I'm exploring through, and it seems to be raining? Does it do this often, or is this an event in the works?

It is a regular thing. Just like the night and day change.

It happens for a random 3 hours every monday. It's the Tempest--she was a part of some event that I wasn't here for. It means you're more likely to get a random event from the Tempest.

If you do get a random event, you must click on it (important!!!), and she will give you one of a selection of storm related items, which may include a Storm Potion.

There's a ping group/discussion board in General Discussion if you want to join! People try to ping when it's raining c: