The avalanche danger rating is CONSIDERABLE. Human triggered avalanches are likely on steep mid and upper elevation north facing terrain in the Mammoth Lakes Basin. Dangerous avalanche conditions exist where newly formed wind drifts and slabs have formed.
Backcountry travel on and below steep slopes should be avoided. There will be areas of sensitive soft and hard wind slabs and human triggered avalanches could be 2’ deep.
Likelihood ?CertainVery LikelyLikelyPossible
Trend ?Decreasing Danger
Yesterday’s storm dropped about 15” of dense new snow above 8,500 ft. Although the avalanche danger peaked yesterday during the storm, fresh wind slabs will be sensitive today and you will be able to trigger avalanches in wind pillows and slabs on north aspects at mid and higher elevations. The combination of dense new snow and strong winds created wind drifts along the high ridgelines- The sensitivity of new snow and wind slabs could be inconsistent depending on the old snow surface.
Where the entire shallow snowpack was faceted – which is everywhere except bare ground, expect whumpfing and collapses today. Whumpfing and slope collapses on flat ground warned me to avoid steeper terrain yesterday during the storm. Ski Patrol at Mammoth Mountain reported 15 foot diameter areas of whumpfing and propagating cracks on steep slopes on ungroomed slopes. The snowpack on the north side of Mammoth Rock produced propagation in some places but not in others. A shooting crack went about 10 feet from the observer.June Mtn ski patrol reported cracking, large slope collapses on undisturbed slopes between 8500 and 9,000 ft yesterday.
There are multiple buried facet layers, now 12-15 inches deep. The new snow will stress the old weak layer but I am not sure the load was enough to cause widespread natural avalanching. There IS enough new snow to increase your chances of strating an avalanche where thin rocky areas are adjacent to thicker more dense snow or on rollovers in north facing terrain right below treeline.
Collapsing is a red flag – do not ignore this "in your face" warning sign. Persistent buried weak layers are unpredictable. Avalanches can be triggered from above by a collapsing snowpack on flat ground. Avoid steep terrain today and slopes connected to and below steep terrain.
Yesterday's storm presented a unique avalanche forecasting problem. Even though 15-16 inches of new snow fell, it fell on a very shallow snowpack- there was not enough snow to link turns. The added snow created more of a base but rocks, logs and terrain features are still visible. Today's snowpack resembles what we would find in November- except it is late winter.
There was very limited data available to assign the Considerable danger rating. The reasons I went with the more conservative danger rating are related to my observations and reports of collapse, cracking, and poor snow structure due to the new load of 2-3.5" of new snow plus the additional effect of wind loading.
<p>Storm totals range from 13-15 inches at Main Lodge and June Mountain Summit. Mammoth Pass picked up 20 inches of snow. Water content of the new snow is high- the storm added 2-3 inches of water added to the snowpack.</p>
<p>Weather conditions varied between June and Mammoth Mountains. Average hourly wind speeds at the top of June Mountain were 12 mph during the storm compared to 48 mph at the top of Mammoth. Mammoth Mountain ski patrol reported swirling winds off the ridges and drifts up to a foot. Off the groomed runs, patrol reported</p>
<p>While 15"of new snow is welcome, rocks, downed logs are just that much more difficult to see. The storm did a good job of covering up bare ground and small plants but northeast winds coming in tonight may strip new snow from many north facing slopes.</p>
<p>Snowpit tests on a slope at 9400 ft in the Lakes Basin yesterdayproduced inconclusive results. ECT tests propagated on the old snow/new snow layer and in the storm snow. Other tests resulted in no propagation. Tests were done on non wind loaded slopes.</p>
|0600 temperature:||18 deg. F.|
|Max. temperature in the last 24 hours:||29 deg. F.|
|Average wind direction during the last 24 hours:||WSW|
|Average wind speed during the last 24 hours:||20 mph|
|Maximum wind gust in the last 24 hours:||40 mph|
|New snowfall in the last 24 hours:||15 inches|
|Total snow depth:||25 inches|
Snow showers continue today with possible accumulations of 2-4” likely above 9,000 ft. After a cold night, daytime highs will only reach the low 20’s today. Overnight lows will be cold with single digits possible. West winds gusting to 50 mph over the ridges are expected today. Saturday will be partly sunny with cold daytime temperatures and increasing northwest winds.