The avalanche danger is MODERATE above 6000 feet on wind loaded terrain steeper than 35º. Winds returned to a southwesterly direction yesterday and increased overnight. Recent winds drifted snow onto a variety of surfaces like surface hoar and sun crusts that can delay the strengthening of these slabs. 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.
East-northeasterly winds followed by increasing west and southwesterly winds created fresh wind slabs over the past few days. These slabs will form on top of a variety of old snow surfaces that include surface hoar, sun crusts, and near surface facets. Wind slabs are likely to be thicker as you approach the Continental Divide where winds speeds were stronger yesterday and expected to be again today. Look for smooth, rounded features on the slope especially on steep convex rollovers, on leeward sides of ridges and cross-loaded gullies. Even with a small amount of new snow, strong winds can create wind slabs over a foot thick.
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 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
Yesterday, winds shifted from a northeasterly direction back to the southwest. A few locations picked up an inch of snow in the last 24 hours. Currently, temperatures above 6000 feet range from 15º-23º F and winds are out of the southwest at 5-13 mph with gusts from 15-23 mph. Today expect light snow showers this morning with temperatures rising to the mid-20s F. Winds will be out of the west and southwest at 5-10 mph with gusts in the mid 20s on the ridgelines and further east in the advisory area.
| 0600 temperature: | 15-23 deg. F. |
| Max. temperature in the last 24 hours: | 16-24 deg. F. |
| Average wind direction during the last 24 hours: | SW |
| Average wind speed during the last 24 hours: | 5-10 mph |
| Maximum wind gust in the last 24 hours: | 14-23 mph |
| New snowfall in the last 24 hours: | 0-1 inches |
| Total snow depth: | 45-58 inches |
| Saturday | Saturday Night | Sunday | |
|---|---|---|---|
| Cloud Cover: | Light snow showers. | Light snow showers. | High pressure building. |
| Temperatures: | 24-31 deg. F. | 14-22 deg. F. | 18-26 deg. F. |
| Wind Direction: | SW | W-SW | W-SW |
| Wind Speed: | 4-6 | 4-7 gusts 20 | 7-12 gusts 17-25 |
| Snowfall: | 0 in. | 0-1 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.
