Moderate to strong northeasterly winds have formed fresh wind slabs on atypical slopes. Continued wind today will add to their thickness and distribution. 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.
Anyone who was conscious yesterday could tell you that our main avalanche problem today is related to the moderate to strong northeasterly winds that battered portions of our advisory area yesterday and overnight. Many of us were able to watch wind slabs develop throughout the day from the comfort of our home. Wind speeds will decrease today but the damage is done. A good example of these windy conditions is the normally benign winds of the WMR ski area where gusts above 40 mph were recorded for 10 consecutive hours yesterday. Wind slabs have formed on atypical slopes and continued wind today will add thickness to these slabs. In some locations the strong winds have deposited snow lower in the starting zone or path than what we are used to seeing. Other areas will exhibit hard slab characteristics. Hard slab can be recognized by your skis or machine not penetrating the surface, a relatively smooth snow surface and hollow sounding snow. Due to their high consequence hard slabs should be avoided at all cost. Wind slabs formed earlier in the week on typical leeward slopes in alpine locations have gained strength but I wouldn't trust them just yet. As a reminder, winds can not only load a slope from the top but also from the side of the path which is referred to as cross loading. 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.
Sunday: Zach toured to Snowshed Mountain in the Flathead Range where he noted atypical loading patterns and cooling temperatures throughout the day. Instabilities in his snowpit were confined to a density change in 2 locations within the top 2 feet of the surface. No other instabilities were noted throughout their tour.
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.
Arctic air infiltrated our area yesterday bringing with it windy conditions and light snowfall. Currently, temperatures above 6000 feet range from -7º to -8º F, and northeast winds are 5-15 mph with gusts from 11-27. Snotel sites across our area have not reported since 3:00 Sunday afternoon but little moisture has been associated with this arctic air mass. Very cold and windy conditions are expected tonight. Winds will decrease starting tomorrow but temperatures will remain very cold through at least Thursday.
| 0600 temperature: | -7 to -8 deg. F. |
| Max. temperature in the last 24 hours: | 10-17 deg. F. |
| Average wind direction during the last 24 hours: | Northeast |
| Average wind speed during the last 24 hours: | 10-25 mph |
| Maximum wind gust in the last 24 hours: | 20-45 mph |
| New snowfall in the last 24 hours: | 0-3 inches |
| Total snow depth: | 56-75 inches |
| Monday | Monday Night | Tuesday | |
|---|---|---|---|
| Cloud Cover: | Cold and breezy. | Very cold and continued breezy. | Continued cold and dry with light winds. |
| Temperatures: | -1 to 12 deg. F. | -19 to -9 deg. F. | -5 to 3 deg. F. |
| Wind Direction: | East-northeast | East-northeast | North-southeast |
| Wind Speed: | 17-19 mph with gusts to 32 | 12-15 with gusts to 31 | 6-7 mph with gusts to 23 |
| Snowfall: | 0-1 in. | 0 in. | 0 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.
