Hear this sound one time, know what it means, and it can send a wave of fear through you:
It sounds like somebody just dropped a sack of potatoes from 50 feet high into 3 feet of powder snow.
On instinct, you will turn to see where it came from. There are no indicators: A moonscape of fresh snow extends as far as you can see up the steep mountain slopes. If you’re not scared by now, then you don’t realize the risk: The daunting power of nature may be about to unleash its ultimate winter force, an avalanche.
Truckee area: avalanche hotline, 530-587-3558, sierraavalanchecenter.org
Mount Shasta: avalanche hotline, 530-926-9613, shastaavalanche.org
Yosemite National Park: 209-372-0200, nps.gov/yose
National Avalanche Center: avalanche.org
The “whumph” noise is a warning sound that an avalanche may be imminent.
It occurs when a deep layer of light, fresh powder is piled high atop a dense layer of frozen ice beneath it — the whumph noise is the sound of that powder compressing, shifting or sliding a bit downhill. That’s how an avalanche often gets started.
Over the coming holidays and into January, anybody who ventures into the snow country away from resorts should use extreme caution for avalanche danger. This includes those who cross-country ski, snowshoe, telemark free-lance style or ride a snowmobile, especially into mountain bowls that feed up into steep slopes.
Last week, there was a shift in force, as we call it.
Just a week ago, the snow line was as high as 9,000 feet, with heavy rain on top of snow in the high country. The moisture soaked in like a giant sponge. When temperatures plummeted, it then froze into a giant ice block.
The snow level then dropped as low as 3,500 to 4,000 feet in the Sierra foothills with 15 inches (and more in some spots) of powder high in the scoop-like cirques. That light powder piled high on the ice block.
If you were up there, you might have heard a whumph or have seen an avalanche, like the one in Lee Vining Canyon east of Yosemite along Highway 120.
On steep slopes, the light powder can slide down across that ice block, gather in strength and eventually unleash a wall of hurtling snow that plunders everything in its path.
In current conditions, Class 2 slab avalanches, which are large enough to bury a person, are possible in the upper reaches of steep-sloped bowls in wilderness.
Class 3s are big enough to break trees. If it keeps snowing – and storms are forecast to pulse on and off into the New Year – there is a chance for that in some spots, especially canyons and steep, bowl-like areas that receive little sunlight.
Typical danger spots include the Old Ski Bowl high on Mount Shasta, the north-facing slope of Unicorn Peak in Yosemite, the canyon below Taboose Pass in the Eastern Sierra, and many other similar spots. Many are great destinations to trek to the ridge and then sail down on a snowboard or skis, do-it-yourself style.
Two feet of powder sitting on top of a sloped ice block can be plenty to create avalanche danger.
If a slope is facing the wind, huge drifts, or what are called wind slabs, can form. This is common at some ski parks, where ski patrol members will inspect the mountain before dawn and throw small charges into the slabs to release the pressure. If you can’t recognize a wind slab when you see one, you should not try to freelance in areas outside of ski parks.
The best advice is to wait until the snowpack stabilizes.
There’s an easy way to determine that: Dig a hole in the snow and see how much powder is piled atop the frozen layer, which is often marked by pine needles and twigs. Powder at least 2 feet deep on an ice block can sound a warning.
Wilderness rangers use a more sophisticated method, but anybody can do this:
On a slope where there is fresh powder, isolate a square of snow from 1 to 6 feet across by digging around it into the ice slab so that a square of snow sits as a separate block. Then step on it. If the snow block slides across the ice, avalanche probability is high.
With a failed test, a 25-degree slope in avalanche-prone areas can be enough for danger; 30- and 40-degree slopes, popular with maverick boarders, can be deadly.
And if you go anyway, hoping for the best, and hear the “whumph” noise, it may be too late.
By Tom Stienstra