Since Thursday, new snow and strong southwest and west winds worked together to create widespread instability. Air temperatures fluctuated a few degrees above and below 32 F yesterday and new snow ranged from 9 to 16% density. Strong winds formed instant slabs in wind exposed areas- though the snow surface may look smooth today, remember slopes will be littered with 3 days worth of wind compacted pillows, drifts and cornices. These wind formed features will likely be sensitive today to the added weight of additional wind loading or a skier.
While we are thankful for new snow and the water contained in the snow, we sure got the short end of the precipitation stick. The Leavitt Lake snow pillow shows over 20 inches of new snow and 5 inches of water content at an average 25% density. The Ski Area study plot and Mammoth Pass picked up 2 inches of water and around 18 inches of snow with average density of 11%. The new snow skis a lot more dense than 11% and digging snowpits takes a lot more effort than the previous storm.
Precipitation accumulated slowly over the last three days and would not necessarily cause a dangerous avalanche situation but the constant wind loaded leeward slopes much faster and with much more intensity than study plot data would lead you to believe. Winds have been blowing for 36 hours at sustained wind speeds of 50 to 70 mph.
While most avalanches occur during storms as a result of heavy wind loads, the avalanche danger remains Considerable today. Even though the snow is dense and conventional thinking would lead one to believe that dense warm snow “sets up” quickly, there is no snowpack information from alpine and subalpine areas to support anything lower than Considerable.
Another unknown is how 2 inches of water has affected the widespread facets and depth hoar that sits at the bottom of over 4 feet of snow.
Yesterday in the Rock Creek area, extended column tests collapsed on depth hoar and facets. The prior week, most tests did not produce results. Yesterday’s test results strongly suggest the new snow loaded the weak layer in Rock Creek. I have no observations from the Mammoth Basin and the snow is different in Rock Creek so one cannot assume similar results will occur in the Mammoth Basin. However, it is safe to say the new snow in the Mammoth Basin added a load to the buried weak layer.