Eastern Sierra Avalanche Advisory - 5/4/17

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THIS AVALANCHE ADVISORY EXPIRED ON May 6, 2017 @ 6:10 am
Avalanche Advisory published on May 4, 2017 @ 6:10 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 possible Wet Slabs.

Loose Wet 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 (where snow is present) as the surface snow thaws. Warm unseasonable weather is forecasted with temperatures climbing into the upper 50’s to low 70’s below 10,000’, mid 40’s and low 60’s above 10,000’ Thursday thru Fri. Forecasted warm overnight temperatures and very warm daytime highs will elevate the concern for Loose Wet releases as the surface snow thaws. The threat will decrease somewhat Friday as daily temperatures cool slightly and winds increase in the mid to upper elevations.

Thurs thru Friday – natural unlikely, triggered avalanches possible, especially on steep solar aspects.

Wet Slab avalanches will be possible in the Mid to Lower elevations due to overnight temperatures remaining above freezing and well-above seasonable daily high temperatures creating isothermal conditions in the upper snowpack or possibly full-depth where the snowpack is shallow, especially south of Mammoth. This is primarily confined to the Low and Mid elevations, mainly on NW-W-SW-S-SE-E aspects in the mid elevations, all aspects for the low elevations (where snow is present) as the surface snow thaws. Though likely isolated in nature, Loose Wet releases may trigger Wet Slabs as they descend a slope.

Thurs thru Friday – natural unlikely, triggered avalanches possible, especially on steep solar aspects.

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 problems for Thursday thru Friday will focus on Loose Wet avalanches and possible Wet Slabs.

Loose Wet 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 (where snow is present) as the surface snow thaws. Warm unseasonable weather is forecasted with temperatures climbing into the upper 50’s to low 70’s below 10,000’, mid 40’s and low 60’s above 10,000’ Thursday thru Fri. Forecasted warm overnight temperatures and very warm daytime highs will elevate the concern for Loose Wet releases as the surface snow thaws. The threat will decrease somewhat Friday as daily temperatures cool slightly and winds increase in the mid to upper elevations.

Thurs thru Friday – natural unlikely, triggered avalanches possible, especially on steep solar aspects.

Wet Slab avalanches will be possible in the Mid to Lower elevations due to overnight temperatures remaining above freezing and well-above seasonable daily high temperatures creating isothermal conditions in the upper snowpack or possibly full-depth where the snowpack is shallow, especially south of Mammoth. This is primarily confined to the Low and Mid elevations, mainly on NW-W-SW-S-SE-E aspects in the mid elevations, all aspects for the low elevations (where snow is present) as the surface snow thaws. Though likely isolated in nature, Loose Wet releases may trigger Wet Slabs as they descend a slope.

Thurs thru Friday – natural unlikely, triggered avalanches possible, especially on steep solar aspects.

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
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Spring continues to be the dominant weather feature, at least through Friday. Overnight temperatures struggled to fall below freezing overnight (Wednesday) throughout the region and are forecasted to remain above freezing Thursday night, which will allow the snow to thaw quickly during the morning. The potential for Loose Wet avalanche will rise thru the day as daytime temperatures warm. As the snow surface thaws, triggered Loose Wet avalanches will be possible in steep terrain, natural will remain unlikely in the Low and Mid Elevations but will be somewhat more likely in the upper elevations as they begin to radically heat-up. 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.

Thurs thru Friday - natural unlikely, triggered avalanches 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.

Avalanche Problem 2: Wet Slab
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Wet Slab avalanches are possible in the Mid to Lower elevations due to overnight temperatures remaining above freezing and well-above seasonable daily high temperatures have produced isothermal conditions in the upper snowpack or possibly full-depth where the snowpack is shallow. This is primarily confined to NW-W-SW-S-SE-E aspects in the Mid Elevations, all aspects for the Low Elevations (where snow is present). Though likely isolated in nature, Loose Wet releases can trigger Wet Slabs as they descend a slope.

Thurs thru Friday – natural unlikely, triggered avalanches possible, especially on steep solar aspects.

- 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.

 

advisory discussion

Spring weather has been the recent dominant theme and will continue for a couple of more days with warm daily temperatures and mild nights. 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.

Forecasts for the weekend shows a dramatic change coming as a cut-off Low sets-up over the western US bring unsettled conditions, snow for the high country, and gusty winds. Stay tuned for a possible late season powder treat.    

 

 

weather

Thurs thru Friday - summer like temperatures today (Thursday) before transitioning Friday with gusty winds as low pressure moves inland. Isolated showers and thunderstorms are possible mid-afternoon over the Sierra by late this afternoon and evening. Strong storms could produce some gusty outflow winds to 40 mph. As the cold spring system nears the California coast on Friday gusty winds of 30-35 mph are possible.

Saturday - a cold low will move into central California on Saturday. Models remain in good agreement in a large area of precipitation developing over the Sierra as early as Saturday morning. These cold spring systems can drop 0.50-1.00 inch of liquid. However, a slight shift in track could lead to little or no precipitation across the region. Snow levels to drop below 7000 feet in the area of precipitation, perhaps as low as 6000 feet. This could lead to several inches of accumulation above 7000 feet, 6-12 inches is not out of the question above 8000 feet in the Sierra.

Sunday thru Monday - operational models and many of the ensemble members are in better agreement with the track of the upper low late in the weekend into early next week. This has boosted confidence in the forecast through at least Monday. The highest pops look to be Sunday closer to the core of the upper low over southern California/Nevada. Mid level convergence provides a focus for potentially moderate to heavy precipitation in any deformation band that forms Sunday. There is also sufficient instability for isolated thunderstorms over eastern and southern parts of the forecast area. Temperatures will be well below average. The low begins to drift east Monday but enough residual moisture and lift for showers through the day. Temperatures should rebound slightly Monday to more seasonal norms with precipitation coverage not be as widespread as Sunday.

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: Sunny then becoming partly cloudy. Isolated showers and thunderstorms in the afternoon. Mostly cloudy then becoming partly cloudy. Isolated showers and thunderstorms in the evening. Partly cloudy.
Temperatures: 61 to 71 deg. F. 39 to 49 deg. F. 57 to 67 deg. F.
Wind direction: Light winds Southwest Light winds becoming South
Wind speed: 10 to 15 mph. Gusts up to 30 mph. 15 to 20 mph in the afternoon. Gusts up to 40 mph.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Thursday Thursday Night Friday
Weather: Sunny then becoming partly cloudy. Isolated showers and thunderstorms in the afternoon. Mostly cloudy then becoming partly cloudy. Partly cloudy.
Temperatures: 51 to 61 deg. F. 34 to 44 deg. F. 46 to 56 deg. F.
Wind direction: Light winds becoming South Southwest Southwest
Wind speed: 10 to 15 mph with gusts to 35 mph in the afternoon. 15 to 25 mph. Gusts up to 45 mph increasing to 60 mph after midnight. 20 to 30 mph with gusts to 60 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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