This is the last advisory for the 2016-17 season. A big Thank You! to all our observers, contributors, sponsors, and the backcountry community for your support, observations, and generous contributions. We literally couldn’t do it without you and the support of the backcountry community. Have a great summer and see you next season!
Bottom Line: Primary avalanche problems for Tues thru Wednesday will focus on Loose Wet avalanches and very isolated Wet Slabs.
Loose Wet will be a concern for all elevations and aspects (Low elevations - where snow is present). The risk for Loose Wet releases continues as overnight Lows remain above freezing while daytime temperatures remain above seasonable. Be especially cautious in complex terrain where sheltered rocky features can promote warming (chutes, couloirs, etc) and inhibit overnight freezing.
Tues thru Wednesday – Natural Wet Loose avalanches possible, triggered likely on all aspects and elevations on steep terrain greater than 35 degrees.
Wet Slab - Weak overnight freezes and unseasonably warm days during the forecast period will cause the snowpack to thaw quickly in the AM and become saturated with water during the day, especially where the snow is thin or where free water runs along an ice lens undermining the snowpack above, potentially producing a Wet Slab release when it fails. These are generally very isolated in nature and size but can be very dangerous if a rider becomes entrained. Use extra caution where the snowpack is thin and/or punchy (unsupportive), ie., Cliff bands, rocky features, shallow snowpack resting on grassy slopes, treed slopes, etc.
Tues thru Wednesday – Natural Wet Slab avalanches unlikely, triggered are possible on all aspects and elevations but especially on NW-W-SW-S-SE-E aspects.
Likelihood ?CertainVery LikelyLikelyPossible
Size ?HistoricVery LargeLarge
The forecast is for continued sunny skies and above seasonable temperatures with overnight Lows struggling to fall below freezing thru Wednesday will maintain the threat of Loose Wet releases as the sun begins to thaw the snow surface, though generally small in nature. 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. Upper elevations, be especially cautious in sheltered locations that promote excessive warming (chutes, couloirs, etc.) Watch for signs of unstable snow such as large roller balls, deep ski penetration, and small point releases.
Tues thru Wednesday – All Elevations aspects (Low elevations - where snow is present): Natural Loose Wet avalanches are possible, triggered releases are likely on steep slopes, all aspects as the surface snow becomes saturated. Loose Wet releases can stress the snow below as they descend potentially triggering larger Wet Slab releases.
- 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.
- Cornices are beginning to fail. Give them plenty of room while traveling along ridgetops and avoid riding on or under corniced slopes.
Likelihood ?CertainVery LikelyLikelyPossible
Size ?HistoricVery LargeLarge
With weak freezes overnight and above seasonable daytime highs, the snowpack can become saturated and loose internal cohesion during the day, especially where the snow is thin or where free water runs along an ice lens undermining the snowpack above, which as result can produce a Wet Slab release. These are generally isolated in nature and small in size but can be very dangerous if a rider becomes entrained. Use extra caution where the snowpack is thin and/or punchy (unsupportive), i.e., Cliff bands, rocky features, shallow snowpack resting on grassy slopes, treed slopes, etc.
Tues thru Wednesday – Natural Wet Slab avalanches unlikely, triggered are possible on all aspects and elevations aspects (Low elevations - where snow is present) but especially on NW-W-SW-S-SE-E aspects.
Late Spring weather will continue to dominate the weather this week with seasonable to above seasonable highs while overnight lows will remain at or above freezing in most locations. The snowpack continues to transition into a late spring snowpack with melt freeze snow grains and the occasional ice lens marking the winter storms of the season. The recent warming trend combined with above freezing overnight temperatures has limited the corn skiing to a few short hours in the AM before becoming saturated. As we have transitioned into the spring, the avalanche hazard has shifted toward classic Loose Wet releases and the occasional Wet Slabs as the snow becomes saturated by the intense sun.
Spring took a brief backseat to Winter when a quick shot of winterish weather moved into the region Monday (5/15) with moderate winds and 1”- 6” of new snow over the higher elevations (5/15 -5/16). Snow amounts varied greatly depending on location due to the squall like convective showers. The new snow has had sufficient time to bond to the underlying but as it warms under the Sierra sun can become saturated and prone to Loose Wet release. Frontal passage winds remained Westerly to Northwesterly producing shallow isolated Wind Slabs on N-E-S-SW aspects. Winds then shifted becoming more Northerly and remained at threshold (15-25 mph) thru Thursday night transporting the dryish new snow on Northerly aspects onto W-S-E aspects, producing an additional round of shallow isolated Wind Slabs in the upper elevations. The Wind Slabs formed during the brief storm cycle have strengthened. Since the storm’s passage, temperatures have begun a steady climb with highs forecasted to reach the mid 70’s in the Sierra Valleys over the next few days and overnight Low will remain at or above freezing, which will increase the threat of Loose Wet releases through the forecast period.
The last significant 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.
A significant storm swept though the region 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.
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.
Tuesday ~0500 Obs Crnt Temp High (Mon)
Tues thru Thursday - An approaching shortwave trough will provide breezy conditions today along with chances for precipitation this afternoon and Wednesday. Winds will increase through the afternoon with wind gusts near 30-35 mph peaking by late afternoon and into early evening as stronger winds aloft arrive across the northern Sierra. A few showers and even an isolated thunderstorm are possible this afternoon from destabilization in afternoon heating and also weak divergent flow aloft associated with the shortwave. Although strong cells are not anticipated, the very dry sub-cloud layers will still promote the potential for gusty outflow winds of 40-45 mph. The shortwave trough axis moves farther inland Wednesday with overall cooler and more seasonable temperatures expected. Instability is quite weak however so not expecting much in the way of thunderstorms on Wednesday. Warmer temperatures and westerly afternoon breezes return for Thursday.
Extended - Broad southwest flow with embedded weak short waves. Friday and Saturday look to be dry with temps a few degrees above normal and typical afternoon west to southwest winds. Sunday and Monday the models show the same pattern with weak southwest flow and embedded short waves with seasonable temperatures.