Issued by Doug Lewis - Eastern Sierra Avalanche Center
Primary avalanche problems for Saturday thru Monday 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 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.
Sat thru Monday – 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.
Thursday thru Friday – 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
With weak freezes overnight and above seasonable day time 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 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.
Late Spring weather has been the dominant weather feature since mid-May with above seasonable highs while overnight lows have remained at or above freezing in most locations. The snowpack over the course of the past few weeks has transitioned into a classic spring time 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 Wet Slabs as the snow becomes saturated by the intense May sun.
With spring coming on in full the snowpack is shrinking rapidly. Outside of the Mammoth Basin, snow is pretty scarce below 8000', with the best coverage above 9000' and up. In Mammoth, snow is still present at 8000' but it's retreating to higher elevations quickly. Best riding can be found where you can access the upper elevation snowpack readily where the road crews have opened the summer roads (Rock Creek, Sabrina Lake or South Lake, Onion Valley, and Virginia Lakes). Tiaoga Pass has yet to announce an opening date.
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.
Saturday ~0500 Obs Crnt Temp High (Friday)
Saturday thru Monday - High pressure will strengthen this Memorial Day weekend and into early next week. This will provide us light winds and a warming trend with highs reaching about 10 to 15 degrees above seasonal averages by Monday with highs in the upper 70s for Sierra valleys. Overall, dry conditions will prevail with mainly cumulus cloud development across the Sierra each afternoon. By Monday, the ridge begins to slide eastward which will provide better low-level instability which may allow some isolated shower and storm development across the Sierra mainly south of the Tahoe Basin.
Extended - The focus for the long term will be on a potential weak upper low Wednesday, if it were to occur. It approaches Tuesday as a split system with SSW flow increasing a bit aloft with the threat of isolated thunderstorms off the Crest with the increase SW flow. Wednesday is a bit of a dilemma with the GFS consistent with the Low splitting off and some deformation across the Sierra during the day. The system will bring some moisture with it, but the forcing is light and the instability is weak. Continued with the slight chance of showers and cooling off 10 degrees or so from Tuesday with light west flow in its wake. Some ridging is expected behind the trough with a more moderate warm-up.