A Special Avalanche Bulletin is in effect for NW Montana. Dense snowfall and strong winds are creating unstable slabs on a variety of weak and slick surfaces. Avalanches breaking in the new snow will be easy to trigger today, and some have the potential to step down and produce very large slides that break near the ground. The safest riding today is on slopes less than 35 degrees or in wind-sheltered terrain that has less than 6" of new snow.
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Above 6500 ft.
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5000-6500 ft.
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3500-5000 ft.
Slabs of new and wind drifted snow will be easy to trigger today and could fail naturally on slopes steeper than 35 degrees. Over a foot of new snow, plus more today, is accumulating on variety of weak layers and crusts. Slabs will be more dangerous in leeward terrain, where gusty southwest winds have been drifting and thickening the snow. Shooting cracks and collapses are warning signs to choose conservative terrain.
Rapid loading is adding stress to faceted crusts buried by hard slabs that are 3 to 5 feet thick. Every major storm cycle this season has produced a handful of very large avalanches that fail near the ground. Feedback from this problem is rare, but it is most likely to be found above 6,000' on steep, windloaded start zones, especially in rocky or alpine terrain. Avalanches breaking in the new snow have the potential to step down and trigger these deadly beasts. Be wary of overhead hazards and avoid traveling in the type of terrain where persistent slab avalanches have been failing.
If 2019 is the year of rain crusts, sun crusts, rime crusts, facets, surface hoar, and other unwelcomed guests, we are happy to show it to the door. But if the year of deep powder, 2020, crosses with 2019 on its way out, there is trouble in the making.
2020 has been a good year so far. Storm totals this morning range from 12" in the Swan Range (2.0" SWE), 9" to 11" in the Whitefish Range (1.1" SWE), 1.4" of SWE in Central Glacier Park, and about 6" of new snow near Marias Pass. South and Southwest winds have been gusting into the 20's and 30's at ridgetops, enough to drift the new snow into thicker and denser slabs. The wet and windy pattern continues today with another 4" to 8" in the forecast. In short, about a foot of dense snow has accumulated in most places - enough to form a soft slab, and these slabs will continue to grow in size through the day.
2019 was not so great. The last few days of the year left us with a mix of weak layers and crusts before this storm: Near-surface facets (example A, example B), surface hoar (example A, example B), and melt-freeze crusts, all topped with a fresh rime crust in some locations. Expect the new snow to bond poorly to these weak layers until proven otherwise. Use small, steep test slopes to get information before considering avalanche terrain. In wind-sheltered areas that saw smaller storm totals, or in areas where the new snow isn't accumulating on these problematic weak layers, you may find better stability. Put upper elevation, leeward terrain on your no-go list today, and carefully assess how well the new snow is behaving in wind-protected terrain before traveling on anything steeper than 35 degrees.
Another warm, wet, and windy one. Air temperatures are hovering near freezing at mid-elevations this morning. Ridgetop wind gusts have been moderate to strong in the past 24 hours and are forecasted to continue through the day, although some stations are showing a downturn in wind speeds this morning. Snowfall continues through the day, with another strong pulse expected this afternoon. The brunt of the storm winds down this evening. Thursday brings a cooler, calmer trend.
| Wednesday | Wednesday Night | Thursday | |
|---|---|---|---|
| Cloud Cover: | Overcast | Mostly Cloudy | Partly Cloudy |
| Temperatures: | 27 to 32 deg. F. | 16 to 21 deg. F. | 21 to 26 deg. F. |
| Wind Direction: | Southwest | Southwest | Southwest |
| Wind Speed: | 10 to 20, G40 | 10 to 20, G35 | 5 to 15, G25 |
| Snowfall: | 3" to 6" in. | 2" to 3" in. | 0 to 1" in. |
| Snow Line: | 4000' | 3000' | 1000' |
This forecast applies only to backcountry areas outside established ski area boundaries. The forecast describes general avalanche conditions and local variations always occur. This forecast expires at midnight on the posted day unless otherwise noted. The information in this forecast is provided by the USDA Forest Service who is solely responsible for its content.
