This photo shows what we mean by isolated slabs in high consequence terrain. Small wind slabs in isolated areas could carry you over cliffs or into terrain traps. Photo courtesy of the Sawtooth Avalanche Center.
I don't have enough information to be confident in saying there is a persisent slab avalanche problem because Iinstabilities seen this weekend are from one specific area. More information is needed from the Mammoth and June Mountain areas to determine if the problem is widespread.
This summary covers the Mammoth and June backcountry.
There are a couple of things to watch for today; small wind slabs will be found in alpine areas-smooth areas on slopes wil have formed in pockets around rocks, entrances to couloirs and on steep slopes. Watch out for places where the snowpack gets thin- around rocks and the sides of gullies. Small avalanches can have big consequences;
Tests done over the weekend show storm snow has bonded to the old snow surface. However, these same tests repeatedly failed on deeply buried faceted grains underneath the rain/dense wet snow layer that formed in early February. Compression and extended column tests failing in the moderate range on fist hard large faceted melt freeze grains is a red flag. It’s difficult to think about deep instability when the fresh snow has created great skiing conditions and even more difficult to take the time to probe to the ground to get an idea of the snowpack structure. These signs of deep instability are very troubling- more information is needed to see whether a similar pattern exists at higher elevations.
Prior to the storm, tests in the Mammoth Basin from 9,000 to over 10,000 feet did not produce any results from stability tests. More information is needed to see if the sudden drop in temperature and new snowfall were the catalyst for this observed change in stability.
Similar to last winter, the snowpack is highly variable depending on elevation and location. The early February storm that saw rain up to 11,000 ft. in the Mammoth area left an indelible, unforgettable thick icy rain crust in the Mammoth Basin and San Joaquin Ridge and a thick, supportable slab in the Negatives and June Mountain area. Over the month of February and despite mild to warm daytime temperatures, the alpine regions are still cold and the snow loses a lot of heat during clear nights. Faceting under the layers and crusts are slowly reducing the strength of the overlying slab as was demonstrated in last weekend’s tests.
Over the weekend, multiple snowpits were dug in the Minaret Summit area. Compression tests and extended column tests showed consistent failures in the moderate to hard range on a layer of large faceted old melt freeze particles near the base of the snowpack. Almost the entire snowpack collapsed on faceted grains about 12 inches above the ground. Fresh cold dry storm snow appeared to have bonded to the old crust and in places was deep enough that skis did not hit the old icy surface.
Reports from the Negatives showed less fresh snowfall and a firm underlying layer. Winds during the storm had moved snow around in steep terrain, forming areas where dry powder conditions abruptly changed to hard sliding surfaces. Gusty east and northeast wind will have loaded small features in the Negatives similar to the second photo.
This photo shows the layering in the snowpack. The arrow points to the weak layer- fist hard grains fell out while the pit was being dug out.
|0600 temperature:||16 deg. F.|
|Max. temperature in the last 24 hours:||24 deg. F.|
|Average wind direction during the last 24 hours:||NE|
|Average wind speed during the last 24 hours:||40 mph|
|Maximum wind gust in the last 24 hours:||60 mph|
|New snowfall in the last 24 hours:||2 inches|
|Total snow depth:||30 inches|
http://unofficialnetworks.com/2015/02/skier-tumbles-250m-down-a-couloir-after-triggering-small-avalanche-tignes-franceMorning temperatures are in the teens. Winds are blowing from the WSW at 20 mph at the top of Mammoth Mountain and light winds are blowing from the south at June Mountain. Scattered upslope snow showers will continue today as a storm system drops in from the Pacific Northwest. The ridge builds back in mid-week bringing a warming trend for the later part of the week.
Two to three inches of snow fell in the last 24 hours in the Mammoth Lakes Basin and on June Mountain. Weekend storm totals are 6 to 12” at locations from Gem Pas to South Lake- precipitation amounts vary widely due to the convective nature of the storm. Strong northeast winds during the storm have come around to the west in the Mammoth area and from the south on June Mountain.
Temperatures will reach the mid 20’s today with overnight lows dipping into the teens. North winds will begin to blow by tonight and will keep temperatures below seasonal temperatures tomorrow.
This snowpack summary applies only to backcountry areas outside established ski area boundaries. This snowpack summary only describes general avalanche conditions and local variations always occur. This snowpack summary expires 24 hours after the posted time unless otherwise noted. The information in this snowpack summary is provided by the USDA Forest Service who is solely responsible for its content.