Winds have slackened, but previous strong northeasterly winds formed wind slabs and hard slabs. Careful snowpack evaluation is essential in wind-loaded terrain today. Hard slabs can exist on north, northeast, and east facing terrain, and this type of avalanche could step down to deeper weak layers. The avalanche danger is MODERATE above 5000 feet, and LOW at the lower elevations today.
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Above 6500 ft.
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5000-6500 ft.
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3500-5000 ft.
Winds have created slabs on all aspects over the past week or so, with the most recent being strong northeast winds on Sunday and Monday that created slabs in terrain located on the southern half of the compass. These winds also formed hard slabs on east, northeast, and north aspects. Yesterday, we found hard slabs that failed and propagated with easy force in some of our our stability tests (video). Hard slabs can be identified by your skis or machine not penetrating the surface and hollow sounding snow. These hard slabs are likely to exist on open terrain at upper elevations. Terrain harboring hard wind slabs should only be approached with caution today. Also, pay attention to terrain that is cross loaded by these recent strong winds. The recent skier triggered avalanche in the Skook Chutes in the southern Whitefish Range is a good example of a cross loaded slope.
In most locations across the advisory area there is weak snow buried in the snowpack. These layers include weak snow (facets) formed during the cold period in mid-December (image), facets surrounding an early December rain crust, and weak snow near the ground in areas with a shallow snowpack. Though these layers have been dormant all season (i.e. no avalanches reported involving these layers) it is extremely important to not let your guard down. There is a lot of uncertainty around how much of a load they can handle before they potentially become a problem. Newly formed hard wind slab avalanches can also step down to these deeper weak layers. 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.
Tuesday: Todd and Seth traveled to Noisy Knob in the Swan Range. Hard wind slabs on all aspects at upper and mid elevations were observed as well as some facets at the ground level in shallow pits. The wind crusts were variable in strength and thickness, but they were able to get an easy failure with propagation on a northwest aspect about 10 inches from the surface.
Monday: We visited the southern Whitefish Range outside the boundary of Whitefish Mountain Resort. They found hard wind slabs on northeast aspects as well as weaker faceted snow 2.5 feet deeper in the snowpack that fractured and propagated in our stability tests.
Sunday: Zach Miller, the FAC intern, toured to Snowshed Mountain in the Flathead Range where he found instabilities in his snowpit confined to a density change in 2 locations within the top 2 feet of the surface.
Saturday: Mark visited Wahoo Creek in the Flathead Range where he 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.
See below for all observations this season.
The arctic air mass continues to hold strong over the advisory area. We have seen light to moderate northeast winds since the stronger winds Sunday/Monday. As of 5:00 am, current temperatures above 6000 feet range from -9º to 1º F, and northeast winds are 2-7 mph with gusts to 14 mph. Continued cold conditions and light winds are expected through tonight.
| 0600 temperature: | -9 to -1 deg. F. |
| Max. temperature in the last 24 hours: | -2 to 6 deg. F. |
| Average wind direction during the last 24 hours: | Northeast |
| Average wind speed during the last 24 hours: | 3-13 mph |
| Maximum wind gust in the last 24 hours: | 6-21 mph |
| New snowfall in the last 24 hours: | 0 inches |
| Total snow depth: | 54-69 inches |
| Wednesday | Wednesday Night | Thursday | |
|---|---|---|---|
| Cloud Cover: | Cold. Partly Sunny. | Very cold. | Cold. Partly Cloudy |
| Temperatures: | 0 to 6 deg. F. | -21 to -8 deg. F. | 6 to 11 deg. F. |
| Wind Direction: | North-Northeast | East-North | South-Southwest |
| Wind Speed: | 5-10 mph | 5-10 mph | 5-10 mph |
| Snowfall: | 0 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.
