What does it mean to drop the PWL?

Graphic courtesy of Joe Stock's 2024 piece in The Avalanche Review.

You'll notice that we dropped the persistent weak layer (PWL)—that's responsible for 4 deaths since mid-February—as a problem yesterday. What we want you to know is that there is a fair bit of uncertainty around dropping a PWL from the forecast. We don't drop a PWL because it's impossible to trigger. In fact, it is literally impossible to be 100% certain that there is not a single lingering weakness. As seen in the graphic, a PWL comes with an inherently higher amount of uncertainty, as do glide avalanches—like the one that ran in Broads yesterday. Know as you travel today that uncertainty lingers in the mountains—and we deal with uncertainty by adding margins. Visiting avalanche pro Lisa Van Sciver put it well in her ob when she wrote, "While the snowpack in this is area is generally stable, in extreme terrain especially unsupportable slopes this structure should be considered..."

Uncertainty is a tough thing to communicate in avalanche forecasting; that's why multiple PhDs are studying it. An important finding so far is that explicitly stating our uncertainty in our forecasts increases trust in the avalanche center. So know: we have some surrounding the PWL. As temps skyrocket next week, know you may find it coming back into the forecast.

This information does not apply to developed ski areas or highways where avalanche control is normally done. This forecast is from the U.S.D.A. Forest Service, which is solely responsible for its content. This forecast describes general avalanche conditions and local variations always occur.