Numerous large to very large avalanches ran during yesterday's wet and windy weather. The avalanche danger is declining now that winds have eased and temperatures have cooled. Human triggers can still get into serious trouble at higher elevations, where fresh wind slabs and deeply buried weak layers are a concern.
?
Above 6500 ft.
?
5000-6500 ft.
?
3500-5000 ft.
After yesterday's snow surface refroze, about 6" of new snow fell on top of the crust, accompanied by extreme southwesterly winds. The new snow has drifted into thicker, stiffer slabs in terrain features that collect drifting snow. These may be further down slope than you might expect, given the extreme wind speeds. Reading the snow surface is the easiest way to manage this problem. Stay off of steep terrain where you find thicker, denser drifts or see cracking underfoot or machine. An older, larger generation of wind slabs also exists at high elevations, exemplified by numerous large naturals that ran yesterday.
Yesterday's warm, windy, and wet weather spurred continued deep slab avalanche activity in the Flathead Range. The slide on Cameahwait failed on weak layers near the ground. The avalanche broke across the entire face and brought massive amounts of debris to the valley floor. This is not the type of avalanche that you can survive. With drastically cooler, calmer weather today, the odds of a natural or triggered deep slab has subsided again. The terrain that worries us has not changed. Alpine bowls and faces at upper elevations, where variable snow depths and rock bands have created shallow spots, are where you may be able to trigger a deadly slide. Although the odds are low, give that terrain a healthy amount of space today if you don't like to play Russian Roulette. Choose routes that reduce your exposure to overhead hazards, especially slopes with dangling cornices, just in case a random cornice fall bombs down onto the slope and triggers another one.
The weather has taken us for a wild ride in the past 36 hours. Freezing levels rose to over 7,000' on Friday night, accompanied by up to an inch of rain (or dense snow in the higher peaks of the Flathead and Park). Southerly and southwesterly winds gusted to over 70 mph. The avalanche danger spiked on Friday night into Saturday morning. Limited observations so far confirm that a number of large wind slabs ran in the Flathead Range, and at least one deep slab pulled out to the ground off of Mt. Cameahwait. In the Whitefish and Swan Ranges, the high freezing line submerged the ranges in rain, limiting wind transport but causing wet avalanche problems. On Saturday afternoon, a cold front passed across the region, refreezing wet surfaces and effectively ending any wet avalanche concerns. While the front brought only a dusting of new snow to the Whitefish and Swan Ranges, it favored the Continental Divide and Rocky Mountain Front with about 6" of new snow. Winds are still gusting into the 30's this morning, but should ease off by mid-morning.
In the Whitefish and Swan Range, the snowpack has quickly stabilized. Riding conditions will be crusty, with only a dusting of new snow on the surface. That dusting of snow was blowing around all night, and might have collected into some small drifts. Those drifts will be small, harmless in size, and easy to spot and avoid. They may not have formed in classic wind-loaded locations due to extreme wind speeds. Rather, they might have formed all the way in Calgary, or random pockets further downslope.
In the Flathead Range and Glacier Park, the danger is also quickly subsiding, but there are larger and more hazardous problems to manage today. Several generations of wind slabs have formed since Thursday. The oldest will be only in the highest terrain, above Friday night's freezing line. Although these should be healing rather quickly now, they will also be the thickest and most dangerous. The 6" of new snow that fell after yesterday's cold front will have drifted into wind slabs that are more common, but smaller. Avoid leeward and crossloaded drifts to manage these problems. Friday's and Saturday's warm, wet, windy, and dramatic weather was a good test on the deep weak layers that we have been advertising in our forecasts all month. They failed yet again. The likelihood of a natural or human triggered deep slab has decreased now that the weather system has passed. With unsurvivable deep slab activity as recently as yesterday, it is best to exercise patience in suspect alpine terrain today.
Cold, calm, and sunny weather is in store for the next two days. Skies will clear today from north to south. Winds are gusting into the 30's this morning at select alpine locations, but should ease through the morning hours. After yesterday's highs reached nearly 40 degrees in the mountains, temperatures have made a welcomed return to winter. Mountain temperatures are currently in the teens and low 20's.
| Sunday | Sunday Night | Monday | |
|---|---|---|---|
| Cloud Cover: | Decreasing clouds | Mostly Clear | Mostly Clear |
| Temperatures: | 18 to 24 deg. F. | 2 to 5 deg. F. | 15 to 21 deg. F. |
| Wind Direction: | West | West | North |
| Wind Speed: | 5 to 10, G20 | 0 to 10 | 5 to 15, G20 |
| Snowfall: | 0" in. | 0" in. | 0" in. |
| Snow Line: | 0' | 0' | 0' |
This forecast applies only to backcountry areas outside established ski area boundaries. The forecast describes general avalanche conditions and local variations always occur. This forecast expires at midnight on the posted day unless otherwise noted. The information in this forecast is provided by the USDA Forest Service who is solely responsible for its content.
