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.