A shift in wind direction, accompanied by increasing wind speeds, has formed fresh wind slabs on atypical slopes. These will continue to form and thicken throughout the day. Wind slabs formed on typical leeward slopes over the past few days are still a concern. Human triggered avalanches are likely in wind loaded terrain. The avalanche danger is CONSIDERABLE on wind loaded terrain above 5000 feet. Cautious route-finding and conservative decision making are essential today.
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Above 6500 ft.
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5000-6500 ft.
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3500-5000 ft.
Over the past week, prevailing west-southwesterly winds drifted low density surface snow onto typical leeward aspects. In many locations wind slabs that formed were deposited on top of weak snow. Around midnight last night the wind direction changed to a northeasterly flow and will remain that way for the remainder of today and tonight. Wind speeds will increase throughout the day and drift the surface snow onto atypical slopes forming fresh wind slabs that will thicken as the day progresses. Choosing safe terrain to recreate on today will be somewhat complicated as we now have wind slabs found on all aspects. It is easy to assume that this problem is confined to ridgetop locations. However, slopes that are roughly perpendicular to the wind direction can be crossloaded. The recent skier triggered avalanche in the Skook Chutes in the southern Whitefish Range is a good example of a cross loaded slope. It is likely that you could trigger a wind slab avalanche today and therefore these slopes should be avoided. Look for smooth, rounded features on the snow surface. Watch for obvious signs of instability like cracking and collapsing while traveling along ridgelines and riding over low consequence wind loaded features.
In most locations across the advisory area there is weak snow buried in the snowpack. These layers include: weak (faceted) snow formed during the cold period in mid-December, weak snow surrounding an early December rain crust, and weak snow near the ground in areas with a shallow snowpack. Though these layers of weak snow have been mostly dormant it is extremely important to not let your guard down. I consider these layers ticking time-bombs. There is a lot of uncertainty around how much of a load they can handle before they become a problem and the transition can occur rapidly. The most common places to trigger these deeper slides are in steep, rocky terrain, and areas with a relatively shallow snowpack. Erich wrote about this in the new Forecaster's Corner.
Saturday: Mark visited Wahoo Creek in the Flathead Range where we found wind affected snow at all elevations. The surface wind slab was sitting on top of weak low density snow and would easily break under the weight of skis. We also noted evidence of numerous thin storm slab avalanches that had occurred Thursday night/Friday day.
Friday: BNSF snow safety observed a natural storm snow avalanche cycle on all aspects at low and mid elevations in John F. Stevens Canyon. These avalanches were thin (4-6") and involved only the new snow that fell overnight. FAC staff visited the site of the skier triggered avalanche that had occurred in the Skook Chutes on Thursday. The avalanche appears to have been a wind slab deposited by cross loading. We also noted natural and skier triggered storm slab avalanches on north and east aspects that involved just the new snow that fell overnight. Riders in McGinnis Creek in the Whitefish Range noted 2 slab avalanches on east facing aspects. Skiers in Tunnel Creek in the Flathead Range reported a upper elevation slab avalanche that was approximately 500' wide and 1 foot in depth.
Thursday: We received a report of a skier triggered avalanche in the Skook Chutes in the southern Whitefish Range. The skier was reported as being partially buried but sustained no injuries. Erich was around Snowslip Mountain in the Lewis Range in southern Glacier National Park and intentionally triggered a very small wind slab avalanche from a safe location above the ridge.
See below for all observations this season.
A change in wind direction occurred around midnight last night with northeasterly winds entering our area. These winds will increase in strength as the day continues ushering in a cold arctic air mass later this morning that will be with us for most of the work week. Currently, temperatures above 6000 feet range from 10º-18º F, and northeast winds are 5-11 mph with gusts from 10-25. Snowfall started this morning with totals of 0-3" recorded so far. Snow should taper throughout the day as the cold arctic air infiltrates. Cold and windy conditions are expected tonight.
| 0600 temperature: | 10-18 deg. F. |
| Max. temperature in the last 24 hours: | 13-21 deg. F. |
| Average wind direction during the last 24 hours: | WSW |
| Average wind speed during the last 24 hours: | 5-15 mph |
| Maximum wind gust in the last 24 hours: | 16-36 mph |
| New snowfall in the last 24 hours: | 0-3 inches |
| Total snow depth: | 57-77 inches |
| Sunday | Sunday Night | Monday | |
|---|---|---|---|
| Cloud Cover: | Snow tapering with windy conditions developing. | Cold and windy. | Cold and windy. |
| Temperatures: | 8-19 deg. F. | -8 to 2 deg. F. | -2 to 10 deg. F. |
| Wind Direction: | Northeast | Northeast | Northeast |
| Wind Speed: | 21-29 mph with gusts to 52 | 20-24 with gusts to 40 | 17-20 mph with gusts to 40 |
| Snowfall: | 2-5 in. | 0-2 in. | 0-1 in. |
| Snow Line: |
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 at 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.
