The avalanche danger is MODERATE above 5000 feet on wind loaded terrain. Northeasterly winds formed wind slabs over old snow surfaces that include surface hoar and sun crusts. Older, lingering wind slabs also exist on other aspects. Human triggered avalanches are possible today. Carefully evaluate the snowpack before committing to any slope, particularly in 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 created fresh wind slabs yesterday into today. 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 in the Flathead Range and Glacier NP where winds speeds were stronger yesterday. Snowslip weather station in southern Glacier Park recorded nearly 8 hours of sustained average winds of 19-25 mph with gusts to 32 mph. Because of these sustained winds, wind slabs could be more sensitive than they were this past week. 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, I went on the hunt in the southern Whitefish Range for recently buried surface hoar, and was successful. I found it about 2 inches below the surface. See observation and video for details. Skiers in the Paola Creek drainage in the Flathead Range yesterday noted wind transport near ridgetops and developing wind slabs in exposed areas (observation). We received no observations from the Swan Range yesterday regarding how the recent northeast wind affected the snowpack.
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).
On Tuesday I traveled to Red Meadow Peak in the northern Whitefish Range. I observed southerly winds loading northerly aspects at upper elevations on that day (video, observation). Also on Tuesday, experienced skiers in the Middle Fork reported sun crust conditions on a southeast aspect of Mt. Penrose. They also noted substantial sluffing of the dry loose surface snow while skiing a steep northeast aspect into Rescue Creek. The quantity of this sluffing surprised them.
Monday, Mark observed a recent wind slab avalanche that appears to have been triggered by a cornice fall (photo, observation) in Rescue Creek in the Flathead Range (observation). We received multiple observations from the Middle Fork corridor in the Flathead Range as well as Glacier National Park last weekend noting wind loading, thin wind slabs, and the presence of surface hoar (observations). Also last Saturday, I witnessed a small, natural wind slab avalanche on a steep, north facing slope as well as another nearby 1-2 day old avalanche on Nyack Peak in the Flathead Range (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
Light snowfall throughout the advisory area continues through mid-day today. 1-3 inches of new snow fell in the past 24 hours with moderate to strong northeast winds. Currently, mountain temperatures above 6000 feet range from 8º-21º F with winds out of the east and northeast at 3-7 mph with gusts to 10 mph. Today, winds will be 5-10 mph with gusts to 15 mph from the east-northeast and shifting to the southwest in some locations. Temperatures will be in the mid 20s F.
| 0600 temperature: | 8 to 17 deg. F. |
| Max. temperature in the last 24 hours: | 12 to 31 deg. F. |
| Average wind direction during the last 24 hours: | Northeast |
| Average wind speed during the last 24 hours: | 9-24 mph |
| Maximum wind gust in the last 24 hours: | 13-32 mph |
| New snowfall in the last 24 hours: | 1-3 inches |
| Total snow depth: | 45-58 inches |
| Friday | Friday Night | Saturday | |
|---|---|---|---|
| Cloud Cover: | Light snowfall. | Continued light snowfall. Decreasing winds. | High pressure building. |
| Temperatures: | 21 to 29 deg. F. | 6 to 16 deg. F. | 16 to 22 deg. F. |
| Wind Direction: | Northeast shifting to South-Southwest | Southwest through Northeast | Southwest |
| Wind Speed: | 5-10 mph. | 4-5 mph. | 5-10 mph. |
| Snowfall: | 1 in. | 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.
