Eastern Sierra Avalanche Advisory - 5/2/17

Avalanche Advisory published on May 2, 2017 @ 6:28 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

Primary avalanche problem for Tuesday thru Wednesday will focus on Loose Wet avalanches.

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. Temperatures are forecasted to climb into the mid 50’s to upper 60’s below 10,000’, 50’s and low 60’s above 10,000’ Tuesday thru Wed combined with forecasted modest overnight freezes and very warm daytime temps will elevate the concern for Loose Wet releases as the surface snow thaws. The threat will increase somewhat Wednesday as daily temperatures rise even further while overnight Lows moderate further, and winds diminish.

Tues thru Wed – 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.

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Above Treeline

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Near Treeline

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Below Treeline
Avalanche Problem 1: Loose Wet
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Very Likely
  • Size ?
    Very Large

Spring continues to dominate the weather picture, at least for the next few days. Overnight temperatures struggled to fall below freezing overnight (Monday) throughout the region, which will allow the snow to thaw quickly during the morning. Loose Wet avalanche concerns will rise thru the day as daytime temperatures climb. 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.

Tues thru Wed - 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.

- Glide Slides – Recent sightings of Glide Cracks on SE aspects below ~10000’ elevate the concern for Glide Avalanches. If encountered, give them wide berth and avoid riding on or under slopes where they are present. 

advisory discussion

Spring will continue to be the dominant theme thru mid-week 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.

Models indicate the weather will change dramatically as we move toward the weekend with a cut-off Low setting up over the western US bring unsettled conditions. Stay tuned.    



weather summary

Tues thru Wednesday – Hi pressure continues to hold over the region with light north to northeast low level flow today should be no match for the higher sun angles of May...so high temperatures should remain well above normal. Easterly flow over the Sierra ridges will continue thru Wednesday with ridge gusts of 35-40 mph range. The ridge axis moves overhead for Wednesday with warming at all levels and increased cumulus. Temps possibly reaching into mid 70’s for highs by Wed.

Thurs thru Friday - Ridge axis starts to slide east Thursday and a weak short wave brings some cooling of temperatures aloft with increasing instability. Some high-based thunderstorms are possible late in the day into the early evening hours Thursday in response to this increased instability. Sierra valley highs could, again, reach the mid 70s. Winds do not seem to be an issue either day as thermal gradient from the Sierra to the basin and range country should not be sufficient to generate much of an early season zephyr. The warmer temperatures are likely to accelerate snowmelt.


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
Temperatures: 56 to 64 deg. F. 35 to 40 deg. F. 59 to 67 deg. F.
Mid Slope Winds: Light winds becoming north North Northeast
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
Tuesday Tuesday Night Wednesday
Weather: Sunny Sunny
Temperatures: 50 to 58 deg. F. 32 to 37 deg. F. 52 to 60 deg. F.
Ridge Top Winds: North North Northeast
Expected snowfall: 0 in. 0 in. 0 in.
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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