The mountains across the Kootenai National Forest received multiple shots of snow above 5,500 feet the past few days. This new snow arrived with fairly high winds and warm temperatures that created a very cohesive slab layer above 5,500 feet on wind loaded aspects. In some locations this slab is overlying a layer of buried surface hoar making it highly reactive. This surface hoar layer is most likely to be found in areas above 5,500 feet that are sheltered from solar radiation and wind. Regardless of where you may or may not find this buried surface hoar the wind slab is still likely to be reactive to a human or weather related trigger.
Exercise a high degree of caution through the weekend on any steep, wind loaded slopes above 5,500 feet in elevation. Use small slopes as tests and dig a pit before riding or sliding in this kind of terrain.
I am including this problem as a blanket of sorts due to the time of year and the consistenly warm temperatures the region has been experiencing these past few weeks. Slopes on elevations below 5,500' are likely to continue releasing small wet slides during warm day time temperatures and on slopes exposed to solar radiation. These small wet slides are unlikely to pose much of a threat to skiers and snowmobilers but they should not be ignored in areas where terrain features such as gullies could increase the consequences of being caught in such a slide.
Pay attention to warming temperatures and intense sunshine when exposed to terrain traps or below large slopes and cornices. There is some potential for slopes to see the sun on Friday if the predicted weather holds true.
Today we traveled to Smith Mountain in the Western Cabinet Range where we performed stability tests on a north-easterly aspect at 6,000 feet. The test failed with easy force (ECTP10) resulting in a clean shear and a 14" thick slab. This slab was on top of a buried surface hoar layer.
Results from extended column test on March 3rd. 14 inch thick on 4 mm surface hoar (ECTP10)
As we traveled into the area we noted extensive wind loading with westerly winds at 10-15 MPH and gusts up to 20. There was abundant fresh snow available for transport so these wind slabs are likely to increase in thickness as forecasted weather has predicted more strong winds through the coming weekend.
Backcountry Forecast from NWS Missoula issued:
210 PM MST Thu Mar 3 2016
DISCUSSION: Another round of light snow showers will cross western
Montana and central Idaho today. Rain will fall below roughly 5000
feet, though both rain amounts of snow amounts will be very light.
Rebuilding high pressure will escort showers east of the region by
tonight, however a strong upper level jet streak will keep gusty
winds prevalent (on average gusts to 30-40 mph, some stronger
along the Continental Divide) in the higher terrain above 6000
feet for most of today into tonight.
--------------------------- 5000-7000 FT ----------------------------
Tonight FRI FRI Night SAT
Cloud Cover 75% 50% 60% 95%
Hi/Lo Temps 26 to 29 40 to 43 33 to 36 41 to 44
Winds(mph) SW 12G24 S 8 S 10 SW 14G29
Precip Chc 30 0 30 100
Precip Type sno/shr none sno/rain sno/rain
Liquid Amt 0.04 0.00 0.02 0.24
Snow Ratio(SLR) 12:1 0 11:1 8:1
Snow Amt(in) 0-1 0 0 0-1
Snow Level 4500 4000 6000 6000
This advisory applies only to backcountry areas outside established ski area boundaries. This advisory describes general avalanche conditions and local variations always occur. This advisory expires midnight on the posted day unless otherwise noted. The information in this advisory is provided by the USDA Forest Service who is solely responsible for its content.
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