Eastern Sierra Avalanche Advisory - 5/6/17

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THIS AVALANCHE ADVISORY EXPIRED ON May 9, 2017 @ 7:03 am
Avalanche Advisory published on May 6, 2017 @ 7:03 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center
bottom line

Primary avalanche problems for Saturday thru Monday will focus on Wind Slabs, Storm Slab, and Loose Wet avalanches.

Wind Slab - Approaching storm is forecasted to deposit up to 12” in some locations by Sunday with moderate S-N-E winds, which will form Wind Slabs in the mid to upper elevations on W-N-E aspects Saturday, shifting to all aspects as winds swing around the compass from North Saturday night to East Sunday.

Sat thru Mon – natural avalanche unlikely, triggered releases possible. All aspects, Mid to Upper elevations.

Shallow Storm Slab avalanches may be possible in the Mid to Upper elevations where snow accumulations exceed ~8” but are not expected to be large in size or widespread.

Sat thru Mon – natural avalanche unlikely, triggered releases possible in the Mid to Upper elevations.

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 surface snow thaws Saturday AM or becomes saturated by rain later in the day. The threat will decrease somewhat Sunday as daily temperatures cool in the Mid and Low elevations but rain below ~8500 may counter any cooling. Monday as skies begin to clear and the sun begins to warm the snow surface the risk may rise again.  

Sat thru Mon – natural unlikely, triggered avalanches possible, especially on steep slopes.

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 Saturday thru Monday will focus on Wind Slabs, Storm Slab, and Loose Wet avalanches.

Wind Slab - Approaching storm is forecasted to deposit up to 12” in some locations by Sunday with moderate S-N-E winds, which will form Wind Slabs in the mid to upper elevations on W-N-E aspects Saturday, shifting to all aspects as winds swing around the compass from North Saturday night to East Sunday.

Sat thru Mon – natural avalanche unlikely, triggered releases possible. All aspects, Mid to Upper elevations.

Shallow Storm Slab avalanches may be possible in the Mid to Upper elevations where snow accumulations exceed ~8” but are not expected to be large in size or widespread.

Sat thru Mon – natural avalanche unlikely, triggered releases possible in the Mid to Upper elevations.

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 surface snow thaws Saturday AM or becomes saturated by rain later in the day. The threat will decrease somewhat Sunday as daily temperatures cool in the Mid and Low elevations but rain below ~8500 may counter any cooling. Monday as skies begin to clear and the sun begins to warm the snow surface the risk may rise again.  

Sat thru Mon – natural unlikely, triggered avalanches possible, especially on steep slopes.

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 has been the dominant weather feature of late. Overnight temperatures struggled to fall below freezing overnight (Friday) throughout the region, 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 and forecasted light rain begins to fall. 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. 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 thru Mon - natural unlikely, triggered avalanches possible on steep slopes in the Mid to Lower elevations, especially Saturday before rain transitions to snow or as the sun breaks through the cloud cover 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 ~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 is forecasted to deposit up to 12” of new snow with moderate winds by Sunday, which will likely produce Wind Slabs in the mid to upper elevations on a variety of aspects due to shifting wind directions as the storm system progresses. Winds are forecasted to be at threshold, or greater, (15 – 25 mph) for the duration of the storm. Wind directions will shift dramatically from day to day shifting the aspects where potential Wind Slabs may form. Sat – winds will be Southerly with shallow Wind Slabs forming on W-N-E aspects in the afternoon as snowfall begins to accumulate. Saturday night – snowfall is forecasted to increase with winds shifting to Northerly, which will likely form Wind Slabs on W-S-E aspects in the Mid to Upper elevations. Sunday snow will decrease but winds are forecasted to veer to the East/Northeast and increasing to 20 to 30mph, which will favor new Wind Slab formation on N-W-S-SE 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. 

Avalanche Problem 3: Storm Slab
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As snow accumulates on a well-developed melt/freeze crust overnight, small shallow Storm Slabs may become an issue in the Mid to Upper elevations Sunday thru Monday where snow accumulations exceed ~8”. The new snow may not bond well to the underlying snow and will likely be easily triggered on steep slopes and may have the potential to carry riders into to hazardous terrain but unlikely to pose a burial risk unless combined with a terrain trap. Potential Storm Slabs are not expected to be large in size or widespread.

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. Since 5/4, temperatures overnight have struggled to fall below freezing with 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

Sat thru Sunday - Low pressure is dropping south along the west coast and will cutoff over north-central California today (Sat). Low level convergence and frontogenesis will combine with broad-scale divergence aloft to produce an area of moderate to heavy rainfall north of the cold core of the low pressure system. Due to the broadscale lift and dynamics of this system there will be precipitation bands that develop and produce locally higher amounts. Generally 0.5"-1.0" will be possible for areas in the eastern Sierra, higher amounts possible beneath stronger precipitation bands this afternoon with slight chances of thunderstorms today (Sat) with the best chances in the morning before the rain bands develop. Snow levels will start out above 8000 feet for much of the day, but then lower to near 7000 feet by tonight and early Sunday morning. Snowfall is likely in the Sierra over 8000 feet with 3 to 8 inches possible in some locations and locally higher amounts around 12 inches if snow levels drop low enough with ongoing shower activity early Sunday morning. The cutoff low begins to slowly meanders southward Sunday afternoon with diminishing showers from north to south. GFS and NAM timing and placement of the low are a bit different by Sunday evening.

Monday - The GFS swings another round of precipitation across the far eastern Sierra, while the NAM is less aggressive with the producing those bands, by afternoon/evening lingering showers in Mono/Inyo county border.

Tuesday thru Wednesday - A shortwave ridge builds over the eastern Sierra and western Nevada Tuesday into Wednesday bringing warmer and drier conditions. High temperatures will rebound to 4-8 degrees above average with passing high-level clouds. This ridging will be short-lived as another trough slowly approaches the Pacific Northwest coast. 

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: Mostly cloudy then becoming cloudy. Chance of rain and snow in the morning. Slight chance of thunderstorms through the day. Rain and snow in the afternoon. Cloudy. Rain in the evening. Snow and slight chance of thunderstorms through the night. Mostly cloudy. Snow showers likely through the day. Slight chance of thunderstorms in the afternoon.
Temperatures: 42 to 50 deg. F. 25 to 30 deg. F. 40 to 46 deg. F.
Wind direction: South North Northeast
Wind speed: 10 to 15 mph with gusts to 30 mph in the morning becoming light. 10 to 15 mph. Gusts up to 30 mph. 15 to 20 mph with gusts to 40 mph.
Expected snowfall: up to 2 inches in. 2 to 5 in. 1 to 3 in.
Over 10,000 ft.
  Saturday Saturday Night Sunday
Weather: Mostly cloudy then becoming cloudy. Chance of rain and snow in the morning. Slight chance of thunderstorms through the day. Snow in the afternoon. Cloudy. Snow and slight chance of thunderstorms. Mostly cloudy. Snow showers in the morning, then snow showers likely and slight chance of thunderstorms in the afternoon.
Temperatures: 35 to 43 deg. F. 19 to 25 deg. F. 33 to 41 deg. F.
Wind direction: South North East
Wind speed: 15 to 25 mph. Gusts up to 45 mph increasing to 55 mph in the afternoon. 15 to 20 mph shifting to the northeast 20 to 30 mph after midnight. Gusts up to 50 mph. 20 to 30 mph with gusts to 50 mph.
Expected snowfall: 1 to 4 in. 2 to 6 in. 1 to 3 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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