Eastern Sierra Avalanche Advisory - 5/13/17

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THIS AVALANCHE ADVISORY EXPIRED ON May 16, 2017 @ 6:47 am
Avalanche Advisory published on May 13, 2017 @ 6:47 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center
bottom line

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

Loose Wet will be a concern in the Low (where snow is present) to Mid elevations, especially south of Rock Creek, as the snow thaws during the AM, primarily on NW-W-SW-S-SE-E aspects thru Monday. Upper elevations the concern is focused on SE-S-SW aspects.

Sat thru Monday – Low and Mid elevation natural avalanche unlikely, isolated 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.

How to read the advisory

Bottom Line

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

Loose Wet will be a concern in the Low (where snow is present) to Mid elevations, especially south of Rock Creek, as the snow thaws during the AM, primarily on NW-W-SW-S-SE-E aspects thru Monday. Upper elevations the concern is focused on SE-S-SW aspects.

Sat thru Monday – Low and Mid elevation natural avalanche unlikely, isolated 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.

Avalanche Problem 1: Loose Wet
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Overnight temperatures fell well below freezing in most locations last night (5/12) and are forecasted to remain cool for the forecast period. As temperatures climb today (Saturday) the surface snow will thaw and increase the potential for Loose Wet avalanche thru the day. Use extra caution 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.

Sat thru Monday: Low to Mid Elevations – Natural avalanches unlikely, triggered Loose Wet Avalanches are possible on NW-W-SW-S-SE-E aspects, Upper Elevation – Natural avalanches unlikely, triggered Loose Wet Avalanches are possible on SE-S-SW aspects, especially where sheltered.

- 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 ~11000’ 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 will continue to rein for the next few days before a change coming early next week when winter will make a brief cameo appearance. A fast moving weather system passed 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 and has transitioned rapidly to corn or near corn snow as the storm moved east and sunny skies returned with temperatures rapidly rebounding to well above normal.

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  resulted in the upper snowpack becoming isothermal where the snowpack is shallow. Glide cracks have been observed on solar aspects below 11,000’ 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

Saturday - Weak disturbance moving into northern CA with scattered clouds and some snow and rain showers across eastern CA from Tahoe northward, gusty winds elesewhere. Saturday night, chilly conditions are again expected.

Sunday thru Monday - A split upper low moving south across northern CA. Moisture looks to be limited, but instability ahead of this low with cool temps aloft looks to be sufficient to produce showers with the possibility of a few afternoon/early evening thunderstorms. For Sunday night and Monday, baggy trough is expected to remain over eastern CA, keeping the threat of showers going into early Monday evening. Instability is weaker compared to Sunday, a few lightning strikes can`t fully be ruled out under the broad cold pool aloft. By later Monday night, the leading edge of the stronger storm system reaching the west coast could spread some precip into northeast CA. Overall precip amounts will be spotty and light, generally less than 0.10 inch. Locally higher amounts are possible near the Sierra where thunderstorms occur. Below average temperatures will continue Sunday and Monday. Snow levels will mainly range from 5500-7000 feet, although snow or graupel could briefly occur below these elevations with stronger showers.

Tuesday thru Friday – A cold low will drop into the region Tuesday, bringing strong winds, mountain snow and valley rains. Strong 100kt jet looks to push this system through fairly quickly Tuesday-Wednesday with good cold air (-10C at 700mb) support. There is a good chance that this storm will be bring snow to the Sierra. This upper low will be very cold with anomalously cold temperatures for this time of year. This will bring snow levels down to 5000 feet with light accumulations by Wednesday morning. Models continue to show potential for strong winds on Tuesday with wind gusts up to 50 mph in the valleys. Winds will peak out Tuesday afternoon before the cold front pushes through late in the day on Tuesday. Brisk north flow is expected on Wednesday behind the front, with high temperatures remaining below normal. Residual showers will remain on Wednesday as cold air aloft keeps the atmosphere unstable. Ridging starts to build back into the region by Thursday, with clearing skies and warming temperatures back to near normal for this time of year. 

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 then becoming mostly cloudy. Slight chance of thunderstorms and snow showers in the afternoon.
Temperatures: 45 to 51 deg. F. 24 to 29 deg. F. 41 to 49 deg. F.
Wind direction: Light winds. Light winds. Light winds.
Wind speed: Gusts up to 30 mph in the morning.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Saturday Saturday Night Sunday
Weather: Sunny. Clear. Sunny then becoming mostly cloudy. Slight chance of thunderstorms and snow showers in the afternoon.
Temperatures: 37 to 45 deg. F. 18 to 24 deg. F. 33 to 41 deg. F.
Wind direction: West Light winds. Light winds.
Wind speed: 15 to 25 mph. Gusts up to 40 mph decreasing to 30 mph in the afternoon. Gusts up to 25 mph in the evening.
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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