Arcata Police Department press release: 

A mountain lion has been spotted off of Fickle Hill road in the Arcata Community Forest. While this is not unusual, we would like residents and users of the forest to be aware and alert. If you do encounter a mountain lion here are some things to do to keep you safe:

Stay calm. Hold your ground or back away slowly. Face the lion and stand upright.

Do not approach a lion.

Give them a way to escape.

Do not run from a lion. Running may stimulate a mountain lion’s instinct to chase. Instead, stand and face the animal. Make eye contact.

If you have small children with you, pick them up if possible so they don’t panic and run. Although it may be awkward, pick them up without bending over or turning away from the mountain lion.
Do not crouch down or bend over.

If the mountain lion moves in your direction or acts aggressively:
Do all you can to appear intimidating. Attempt to appear larger by raising your arms and opening your jacket if you are wearing one. Wave your arms slowly and speak firmly in a loud voice.

If looking bigger doesn’t scare the mountain lion off, start throwing stones, branches, or whatever you can reach in its direction without crouching or turning your back.