid and Lower elevations (where snow is present) as the new snow thaws today (Thursday) AM. The threat will decrease a bit Friday as daily temperatures cool somewhat in the Mid to Upper elevations as a cold front brushes by northern CA.

Thursday thru Friday – Low and Mid elevation natural avalanche unlikely, triggered releases possible on steep terrain 35 degrees and steeper on all aspects. Upper Elevations – NW-W-SW-S-SE-E aspects, natural avalanche unlikely, triggered releases possible on steep terrain 35 degrees and steeper.

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.

- 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 solar aspects below ~11000’ elevate the concern for Glide Avalanches. Where encountered, give them wide berth and avoid riding on or under slopes where present.

 

Avalanche Problem 1: Loose Wet
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  • Size ?
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Spring continues its resurgence this week after a weak storm deposited brought the region 4” to 8” of snow above 9000’ on 5/6. Overnight temperatures have struggled to fall below freezing in most locations last night (5/10) and are forecasted to remain mild for the forecast period resulting in the surface snow warming quickly during the AM hours. As temperatures climb rapidly today (Thursday) the surface snow will warm quickly increasing the potential for Loose Wet avalanche thru the day for all elevations, as the snow surface thaws, the threat of triggered Loose Wet avalanches will become increasingly possible in steep terrain, natural releases are 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. Mid and Lower elevations are showing signs the snow is going isothermal, especially where the snowpack is approximately ~4’ or less. As a result, it is possible that a Loose Wet release could trigger a larger more dangerous Wet Slab avalanche.

Thurs thru Friday – Low to Mid Elevations, all aspects - natural unlikely, triggered avalanches possible on steep on terrain 35 degrees and steeper on all aspects. Upper Elevations (NW-W-SW-S-SE-E) – natural Loose Wet avalanches are unlikely, triggered releases possible on terrain 35 degrees and steeper on all aspects.

- 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 solar aspects below ~11000’ elevate the concern for Glide Avalanches. Where encountered, give them wide berth and avoid riding on or under slopes where present.

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.

 

advisory discussion

Spring weather has returned this week after winter made a brief cameo appearance last weekend with fast moving weather system passing 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. Temperatures post storm passage have rebounded quickly with highs in the 50’s and 60’s and low’s remaining quite mild, mostly struggling to fall below freezing. The new snow has for the most part transitioned to corn or nearly so, improving the skiing. 

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 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.

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.

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.

 

 

weather

Thurs thru Friday - An upper-level low is expected to remain off the Pacific Northwest today as a shortwave trough moves into northern CA kicking up winds across the region this afternoon and evening, which is expected to peak by late this evening and overnight. Upper level winds are projected to peak a few hours later, more toward Friday morning, could even briefly approach advisory criteria by early Friday morning when the upper trough and surface cold front pass through. Moisture is limited with this trough passage with a few showers limited to eastern CA from Tahoe northward and far northwest NV overnight. Friday, much cooler air will spread across the region, with highs dropping to 50s near the Sierra with brisk West to Northwest winds with gusts 35-40 mph.

Saturday thru Sunday - Another shortwave will push across northern CA-NV with 700 mb temps dropping to between -8 and -10 C which is well below typical levels for mid-May with highs mainly in the 50s-lower 60s and mainly dry. A broad scale trough remains over the west Sunday with multiple weak shortwaves rotating through the flow. 

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. Partly cloudy. Sunny.
Temperatures: 58 to 66 deg. F. 36 to 42 deg. F. 46 to 56 deg. F.
Wind direction: Southwest Southwest Southwest
Wind speed: 15 to 20 mph with gusts to 35 mph. 15 to 25 mph with gusts to 45 mph. 15 to 25 mph. Gusts up to 50 mph.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Thursday Thursday Night Friday
Weather: Partly cloudy then becoming sunny. Partly cloudy. Sunny.
Temperatures: 48 to 56 deg. F. 30 to 36 deg. F. 38 to 48 deg. F.
Wind direction: South Southwest Southwest
Wind speed: 15 to 25 mph with gusts to 45 mph. 20 to 35 mph with gusts to 70 mph. 30 to 45 mph decreasing to 25 to 35 mph in the afternoon. Gusts up to 75 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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