Natural and/or skier triggered avalanches occurred each of the past three days. Warming temperatures, strong winds, heavy dense snow at mid/upper elevations with rain at lower elevations has created dangerous avalanche conditions. Storm slabs will be more reactive and widespread than yesterday with wet loose activity increasing. A variety of crusts/weak layers exist in our snowpack which are being stressed from this weeks loading, possibly resulting in a large avalanche. Choosing lower angle sheltered terrain is recommended today.
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Above 6500 ft.
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5000-6500 ft.
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3500-5000 ft.
Between 0.5-0.9" of precipitation (in the form of SWE) and associated warm temperatures entered our area yesterday afternoon and overnight depositing a rain/snow mix at lower elevations and dense snow at upper elevations. Yesterday's storm slab formed an inverted snow surface which lead to natural and human triggered avalanches. Today's storm slab will be even more "upside down" creating dangerous avalanche conditions. Moderate to strong west to southwest winds will thicken these slabs on leeward aspects and crossloaded slopes. Cracking, shooting cracks and collapsing (whoomphing) will be obvious red flags in the snow surface. Avoid riding above terrain traps and convexities while testing small slopes before going into steeper terrain.
Prior to yesterday/last nights rain and wet snowfall, deep dry unconsolidated snow was found on sheltered aspects at all elevations. A wet loose natural avalanche cycle likely started overnight and will continue through today at low and mid elevations. With warm upper elevation temperatures expected today, tree bombs may occur starting wet loose slides. The first sign of unstable surface snow will be rollerballs and pinwheels. At low elevations the surface snow is resting on a stout crust which may allow loose wet snow to travel long distances and entrain a fair bit of snow. We recommend not recreating on slopes steeper than 35º or above terrain traps if the surface snow that you are on is becoming wet.
This weeks snowfall has buried weak faceted snow, surface hoar and crusts 3-5'+ below the snow surface. We received a report of a large avalanche (~5' deep) in Rescue Creek this weekend which likely failed on one of these layers (observation). On Friday, also in Rescue Creek, a large avalanche occurred which may have failed on a deeply buried layer. These red flags confirm that we still have a persistent slab problem. Stability tests still indicate that some of these weak layers have the ability to propagate with potential avalanche size increasing with continued snowfall (see observation). The slabs overlying these layers are not uniformly thick with thin areas, such as rock outcrops and the edges of the slab, being most susceptible to human triggering. Digging is the only way to identify which slopes contain these persistent weak layers and when in doubt it is best to default to lower angle slopes and less consequential terrain.
Deep slab avalanches failing near the ground on the Thanksgiving and December 17 rain crust/faceted layer remains a low likelihood, high consequence concern in the Flathead and northern Whitefish Ranges. This is a difficult problem to forecast and managing terrain choices is your best weapon to avoid a very large and destructive avalanche. Deep slabs are most likely to be triggered on upper elevation slopes containing variable snow depths and rocky terrain. They can be triggered from less steep areas on adjacent slopes and below steep terrain. Snowpack assessment for deep weak layers is challenging and careful terrain selection is important. Deep slab avalanches are found at upper elevations on all aspects with the likelihood of triggering rated unlikely.
As I said yesterday and will reiterate today, January is going out with a bang...24 hour precipitation totals: WMR 5”/ 0.47” SWE in the southern Whitefish Range, Stahl Peak 3"/0.5" SWE in the northern Whitefish Range, 4”/ 0.9” SWE at Noisy Basin in the Swan Range and Flattop 5"/0.8" SWE. Snow water equivalent (SWE) totals for the past 5 days: Noisy Basin 4.6", Flattop 3.4" and Stahl 2.8". Southwest winds increased overnight with Hornet, in the northern Whitefish Range, recording sustained winds up to 22 mph gusting to 33 and Snowslip, in John F. Stevens, reporting a maximum of 24 mph sustained gusting to 42.
Yesterday, natural and human triggered storm slab avalanche activity was noted in the southern Whitefish Range and involved dense new snow overlying low density snow. With warming temperatures and increasing wind speeds overnight, today's storm slab will be more reactive than yesterday. Warming temperatures, rain and wet snow have created a wet loose problem at low and mid elevations. This problem may creep up into the upper elevations as the day progresses and trees start to shed their heavy load of snow. Wet loose slides will have the ability to entrain a lot of snow and deposit large debris piles.
Friday, we received a report of a large natural slab avalanche on Nyack Mountain in Rescue Creek, Flathead Range. This avalanche traveled 3000'+ to the valley floor and occurred in the same avalanche path where a large persistent slab avalanche failed January 12 or 13 (see observation). Weather stations and field observations confirm moderate winds with strong gusts produced substantial transport of snow which likely triggered this slide. The failure layer is unknown but we are assuming this slide ran on the bed surface of the previous slide which was the Thanksgiving crust/facet layer.
A short lived drying and warming will occur today before precipitation enters our area late this afternoon with temperatures plummeting. Snow is on tap tonight into tomorrow with much colder temperatures.
| Monday | Monday Night | Tuesday | |
|---|---|---|---|
| Cloud Cover: | Warmer with precipitation arriving this afternoon. | Cooling with snow | Cooler with snow |
| Temperatures: | 36 to 41 deg. F. | 22 to 27 deg. F. | 29 to 34 deg. F. |
| Wind Direction: | SW | SW | SW |
| Wind Speed: | 10 to 20 mph, gusting to 45 | 10 to 20 mph, gusting to 50 | 10 to 20 mph, gusting to 55 |
| Snowfall: | 0 in. | 2 to 4 in. | 1 to 3 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.
