Recent snowfall and winds have continued to develop wind slabs. Increasing wind speeds today will add to this problem. Human triggered avalanches are likely, particularly 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.
?
Above 6500 ft.
?
5000-6500 ft.
?
3500-5000 ft.
I realize that we are starting to sound like a broken record but wind slabs remain our #1 avalanche problem today. Over the past week we have enjoyed copious amounts of new low density snow. Wind speeds over this period have easily been strong enough to drift this low density snow, particularly in the alpine. Friday morning's snowfall was a bit denser than what we have seen over the past two weeks, but overnight winds were strong enough to move this snow. Increasing wind speed today will continue to form fresh wind slabs and add weight to existing slabs. It is easy to assume that this problem is confined to ridgetop locations. However, slopes that are roughly perpindicular to our prevailing westerly-southwesterly flow 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. Wind loaded terrain is easily identifiable and should be avoided today. 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.
An intense period of warmer denser snowfall early Friday morning was responsible for a widespread, but small storm slab avalanche cycle that occurred during the day on Friday. The majority of these avalanches were observed at low and mid elevations and generally were 4 to 6 inches deep. The warmer denser snow accumulated onto cold drier snow that fell on Thursday and made for a sensitive upside-down storm layer. BNSF avalanche safety reported natural avalanches up to 100 feet wide in John F. Stevens Canyon. FAC staff and snowmobilers noted both natural and triggered avalanches in the southern Whitefish Range while other snowmobilers in the northern Whitefish Range reported small natural activity. This storm layer will not be as reactive today and I am anticipating the natural activity to have diminished. However it will still be possible to trigger a storm slab avalanche today. These will be thin and relatively small but could knock you off your feet or machine and ruin your day. Cracking in the snow surface is a tell tale sign of a surface slab.
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.
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.
Enjoy today's seasonal temperatures before bitterly cold temperatures intrude our area for most of the coming week. Currently, temperatures above 6000 feet range from 10º-19º F, and southwest winds are 6-14 mph with gusts from 13-28. Today we should see partly cloudy skies with temperatures being cooler than yesterday. Winds will increase in strength through the day.
| 0600 temperature: | 10-19 deg. F. |
| Max. temperature in the last 24 hours: | 18-25 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: | 15-36 mph |
| New snowfall in the last 24 hours: | 0-1 inches |
| Total snow depth: | 58-74 inches |
| Saturday | Saturday Night | Sunday | |
|---|---|---|---|
| Cloud Cover: | Partly cloudy with gusty winds. | Snow with windy conditions and cold temperatures. | Cold and windy. |
| Temperatures: | 19-26 deg. F. | 4-13 deg. F. | 7-18 deg. F. |
| Wind Direction: | Southwest | North and East | Northeast |
| Wind Speed: | 10-14 mph with gusts to 31 | 12-16 with gusts to 39 | 22-28 mph with gusts to 47 |
| Snowfall: | 0 in. | 2-5 in. | 0-2 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.
