Issued by Doug Lewis - Eastern Sierra Avalanche Center
Primary avalanche problems for Tuesday thru Thursday will focus on Loose Wet avalanches and very isolated Wet Slabs.
Loose Wet will be a concern for all elevations and aspects (where snow is present). The risk for Loose Wet releases may become somewhat more widespread as overnight Lows remain near or above freezing and 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 Thursday – All elevations and aspects - Natural Loose Wet avalanches unlikely, triggered are possible on steep terrain greater than 35 degrees.
Wet Slab - Weak overnight freezes and unseasonably warm days are forecasted, which can cause the snowpack to 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, producing a Wet Slab release when it fails. These are generally very isolated in nature and limited in 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 Thursday – All elevations and aspects - Natural Wet Slab avalanches unlikely, triggered are possible.
Late Spring weather continues to be the dominant weather feature this past week with above seasonable highs and warm overnight lows with modest freezes. 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 (Monday)
Tues thru Thursday - Winds will increase Wednesday and Thursday as a cold front dips into the region. Expect breezy to gusty winds along the Sierra Front into the Basin and Range. Gusts will be in the 30 to 40 mph range while wind prone locations could see gusts as high as 50 mph. Tues - temperatures will warm to the mid/upper 70s for Sierra valleys. Expect temperatures to drop to around the upper 60s for Sierra Valleys by Thursday with lows dipping into the low 30s Thursday night. Isolated thunderstorms are possible today along the Sierra crest in Mono County as surface instability and convergence drive forcing for ascent. More dynamic forcing will be present Wednesday as a trough moves towards the region. Expect isolated thunderstorms for Mono Counties.
Friday thru Monday - a trough will pass by to the north Friday followed by a ridge for the weekend into Monday. Moisture and instability with the trough will be north and east of the region so it will likely stay dry. Temperatures will be closer to seasonable Friday after a cold start with some colder Sierra valleys dropping to freezing. Saturday morning will once again start cold for valley locations as upper slopes and ridges begin to warm under building ridge. Temperatures will then stay well above normal from Saturday afternoon through Monday with highs well into the 70s Sierra valleys, which will accelerate creek and stream flows.