Small, soft, shallow wind drifts may pose an avalanche problem above tree line. I do not expect these small wind slabs will extend very far downslope. If you do get out and brave the subzero wind chills today, exercise normal caution and be alert for areas of wind drifted snow.
Observations made over the past two days show a snowpack in transition from one composed of rounded, well bonded forms to facets. The widespread pre-Christmas “crust” that formed on December 20 and strengthened during the mild days before Christmas Day is crumbling as facets form above and below the layer. The rounded grains that formed a supportive layer for skiing and riding is composed of facets and rounds turning into weak cohesion-less grains.
Cold days, colder nights and a shallow snowpack are the perfect ingredients for facets to form. If the ground temperature is 0C (32F) and the snow surface is – 18C (0 degrees F), there is a huge difference in temperature over a short distance of 50 to 60 cm (19 to 23 inches). In less than a week, a snowpack that could withstand the loading of a real Sierra storm, measured in footage, is changing to a structure similar to last December and January. When storms finally reached our area at the end of January and early February, there was a widespread natural avalanche cycle.
While we don’t know what the weather will bring in the next few weeks, a faceted shallow snowpack is the setup for a persistent weak layer avalanche problem.
For now, small, soft, shallow wind drifts may pose an avalanche problem above tree line. I do not expect these small wind slabs will extend very far downslope. If you do get out and brave the subzero wind chills today, exercise normal caution and be alert for areas of wind drifted snow.
Snowpack observations received from an avalanche class yesterday reported 25 inches (65 cm) of snow on north facing aspects near Minaret Summit. The prominent crust that formed on Dec. 20 was about 15 cm (6 inches) down from the surface. The snow above and below the crust was faceting. The class did a few compression tests with moderate results on the facets below the crust.
On Monday, another observer reported variable conditions ranging from scoured surfaces on exposed above tree line slopes and sastrugi on White Wing. A shallow weak snowpack was observed on northeast facing slopes in the trees.
Yesterday, I toured to the upper Mammoth Lakes Basin and found variable snow conditions on Lake Mary Ridge. There was an inch or two of new snow that was blowing over the ridge and depositing on the small leeward, south facing slope opposite the ridge. Probing revealed the crust was widespread on the north slopes of the ridge but the crust was discontinuous and was falling apart due to faceting.
|0600 temperature:||0 deg. F.|
|Max. temperature in the last 24 hours:||11 deg. F.|
|Average wind direction during the last 24 hours:||NE|
|Average wind speed during the last 24 hours:||45 mph|
|Maximum wind gust in the last 24 hours:||70 mph|
|New snowfall in the last 24 hours:||1-2 inches|
|Total snow depth:||23 inches|
Yesterday was a cold windy day with daytime highs reaching 18F in the town of Mammoth and 11F at the ski patrol study plot at 9,000 ft. on Mammoth Mountain. Northeast winds favored upslope snow showers and June Mountain picked up 3 inches of new cold dry snow thanks to the Mono Lake effect.
Clear skies today mean colder temperatures with highs in the single digits above 9,000 ft. and dangerous wind chills as low as -30F. The coldest morning of the week will be New Years Day with temperatures below 0 degrees F.
High east/northeast winds continue through today with sustained winds of 35 to 45 mph and gusts to 70 mph. The National Weather Service in Reno warns that the strong east wind could cause in larger tree falls due to strong winds on December 11. The NWS sees some similarities with the wind event of December 2011 that cause widespread blow downs in the Devils Postpile area.
The 5 day forecast calls for a gradual warmup after New Years with the high pressure ridge moving east through the weekend.