Weak layers that continue to produce avalanches for several days or weeks after a storm.
Certain weak layers tend to stabilize quickly after a storm while other kinds of weak layers take much longer to stabilize. The three most notorious, persistent weak layer include: faceted snow, depth hoar and surface hoar. As you can imagine, persistent weak layers cause most avalanche accidents because the avalanche danger can linger several days after a storm, just waiting for a trigger.
The presence of a persistent weak layer, alone, doesn’t necessarily mean danger. But If a buried, persistent weak layer also produces a high quality shear and is also weak on your strength tests, you should definitely avoid avalanche terrain where those conditions exist.