Dangerous conditions exist on or below steep slopes today. Dense snow and gusty winds have created slabs 1-3 feet thick that will be reactive to the weight of a person or snowmachine. Natural avalanches are also possible. Both triggered and natural slides could break into deeply buried weak layers. The likelihood of triggering slides – and their potential size – will be greatest at upper elevations, and in the Lake McDonald area of Glacier National Park.
?
Above 6500 ft.
?
5000-6500 ft.
?
3500-5000 ft.
On slopes steeper than about 30 degrees, you can trigger slabs that break in the new snow or at the interface with the old snow. These will be thickest and most reactive on wind-loaded and cross-loaded slopes. As an example, a near-miss Tuesday in the southern Whitefish Range involved a remotely-triggered wind slab 3-feet thick on an easterly slope. Gusty winds today will continue to drift snow and thicken slabs, so the likelihood of triggering slides will remain elevated through the day.
Some triggered or natural slides could break on old weak layers. You can trigger these slides from adjacent slopes or from below, and they can break across terrain features. While this hazard is more isolated than that involving only new and drifted snow, any triggered slides will be wider and involve surprising amounts of debris. Avoid stopping or riding on or under slopes steeper than about 30 degrees.
Another warm, windy storm swept across the forecast region on Tuesday and Tuesday night. Snowfall favored the northern swath of the forecast region. The highest snowfall totals look to be localized along the Continental Divide in Glacier National Park, where Flattop Mountain SNOTEL has recorded 2 inches of Snow Water Equivalent – about 15 inches of snow - since early Tuesday morning. Whitefish Mountain Resort is reporting 1.95” of SWE and about 15 inches of new snow. The southern parts of the region – John F. Stevens Canyon, Essex, and the Swan Range – received about half the snow and SWE.
Gusty southwesterly winds accompanied the snowfall. These were most sustained at upper elevations near summits and ridgelines. The winds drifted snow and cross-loaded southerly slopes Tuesday. This loading was most extensive on leeward (easterly and northeasterly) slopes near ridgelines, though observers reported some cross-loading at mid elevations.
The combination of new and drifted snow has increased the avalanche danger on and below steep slopes. Freshly-formed slabs pose a danger in themselves. The most widespread danger will be Storm Slab avalanches at upper elevations. Slopes where drifted snow has thickened and stiffened the recent snow will be most dangerous.
Freshly-formed slabs have also increased the load on various old weak layers buried mid-pack or deeper. These include basal facets, surface hoar, and facets around crusts. Various combinations exist on all aspects, but they’re reactive on isolated slopes. So leave a wide margin for surprises.
To ride safely today, avoid steep slopes near areas where you see shooting cracks or feel collapses. Avoid crossing on or under slopes steeper than about 30 degrees. Clear signs of drifted snow include hollow-feeling snow and areas with dense snow that doesn’t allow your boards or machine to sink more than a few inches below the snow surface.
Our next Avalanche Awareness talk is Thursday, December 20th at Penco Power Products at 6:30 p.m.!
We are offering an Avalanche Awareness talk Thursday, December 27 at Stumptown Snowboards at 7:00 p.m.!
We are offering a Motorized Level 1- Avalanche Fundamentals course January 11-13!
Gusty winds will continue to blow from the southwest and west today. Wind speeds should be highest at upper elevations, though they'll mix down to mid elevations at times. Snow showers today will bring a few more inches of snow. Thursday, expect warmer temperatures and lighter winds.
| Wednesday | Wednesday Night | Thursday | |
|---|---|---|---|
| Cloud Cover: | Broken | Broken | Broken |
| Temperatures: | deg. F. | deg. F. | deg. F. |
| Wind Direction: | 25-31 | 17-25 | 30-35 |
| Wind Speed: | SW/W 15-20G30 | S/SW 10-15 G25 | S/SW 10-15 G25 |
| Snowfall: | 1-2" in. | 0-2" in. | 0-2" in. |
| Snow Line: | 3000 | 2500 | 4000 |
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.
