The avalanche danger is MODERATE above 6000 feet on wind loaded terrain steeper than 35º. Recent winds drifted snow onto a variety of surfaces like surface hoar and sun crusts that can cause instability to linger. Human triggered avalanches are possible today. Carefully evaluate the snowpack before committing to any slope, particularly in steep, wind loaded terrain. The avalanche danger is LOW on all other terrain.
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Above 6500 ft.
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5000-6500 ft.
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3500-5000 ft.
There is a lot of variability across the advisory area in regard to wind affected snow. Recently, we have found several age groups of wind slab that have not been especially reactive. Yesterday, Mark found wind-drifted snow that did not even form a cohesive slab. The main concern at this point is in the areas where these slabs formed on top of a variety of weak snow surfaces that include surface hoar, sun crusts, and near surface facets. These slabs will be slower to gain strength so it remains important to carefully assess windloaded areas before committing to a slope. Wind slabs are likely to be thicker in areas near the Continental Divide where wind speeds have been stronger. Look for smooth, rounded features on the slope especially on steep convex rollovers, on leeward sides of ridges and cross-loaded gullies.
Today's snow surface is tomorrow's weak layer. Surface hoar and near surface facets (small, weak snow grains just under the snow surface) exist throughout the advisory area. Yesterday's snowfall buried these weak layers, but they should not become an "out of sight, out of mind" issue. This round of snow is unlikely to produce storm slabs, but it's important to monitor the reactivity of these weak layers as new snow continues to pile up on top of them. On south aspects, a sun crust formed that could become a potential bed surface as more snow falls as well.
On another note, there are two deeper layers in the snowpack that were a concern earlier in the season, but we haven't observed or received reports of avalanches on these layers or reactiveness in stability tests in over 3 weeks. These layers include facets surrounding the December 9 rain crust and larger facets near the ground in areas with a shallow snowpack. It is unlikely you'll trigger an avalanche on these layers, but not impossible. It is still worth taking the time to dig into the snow and see how these layers react in the areas that you are skiing or riding, especially in areas with a shallow snowpack (less than 3-4 feet deep).
Yesterday, Mark and Guy traveled to Baldhead Mountain in the Flathead Range. They found soft, cohesionless wind drifted snow on leeward aspects and buried surface hoar 2-4 inches deep (photo). They also noted weak snow near the ground that was unreactive in stability tests (observation).
Skiers in the Marion Lake area, also in the Flathead Range found a shallow snowpack (about 3 feet deep). On sunny aspects they found a 1 inch crust with 2 inches of recent snow on top. Below the crust they noted weak snow that failed and propagated in stability tests.
Friday, Erich and I traveled up a ridge that divides the Lost Johnny and Wounded Buck drainages in the Swan Range. We observed buried surface hoar and near surface facets in the top 2-3 inches of snow. The snowpack in this area was deep and uniform (photo, video).
Thursday, Erich went on the hunt in the southern Whitefish Range for recently buried surface hoar, and was successful. He found it about 2 inches below the surface. See observation and video for details. Skiers in the Paola Creek drainage in the Flathead Range noted wind transport near ridgetops and developing wind slabs in exposed areas (observation).
On Wednesday, skiers in the Marion Lake area observed a wind slab in a cross-loaded gully on nearby Mt. Adams that appeared to be 1-2 days old, around 150 feet wide and up to 1 foot deep (photo). Also on Wednesday, on Sub-Shields in southern Glacier National Park, GNP rangers observed surface hoar (though melting on southerly aspects) and a snowpack that has settled considerably since last week. That same day nearby on Snowslip and Running Rabbit Mountains, skiers observed unreactive wind slabs, plenty of surface hoar, and a decomposing December 9 rain crust near the ground (observation).
Thanks to everyone for submitting observations. They are extremely useful for everyone.
Visit our Observations page and our You Tube channel for more observations from the entire season.
Please let us know what you are seeing out there. Your observations are important and valued.
HOW TO SUBMIT OBSERVATIONS:
Email: [email protected]
Call and leave a message: 406.387.3821
You can also submit quick observations via text: 406.241.4571 (FAC mobile)
OR
Submit Snowpack Observations: http://www.flatheadavalanche.org/node/add/snowobs
Submit Avalanche Observations: http://www.flatheadavalanche.org/node/add/avyobs
Moderate west and southwest winds with strong gusts continued yesterday and are expected again today. Currently, mountain temperatures range from 13º-21º F and winds are out of the west and southwest at 6-11 mph with gusts from 15-30 mph. Today, a ridge of high pressure builds over the area. Expect temperatures in the mid 20s and winds to continue out of the west and southwest at 5-10 mph with gusts from 25-30 mph in upper-elevations and in the east part of the advisory area.
| 0600 temperature: | 13-21 deg. F. |
| Max. temperature in the last 24 hours: | 16-24 deg. F. |
| Average wind direction during the last 24 hours: | WSW |
| Average wind speed during the last 24 hours: | 5-10 mph |
| Maximum wind gust in the last 24 hours: | 18-30 mph |
| New snowfall in the last 24 hours: | 0 inches |
| Total snow depth: | 46-58 inches |
| Sunday | Sunday Night | Monday | |
|---|---|---|---|
| Cloud Cover: | High pressure building. | Partly cloudy. | Partly cloudy. |
| Temperatures: | 21-32 deg. F. | 13-20 deg. F. | 21-29 deg. F. |
| Wind Direction: | W-SW | W-SW | SW |
| Wind Speed: | 7-10 gusts 20-23 | 6-9 | 7-9 gusts 21 |
| 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.
