Persistent Slab Avalanches are on a weak layer buried by one or two storms or by wind deposited snow which produces after storms. With surface-hoar weak layers, can run on surprisingly gentle slopes.Persistent slabs cause more avalanche fatalities than their big brother, deep persistent slabs: 68 percent in Colorado, 47 percent in Utah, and 43 percent in Canada.
Persistent Slab week layer Avalanches
Persistent slabs are especially tricky and deadly for the following reasons:
• Instability persists long after other areas have stabilized.
• In elevated avalanche-danger conditions, are commonly triggered remotely from an adjacent slope and are especially bad news when triggered from the bottom.
• Can break wide and large, crossing terrain features and breaking into adjacent slide paths.
• No visible clues from the surface, requiring other clues or snowpits to detect them.
• Because the slabs are harder, the crown face tends to fracture above you, leaving you on the wrong side of the fracture line, unless the slab is very soft and shallow.
• With surface-hoar weak layers, can run on surprisingly gentle slopes (sometimes less than 30 degrees).
• With stiffer, older slabs, or if several days have passed since a storm, they are the classic low-probability, high-consequence avalanches—difficult to trigger but large and dangerous.
• Often produce alarm signals (collapsing or cracking) after buried by the first storm, but may produce fewer alarm signals when reactivated by subsequent storms.
• Can vary widely in both hardness and depth, so snowpit and other active tests are reliable with thinner and weaker slabs but less so with thicker or stiffer slabs; thinner slabs are more predictable than thicker slabs.
Advice and Avoidance
Persistent slabs are your basic nightmare and account for more avalanche fatalities than any other kind. The only way to deal with them is by avoidance or making very conservative terrain choices, such as staying on slopes less than 30 degrees that are not locally connected to steeper terrain. Give them plenty of extra time to stabilize after a storm—days or weeks which goes against human nature.
The cookie tastes best fresh out of the oven, but you need to be patient and wait a few days, weeks, or longer. Most people don’t. So it’s common to trigger persistent slabs that have previous tracks on them. Persistent slabs require equally persistent patience.
Deep Persistent-Slab Avalanches
If you think persistent slabs are scary, just wait until you encounter their big brother. Deep persistent slabs account for 24 percent of fatalities in Colorado, 22 percent in Utah, and 28 percent in Canada. They are simply persistent slabs that have been buried by several storms instead of just one or two; the weak layer is formed within the snowpack in the mid- or lower layers near the ground, and the slabs are stiff, hard, and deep, making them difficult to trigger but monstrous and usually unsurvivable. They commonly break to the ground—removing the entire season’s snowpack. Yikes!
Most of the bullet points from persistent slabs are true with deep persistent slabs as well, but think of them more like persistent slabs on steroids. Most of the time they are not triggered from the thick part of the slab, where extra weight is relatively insignificant, but rather from a shallow area such as near a windblown ridge, near rocks on a slope, or from the bottom, where a person’s weight can more easily initiate the collapse of the weak layer.
These are the bad boys that take out numerous previous tracks when someone tickles the right spot. Deep persistent slabs are the poster children for low-probability, high-consequence avalanches. It’s like playing Russian roulette with a pistol that has ninety-nine empty chambers and one chamber loaded with a nuclear warhead.