The avalanche danger remains CONSIDERABLE at middle and upper elevations today. Natural avalanches are possible and human-triggered avalanches are likely today. The combination of two feet of new snowfall and strong winds created widespread instability by overloading wind drifted snow from Friday, Saturday and Sunday. Wind loaded slopes are certain to have deeper accumulations of storm slabs and wind slabs over buried weak layers at the bottom of the snowpack.
CONSIDERABLE means dangerous avalanche conditions exist on all wind loaded terrain. Winds in excess of 100 mph have created unusual wind loading patterns. Careful snowpack evaluation and cautious route-finding is essential today. Avoid slopes steeper than 30 degrees and avalanche run out zones.
Likelihood ?CertainVery LikelyLikelyPossible
Trend ?Decreasing Danger
Steep slopes that have been wind loaded will be the primary concern. North and east facing slopes received the brunt of wind loading and will contain fresh, dense wind slabs. However, when wind speeds exceed 50 and 60 mph for three days in a row, swirling winds have likely deposited snow on all aspects in treeline and in alpine zones. Two feet of new wet snow has fallen over the last two days above 9,000 ft- last night another few inches fell with winds merely 30-40 mph but strong enough to form new wind slabs.
Over 2 inches of water added weight to the buried weak layers. Thick facet and depth hoar layers are found on NW-N-NE aspects but could be more widespread on other aspects-there is no information from east, south and west aspects. There is a lot of wind loaded, thick dense slabs on top of 3-6 inches of weak faceted snow. Large avalanches could occur if avalanches step down to the buried facets and depth hoar layer at the base of the snowpack.
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.
|0600 temperature:||27 deg. F.|
|Max. temperature in the last 24 hours:||32 deg. F.|
|Average wind direction during the last 24 hours:||sw|
|Average wind speed during the last 24 hours:||60-70 mph|
|Maximum wind gust in the last 24 hours:||80 mph|
|New snowfall in the last 24 hours:||8 inches|
|Total snow depth:||40 inches|
Skies are clearing this morning. The relentless west and souithwest winds have calmed down to the 30-45 mph range over the ridgetops. Dry and mild conditions return today. Another Pacific storm will brush our area mid week, creating windy conditions at the higher elevations. The next chance for precipitation is next weekend.