Likelihood ?CertainVery LikelyLikelyPossible
Size ?HistoricVery LargeLarge
Moderate temperatures have been slowly bringing the snowpack together over the past week. The persistent weak layers such as facets and patches of buried surface hoar are still lurking out there; but, they are rounding out and coming together. Backcountry riders are most likely to trigger a slide on one of these weak layers in an area with a thinner snowpack. These weak layers are becoming stubborn, but they are deep and will result in large avalanches if triggered.
This past week I have had the opportunity to travel into the deep snowpack of the Cabinets and up into the Purcell Mountians via Spread creek. In the Cabinet mountains the snowpack is quite deep and the main avalanche concerns are arising from the surface layers brought in by the storm that left on December 30th. This storm left us with a layer of dense snow 6-10" thick that sat on top of lighter and colder snow. I also observed extensive evidence of battling winds that left thin slabs and pillows on all aspects. With a couple of days to settle out these layers were already proving to be fairly stable in pit tests and showing very little reactivity to ski cuts as well.
In the Purcells the snowpack is much thinner, it is also growing another pretty layer of surface hoar up to 1cm tall. The same structure and benchmark layers are there (Thanksgiving rain crust/facets/more crust) but the weak layers are not as deep. Stability tests are showing good strength and the facets all appear to be rounding out and coming together with the moderate mountian temperatures at higher elevations. Primary concern is that with this thinner snow a rider may be more likely to trigger one of these weak layers resulting in a slab that would be 2' thick. Also, colder air has been trapped in the drainage bottoms which is causing these weak layer to heal at a slower rate. So, just because you are at a drainage bottom you should still be giving steep terrain respect and practicing safe travel habits.
Backcountry Forecast from NWS Missoula issued: 330 AM MST Fri Jan 5 2018 DISCUSSION: Observations across the region area already coming in with above freezing temperatures at high elevations. This is expected to continue through Saturday morning when a cold front will drop snow levels back down to the valleys again. Rain and snow are expected in the mountains through Sunday morning when it will decrease to mountain showers. Next week the pattern transitions back to more active and cooler. Kootenai: --------------------------- 5000-7000 FT ---------------------------- Today Tonight Sat Cloud Cover 85% 90% 80% Hi/Lo Temps 29 to 37 26 to 28 29 to 36 Winds(mph) SW 8 SW 8G20 SW 9G20 Precip Chc 40 80 70 Precip Type snow sno/fzra snow Liquid Amt 0.02 0.06 0.05 Snow Ratio(SLR) 10:1 11:1 13:1 Snow Amt(in) 0 0-1 0-1 Snow Level 3000 4500 3500
Avalanche conditions change for better or worse continually. Backcountry travelers should be prepared to assess current conditions for themselves, plan their routes of travel accordingly, and never travel alone. Backcountry travelers can reduce their exposure to avalanche hazards by utilizing timbered trails and ridge routes and by avoiding open and exposed terrain with slope angles of 30 degrees or more. Backcountry travelers should carry the necessary avalanche rescue equipment such as a shovel, avalanche probe or probe ski poles, a rescue beacon and a well-equipped first aid kit. For a recorded version of the Avalanche Advisory call (208)765-7323.
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